SOILS, SEC 2 • GLOBAL CHANGE, ENVIRON RISK ASSESS, SUSTAINABLE LAND USE • RESEARCH ARTICLE

Contrasting decomposition rates and nutrient release patternsin mixed vs singular species litter in agroforestry systems

Yikun Wang & Scott X. Chang & Shengzuo Fang & Ye Tian

Received: 26 August 2013 /Accepted: 13 January 2014 /Published online: 11 February 2014# Springer-Verlag Berlin Heidelberg 2014

AbstractPurpose The rate of litter decomposition can be affected by asuite of factors, including the diversity of litter type in theenvironment. The effect of mixing different litter types ondecomposition rates is increasingly being studied but is stillpoorly understood. We investigated the effect of mixing eitherlitter material with high nitrogen (N) and phosphorus (P)concentrations or those with low N and P concentrations onlitter decomposition and nutrient release in the context ofagroforestry systems.Materials and methods Poplar leaf litter, wheat straw, peanutleaf, peanut straw, and mixtures of poplar leaf litter-wheatstraw, poplar leaf litter-peanut leaf, and poplar leaf litter-peanut straw litter samples were placed in litter bags, and theirrates of decomposition and changes in nutrient concentrationswere studied for 12 months in poplar-based agroforestry sys-tems at two sites with contrasting soil textures (clay loam vssilt loam).Results and discussion Mixing of different litter types in-creased the decomposition rate of litter, more so for the sitewith a clay loam soil texture, representing site differences, andin mixtures that included litter with high N and P concentra-tions (i.e., peanut leaf). The decomposition rate was highest inthe peanut leaf that had the highest N and P concentrationsamong the tested litter materials. Initial N and P

immobilization may have occurred in litter of high carbon(C) to N or C to P ratios, with net mineralization occurring inthe later stage of the decomposition process. For litter mate-rials with a low C to N or P ratios, net mineralization andnutrient release may occur quickly over the course of the litterdecomposition.Conclusions Non-additive effects were clearly demonstratedfor decomposition rates and nutrient release when differenttypes of litter were mixed, and such effects were moderated bysite differences. The implications from this study are that itmay be possible to manage plant species composition to affectlitter decomposition and nutrient biogeochemistry; mixedspecies agroforestry systems can be used to enhance nutrientcycling, soil fertility, and site productivity in land-use systems.

Keywords Agroforestry . Litter decomposition .Mixing .

Non-additive effect . Nutrient release

1 Introduction

Litter decomposition is a critical step linking ecosystem pro-cesses with plant productivity (Edmonds and Tuttle 2010).Nutrients released through litter decomposition become avail-able for plant uptake and thus support ecosystem productivity;as such, litter decomposition is a critical part of the biogeo-chemical cycle of elements in terrestrial ecosystems(Mcclaugherty et al. 1985; Berg 2000). The rate of litterdecomposition also affects ecosystem carbon (C) balanceand storage (Rubino et al. 2007; Edmonds and Tuttle 2010),and ultimately the global C cycle and the climate system. Dueto the importance of litter decomposition in the function ofterrestrial ecosystems, factors affecting litter decompositionhave been extensively studied (Melillo et al. 1982; Coûteauxet al. 1995; Aerts 1997). For example, litter decomposition isknown to be affected by litter quality, such as the initial

Responsible editor: Chengrong Chen

Y. Wang : S. Fang (*) :Y. TianCollege of Forest Resources and Environment, Nanjing ForestryUniversity, 159 Longpan Road, 210037 Nanjing, People’s Republicof Chinae-mail: fangsz@njfu.edu.cn

S. X. Chang (*)Department of Renewable Resources, University of Alberta,Edmonton, Alberta T6G 2E3, Canadae-mail: scott.chang@ualberta.ca

J Soils Sediments (2014) 14:1071–1081DOI 10.1007/s11368-014-0853-0

nutrient concentrations, lignin content, and C to nitrogen (N)ratio (Berg 1986; Melillo et al. 1982; Song et al. 2009), andenvironmental conditions such as soil temperature, aeration,and moisture availability (Swift et al. 1981; Cortez 1998;Coulis et al. 2013).

Most litter decomposition studies have been conductedusing litter of the same species (Fogel and Cromack 1977;Kasurinen et al. 2006; Duboc et al. 2012). The decompositionrates of litter with different initial N concentrations can be verydifferent under the same environmental conditions (Zhanget al. 2008a). In natural ecosystems, litter from different spe-cies returns to the ground and forms a mixture. Such littermixtures may dramatically change the quality of litter and canmarkedly change the decomposition rate as well as the nutri-ent release pattern, and those ultimately affect the availabilityas well as the timing of the release of nutrients in the soil (DeMarco et al. 2011; Lecerf et al. 2011). However, the effect oflitter mixing on litter decomposition will likely be influencedby the kind of litter involved which will be site specific. Assuch, understanding the rate of decomposition of mixed litterbecomes very important and the decomposition of mixed litterhas increasingly become a subject of research in the lastseveral years (Lecerf et al. 2011; Pérez-Suárez et al. 2012;Jiang et al. 2013). In forest plantation systems, opportunitiesfor mixing of litter of different quality arise when mixedspecies plantations are established or when litter from treesis mixed with that of understory vegetation. Opportunities formixed species litter to form are planting of trees of differentspecies and planting of tree species with different crop speciesin agroforestry systems.

Agroforestry systems are considered an alternative land-use system that increases the land-use intensity and diversifiesthe farm economy as compared with either monoculturalagricultural or plantation systems (Cubbage et al. 2012). Ag-roforestry systems are widely practiced in temperate Chinaand in northern Jiangsu province. One such system uses fast-growing hybrid poplars as the tree species to intercrop withannual crops (Fang et al. 2010). Crops grown in agroforestrysystems can include legumes and non-legumes. To make thesystem sustainable, it is essential to maintain the rate of litterdecomposition and use that as one of the means to meet part ofthe nutrient requirement of crops and overstory tree species(Zeng et al. 2010). However, the effect of mixed forest treelitter and annual crop litter on the rate of litter decompositionand nutrient release has not been studied in those agroforestrysystems.

The effect of mixing litter of different species or qualitymay also be strongly affected by the soil type, such as the claycontent in the soil, as the surface area of soil particles cansubstantially affect biological activities (Kooijman et al.2009). In order to better understand the interactive effects oflitter type mixing and soil condition on litter decompositionrates, and thus the release of nutrients, we conducted a field-

based litter decomposition study at two sites with different soiltextures to test the following three hypotheses: (1) Leaf litterdecomposition rates will be increased when tree litter is mixedwith litter from legume crops that has a higher N concentra-tion; in other words, the decomposition rate in a balanced,two-species mixture will be greater than the mean rate of thetwo species when studied separately; (2) soils with a higherclay content will support a higher litter decomposition rate dueto the beneficial effect of clay particles on increasing thesurface area for microbial activities and on increasing waterretention and soil aggregation; such effects will help increasethe rates of litter decomposition; and (3) the pattern of nutrientrelease follows that of litter decomposition, i.e., the litter withhigh N concentrations or the litter mixed with another litterwith a high N concentration will have a higher nutrient releaserate as compared with the mean release rate of the two specieswhen studied separately.

2 Materials and methods

2.1 Study sites

The research reported in this paper was conducted in twolocations in northern Jiangsu province: (1) Chenwei forestfarm (118°36′ E, 33°32′ N) in Sihong county. This locationhas a humid-subhumid warm temperate climate, with a meanannual temperature of 14.4 °C, mean annual precipitation of972.5 mm, 197 days of frost-free period, and 2,095.2 sunshinehours per year (Wang et al. 2010). The soil at this location wasformed on fine sediments of the Hongzhe Lake and has a clayloam texture (Table 1, based on the International Union of SoilScience classification system, same below); (2) Dafeng forestfarm (120°45′ E, 33°02′ N) in Dafeng county. This locationhas a subtropical coastal monsoon climate, with a mean an-nual temperature of 14 °C, a mean annual precipitation of1,058.4 mm, 213 days of frost-free period, and 2,255.6 sun-shine hours per year (Zhang et al. 2010). This site was formedabout 50 years ago from sea sediments and has a silt loamtexture (Table 1). The clay loam soil in Sihong had a lowermineral N concentration, but otherwise, the other soil proper-ties such as organic C and total N, phosphorus (P), andpotassium (K) contents were rather similar among the twosites (Table 1).

The planted hybrid poplar, Nanlin-95 (Populuseuramericana cv. “Nanlin95”), was the same at both sites.This hybrid poplar was a hybrid (F1 clonal) of poplar I-69(Populus deltoides Bartr. cv. “Lux”) and poplar I-45(P. euramericana (Dode) Guineir cv. “I-45/51”). The treeswere planted at a 6×6 m spacing and were intercropped withpeanut (Arachis hypogaea var. Tai-Hua No. 3) or wheat(Triticum aestivum Linn var. Qian-Mai No. 1). When theexperiment began, the trees were 5 years old.

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2.2 Soil and plant sampling

Soil samples were collected using a soil auger (3.5-cm diam-eter). In each plot, a composite soil sample was collected fromthe 0–20-cm depth by sampling from 15 random points fol-lowing an S curve/pattern. Each soil sample was air-dried,sieved to pass a 2-mm screen, and stored for the analysis ofsoil physical and chemical properties. Wheat straw was col-lected in June 2010, peanut leaf and straw samples werecollected in September 2010, and poplar leaf litter was col-lected in November 2010. All plant materials used for thisexperiment were collected from the Chenwei forest farm; theywere air-dried for use for this experiment. Differences be-tween poplar leaf litters from the two sites were statisticallynon-significant, and using the same plant material allowed usto focus this study on the effects of the site conditions on litterdecomposition.

2.3 Litter decomposition experiment

Litter decomposition rate was determined using the litterbag method. Three blocks of areas were selected in eachstudy site (Chenwei and Dafeng), and within each block,seven litter composition treatments were installed: (1)poplar leaf litter (PL), (2) wheat straw (WS), (3) peanutleaf (PeL), (4) peanut straw (PeS), (5) poplar leaf litter-wheat straw mixture (PL-WS), (6) poplar leaf litter-peanutleaf mixture (PL-PeL), and (7) poplar leaf litter-peanutstraw mixture (PL-PeS). In the experiment, each of themixed litter treatment was based on a 1:1 (mass to mass)ratio. The poplar leaf litter had the highest C and inter-mediate N contents, the peanut leaf had the lowest C andthe highest N contents, while the wheat straw had thelowest N content (Table 1). Those litter materials thus hadthe following orders in terms of their C to N ratio: wheat straw(91) >> peanut straw (40) ≈ poplar leaf litter (38) >> peanut leaf(18). The peanut leaf and straw had higher P contents while thewheat straw had a higher K content than the other litter types(Table 1).

Litter bags 15×200 cm in size with a 2-mm mesh openingsize were used for this study. Each bag contained 15 g (oven-dry weight at 60ºC) of litter. Litter bags were installed in thefield in January 2011. Litter bags were randomly distributedwithin each block at each site. Each litter bag was securedusing a metal peg to ensure that the litter bag was in fullcontact with the soil surface. Within each block, five bagsfor each treatment were installed to accommodate the fivesampling times described below.

One litter bag for each treatment was retrieved from eachblock and each site in April, July, and November 2011 andJanuary 2012 to represent litter after 3, 6, 10, and 12months ofdecomposition, respectively. The initial litter was assumed tobe 100 % undecomposed at time zero. After the litter bagswere retrieved from the field, they were cleaned of soils bybrushing. The remaining litter in each litter bag was dried in anoven at 60 °C until constant weight and weighed. The litterwas then ground to pass a 0.5-mm sieve to determine theconcentrations of N, P, and K as described below.

2.4 Soil and plant chemical analysis

Soil pH was measured using a pH meter based on a 1:2.5(mass to mass) soil to water ratio. Soil particle size distributionwas determined by a laser soil particle size analyzer (Rise-2008). Soil mineral N concentrations were measured with aflow-injection autoanalyzer (Bran+Luebbe AA3) after the soilsamples were extracted with a 2 mol L−1 KCl solution. Or-ganic C contents in soil and plant materials were determinedusing a total organic carbon analyzer (Elementar LiquiTOC,Germany).

Soil and plant materials were digested using 5 mL concen-trated H2SO4 and 1 mL concentrated HClO4 for 30 min at120 °C. The digestion continued at 360 °C until the digest isclear. After digestion, the N and P concentrations in the digestwere determined using a flow-injection analyzer (Bran+Luebbe AA3). The K concentration in the digest was analyzedusing a Hitachi 108-80 atomic adsorption spectrometer(Hitachi Ltd., Japan).

Table 1 Basic physical and chemical properties of the tested soils and plant litter materials from Sihong and Dafeng (mean±SD, n=3)

Site orlitter type

Soiltexture

pH Clay content(%)

Silt content(%)

Mineral N(mg kg−1)

Organic C(g kg−1)

Total N(g kg−1)

C:Nratio

Total P(g kg−1)

Total K(g kg−1)

Sihong Clay loam 7.38±0.15 24.84 41.56 36.45±2.11 13.84±1.37 1.48±0.12 9.5±2.0 0.51±0.01 11.24±0.22

Dafeng Silt loam 8.02±0.09 14.76 76.30 47.62±1.46 12.65±0.66 1.67±0.04 7.6±0.1 0.73±0.01 14.05±0.14

Poplar leaf litter /a / / / / 482.65±7.8 12.85±0.18 37.6±1.2 0.94±0.04 6.88±0.25

Wheat straw / / / / / 448.94±12.5 4.94±0.51 91.8±9.9 0.77±0.08 21.13±0.57

Peanut leaf / / / / / 373.89±15.7 21.24±0.76 17.7±1.0 1.75±0.02 4.77±0.11

Peanut straw / / / / / 418.12±8.2 10.37±0.24 40.5±3.6 1.24±0.06 2.84±0.14

a Not applicable or determined

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2.5 Calculations and statistical analysis

The percent of litter remain (Y) was calculated as

Y ¼ X t=X 0 � 100

where X0 is the initial litter sample weight and Xt is the amountof litter remaining after tmonths of incubation in the field.

The litter decomposition process was modeled using thenegative exponential decay model of Olson (1963), in theform of Xt/X0=e

−kt, which in practice is usually expressed asY=ae−kt, where a is a fitting parameter and k is the annualdecay rate (gram per gram per year).

The effect of mixing of litter material from different specieson litter decomposition was assessed by calculating the ex-pected value for the percentage of litter remaining (Ye) fol-lowing the method used by Lin et al. (2006):

Ye ¼ measured Y for PL aloneð Þ � Aþ measured Y for crop straw=leaf aloneð Þ � B½ �= Aþ Bð Þ

whereAandB refer to the proportion of PL and crop straw/leaf inthe litter bag with mixed litter, in this study both were 50 %.WhenYe=Y, there was no interaction between the two litter typesmixed;whenYe>Y, therewas a suppressing effect bymixing twodifferent litter types; and when Ye<Y, there was a positive effectof mixing two litter types on litter decomposition rates.

In assessing nutrient release through litter decomposition,the percentage of nutrient loss was calculated as (initial nutri-ent content−sample nutrient content at a sampling time)/initial nutrient content×100. Nutrient content is the productof nutrient concentration and the amount of litter.

Statistical analysis was performed using the SPSS (version11.5) software. The analysis of the relationship between theweight of litter remaining and time since the beginning of litterdecomposition was based on Olson’s (1963) exponential mod-el, with regression equations developed for each site (n=15).The predicted rate of litter decomposition for the mixed littertreatments was calculated based on an additivemodel; when thepredicted value was significantly different from the observedvalue, then there was a non-additive effect of litter mixing onlitter decomposition rate. The observed and predicted values ofthe percent weight remaining for mixed residue types in theinitial and late stages of litter decomposition were subject to ananalysis of variance (ANOVA) using a two-way ANOVA totest the differences between the observed and predicted valuesof the percent weight remaining and among the mixed residuetreatments. Linear regression analysis was performed to under-stand the relationship between the rate of litter decompositionand N or P concentrations in the litter.

3 Results

3.1 Litter decomposition rate

The temporal dynamics of the amount of litter mass remain-ing was rather consistent between the two sites (Fig. 1),

where the percentage of litter mass remaining of the peanutleaf decreased the fastest over time, while that of the wheatstraw decreased the slowest, with poplar leaf litter and PL-WS having the second slowest decrease rate. Between thetwo sites, the percentage of litter mass remaining at the endof the 12-month incubation was lower in Dafeng (silt loam)than in Sihong (clay loam). The decomposition rates werevery well described by Olson’s (1963) exponential model,for both site, with R2 values ranging from 0.94 to 0.99(Table 2). The decomposition rate (per year) was slowestfor the poplar leaf litter and wheat straw samples; peanutstraw had a decomposition rate similar to that of poplar leaflitter and wheat straw, while peanut leaf had the highestdecomposition rate (Table 2). As a result, the time (in years)for 50 and 95 % decomposition was longest for poplar leaflitter, wheat straw, and peanut straw and was shortest forpeanut leaf. When the slow-to-decompose poplar leaf litterwas mixed with either the peanut straw or peanut leaf, thedecomposition rate of the mixture substantially increased.Our data indicated that when the poplar leaf litter wasmixed with either wheat straw, peanut leaf, or peanut straw,the time to 50 or 95 % decomposition of the litter mixtureall decreased, particularly when the poplar leaf litter wasmixed with peanut leaf (Table 2).

When evaluating the effect of mixing wheat straw, peanutleaf, or peanut straw with poplar leaf litter on litter decompo-sition, it was shown that in the initial stage of litter decompo-sition (on or before April 2011), the observed and predictedvalues of the percent weight remaining were not differentregardless of the type of litter mixed with the poplar leaf litteror the study site (Table 3). However, in the late stage of litterdecomposition (in early January 2012), the observed percentweight remaining was always lower than the predicted valuesregardless of the type of litter mixed with poplar leaf litter orthe study site (Table 3). The predicted values were based onthe assumption of no synergistic effect of mixing the differenttypes of litter. Therefore, the significantly lower-than-

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observed predicted values indicated that there was a positiveeffect of mixing on the rate of litter decomposition.

The rate of litter decomposition was significantly correlat-ed with the N and P concentrations (Fig. 2), but not with the Kconcentration in the litter (data not shown). The slope of theregression lines was steeper in Dafeng than that in Sihong,regardless of the nutrient involved (Fig. 2). Figure 2 showsthat if we remove the PeL and PL + PeL samples, there was norelationship between litter decomposition rate and either theconcentration of N or P in the litter when the initial N and Pconcentrations were in a narrow range.

3.2 Release of nutrients during litter decomposition

In terms of the release of N during litter decomposition, thecumulative rate of N release in the first 3 months was higher in

the PeL, PL-PeL, PeS, and PL-PeL treatments than in theother three treatments regardless of the study site; there wasa net immobilization of N in the PL, WS, and PL-WS treat-ments (Fig. 3). Between the 10th and 12th month, the cumu-lative N release rates were markedly different between thePeL, PL-PeL, and the other treatments; at the Sihong site, thecumulative N release rates were still separated between thePeS, PL-PeS, and the other treatments, but at the Dafeng site,the differences between the other five treatments diminishedtowards the end of the 12-month study period.

The cumulative release rate (and dynamics) of P (Fig. 4)was very similar to that of N (Fig. 3), with the highest releasein the PeL and PL-PeL treatments, intermediate in the PeS andPL-PeS treatments, and lowest in the WS, PL, and PL-WStreatments. Similar to the dynamics of N release during litterdecomposition, there was a net immobilization of P in the first3 months in the PL, WS, and PL-WS treatments.

For the cumulative release of K, the general trend wassimilar to that of N or P, with the highest cumulative K releaseobserved in the PeL and PL-PeL treatments (Fig. 5). The mostinteresting observations were the lack of any net immobiliza-tion of K in any of the treatments throughout the study periodand that the lowest cumulative K release was in the PeStreatment rather than in the WS treatment as was the case forN and P (Figs. 3, 4, and 5).

4 Discussion

The higher decomposition rates of peanut leaf and PL-PeLthan in the other litter types indicate that the rate of decompo-sition was related to the N and P concentrations in the littersamples because peanut leaf samples had high N and P con-centrations (Melillo et al. 1982; Coûteaux et al. 1995; Aerts1997). The rate of litter decomposition is directly affected bylitter quality, such as the N and P concentrations in the litter, ashigher litter N and P concentrations enhance microbial activ-ities and litter decomposition (Berg 1986; Polyakova andBillor 2007). In this study, litter decomposition rates weresignificantly correlated with the N and P concentrations inthe litter (Fig. 2), indicating the influence of litter N and Pconcentrations on the rate of litter decomposition. However,using litter quality to predict litter decomposition has itslimitations; for example, Prescott et al. (2004) showed thatthe initial nutrient concentration of litter was closely relatedwith the first year decomposition rate of litter, and as the litterdecomposition progressed, the relationship weakened. Whenthe lignin content in the litter is greater than 20 %, the C to Nratio of the litter does not reflect the litter decomposition rate(Moretto et al. 2001). A recent study suggested that, in theearly stage of litter decomposition, the initial litter qualitycontrols the rate of litter decomposition and microbial com-munity composition, while in the later stage of litter

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Fig. 1 Changes in the dry weight of litter remaining (mean±SD) overtime under different treatments at Sihong andDafeng.PLpoplar leaf litter,WS wheat straw, PeL peanut leaf, PeS peanut straw, PL-WS poplar leaflitter-wheat straw, PL-PeL poplar leaf litter-peanut leaf, PL-PeS poplarleaf litter-peanut straw

decomposition, the composition of the microbial communityplays a greater role in controlling litter decomposition rate(Bray et al. 2012). In the early phase of litter decomposition,water-soluble substances and holocellulose unshielded bylignin are degraded; however, decomposition of celluloseshielded by lignin is ruled by lignin degradation in the laterstage of litter decomposition (Berg 1986, 2000).

In this study, all litters had the highest decomposition ratesin the period April–November, suggesting more favorableenvironmental conditions in that period than in the other twoperiods (January–April; November–January), as environmen-tal conditions can markedly influence the rate of litter decom-position (Coûteaux et al. 1995; Aerts 1997). This difference indecay rates during different times of the year accounts for the

anomalously high intercepts, above 100 %, in the regressionequations (Table 2). The rate in the initial stage of litterdecomposition (i.e., the first 3 months) was relatively lowacross the treatments and study sites (Fig. 1); this was likelyaffected by a range of factors: (1) the breakdown of the litter toform smaller-sized litter particles takes time and the decom-position rate increases with the decreasing litter particle size(Loecke and Robertson 2009); (2) it takes time for the micro-bial populations to colonize the litter; and (3) the low temper-ature in the initial stage of the litter decomposition in thisstudy would be the key factor affecting the initial rate of litterdecomposition as the litter bags were installed in January, thecoldest month of the year in the study region; temperature isknown to be an important factor regulating litter

Table 2 Regression equations based on Olson’s (1963) exponential model to describe litter decomposition at Sihong and Dafeng (n=15)

Residuetype

Regressionequation

Correlationcoefficient (R2)

Decompositionrate (year−1)

Time for 50 %decomposition(year)

Time for 95 %decomposition(year)

Sihong

Poplar leaf litter (PL) Y=111.9e−0.83ta 0.98 0.83 0.97 3.74

Wheat straw (WS) Y=127.3e−0.91t 0.95 0.91 1.03 3.56

Peanut leaf (PeL) Y=150.5e−2.19t 0.99 2.19 0.50 1.55

Peanut straw (PeS) Y=115.3e−1.03t 0.99 1.03 0.81 3.03

Poplar leaf litter-wheat straw (PL-WS) Y=129.5e−1.14t 0.98 1.14 0.84 2.86

Poplar leaf litter-peanutleaf (PL-PeL)

Y=143.7e−1.87t 0.99 1.87 0.57 1.80

Poplar leaf litter-peanut straw (PL-PeS) Y=113.8e−1.05t 0.99 1.05 0.78 2.97

Dafeng

Poplar leaf litter (PL) Y=107.8e−0.88ta 0.99 0.88 0.87 3.48

Wheat straw (WS) Y=131.8e−1.06t 0.97 1.06 0.92 3.09

Peanut leaf (PeL) Y=221.4e−3.47t 0.99 3.47 0.43 1.09

Peanut straw (PeS) Y=108.6e−1.06t 0.99 1.06 0.79 3.12

Poplar leaf litter-wheat straw (PL-WS) Y=122.1e−1.15t 0.99 1.15 0.77 2.77

Poplar leaf litter-peanut leaf (PL-PeL) Y=143.3e−2.10t 0.99 2.10 0.50 1.60

Poplar leaf litter-peanut straw (PL-PeS) Y=111.1e−1.12t 0.94 1.12 0.71 2.77

a Y is the weight of the litter remaining after a certain period of litter decomposition and t is the time since the beginning of litter decomposition

Table 3 Observed and predicted values of the percent weight remaining (mean±SD) for mixed residue types in the initial and late stages of litter decomposition

Site Mixing treatment Initial stage of decomposition (early April 2011) Late stage of decomposition (early January 2012)

Observed Predicted Observed Predicted

Sihong PL-WS 94.7±0.82a 93.6±1.33a 42.3±3.28a 50.9±1.52b

PL-PeL 84.9±5.82a 85.8±1.67a 20.9±3.68a 32.9±1.19b

PL-PeS 87.3±2.10a 90.2±1.48a 39.9±2.71a 45.5±0.61b

Dafeng PL-WS 92.2±0.25a 91.5±2.48a 39.4±1.91a 45.5±1.33b

PL-PeL 88.2±2.37a 85.3±2.76a 17.6±0.97a 25.7±1.73b

PL-PeS 91.8±1.47a 86.7±3.12a 38.8±1.00a 41.5±1.15b

PL poplar leaf litter, WSwheat straw, PeL peanut leaf, PeS peanut straw

Different lowercase letters indicate significant differences between the observed and predicted values at P<0.05 for each mixing treatment

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decomposition rates (Aerts 1997; Cortez 1998). As the litterdecomposition process progresses, the rate of litter decompo-sition would become controlled by other factors such as litterquality to a greater degree and thus, the differences betweenthe different types of litter becamemore pronounced at the endof the 12-month study (Fig. 1; Table 3); this was consistentwith many other experiments reported in the literature(Coûteaux et al. 1995; Berg 2000; Prescott et al. 2004).

This study reveals that when a litter with low N concentra-tion was mixed with another type of litter with high N con-centration, the rate of decomposition of the mixed litter wasincreased and was greater than the predicted rates based on theadditive model; those litter were from species commonly usedin agroforestry systems in temperate to subtropical China(Fig. 1; Tables 2 and 3). This supports our first hypothesis.Such findings are consistent with several other studies thatevaluated the effect of mixed litter type on litter decomposi-tion (Sanborn and Brockley 2009; Zeng et al. 2010; DeMarcoet al. 2011). The number of reports on decomposition rates ofmixed litter has markedly increased in recent years as re-searchers began to realize the importance of studying litterdecomposition in a state closer to natural conditions (DeMarco et al. 2011; Pérez-Suárez et al. 2012; Montané et al.

2013; Tan et al. 2013). Therefore, this study and others dem-onstrated the potential synergistic effect of mixing litter ofdifferent types. Mixing of different types of litter effectivelychanges the physical and chemical properties of the littermaterial, and such changed properties may directly affect litterdecomposition rates. The heterogeneity created by mixingdifferent litter types may also change soil faunal, microbial,and enzyme activities and indirectly affect litter decomposi-tion (Hector et al. 2000).

What is most interesting from this study is the enhancedlitter decomposition when poplar leaf litter was mixed withwheat straw (Table 3). There have been few reports of suchsynergistic effects of mixing two litter types that are both lowin N concentrations (Montané et al. 2013). This finding indi-cates that the synergistic effect of mixing different types oflitter together is not limited to mixing litter types with a high Nconcentration, at least in the first year of litter decomposition.

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Fig. 2 Relationship between decomposition rate and N and P concentra-tions in litter samples at both Sihong and Dafeng

Fig. 3 The cumulative loss rate (in percent) of total N in different littertreatments over time during decomposition at both Sihong and Dafeng.PL poplar leaf litter, WSwheat straw, PeL peanut leaf, PeS peanut straw,PL-WSpoplar leaf litter-wheat straw, PL-PeLpoplar leaf litter-peanut leaf,PL-PeS poplar leaf litter-peanut straw

In this study, the poplar leaf litter had a higher N concentrationand were thus subsidizing the decay of the wheat straw whenthey were mixed together; because the accelerated decay wasmeasured as a departure from the mean decomposition rate ofeach litter type in the mixture, it was most likely the wheatstraw that decomposed more quickly in the mixture as com-pared to its decomposition as a single litter. The implication ofthis is for planting different plant species that are not N-fixing(and thus have relatively low N concentrations in the litter) inagroforestry systems to increase litter decomposition rates andthus nutrient cycling in such systems. However, the greaterincrease in the decomposition rates when the poplar leaf litterwas mixed with peanut leaf (Table 3) that has a higher Nconcentration indicates that it is more beneficial to mix litterwith low N concentrations with those with high N concentra-tions to increase litter decomposition and nutrient cycling inagroforestry systems.

Between the two studied sites, the rate of litter decomposi-tion was higher in the silt loam than in the clay loam soil(Table 2), indicating that the site condition in Dafeng (the siltloam soil) was more conducive for litter decomposition. Thisresult rejects our second hypothesis. One of the potentialfactors affecting this difference maybe that soils with a higherclay content may retard litter decomposition through a greaterimmobilization capacity for N and thus reducing soil N avail-ability for microbial populations decomposing the litter, atleast in the early stage of litter decomposition when the C toN ratio of the litter being decomposed is still very high; forexample, Hassink (1995) and Ladd et al. (1985) also foundthat soil microbial activities were higher in a coarse-texturedthan in a finer-textured soil. They suggested that the net Cmineralization rate in soils is dependent on the turnover ofbiomass C and that decay rates are lower in soils with a

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Fig. 4 The cumulative loss rate (in percent) of total P in different littertreatments over time during decomposition at both Sihong and Dafeng.PL poplar leaf litter, WSwheat straw, PeL peanut leaf, PeS peanut straw,PL-WSpoplar leaf litter-wheat straw, PL-PeLpoplar leaf litter-peanut leaf,PL-PeS poplar leaf litter-peanut straw

Fig. 5 The cumulative loss rate (in percent) of total K in different littertreatments over time during decomposition at both Sihong and Dafeng.PL poplar leaf litter, WSwheat straw, PeL peanut leaf, PeS peanut straw,PL-WSpoplar leaf litter-wheat straw, PL-PeLpoplar leaf litter-peanut leaf,PL-PeS poplar leaf litter-peanut straw

heavier texture that have a greater capacity to protect decom-poser populations which would lower the turnover rate ofbiomass C (Ladd et al. 1985). Another explanation is relatedto the ability of the decomposer community in adjusting andtaking advantage of the new substrate; when a high-qualitylitter is placed in a low-quality site, the decomposing commu-nity can adapt to the better substrate (high-quality litter) andthrive to increase the decomposition rate of the added litter (ascompared to the decomposition rate at its original high-qualitysite); on the other hand, when a low-quality litter is added to ahigh-quality site, the decomposer community in the high-quality site is more difficult to be adapted to the low-qualitylitter and as a result, the litter decomposition rate is decreased(Zhang et al. 2008b). In Dafeng, the silt loam soil had aslightly higher C to N ratio in the soil organic matter andlower N and P concentrations (Table 1)—the decomposercommunity at that site might be more adapted to utilizingthe substrate added to the site to increase the decompositionrate as compared with the richer Sihong site (Table 2); this isconsistent with the observation in Zhang et al. (2008b).

When the rate of litter decomposition and change in nutri-ent concentrations over the course of litter decomposition areconsidered together, the dynamics of nutrient release or nutri-ent immobilization can be studied. Our data indicate thatdifferences in nutrient dynamics exist between the differentnutrients studied and between the treatments (Figs. 3, 4, and5), suggesting that the litter quality and the characteristics ofthe particular nutrient affected nutrient release from or immo-bilization by the decomposing litter (Edmonds 1987;Watanabe et al. 2013). When the initial N concentration inthe litter is low (and thus the C to N ratio is high), soil N oftengets immobilized and that affects the soil N dynamics (Li et al.2007). The significantly higher cumulative N and P releaserates in the PeL and PL-PeL treatments at the end of the litterdecomposition study (Figs. 3 and 4) indicate again the ease ofN and P release when their concentrations were high in theinitial litter material (Table 1). Therefore, the last hypothesis isaccepted. Rates of N and P release and immobilization and Krelease vary consistently among the treatments across the twostudy sites and across the periods studied, again illustrating thedominant control of litter quality and season on the release ofnutrients through litter decomposition (Swift et al. 1981; Berg1986).

Differences in the dynamics of N, P, and K release duringlitter decomposition indicate that N and P were limiting formicrobial activities in the PL, WS, and PL-WS samples and anet immobilization occurred in the early stages of litter de-composition until the N and P limitation was removed throughthe enrichment of N and P during litter decomposition (Figs. 3and 4). On the other hand, microbial requirement for K duringlitter decomposition was likely low and no net K immobiliza-tion was observed (Fig. 5). The implications of those resultsare that (1) N and P fertilization may be needed when the litter

material returning to the soil surface has high C:N and C:Pratios and (2) it is possible to alter the rate and timing of N andP release from litter decomposition by altering the composi-tion of the decomposing litter material, which can be achievedthrough deploying different species in an agroforestry system.The reader is cautioned that the study reported here representsa simplified scenario, and in reality, the dynamics of litter fallis complex; for example, the litter of annual crops and that ofpoplar tree may fall in different times of the year. Litterdecomposition is a continuous process, and the interactionsbetween litters of different quality in an agroforestry systemwould be more complex than what was studied in this paperdue to the different timing of litter fall of the different plantspecies. In addition, the 1:1 ratio of litter mixing studied in thispaper reflects one of the many mixing ratios that could occurin the field and the relative amount of the component speciescan affect the magnitude of the non-additive effect in littermixtures (Ward et al. 2010; De Marco et al. 2011).

5 Conclusions

Non-additive (synergistic) effects were observed when differ-ent litter types were mixed in this decomposition study; thenon-additive effect was most pronounced when a litter typewith high N and P concentrations was mixed with anotherwith low N and P concentrations. In addition, synergisticmixing effects were also observed even when two litter typeswith low N and P concentrations were mixed together, sug-gesting positive effects of nutrient enrichment after mixingdifferent litter types and possibly a positive effect of thediversity of litter type, which may increase the heterogeneityof the habitat for litter decomposition. The decomposition oflitter and the non-additive effect of litter mixing were sitespecific. We also conclude that the dynamics of nutrientrelease from litter decomposition depends on the litter qualityand the particular nutrient of concern. Litter quality, litterdecomposition, and nutrient release are closely linked. Nutri-ents such as K that are in abundant supply would not likely beimmobilized during litter decomposition, while nutrients suchas N and P that are most often limiting to microbial activitieswill likely be immobilized in the early stage of litter decom-position when the decomposing litter has high C:N and/or C:Pratios. However, the synergistic effect of mixing litter withhigh N and P concentrations on nutrient release was found forN, P, and K. Findings from this study have significant impli-cations for nutrient management in agroforestry systems intemperate and subtropic regions.

Acknowledgments This work was supported by the National BasicResearch Program of China (973 Program, 2012CB416904), the NationalForestry Public Welfare Research Project of China (No. 201004004), andthe Priority Academic Program Development of Jiangsu Higher

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Education Institutions (PAPD). We would like to acknowledge Dr.Luozhong Tang, Professor Xizeng Xu, and Mr. Xiaoliang Lu for theirable assistance in establishing the experimental plantation and in datacollection. We thank two anonymous reviewers for their comments thatimproved the quality of an earlier version of the manuscript.

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