Litter decomposition and nutrient release via litter decomposition in New Zealand eucalypt short rotation forests

Agriculture, Ecosystems and Environment 75 (1999) 133–140

Short Communication

Litter decomposition and nutrient release via litter decomposition in NewZealand eucalypt short rotation forests

L.B. Guo∗, R.E.H. SimsInstitute of Technology and Engineering, College of Science, Massey University, Palmerston North, New Zealand

Received 1 December 1998; received in revised form 19 April 1999; accepted 20 April 1999

Abstract

Litter decomposition plays a major role in the cycling of energy and nutrients in woodland ecosystems. The rates of leaflitter decomposition and the resulting nutrient (nitrogen and phosphorus) release were monitored over a 12-month periodfor Eucalyptus brookeranaand two types ofE. botryoidesleaf litter under one population density trial of eucalypt shortrotation forests. The results showed that the tree density had little influence on the rates of litter decomposition and nitrogenrelease, but had a significant effect on phosphorus release. The higher the population density, the slower the release. The totalnitrogen and phosphorus retention in the litter increased at first, particularly for phosphorus under the highest tree density(9803 trees ha−1). There were significant differences between the rates of both litter decomposition and nutrient release amongthe three studied leaf litter types. Litter dry weight loss and nutrient release were faster fromE. brookeranalitter than fromE. botryoideslitter. Autumn was the main season for litter decomposition and nutrient release. Overall, short rotation forestsshould be managed rationally based on the fluctuation of litter decomposition, and nutrient cycling in the system to ensure asustainable production system of land use. ©1999 Elsevier Science B.V. All rights reserved.

Keywords:Litter decomposition; Nutrient cycling;Eucalyptus; Short rotation forests

1. Introduction

Trees in forests absorb nutrients from the soilto support their growth. At the same time somepart of the nutrient uptake is returned to the for-est floor via litter fall. The faster the tree grows,the more litter it produces (Penfold and Willis,1961). In eucalypt short rotation forests (SRF), upto more than 10 Mg ha−1 year−1 of litter was pro-

∗ Corresponding author. Tel.: +64-6-350-4357; fax: +64-6-350-5640.E-mail address:[email protected] (L.B. Guo)

duced, and up to 140 kg ha−1 year−1 of nitrogen and8.2 kg ha−1 year−1 of phosphorus were returned tothe soil surface via litter fall (Guo and Sims, 1999).These nutrients will be released to the soil via litterdecomposition.

The litter accumulating on the forest floor providesenergy, nutrients and a living environment to the soilfauna and micro-organisms. Bargali et al. (1993) indi-cated that decomposition processes play an importantrole in soil fertility in terms of nutrient cycling andformation of soil organic matter. Litter, therefore,plays a major role in the transfer of energy and nutri-ents within a woodland ecosystem. The rate of cycling

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134 L.B. Guo, R.E.H. Sims / Agriculture, Ecosystems and Environment 75 (1999) 133–140

Fig. 1. Monthly rainfall (R) and average soil temperature (T) at 10 cm depth for the study site at Massey University, Palmerston North,New Zealand.

of nutrients through the decomposer subsystem is animportant regulator of ecosystem productivity (Swiftet al., 1979). In highly-productive plantation forestry,it becomes important to effectively manage soildevelopment, nutrient cycling and manipulation oflitter decomposition productivity (Adams and Attiwill,1986).

The slow rate of decomposition of eucalypt forestlitter, resulting in the storage of significant amountsof nutrients in the soil, has been recognised as a fea-ture of this genus (Adams and Attiwill, 1986). Amongthe factors that control rates of litter decomposition inforests, the influences of climate (temperature, humid-ity and moisture) and litter quality (e.g. lignin-to-N,C-to-N, the availability of N and P) have been welldocumented (Nagy and Macauley, 1982; Taylor et al.,1991; Attiwill and Adams, 1993; Prescott, 1996).

Therefore, the litter decomposition and nutrient re-lease via the decomposition, and their seasonal varia-tions need to be clearly understood in order to managenutrient balances for long-term sustainable land usewhen growing SRF. Little information has been pub-lished about the tree density effect on eucalypt litterdecomposition and nutrient release via litter decom-position. The objectives of this study were to inves-tigate litter decomposition and the resulting nitrogenand phosphorus release rate between various leaf lit-ter types under different tree planting densities; andto ascertain seasonal variations of litter decompositionover a 12-month period.

2. Methods

2.1. Study site

The study was conducted at Massey University,Palmerston North, New Zealand (latitude 40◦23′S,longitude 175◦37′E) where the 10-year average an-nual rainfall is 995 mm, the mean annual soil tem-perature (10 cm) is 13◦C. Monthly distributions ofrainfall and soil temperature are shown in Fig. 1.Temperatures during the period of study from July1994 till June 1995 were close to the average, butrainfall was slightly above average.

The soils are recent belonging to the Rangitikei soilsseries (Eutric Fluvisol) made up of well to excessivelydrained and gravely soils. The soil profiles are weaklydeveloped, and variable in both depth and texture.

One radial trial ofEucalyptus brookeranahad beenestablished in November 1991 after Nelder (1962)with plant densities ranging from 2000 to 20 000 treesha−1. The trial was ready for its first harvest at 4 yearsof age on completion of the monitoring period.

2.2. Experimental design and materials

A factorial experimental design with repeated mea-surements (four 3-monthly litter bag collections) wasconducted to evaluate the rates of eucalypt litter de-composition and nutrient release on the forest floor

L.B. Guo, R.E.H. Sims / Agriculture, Ecosystems and Environment 75 (1999) 133–140 135

underE. brookeranaSRF. Two factors were evaluated:tree density (2340, 4130, 9803 trees ha−1 in the radialtrial); and litter type (E. brookerana, and two types ofE. botryoidesleaf litter, one from trees irrigated withmeatworks effluent, and the other from trees withoutirrigation). The nylon mesh bag technique (Bocockand Gilbert, 1957) was used to monitor litter decompo-sition. Nylon bags (10× 15 cm) with 1 mm mesh wereused after Gallardo and Merino (1993) as they con-sidered the mesh was small enough to prevent majorlosses of the smallest leaves, yet large enough to per-mit aerobic microbial activity and free entry of smallsoil animals.

In April 1994 at Oringi, Dannevirke, New Zealand,E. botryoidesfresh litter leaves were collected from theforest floor under two 6-year-old stands, one irrigatedwith meatworks effluent at 20 mm per week and onewithout irrigation. E. brookeranafresh litter leaveswere taken from under the radial trial at the same time.Two grams of the sample of air-dried leaf materialwere weighed (to the nearest 0.01 g) and placed in eachlitter bag. The filled litter bags were placed in plasticbags for transport and storage in order to minimiseany error through spillage (Wieder and Lang, 1982).In June 1994, the litter bags were anchored on theforest floor using three replicates of four bags for eachtreatment to allow 4× 3 monthly collection during the12-month period. Overall, 108 litter bags were used.Five extra bags from each of the three litter typeswere retained in the laboratory to determine the initialmoisture content (100◦C) and chemical compositionof the litter.

2.3. Laboratory analysis

After each bag collection in September 1994,December 1994, March 1995 and June 1995, the lit-ter from the bags was carefully brushed and madefree of weed leaves, seeds and roots, tree roots,fauna and other foreign materials by hand. The litterwas weighed after overnight oven drying at 100◦C(Nicholson, 1984), then ground to pass through 1 mmsieves.

Nitrogen (N) and phosphorus (P) were analysed asfollows (Bolan and Hedley, 1987): weigh accurately0.1000 to 0.1010 g ground sample and place in a pyrestube; add 4 ml of digest mixture (consisting of 250 g

K2SO4 and 2.5 g selenium powder into 2.5 l H2SO4,heated over a gas ring for 2 h) and heat it in an alu-minium block at 350◦C for 4 h; cool the tubes anddilute with 50 ml of distilled water; use a Techni-con Auto Analyzer to determine total Kjeldahl N andtotal P.

Litter dry weight loss and nutrient release were cal-culated as follows:

L (%) = W0 − Wt

W0× 100 (1)

and

R (%) = W0C0 − WtCt

W0C0× 100 (2)

whereL is litter dry weight loss;R is nutrient release;W0 is the initial litter dry weight;Wt is the dry weightof the remaining litter in litter bag when it was col-lected;C0 is the nutrient concentration (mg g−1) in theinitial litter; Ct is the nutrient concentration (mg g−1)in the remaining litter.

2.4. Statistical analysis

The data was analyzed to show seasonal effects onlitter decomposition and nutrient release under variouspopulation densities using the SAS GLM procedure(SAS Institute, 1990).

The litter decomposition was modelled exponen-tially (Olson, 1963):

Wt = W0 e−kt (3)

whereWt is the dry weight at timet, W0 the initialleaf litter dry weight, andk the annual instantaneousdecay constant.

3. Results and discussion

3.1. Litter decomposition

In litter decomposition studies, instantaneous decayconstant (k) is commonly used to compare litter de-composition rates between species or between variousenvironments. Thek values of different litter types un-der various tree densities in the current study are listedin Table 1. The values ofkR, estimated from the re-


Table 1Annual instantaneous decay constants (k) for the decompositionrates of three leaf litter types of two eucalypt species

Litter typea Tree density k (year−1)b kRc r2

(trees ha−1)b

E. brookerana 2340 0.94 a 0.77 0.734130 0.80 a 0.67 0.769803 0.76 ab 0.61 0.71

E. botryoides-C 2340 0.35 bc 0.30 0.874130 0.24 c 0.22 0.899803 0.29 c 0.26 0.94

E. botryoides-E 2340 0.36 bc 0.31 0.964130 0.34 bc 0.28 0.679803 0.37 bc 0.29 0.93

a C = litter from trees without irrigation; E = litter from trees irri-gated with meatworks effluent.b The constantk was calculated from decomposition data at theend of the 12-month period (n= 3; means with the same letterin the column are not significantly different by Duncan grouping,P< 0.05 ).cThe constantkR was calculated from the slope of the regressionof ln (proportion of dry weight remaining) against time.

gression equations, were markedly biased by the datafrom the summer period, and hence were smaller thank calculated using data from the end of the year.

Tree density had little effect on the rates of litterdecomposition as there was no significant differencebetween the decay constants (k) of each litter typeunder the three densities monitored (Table 1). How-ever, leaf litter from the various sources gave differ-ent decomposition rates (Fig. 2). In the first 9 months,the leaf litter decomposed relatively slowly, especiallyduring summer when virtually no weight change wasmeasured during those 3 months. The decompositionrates increased in autumn, being particularly evident inE. brookeranalitter. This increase may be due to thetemperature and moisture in the season being moresuitable for the microbes which consume the litter.Overall, autumn was the season for most rapid leaflitter decomposition.

After 12 months, more than half of the original leaflitter weight was lost fromE. brookeranaleaf litter.Less than one third of the dry weight was lost fromtheE. botryoidesleaf litter both from trees without ir-rigation and irrigated with effluent. The above resultswere comparable to Ericsson et al. (1992). They re-ported decomposition and mineralisation of eucalyptleaves proceeds remarkably fast, 30% to 50% weightbeing lost during the first year, despite the sclerophyll

nature, while the soil moisture content was favourablefor microbial activity.

The k values reported for eucalypt leaf litter de-composition in other studies were 0.47 forE. dives,0.53 for E. pauciflora and 0.68 forE. delegatensis(Woods and Raison, 1983); 0.39 and 0.59 forE. obli-qua (Baker and Attiwill, 1985); and 0.54 forE. di-versicolor(O’Connell, 1987). In the current study, thek value for E. botryoideswas lower (k< 0.4) thanthe above results, whereas that forE. brookeranawashigher (k> 0.7) (Table 1). Leaf litter fromE. botry-oidestrees irrigated with meatworks effluent decom-posed faster than litter from trees without irrigation,but only significantly for the first 6 months of the pe-riod monitored (Fig. 2).

Litter decomposition can be controlled by the inter-nal physicochemical properties of the substrate and bythe external factors of the environment under whichdecay takes place (Williams and Gray, 1974; Gillonet al., 1994). Therefore, consideration of environmen-tal factors must include those which may be regardedas ‘external’ to the decomposition process (e.g., soilmoisture, temperature) together with ‘internal’ in-fluences such as the chemical composition of theleaves.

Tree population density is an external factor whichcan cause environmental change on the forest floor,such as light penetration, moisture and temperature.These in turn can affect the activity of soil faunaand micro-organisms. In the present study, the densitychange appeared to have little effect on litter weightloss even though the trees in the lowest density hadnot reached canopy closure at that stage.

Adams and Attiwill (1986) indicated the slowrate of decomposition ofEucalyptusforest litter hasbeen recognised as a feature of this genus. How-ever, Briones and Ineson (1996) did not confirm thisreputation ofEucalyptusas a recalcitrant litter sincethe decomposition of the eucalypt paralleled that ofbirch litter in terms of mass loss. In the present study,a significant difference was found between the twospecies, and even between various sources of onespecies during the first few months. The variationof leaf litter decomposition should be caused by theinternal factors, the characteristics of the litter itself.These include nutrient concentration, lignin contentand the ratios of lignin to the nutrients in the litterwhich are affected by species, and by the environ-


Fig. 2. Dry weight loss (%) from the bagged leaf litter of two eucalypt species over the 12-month period (C = litter from trees withoutirrigation; E = litter from trees irrigated with meatworks effluent;n= 9; vertical bars indicate LSD0.05).

Fig. 3. Nitrogen release (%) from the bagged leaf litter of two eucalypt species over the 12-month period (C = litter from trees withoutirrigation; E = litter from trees irrigated with meatworks effluent;n= 9; vertical bars indicate LSD0.05).

ment of the growing trees. There are over 500 speciesof Eucalyptus(Brooker and Kleinig, 1990; Boland etal., 1992), and many of them are suitable for biomassproduction. Since a variation of litter characteristicsdoes exist between eucalypt species, the species tobe planted could be selected according to the needfor a specific rate of nutrient cycling. The slow de-composition rate and resulting nutrient cycling couldbe increased by selecting a species with more easilydecomposed leaf litter.

3.2. Nutrient release from leaf litter

Nitrogen and phosphorus are usually the major nu-trients needed for plantation forests. Only the loss ofthese two nutrients from the leaf litter was monitoredin the current study.

3.2.1. NitrogenTree density had little effect on N loss from the leaf

litter during decomposition there being no significant


Fig. 4. Phosphorus release (%) under three tree population densities from the bagged eucalypt leaf litter: (a)E. brookeranalitter, (b) E.botryoideslitter from trees without irrigation, and (c)E. botryoideslitter from trees irrigated with meatworks effluent (n= 3; vertical barsindicate LSD0.05).


difference between the N loss from the bagged litterunder the three densities. The N retention in the litteractually increased in the first 9 months instead of anyloss (Fig. 3). At the end of the year, the order of Nrelease rates from the bagged litter wereE. brooker-ana> E. botryoides-E >E. botryoides-C. More N wasstill found in the bagged litter than the initial amount inE. botryoidesleaf litter from trees without irrigation.Nitrogen was released fromE. brookeranalitter par-ticularly rapidly in the autumn, dropping from 140%to 65% of the initial N amount, which was closely re-lated to litter dry weight loss (Fig. 2).

3.2.2. PhosphorusEven though tree density had no evident effects on

litter weight loss and N release rates, it did have asignificant influence on the P release rates from theleaf litter (Fig. 4). The higher the tree density, thelower the P release rate.

During the first 9 months, P was accumulated inmost of the bagged litter, especially under the high-est tree density (9803 trees ha−1). After 12 monthsof exposing the litter on the forest floor under thehighest density, only some P had been released fromthe E. brookerana leaf litter, while E. botryoidesleaf litter had accumulated more P than the initialamount. Under the lower population densities, P wasreleased in all three litter types by the end of the12-month period. Across the population densities, Pwas released faster fromE. brookeranaleaf litter thanfrom E. botryoidesleaf litter. There was little dif-ference between the two litter types within the latterspecies.

Will et al. (1983) reported tree population densityhad little or no effect onPinus radiatalitter decompo-sition rate or loss of nutrient. The results in the presentstudy also showed similar trends, but the populationdensity did have an effect on P loss: the higher thedensity, the slower the P release and the greater the Pretention.

Increases of N and P retention in the litter are fre-quently reported which can be attributed partly to mi-crobial immobilisation hence leading to redistributionof nutrients between the litter layer (Baker and Atti-will, 1985). The nutrients can be imported into the lit-ter layer from other sources, such as rainfall, through-fall, stemflow, frass from herbivores or translocation

in fungal hyphae from surface soil and lower strataof the litter layer (O’Connell and Grove, 1996). If thelitter is to be removed from the forest floor in orderto deplete N and P from the site, such as SRF whenproduction is linked with effluent land treatment, thelitter should be collected when maximum N and P re-tention occurred. If litter is to be removed from thesite and used for other purposes, such as energy con-version or composting, the litter should be collectedsoon after litter fall to minimise nutrient removal, e.g.,after peak fall during summer time in eucalypt SRF(Guo and Sims, 1999).

4. Conclusions

Tree population density had little influence on lit-ter decomposition and N release from litter, but hadsignificant effects on P release. The higher the den-sity, the slower the P release. The N and P retentionin the litter increased at first, especially for P underthe highest density. The litter from the two differentspecies and two sources ofE. botryoideshad differ-ent decomposition and nutrient release rates. Autumnwas the main season for litter decomposition and nu-trient release. An ideal species for a SRF should beselected according to the main objective of growingthe SRF to ensure a more sustainable energy produc-tion and optimum land use depending on whether thespecies had rapid or slow litter decomposition rates.Hence, the SRF should be managed rationally basedon the fluctuation of litter decomposition, and nutrientcycling within the system.

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Litter decomposition and nutrient release via litter decomposition in New Zealand eucalypt short rotation forests