Plant-Soil Interactions in Multi Strata Agroforestry in the Humid Tropics

8/3/2019 Plant-Soil Interactions in Multi Strata Agroforestry in the Humid Tropics

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Plant-soil interactions in multistrata agroforestry in the humid tropics

G. Schroth1, *, J. Lehmann2, M. R. L. Rodrigues3, E. Barros4 and J. L. V. Macdo31Biological Dynamics of Forest Fragments Project, National Institute for Research in the Amazon (INPA),

C.P. 478, 69011-970 Manaus-AM, Brazil; 2 College of Agriculture and Life Sciences, Department of

Crop and Soil Sciences, Cornell University, 909 Bradfield Hall, Ithaca, NY 14853, USA; 3Embrapa

Amaznia Ocidental, C.P. 319, 69011-970 Manaus-AM, Brazil; 4National Institute for Research in the

Amazon (INPA), C.P. 478, 69011-970 Manaus-AM, Brazil (*Author for correspondence: E-mail:

[email protected])

Key words: fertility management, nutrient cycling, spatial and temporal patterns, sustainability, tree crops

Abstract

Multistrata agroforestry systems with tree crops comprise a variety of land use systems ranging fromplantations of coffee, cacao or tea with shade trees to highly diversified homegardens and multi-storeytree gardens. Research on plant-soil interactions has concentrated on the former. Tree crop-based landuse systems are more efficient in maintaining soil fertility than annual cropping systems. Certain tree cropplantations have remained productive for many decades, whereas homegardens have existed in the sameplace for centuries. However, cases of fertility decline under tree crops, including multistrata agroforestrysystems, have also been reported, and research on the causal factors (both socioeconomic and biophys-ical) is needed. Plantation establishment is a critical phase, during which the tree crops require inputsbut do not provide economic outputs. In larger plantations, tree crops are often established together with

a leguminous cover crop, whereas in smallholder agriculture, the initial association with food crops andshort-lived cash crops can have both socioeconomic and biological advantages. Fertilizers applied to,and financed by, such crops can help to recapitalize soil fertility and improve the development condi-tions of the young tree crops. Favorable effects on soil fertility and crop nutrition have been observed inassociations of tree crops with N2-fixing legume trees, especially under N-deficient conditions. Dependingon site conditions, the substitution of legume service trees with fast-growing timber trees may lead toproblems of competition for nutrients and water, which may be alleviated through appropriate plantingdesigns. The reduction of nutrient leaching and the recycling of subsoil nutrients are ways to increasethe availability of nutrients in multistrata systems, and at the same time, reduce negative environmentalimpacts. These processes are optimized through fuller occupation of the soil volume by roots, allowinga limited amount of competition between associated species. The analysis of temporal and spatial patternsof water and nutrient availability within a system helps to optimize the use of soil resources, e.g., byshowing where more plants can be added or fertilizer rates reduced. Important research topics in multi-

strata agroforestry include plantation establishment, plant arrangement and management for maximumcomplementarity of resource use in space and time, and the optimization of soil biological processes, suchas soil organic matter build-up and the stabilization and improvement of soil structure by roots, faunaand microflora.

Agroforestry Systems 53: 85102, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands.


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Introduction

In this paper, the term multistrata agroforestrycomprises all tree crop-based land use systems

that have two or more vegetation layers andinclude more than one tree species. This includessimply-structured plantations of coffee (Coffeaspp.), cacao (Theobroma cacao), or tea (Camelliasinensis), often with a monospecific overstoreyof shade trees (Beer et al., 1998), as well ashighly diversified systems with a forest-like struc-ture, such as homegardens and agroforests(Michon and de Foresta, 1999; Torquebiau, 1992).Multistrata agroforestry systems are wide-spreadin the humid and subhumid tropics of the world,including lowland and mountainous regions (Nair

and Muschler, 1993). The simpler systems are ofmajor importance for the world markets. The morecomplex, homegarden-like systems are locallyimportant for the subsistence of smallholderfarmers and contribute to the conservation ofgermplasm diversity of tree crop species and tothe maintenance of niches for fauna and flora inthe agricultural landscape (Michon and de Foresta,1999; Perfecto et al., 1996).

Investigation of plant-soil interactions inmultistrata agroforestry systems has largely con-centrated on the less complex systems, presum-ably because of their economic importance andtheir relatively simple structure, which facilitatesresearch. The complex nature of homegarden-likesystems poses major challenges for the study ofplant-soil interactions. In his review of tropicalhomegardens, Torquebiau (1992) concluded thatthese systems possess several attributes of bio-physical and social sustainability, although manyassumed benefits are poorly quantified.

In this review, we attempt to provide anoverview of the available information on plant-soilinteractions in multistrata agroforestry systemsbased on tree crops in the humid tropics. We

analyze their benefits in comparison with lessdiverse systems, such as tree crop monoculturesand systems based on annual crops. We identifyproblems for the maintenance of soil fertilityunder tree-based land use systems and proposeways of optimizing these systems and maximizingtheir environmental benefits.

Soil fertility in tree-based cropping systems in

the humid tropics

Maintenance and decline of soil fertility under tree

crops

There is little doubt that under humid tropical con-ditions, tree crops are better suited for the main-tenance of soil fertility than annual crops (Ewel,1986). This is a consequence of the permanent soilcover and perennial root systems of the trees,which provide continuous soil protection, a favor-able environment for soil biological processes andmore efficient nutrient cycling than systems basedon annual crops. Evidence for the sustainability(including the maintenance of soil fertility) of

tropical homegardens is provided by their long-term existence on some sites (e.g., over 200 yearsin some parts of Thailand; Gajaseni and Gajaseni,1999). Although the mechanisms responsible forthis stability are poorly studied, they certainlyinclude permanent soil protection as well as addi-tions of nutrients and organic matter in domesticwastes, ashes, manure, etc. (Torquebiau, 1992).

In contrast to the complex homegardens, thedevelopment of soil fertility under simpler, tree-based systems, such as monoculture plantations oftree crops, shaded coffee and cacao plantations,has repeatedly been studied, with variable results.Kowal and Tinker (1959) found no decrease ofchemical soil fertility during 16 years under oilpalm (Elaeis guineensis) after secondary forest ona Nigerian Alfisol, except for losses of K and Mgwhich would have to be replaced by fertilizer. Soilorganic matter tended to increase. In contrast,Ollagnier et al. (1978) found decreases of total soilC to 60% and of total N to 75% of the levels underadjacent forest in oil palm plantations of up to 14years age on an Ultisol in the southern CtedIvoire. The reduced fertility levels were con-sidered adequate for oil palm, since these sites

still supported productive plantations after 60years. However, problems with the long-term pro-ductivity of oil palm were encountered on thesandy ferralitic soils of the Dabou savanna inCte dIvoire, where yields decreased by 20 to30% from the first to the second rotation. Poordevelopment of palms and a cover crop wereexplained by soil compaction and consequentlyreduced water availability and root development.

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The structural degradation was related to thereplacement of divalent by monovalent cations onthe exchange complex, caused by the heavy K fer-tilization, as well as the use of heavy machinery

in plantation management, felling and site prepa-ration for the second rotation (Caliman et al.,1987). Obviously, this degradation was morerelated to the management of the plantation thanto the crop itself.

Ayanlaja (1987) attributed declining produc-tivity of most cacao plantations in Nigeria afterabout 10 years to inadequate site selection of manyfarms. Other authors working in the same regionfound that the soil under cacao degraded progres-sively even in shaded plantations. In a 15-year-oldcacao plantation with cola (Cola nitida) shade

trees on an Alfisol in SW Nigeria, the soil undercacao and cola trees tended to have less C, P, K,Ca and Mg, and a lower pH than the soil betweenthe trees, but there was no difference between thetree species (Ekanade, 1987a). After 40 years, thesoil under cacao showed significantly lower fer-tility values than the soil under the shade trees andbetween the two tree species (Table 1). The sameauthor found less favorable physical and chemicalsoil properties on cacao farms than adjacent forest(Ekanade, 1987b). Similarly, Ahenkorah et al.(1987) reported decreasing soil C contents during16 years under cacao with different shade densi-ties (no shade or two levels ofTerminalia ivorensis

shade) and two levels each of N, P, and K fertil-izer on an Alfisol in Ghana. They mentioned thathumus loss was independent of shade intensity andfertilization (detailed data were not given),

although the shade trees seemed to have a positiveeffect on the soil P status. The reported humuslosses under cacao in Ghana were in contrast toincreasing soil organic matter contents over 10years under shaded cacao on a Typic Humitropeptin Costa Rica (Beer et al., 1990). The reasonfor the contrasting findings was certainly thatAhenkorahs experiment had been establishedafter forest, and Beers experiment after sugarcane(Saccharum sp.), indicating that shaded perennialcrops are more favorable for the soil organicmatter balance than (regularly burned) sugarcane

but less so than forest. This is consistent withsevere C losses under sugarcane in comparison toprimary forest in a Brazilian Oxisol (Woomer etal., 1994).

Do decreasing fertility parameters always

indicate unsustainability?

These examples demonstrate that, compared withthe soil under forest, certain fertility parameterscan decrease under tree crops when planted eitherin monoculture or in multistrata agroforestrysystems. Such decreases do not always indicateunsustainable soil use. When forest is converted

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Table 1. Chemical characteristics of an Alfisol under a 40-year-old plantation of cacao (Theobroma cacao) with shade trees ofcola (Cola nitida) in the Nigerian cacao belt.a

Depth Under shade Between shade Under(cm) trees trees and cacao cacao

Organic matter 0015 003.1 a 003.0 a 002.2 b(%) 1545 002.1 a 002.3 a 001.7 a

Available P 0015 009.4 a 009.2 a 005.2 b(Bray 1, mg kg1) 1545 008.0 a 008.3 a 006.0 a

Exchangeable K 0015 089 a 104 a 061 b(mg kg1) 1545 060 b 079 a 055 b

Exchangeable Ca 0015 260 a 256 a 151 b(mg kg1) 1545 237 a 241 a 188 b

Exchangeable Mg 0015 120 a 128 a 100 b(mg kg1) 1545 088 a 093 a 085 a

a From Ekanade (1987a). Values followed by the same letter in a row are not significantly different at P < 0.05 (LSD test).


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to plantation agriculture, a fertility parameter, suchas soil organic matter content, may equilibrate ona lower level, which may, however, still beadequate for the respective crop. A limited

decrease in soil fertility may not threaten long-term productivity, especially of relatively un-demanding crops such as oil palm, as long as thenutrient requirements of the crop are satisfiedthrough fertilizer application (Ollagnier et al.,1978). In contrast, fertility loss under relativelydemanding crops, such as cacao, may be moreproblematic. If fertility decline causes severeyield decreases, the area may be abandoned ortransformed into a less demanding land use, suchas extensive pasture, and a new forest plot maybe cleared for the second rotation instead of

replanting the same site. For a number of reasons,of which declining soil fertility is apparently one(Ekanade, 1987b), this pioneer strategy hastraditionally been practiced for cacao and hasmade a substantial contribution to deforestationin Latin America and Africa (Jarrige and Ruf,1990).

Thus, the sensitivity of a crop species toreduced soil fertility is one factor that determineswhether, or at which stage, fertility decline causesunsustainability. Another factor is obviously whichfertility parameters decrease. Decreasing nutrientavailability in the soil is relatively easy to correctwith mineral fertilizers, provided that these areavailable and that their application is economic.This is often not the case, especially in remoteregions, when credits are not available, and whenfluctuating weather conditions and market pricesincrease the financial risks. Decreasing soilorganic matter contents indicate loss of cationexchange capacity and organically bound nutrientsand may result in increased P fixation by Fe andAl oxides, destabilization of the soil structure andreduced soil biological activity (von Uexkll,1986). Once soil organic matter levels have

decreased, they are generally slow to recover(Webster and Wilson, 1980). Also, soil physicaldegradation is generally difficult to correct andmust be considered a threat to sustainability. Thesame is true for the build-up of nematode andother disease and pest populations. This lattertopic is discussed in a separate review (Schrothet al., 2000a).

Approaches to optimizing tree-based land use

systems

Although certain tree-based, tropical land use

systems have remained stable and productive overtime periods ranging from decades to centuries,the maintenance of soil fertility under tree cropsin the humid tropics can clearly not be taken forgranted. It is, therefore, important to identify thefactors which contribute to fertility maintenanceor decline in such systems. The equilibration ofthe nutrient balance with mineral fertilizers isan economic rather than a technical problem.However, the efficiency with which availablenutrients are taken up by crops can be improved,and unproductive nutrient losses reduced, through

the design and management of land use systems.Where suitable crop associations increase farmersincome, fertilizers become more easily accessible.The management of soil physical and biologicalproperties is more complex and much less under-stood than nutrient management, but some prin-ciples are nevertheless outlined below. The choiceof the principal crop species in a land use systemis based on socio-economic considerations andcannot easily be altered, even if the species are notoptimal from a soil fertility perspective. Moreflexibility exists with secondary species, such asshade trees, and with respect to system design andmanagement. These are, therefore, the focus of thefollowing discussion.

Maintaining and replenishing soil fertility

during plantation establishment

Tree crops usually take several years to reach com-mercial production. In particular during establish-ment, many tree species are slow-growing andrequire intensive protection against weeds, pestsand diseases. Unless the tree crops are established

under an existing tree stand, rapid soil protectionby herbaceous plants is important to reduceerosion and surface runoff, compaction of the soilsurface by direct rain impact, and humus losscaused by high soil temperature, increased decom-position rates, and reduced litter input (Sanchez etal., 1985). For tropical homegardens, the bridgingof this lag phase during which the plantation

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requires inputs but does not provide outputs hasbeen identified as an important research topic(Torquebiau, 1992).

Cover crops vs. temporary intercrops

In larger commercial plantations, the most effi-cient way to protect the soil and provide favorablegrowth conditions for young tree crops is usuallyto establish a leguminous cover crop, such asPueraria phaseoloides or Centrosema pubescens.These species rapidly cover the soil, fix up to 150kg ha1 yr1 of atmospheric N2 (Giller and Wilson,1991), suppress weeds and increase the activity ofsoil organisms such as earthworms (Agamuthu andBroughton, 1985; Sanginga et al., 1992). However,

the use of cover crops requires a certain labourinput, first for their establishment and then forprotecting the tree crops from climbing legumes.Also, vigorous cover crops may compete withthe tree crops for nutrients and water, especiallyduring the dry season (Lehmann et al., 2000b).

Smallholder farmers usually prefer to associatetheir young tree crops with annual and semi-perennial food crops such as rice (Oryza sativa),cassava ( Manihot esculenta) and bananas (Musasp.), until either the shading by the trees becomestoo intensive or the fertility of the soil, which istemporarily increased by burning the previousforest or fallow vegetation, has been exhausted(Sanchez et al., 1985; Sousa et al., 1999). Theseintercrops help to bridge the lag phase (Rodrigoet al., 2001) and can also have direct advantagesfor the initial development of the tree crops. In thecentral Cte dIvoire, the association of (fertilized)coconut palms (Cocos nucifera) with food cropssuch as cassava and yam (Dioscorea sp.) duringthe first two years, and subsequent installation ofthe cover crop Pueraria javanica, led to fasterpalm growth than when the cover crop was plantedin the first year (Zakra et al., 1986). Apparently,

the palms profited from soil tillage and weedingof the food crops, especially through improvedwater supply. Similar effects have been observedwith rubber trees on a ferralitic soil in the sameregion (Kouadio et al., 1996) and with peach palm( Bactris gasipaes) on a Typic Paleudult in thePeruvian Amazon (Szott et al., 1991). Theseobservations are, to some extent, parallel to results

from timber plantations established with theTaungya method (Kapp and Beer, 1995).

Intercrops for recapitalization of soil fertility

On acid, nutrient-poor soils, a critical period inthe development of a young tree crop plantationis often a few years after planting, when thedeclining soil fertility prevents annual croppingbefore the tree crops have attained their produc-tive phase. Sousa et al. (1999) report that oncentral Amazonian Oxisols, young tree crops suchas cupuau (Theobroma grandiflorum) are oftenplanted into a cassava crop which is grown afterslashing and burning the forest. Without fertilizingand liming, annual cropping has to be given up

after about two years, and the land is then tem-porarily abandoned. The developing fallow vege-tation is only cleared several years later when thetree crops start flowering. Such poor developmentconditions for the trees slow down their vegeta-tive growth, delay the beginning of the productionphase and reduce yields.

In order to avoid this temporary abandonmentof the area, some external inputs (especially P fer-tilizer) are required, either to prolong the annualcropping or to establish a leguminous cover crop.Under suitable market conditions, such inputs canbe financed with the products of semi-perennialspecies such as papaya (Carica papaya), passionfruit (Passiflora edulis) and banana, which canbe associated with the young tree crops. Onsome soils, fertilizers that are applied to suchdemanding, high-value intercrops can recapi-talize soil fertility for several years after thesecrops have been removed from the plot. On acentral Amazonian Oxisol, cupuau planted inassociation with papaya during the first three yearsgave earlier and greater yields than when plantedin monoculture (Figure 1). This was partly becauseof the residual fertilizer in the interrows between

the trees (Schroth et al., 2001a) and partly becauseof the improved development of the leguminouscover crop, which also profited from the increasedfertility and effectively suppressed competitivegrass weeds (G. Schroth, pers. observation). Therelatively large mineral and organic fertilizer ratesapplied to papaya (see legend to Figure 1) aremore realistic for intensive homegardening than

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for larger-scale agriculture. Thus, it may often bemore efficient to establish tree crops in small,intensively managed plots, whose area is progres-sively increased, than in larger plots with moreextensive management.

Permanent associations with food crops

Annual intercrops in young tree plantations canmake an important contribution to food produc-tion in regions dominated by tree crop agriculture.In pioneer areas, where many new plantations areestablished, the food produced in this way canexceed the demand from plantation owners and

local markets (de Rouw, 1987). On the other hand,in areas where the land is largely occupied by treecrops and few new plantations are established,food production can be limited by the availabilityof land for annual crops (Lawrence et al., 1998;Vaast, 1988). Modified planting designs for treecrops, which allow permanent intercropping withfood crops, have been proposed to avoid this sit-uation. These include cacao rows alternating with

strips of annual crops (Decazy et al., 1988) and oilpalm planted at increased spacing so that the inter-spaces can be used for food crops during the rainyseason (Daniel et al., 1996). The latter techniquealso increased tree survival during drought years.The problems of soil fertility maintenance underpermanent annual cropping will not be discussedin this review (see Smyth and Cassel, 1995).

Effects of shade trees on soil fertility and crop

yields

Tree legumes for shade and soil improvement

N2-fixing legume trees are most commonly usedas shade trees for coffee, cacao and tea (Beer etal., 1998). Some species provide useful products,e.g., fruits and firewood from Inga spp. Otherspecies, such as Gliricidia sepium and Erythrinaspp., are used mainly for their service functions,especially biological N2-fixation, production ofnutrient-rich litter and shade. According to Beer

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Figure 1. Annual yields of cupuau (Theobroma grandiflorum) in a multistrata agroforestry system and in monoculture on acentral Amazonian Oxisol. In the agroforestry system, the cupuau was associated with peach palm (Bactris gasipaes) and rubbertrees (Hevea sp.). Papaya (Carica papaya) was grown between the tree crops in the agroforestry plots from planting in 1993 until1995 and was fertilized with 420 kg N, 93 kg P, 175 kg K, 1.5 t of dolomitic lime and 36 m3 of chicken manure ha1 (totals ofthree years). Asterisks indicate significant differences at P < 0.05.


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(1988), the biological N2-fixation of legume shadetrees can reach about 60 kg N ha1 yr1, which isa relevant amount in smallholder plantations withlittle or no N fertilization. On a Tropudalf in

Brazil, total soil N was higher within about 4.5 mdistance from Erythrina glauca and E. poeppi-giana shade trees than under the surroundingcacao. Accordingly, the soil under unshaded cacaoand under cacao shaded by non-leguminous treeshad much lower total N than the soil under

Erythrina shade (Santana and Cabala-Rosand,1982). On a hydromorphic soil in the same region,significant yield increases of 10 to 19-year-oldcacao with decreasing distance from the shadetrees (8.4 to 2.5 m) could partly be explained byhigher total C, N, and exchangeable bases in the

soil close to the trees, although extractable Ptended to be higher at greater distance. The cacaogrowing close to the shade trees also had more fineroots in the topsoil and a deeper root system thanthat growing at greater distance, probably becausethe shade trees locally improved subsoil aerationwith their roots (Zevallos and Alvim, 1967).

Pronounced favorable effects of N2-fixing treeson associated tree crops have also been observedwhen regenerating a degraded coconut plantationon a poor coastal sand in the Cte dIvoire (Zakraet al., 1996). After long-term coconut cropping inassociation with cassava, without use of fertilizersand with regular removal of harvest residues, thesoil was degraded to an extent that herbaceouscover crops could not be established. When

replanting the site, Acacia mangium and Acaciaauriculiformis were arranged in double-rows alter-nating with two palm rows. TheAcacia trees werepruned every four years. Leaves and small

branches were used as mulch and larger branchesand trunks were exported from the system asfirewood. When N-fertilization of the palms wasdiscontinued in the fifth year, the associationimproved the N status and coconut production ofthe palms compared with the unfertilized control(Table 2). There were, however, concerns aboutK exports in the firewood. Casuarina equisetifolia,another N2-fixing species, was less efficient thanthe two Acacias in improving coconut yields,although it was highly suitable as a windbreak toprotect the sensitive legume trees from sea spray

(de Taffin et al., 1991).N2-fixation by legume trees, and thus the poten-tial for N transfer to the crops, is influenced bythe management of the trees, especially by shootpruning. The frequency and intensity of shootpruning of shade trees are determined by micro-climatic requirements of the crop, site fertility,pest and disease pressures, and managementintensity (including fertilizer inputs) (Beer et al.,1998; Schroth et al., 2000a). Shoot pruning andmulching recycle tree biomass and reduce theimmobilization of nutrients in the trees. Pruningalso reduces tree transpiration and thereby com-petition with the crops for soil water. However,drastic pruning, with removal of the entire leafbiomass, that is practiced in some coffee-shade

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Table 2. Effect of the N2-fixing tree species Casuarina equisetifolia, Acacia mangium and Acacia auriculiformis on theproduction of associated coconut palms (Cocos nucifera) planted in 1986 on a degraded coastal sand in the Cte dIvoire.a

Year Coconut Coconut Coconut Coconut Coconutmonocrop; monocrop; + C. equisetifolia; + A. mangium; +A. auriculiformis;no N fertilizerb with N fertilizerc no N fertilizerb no N fertilizerb no N fertilizerb

(nuts tree1)

19891990 0004.7 ab 0005.8 a 0002.1 ab 0003.4 ab 0001.1 b19901991 0038.5 a 0043.4 a 0021.4 bc 0023.6 b 0013.1 c19911992 0054.5 b 0091.5 a 0063.0 a 0067.3 a 0070.3 a19921993 0016.7 b 0053.8 a 0048.9 a 0052.4 a 0049.7 a19931994 0021.9 c 0052.3 b 0056.6 b 0071.3 a 0076.8 aTotal (per ha) 3504 8368 6113 7700 8294

a From Zakra et al. (1996) and N. Zakra, personal communication. Values followed by the same letter in the same year are notsignificantly different at P < 0.05.b In these treatments, the last N fertilization of the coconut palms was in 1990.c In the treatment with N fertilizer, the coconut palms received 225 g N tree1 year1 (as urea) from 1991 to 1994.


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tree systems, may cause nodule death and stronglyreduce nodule activity, as has been observed for

E. poeppigiana in Costa Rica (Nygren andRamrez, 1995) and for Leucaena diversifolia in

Burundi (Snoeck, 1995). More frequent partialpruning of the trees may be preferable to biannualcomplete pruning (Snoeck, 1995), although thelabour requirements may also be higher (Muschler,1999).

N2-fixation by legume trees is most likely toconstitute a relevant input to a cropping systemwhen the site is poor in N and when N fertiliza-tion is low. At high N fertilization rates (e.g., upto 270 kg N ha1 yr1 for coffee in Costa Rica), N2-fixation is insufficient to justify the selection ofeconomically worthless leguminous shade tree

species (Beer, 1988), especially as high nitratelevels in the soil typically inhibit biological N2-fixation (Giller and Wilson, 1991). Under theseconditions, legume shade trees may be selectedmainly because of their ease of pruning and rapidregrowth. High biomass production, which helpsto maintain adequate soil organic matter levels,may then be more important than N2-fixation forthe overall effect of shade trees on soil fertility(Beer, 1988).

Timber trees as shade

The legume shade trees in coffee and cacao plan-tations often do not have commercial products,although they require labour investments for estab-lishment, pruning and mulch distribution. In manyregions, coffee and cacao are traditionally grownunder the shade of timber and fruit tree species,which are either retained when clearing the forestor specifically planted (especially fruit trees; seede Rouw, 1987, on cacao plantations in the CtedIvoire). In Bahia, Brazil, the shade tree layer oftraditional cacao plantations includes valuabletimber species such as Dalbergia nigra and

Cedrela odorata as relics of the Atlantic rainforest(Johns, 1999). Non-leguminous shade trees arealso used in intensively managed plantations.Many Costa Rican coffee farmers have recentlyreplaced N2-fixing legume shade trees with fast-growing timber trees, especially Eucalyptusdeglupta. Among the advantages of this species,farmers mentioned fast growth (leading to rapidestablishment of the shade), relatively light shade,

reduced pruning requirements and (to a lesserextent) the additional value of the timber (Tavareset al., 1999).

As mentioned above, many Costa Rican coffee

plantations receive high rates of N (and other) fer-tilizer, and negative effects of the substitution oflegume by timber trees on the N balance of thesystem would not be expected. This argument doesnot hold in other coffee and cacao producingregions, where little or no fertilizer is applied(Beer, 1988). In contrast to regularly prunedlegume trees, timber trees accumulate nutrients intheir growing biomass and this accumulation tendsto be highest for the economically attractive, fast-growing species. If these nutrients are not takeninto consideration when determining fertilizer

rates, or where no fertilizer is used, nutrient avail-ability for the crops may be reduced (Beer, 1988).Negative effects of timber trees on yields of

associated tree crops have repeatedly beenreported. Under conditions of moderate fertilizerinputs and little or no water limitation in CostaRica, cocoa yields were 20% greater under E.

poeppigiana than under Cordia alliodora shadeduring the first five years, but were similar duringthe second five-year period (Beer et al., 1990).The reason for the initial yield difference is notknown, although cocoa yields seemed to be mainlylimited by light at the end of the experiment (J.Beer, pers. comm.). In the aforementioned shade-and-fertilizer experiment in Ghana, Bonaparte(1967) measured significant yield decreases of sixto seven-year-old cacao with decreasing distancefrom T. ivorensis shade trees (from 6.5 to 2.2 m),in contrast to the aforementioned yield increasesof cacao in the proximity of leguminous shadetrees (Zevallos and Alvim, 1967). Competitionwith the shade trees for soil K may have occurred,since cacao responded to K fertilization. However,Bonaparte (1967) suggested that competition forlight rather than soil resources was the main

reason for the observed yield depressions sinceyields were closely correlated with light incidence,the water supply was relatively abundant, and theyield gradient with increasing tree distancewas not influenced by fertilization. It should bementioned that this latter argument does notexclude the possibility of competition for nutrients(Schroth, 1999).

In regions with a pronounced dry season, com-

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petition for water between shade trees and treecrops can become limiting. Soil water contentsduring the five months dry season in the CentralValley of Costa Rica were lower under coffee with

E. deglupta shade than under coffee withE. poep-pigiana shade or unshaded coffee, and the coffeeassociated withE. deglupta visibly suffered morefrom drought stress than in the other two treat-ments (Jimnez and Alfaro, 1999). Similarly, inthe Abengourou region near the northern (dry)limit of the cacao-growing area of the CtedIvoire, several planters reported that, duringdrought periods, their cacao suffered more underforest tree shade than without shade. This effecthad been especially pronounced in 1983, a verydry year (G. Schroth, unpublished results of a farm

survey 1992).Competition between shade trees and crops canbe reduced through tree spacing and pruning,although pruning may be difficult for large trees.If microclimatic protection by the trees is a goal,but strong above- and belowground interactionswith the crops are to be avoided, shade trees canbe planted on the plot boundary, in contour rowsbetween plots, etc. (Schroth, 1999). For example,cacao has been planted in hollow square plant-ings or box-plots in association with timber treesor other tree crops in West Africa and Sabah(Egbe and Adenikinju, 1990; Lim, 1980), and treeplanting in shelterbelts around coffee plots hasbeen recommended for seasonally dry sites in EastAfrica (Foster and Wood, 1963).

Such arrangements may reduce, but not alwayseliminate negative root interactions. Murray(1975) noted that the use of mahogany trees(Swietenia mahagoni) as windbreaks for cacao inthe West Indies led to root competition andreduced cocoa production, extending for two orthree rows into the plantation. Such problemscould possibly be reduced by controlling thelateral root development of the trees. Attempts to

achieve this by planting grass strips with dense butlaterally restricted root systems, similar to thoseused in erosion control, along tree rows, havegiven promising results for young timber trees inCosta Rica, although the effect on older treesrequires further study (Schaller et al., 1999).

Through their litter, throughfall and stemflow,timber trees contribute to the cycling of organicmatter and nutrients; e.g., five tons of leaf litter

ha1 yr1, containing 79 kg N and 4.5 kg P, in atypical cacao farm shaded by forest trees in WestAfrica (Murray, 1975) and 5.7 t ha1 yr1 of litter,containing 114 kg N, 7 kg P and 54 kg K, fromC. alliodora shade trees in a Costa Rican coffeeplantation (Beer, 1988). However, the return oforganic material and nutrients from leguminousshade trees can be much higher: up to 20 t ha1 yr1

of biomass obtained from biannual pruningof E. poeppigiana shade trees (Beer, 1988).Accordingly, Beer et al. (1990) reported a morefavorable effect of E. poeppigiana than of C.alliodora on the soil organic matter balance ofshaded cacao plantations after 10 years.

In summary, fast-growing timber trees shouldnot be associated with (tree) crops on sites where

water and nutrient availability strongly limit cropgrowth, although they can be an option for diver-sifying the production of tree crop plantations inareas with sufficient rainfall and fertile soils, orwith adequate fertilization. In order to reduce therisk of competitive effects on the crops, e.g.,during drought years, specific planting arrange-ments which reduce interspecific interactionsshould be considered, especially for fast-growing,aggressive species.

Reduction of nutrient leaching

Nutrient leaching in tree-based land use systems

Nutrient leaching reduces the availability of nutri-ents to crops and trees. Nitrate leaching is also amajor cause of soil acidification and may causegroundwater contamination. When soil moisture isabove field capacity, leaching of soluble nutrientscan occur wherever the sum of nutrients appliedas fertilizer, released from litter and mobilized inthe soil (especially N mineralization), temporarilyor permanently exceeds the capacity of the

vegetation to assimilate these nutrients. Nutrientleaching is particularly a problem on soils withhigh infiltration rates and low nutrient retention,such as sandy soils and well-structured ferraliticsoils with low-activity clays (von Uexkll, 1986).

Nutrient leaching is not restricted to systemswith annual crops, but also occurs in tree crop-based systems. Under a fertilized coffee plantationin Kenya, accumulations of 2200 kg nitrate-N ha1

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have been found at 1 to 5 m soil depth (Michori,1993, cit. in Buresh and Tian, 1998). Nitrate accu-mulations in the subsoil have also been reportedfrom different tree crop plantations on central

Amazonian Oxisols (Schroth et al., 2000b; Schrothet al., 1999a).

The factors which influence nutrient leachingfrom tree crop-based systems in the tropics haverarely been studied in detail. Babbar and Zak(1995) reported lower N leaching rates fromshaded than from unshaded coffee cultivation inCosta Rica (9 vs. 24 kg ha1 yr1), a differencecaused by higher nitrate concentrations in the soilsolution of the unshaded plantation. However, theplantations differed also in the density and pruningregime of the coffee plants, and the data are there-

fore difficult to interpret. Imbach et al. (1989)measured lower concentrations of K, Ca, and Mgin the percolating soil water at 1 m depth in acacao plantation with C. alliodora instead ofE.

poeppigiana shade trees on a Humitropept inCosta Rica; a finding possibly due to the afore-mentioned nutrient accumulation in the timbertrees vs. a high level of nutrient recycling through

Erythrina prunings. Although the authors stressedthat there were no technical or economic problemsto the replacement with fertilizers of the relativelysmall nutrient losses, their finding illustrates thepossible influence of individual tree componentsand their management on the efficiency of nutrientcycling in multistrata systems.

Stabilizing nutrient uptake over time

Other factors which affect nutrient leaching fromcropping systems have been identified in a com-

parison of a multistrata system of cacao, bananaand C. alliodora with an annual cropping systemof maize ( Zea mays) on a Typic Dystropept inCosta Rica (Seyfried and Rao, 1991). The multi-strata system had lower nutrient concentrations inthe soil solution at 90 cm depth and lower nutrientleaching than the annual crop (Table 3), despiteapproximately similar nutrient inputs to the soilfrom fertilizer (higher under maize) and litterfall(higher under tree crops). The superior efficiencyof the multistrata system in retaining nutrients wasattributed to the perennial root systems, the con-

tinuous nutrient uptake, the presence of a litterlayer where nutrients could be adsorbed ormicrobially immobilized, and the lower percola-tion due to the continuous presence of a tran-spiring canopy. A higher soil organic mattercontent and consequently cation exchangecapacity in perennial systems should possibly beadded to this list.

These system properties indicate how tree crop-based systems should be designed to reduceleaching losses. The most crucial point may notbe that multistrata systems with various treespecies have denser root systems than monocul-tures, as the root systems of annual and perennialmonocultures can also be quite dense (Ewel et al.,1982). More important may be the fact that, at anygiven time, at least some of the plants in thesystem should be actively growing and taking up

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Table 3. Water balance and nutrient leaching in a maize (Zea mays) monocrop and a multistrata agroforestry system with cacao(Theobroma cacao), plantain (Musa sp.) and Cordia alliodora on a Typic Dystropept in Costa Rica.a

Maize fieldb Multistrata systemc

Total rainfall (cm yr1) 111 111

Actual evapotranspiration (cm yr

1

) 045 055Deep percolation (cm yr1) 066 057Leaching losses (kg ha1 yr1):

N 057 001K 003 001Ca 043 003Mg 021 003

a From Seyfried and Rao (1991).b Fertilized with 240, 55 and 40 kg ha1 yr1 of N, P and K, respectively.c Fertilized with 140, 30 and 20 kg ha1 yr1 of N, P and K, respectively.


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nutrients, thereby reducing the nutrient concen-tration in the soil solution, and should have atranspiring canopy, thereby reducing water perco-lation. This situation is probably more difficult to

achieve in a monoculture plantation than in anassociation of several tree species that differ intheir phenology, growth rhythms and responseto changing weather conditions (Ewel, 1986).Modest competition between species, leading toincreased nutrient uptake of one species when thatof competitors is reduced, could help to stabilizenutrient uptake over time and could then be anadvantage (see Doak et al., 1998). Although con-trasting root dynamics of associated cacao andshade trees have been demonstrated (Muoz andBeer, 2001), there seem to be no data in the liter-

ature to test the hypothesis of more continuousnutrient uptake in systems composed of severaltree species than in tree crop monocultures.However, the question deserves studying.

In many multistrata systems, including the onestudied by Seyfried and Rao (1991), a large per-centage of the nutrients entering the soil is derivedfrom organic sources, such as tree litter andprunings, rather than from mineral fertilizer, andthis can help to stabilize the nutrient supply overtime and further reduce nutrient losses (Lehmannet al., 1999). Snoeck (1995) applied 15N-enrichedurea or biomass from L. leucocephala or

Desmodium intortum to coffee plants on an Oxisolin Burundi and measured the distribution of theN in undecomposed biomass, coffee plants andsoil after one year (Table 4). The percentage of theapplied N that was taken up by the coffee wasmuch smaller for the biomass than for the fertil-izer, which partly may have been due to the dif-ferent amounts applied. However, whereas almost

half of the urea-N was lost from the system, pre-sumably by gaseous losses and leaching below 30cm, most of the N released from biomass was stillencountered in the topsoil, with only a minor

fraction missing.

Optimizing spatial patterns of nutrient uptake

Another possible way to reduce nutrient leachingin tree crop-based systems is by making optimumuse of the spatial patterns of nutrient availabilityin the soil. The aforementioned studies treated theinvestigated multistrata systems essentially ashomogeneous entities with respect to the quantityand nutrient load of the percolating soil solution.However, nutrient leaching from a system can

occur at spots with a temporary or permanentnutrient surplus, even if the same nutrients are inshort supply at some other spot within the system.In this situation, spatial averaging may suggest anequilibrium between supply and demand which inreality does not exist. Some degree of spatial het-erogeneity of water and nutrient availability in thesoil, and thus of nutrient leaching, is inevitable inany agricultural system, but especially so in treecrop-based systems because of the wider plantspacing compared with annual crops and the oftenlocalized fertilizer application. In agroforestrysystems composed of several different tree andcrop species, spatial patterns of nutrient cyclingalso arise from differences between species withrespect to growth, fertilizer rates, pruning inten-sities, litter quantity and quality, root distribution,water requirements, and effects on soil physicalproperties such as macroporosity.

The effect of spatial patterns of nutrient avail-ability and uptake on nutrient leaching is illus-

95

Table 4. Distribution of15N labelled nitrogen one year after application to coffee (Coffea arabica) plants, as urea or biomass ofLeucaena leucocephala or Desmodium intortum, on an Oxisol in Burundi.a

N source N applied N remaining N taken up N in soil N lostin biomass by coffee (g) (030 cm) from system

L. leucocephala 34.4b 5.9 (17%) 2.1 (6%) 25.6 (74%) 0.8 (2%)D. intortum 20.5c 2.4 (12%) 1.2 (6%) 16.4 (80%) 0.5 (2%)Urea 09.2 0 (0%) 2.1 (23%) 02.9 (31%) 4.2 (46%)

a From Snoeck (1995).b As 1225 g biomass with 2.81% N.c As 682 g biomass with 3.01% N.


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trated by the distribution of nitrate in a centralAmazonian Oxisol under a 14-year-old oil palmmonoculture (Figure 2). The mineral N in the soilwas incompletely utilized by the system, as evi-

denced by the nitrate accumulation in the subsoillikely resulting from nitrate leaching between thepalms, although the nitrate concentration in thesoil under the palms was close to the detectionlimit. This nitrate was mainly derived from themineralization of soil organic matter, as the plotshad never been fertilized with N (Schroth et al.,2000b). It seems likely that associating the palmseither with a shade-tolerant understorey crop orsmall-crowned trees would have increased N-uptake from the soil in the interspaces, reduced Nleaching, and increased the productivity of the

plot without any additional N inputs (althoughprobably at the price of additional inputs of othernutrients such as P). Non-competitive specieswould have to be used, as oil palm is sensitiveto both shading and competition for water(Jacquemard, 1995).

Spatial patterns of nitrate accumulation in thesubsoil have also been found under four tree cropspecies (cupuau T. grandiflorum, peach palm B. gasipaes, Brazil nut Bertholletia excelsaand annatto Bixa orellana) and a leguminouscover crop (P. phaseoloides) in a multistrata

system on a central Amazonian Oxisol (Schroth etal., 1999a). Despite modest rates of N fertilization,nitrate had accumulated in the subsoil under allspecies. However, significantly more nitrate was

found under the cover crop, which received no Nfertilizer, than under all tree species exceptcupuau. The inefficient N use in the system couldbe explained by analyzing the spatial patterns ofN mineralization and N uptake by the trees. Figure3 shows N mineralization rates in the topsoil of aslightly different multistrata system at the samesite. The N mineralization rates were high in allpositions of the system; however, approximatelytwice as much N was mineralized under the covercrop than under cupuau and peach palm, underwhich the soil was kept free of vegetation. Under

the rubber trees, where the soil was covered by P.phaseoloides, intermediate N mineralization rateswere found. Tracer studies indicated that cupuauand peach palm took up little N from the soilunder the cover crop, presumably because of thegenerally high N availability in the soil (Lehmannet al., 2000a). These data suggest that N leachingcould be reduced by omitting N fertilization of thetrees to increase their demand for mineral N underthe cover crop, and planting semi-perennial cropsin the spaces between the trees for as long aspossible to make use of surplus mineral N in the

96

Figure 2. Distribution of mineral N in a central Amazonian Oxisol under a 14-year-old oil palm plantation (Elaeis guineensis)at three tree distances. The palms were planted at 9 by 9 m triangular spacing. The soil was covered by spontaneous vegetationafter the leguminous cover crop had been shaded out. The plots had never been fertilized with N (from Schroth et al., 2000b).


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soil (Schroth et al., 1999a). At sites with lower Navailability in the soil, the cover crop could beallowed to grow right to the stem of the tree cropsto increase N mineralization in their rooting zone,although the effort of protecting small trees fromthis climbing legume would also increase. Thesemeasures could increase plot yields while econo-mizing N fertilizer (although, again, other nutri-ents would have to be applied to additional crops).

The spatial distribution of fertilizers (e.g., local-ized or broadcast) may influence the uptake offertilizer nutrients by tree crops, but may also havesecondary effects on tree root distribution, andtherefore on the spatial uptake patterns of waterand other nutrients from the soil. Fertilizers are

commonly applied on a weeded circle around thestem of tree crops to maximize benefits and reducecompetition from weeds or cover crops. Over theyears this practice can lead to steep fertilitygradients with increasing distance from the stem.In the oil palm plantation of Figure 2, the avail-able P concentration (Mehlich 3) between 0 and10 cm soil depth decreased from 165 mg kg1 at1 m from the stem, where the fertilizer had always

been applied, to 4 mg kg1 at 2.5 m from the stem.Over the same distance, exchangeable acidityincreased from 27 to 84% of the effective cationexchange capacity. These steep gradients wereprobably a reason for limited lateral root devel-opment of the palms and therefore inefficientexploration of the soil. A gradual increase of thefertilized area during the early years of the plan-tation would have presumably increased lateralroot spread of the palms and thus the utilizationof nitrate and water from the soil between thetrees, thereby reducing nutrient leaching (Schrothet al., 2000b).

The application of soluble fertilizer too close tothe stem of certain tree species is also inappro-

priate because of increased water infiltration, andthus leaching risk, in this zone. In the aforemen-tioned multistrata system in Amazonia, averagewater input into the soil per unit area in through-fall and stemflow in the proximity of peach palmsand Brazil nut trees was, respectively, 152% and145% of rainfall, due to the high stemflow of thesespecies (Schroth et al., 1999b). High stemflow isa common feature of palms (Lloyd and Marques,

97

Figure 3. Net soil N mineralization (010 cm) during one year under three tree crops and a cover crop in a multistrataagroforestry system (age five years) at two fertilization levels on a central Amazonian Oxisol. Pueraria fertilized had a morefertile soil than Pueraria unfertilized due to a previous papaya intercrop (see legend to Figure 1). The letters above the columnpairs indicate significant differences between sampling positions (from Schroth et al., 2001a).


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1988) which are components of many homegar-dens and other multistrata systems.

The analysis of spatial patterns of water andnutrient availability in the soil thus provides

insights into the effects of individual plant species,their spatial arrangement and management onnutrient cycling within a system, and therebyshows where nutrients are used inefficiently andare in risk of being leached. Based on such infor-mation, fertilizer can be economized or distributedmore efficiently, additional crops added or thespacing of the existing crops adjusted to achievebetter exploration of the soil volume withoutcreating severe competition. The objective wouldbe a system where modest belowground competi-tion between species guarantees efficient uptake

of water and nutrients over the whole plot area andthus little nutrient leaching. Severe root competi-tion, which depresses crop yields and may causethe death of tree crops, obviously has to beavoided. In strongly seasonal climates, this maybe achieved by including annual crop species thatare harvested before the onset of the dry season,thereby avoiding competition with the trees forwater (Daniel et al., 1996).

Nutrient recycling from the subsoil

It is a common (though poorly quantified) agro-forestry hypothesis that, in annual croppingsystems, nutrient recycling from the subsoil canbe increased by the integration of deep-rootingtrees. If the subsoil contains substantial amountsof nutrients (such as leached nitrate), is suffi-ciently moist to allow nutrient transport anduptake, and the trees have sufficient amounts offine roots in the respective depths, nutrient uptakefrom the subsoil is likely to occur (van Noordwijket al., 1996). In systems with tree crops, the inte-gration of deep-rooting trees may however not

always be an advantage. Depending on cropspecies, climate and soil conditions, the crops mayhave sufficiently deep roots themselves. Rootingdepths of 4.5 m or more have been reported forArabica coffee (Webster and Wilson, 1980) and 5to 6 m for tea (Willey, 1975). In the latter study,the tea bushes wilted later and seemed to bedeeper-rooted than the associated shade trees. Inother cases, however, there is a clear potential for

deep-rooted trees to take up nutrients from thesubsoil under tree crops, as has been shown above.

Deep nutrient capture by trees increases totalnutrient availability in the system. However, an

important difference between fallow systems andtree-crop associations is that, in the latter, thenutrients in the tree biomass do not necessarilybecome available to the crops. Nutrients arereleased from trees into the soil through prunings,litter and dying roots, or are leached from thecrown by throughfall and stemflow (Schroth et al.,2001b). However, the trees compensate theselosses through nutrient uptake from the soil toproduce new leaves and roots. Therefore, the inte-gration of deep-rooting trees into a system onlyincreases the nutrient availability in the topsoil if

the quantity of nutrients taken up from below thecrop rooting zone is greater than the quantitystored in the tree biomass (and in undecomposedtree litter) (Willey, 1975). This is most likely tooccur in soils with high subsoil fertility (which arerare in the tropics) or if the crops compete effi-ciently with the trees for nutrients in the topsoil.Competitive crops may force the trees to take upa large part of their nutrients from the subsoil andmay then scavenge these nutrients from thedecomposing tree litter.

A favorable effect of deep nutrient uptake ismost likely to occur in the case of leguminousservice trees, whose nutrient-rich leaf and branchbiomass is regularly returned to the soil asprunings, and the net nutrient accumulation in thetrees is small. For timber and fruit trees, whichare not regularly pruned, low competitiveness isparticularly important. If these trees are morecompetitive in the topsoil than the (tree) crops,their integration into a cropping system willreduce rather than increase nutrient availability,even if the trees are deep-rooting. Additional fer-tilizer may then be necessary to prevent yielddepressions.

Even if deep-rooting trees do not increasenutrient availability for associated crops, theirintegration into a system can nevertheless beuseful, because by increasing the total rootingvolume of the association, the total nutrient uptakeby the system can increase. In this case, the advan-tage of deep nutrient capture comes from addi-tional tree products (e.g., wood, fruits) and effectsnot related to nutrient cycling (e.g., improved

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microclimate or lowering of shallow water tables).Management measures for increasing the rootingdepth of trees have been discussed elsewhere(Schroth, 1999).

Conclusions

Land use systems based on tree crops, such asmultistrata agroforestry systems, have clearadvantages over annual cropping systems for themaintenance of soil fertility in the humid tropics.These include permanent soil protection, a morefavorable environment for soil biologicalprocesses which affect litter decomposition andsoil structural improvement, and more efficient

nutrient cycling. However, soil degradation undersuch systems has repeatedly been observed. Well-designed, medium- to long-term, process-orientedsoil fertility studies are necessary to enable us todefine more clearly the conditions for the sus-tainability and productivity of tree crop-based landuse systems in the humid tropics.

Soil fertility maintenance in multistrata (andother land use) systems depends on both socio-economic and biophysical factors. During theestablishment and early development of tree crops,an important socioeconomic objective is to reduceor avoid the lag phase, during which the planta-tion requires inputs but does not provide outputs.This can be achieved through appropriate associ-ations of trees with annual and semiperennialintercrops, which can be replaced after a few yearsby leguminous cover crops. Under suitable marketconditions, the returns from such intercrops can beused to finance an initial recapitalization of soilfertility through fertilizer inputs, from which thetree crops may profit for several years. Duringlater years, the diversification of the outputthrough association of different species fordomestic use and commercialization, including

valuable timber trees, is an important objective.Such measures can increase the access to externalnutrient sources and thereby help to equilibrate thenutrient balance of the system.

On the biophysical side, there are a numberof measures which could help to maintain andimprove soil fertility under tree crop-basedsystems in the humid tropics, although the effectsof few of them have been quantified. N 2-fixing

legumes have been shown to improve the N statusof the soil, as well as N nutrition and yields ofassociated tree crops on N deficient sites andunder conditions of low N fertilization. On sites

with sufficient availability of water and nutrients,legume trees can often be complemented orreplaced by timber trees, although this will oftenrequire additional fertilizer inputs and specificplanting designs to avoid competition with thecrops, especially during drought years. The reduc-tion of nutrient leaching and recycling of subsoilnutrients by deep-rooting trees can improve theavailability of nutrients in the system and reducenegative environmental impacts. However, infor-mation about the spatial and temporal patterns ofwater and nutrient availability is required to design

effective measures; e.g., optimal spacing of thetrees, inclusion of additional crop species, alteredmanagement of a cover crop, or reduced quanti-ties and altered distribution of fertilizer. Suchmeasures can be combined to create a limitedamount of competition between the plants in asystem, which is necessary to achieve optimumuse of soil resources in both space and time.

Measures which help to equilibrate the nutrientbalance of a system will not necessarily avoidproblems of soil organic matter loss and soil struc-tural degradation. The latter can be addressedthrough measures such as permanent soil cover, alow degree of mechanization, stimulation of plantroot development, and the production of largequantities of litter and prunings, which help tomaintain a high level of soil microbial and faunalactivity. In many cases, the negative effects ofcertain tree crops on the soil can probably bereduced or avoided by associating them with treesand cover crops with a more favorable effect(Vohland and Schroth, 1999).

The conduction of soil research in multistrataagroforestry systems poses methodological diffi-culties, which are partly related to the spatial and

temporal complexity and heterogeneity of thesesystems. A synthesis of two complementaryapproaches seems promising: the comparison ofcomplex multistrata systems with simpler annualand perennial cropping systems on the whole-plot level; and the analysis of within-plot,spatio-temporal patterns of soil fertility andnutrient cycling in complex associations, similarto the analysis of single-tree effects in forest and

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savanna ecology (Kellman, 1979; Zinke, 1962).The first approach is necessary for evaluating thetotal economic and environmental costs andbenefits of a certain system type, but the results

are very specific to the systems studied and givelittle information on how their characteristicswould be affected if one crop species or manage-ment measure was exchanged for another, or howthe systems could be improved with respect to acertain characteristic. The second approach(single-tree effects) provides insights into theeffects of certain plant species or managementmeasures on plant-soil processes within tree crop-based land use systems. Species and managementpractices can then be identified to manipulate thesystems properties into a certain direction; e.g.,

towards more efficient nutrient cycling or soilcarbon enrichment in order to improve produc-tivity and sustainability. The results may some-times be more difficult to scale up than those fromthe first approach, but they are easier to transforminto practical recommendations.

Thematically, the following questions deserveparticular attention:

1. How can multistrata systems be establishedmost efficiently under conditions of infertilesoils, especially when limited availability ofmarkets and credits makes access to fertilizersdifficult?

2. How can crop and tree species be arranged andmanaged for maximum complementarity ofresource use in space and time, especially inseasonal climates and under conditions ofperiodic drought?

3. How can soil biological processes, such as soilorganic matter build-up and the improvementand stabilization of soil structure by roots,fauna and microflora, be optimized in tree-based land use systems?

Acknowledgements

Much of the research, on which this review isbased, has been funded by the German Ministryof Education and Research (BMBF) togetherwith the Brazilian Conselho National de Desen-volvimento Cientfico e Tecnolgico (CNPq) aspart of the SHIFT programme at Manaus, projects

0339457/ENV23 and 0339641/ENV45, and by theGerman Society for Technical Cooperation (GTZ)under its Tropical Ecology Support Programme(BN 96.2151.7-00.107) at CATIE, Costa Rica.

John Beer, PK Nair and two anonymous refereesmade useful comments on earlier versions of themanuscript.

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Plant-Soil Interactions in Multi Strata Agroforestry in the Humid Tropics