High quality litters with faster initial decomposition produce more stable residue remaining in a subtropical forest ecosystem

Litter decomposition is a vital link between material circulation and energy flow in forest ecosystems and is intensely affected by global change factors, such as increased nitrogen (N) deposition and altered precipitation regimes. As essential nutrients, calcium (Ca), magnesium (Mg), and manganese (Mn) play crucial roles in plant energy metabolism, photosynthesis, and membrane transport of plants, and the major source of these nutrients is litter decomposition. However, the dynamics of Ca, Mg, and Mn during decomposition have been largely ignored. Thus, to better understand Ca, Mg, and Mn dynamics during leaf litter decomposition in the scenario of increasing N deposition and decreasing precipitation, we carried out a two-year field litterbag experiment in a natural evergreen broad-leaved forest in the central area of the rainy area of Western China. Two levels of N deposition (ambient N deposition and 150 kg·N·ha−1·y−1) and precipitation reduction (no throughfall reduction and 10% throughfall reduction) were set, i.e., control (Ctr: without nitrogen deposition or throughfall reduction), N deposition (N, 150 kg·N·ha−1·y−1), throughfall reduction (T, 10% throughfall reduction), and N deposition and throughfall reduction (NT, 150 kg·N·ha−1·y−1 and 10% throughfall reduction). We found that leaf litter Ca concentration increased in the early decomposition stage and then decreased, while Mg and Mn concentrations generally decreased during the whole period of decomposition. The amount of Ca showed an accumulation pattern, while Mg and Mn generally showed a release pattern. N deposition and throughfall reduction affected the Ca, Mg, and Mn dynamics, varying with different decomposition stages; i.e., N deposition significantly affected the concentration and amount of Ca, regardless of the decomposition stages, while throughfall reduction significantly affected the Ca concentration in the whole and early decomposition stages. N deposition significantly affected the concentration and amount of Mg in the whole and early decomposition stages, while throughfall reduction had no significant effects. Throughfall reduction significantly affected the concentration and amount of Mn in the whole and late decomposition stages, while N deposition had no significant effects. Ca concentration generally showed a significant positive linear relationship with mass loss in the early decomposition stage; Mg concentration showed a significant positive linear relationship with mass loss in the Ctr and N treatments in the early and late decomposition stages; Mn generally showed a significant negative linear relationship with mass loss, regardless of the decomposition stage. Overall, the results suggest that Ca accumulation is more likely affected by N deposition, while Mg and Mn releases are more likely affected by N deposition combined with throughfall reduction, particularly in the early decomposition stage.

We conducted a 512-day incubation experiment to study the dynamics of microbial necromass and soil carbon fraction in the 'litter-soil' transformation interface soil layer (TIS) during litter decomposition, using a perennial C3 herb, Stipa bungeana, in the loess hills. The results showed that soil microbial necromass was dominated by fungi in the early and middle stages, and by bacteria in the late stage. The contribution of fungal necromass C to mineral-associated organic C (MAOC) was significantly higher (38.7%-75.8%) than that of bacteria (9.2%-22.5%) and 2-3 times more than the contribution rate of bacterial necromass. Soil organic C (SOC) content was decreasing during litter decomposition. The input of plant C resources stimulated microbial utilization of soil C fractions. The continuous decrease in particulate organic C during the early and late stages of decomposition was directly responsible for the decrease in SOC content. In contrast, the fluctuating changes in microbial necromass C and MAOC played an indirect role in the reduction of SOC. The increase in soil microbial necromass C caused by a single exogenous addition of litter did not directly contribute to SOC accumulation.

Effect of litter substrate quality and soil nutrients on forest litter decomposition: A review

Soil organisms influence organic matter turnover and nutrient cycling via processing of organic matter entering the soil as litter and root-derived resources. Plant species differ enormously in the quality and quantity of litter and roots that they produce, and this diversity strongly modifies decomposition of litter by decomposer organisms. Higher plant diversity is generally assumed to improve habitat conditions and availability of resources, thereby improving the abundance and activity of decomposer organisms. Tropical Andean montane rainforest ecosystems harbor an exceptional diversity of plant and animal species. However, little is known on how the huge diversity of plants and root resources affect the activity of soil communities and the overall decomposition rates, particularly during early stages of decomposition. This thesis aims to contribute to our understanding of the effects of leaf litter diversity and root resources on microorganisms and decomposer microarthropods during the early stages of litter decomposition in Andean tropical montane rainforest ecosystems. The studies were performed as field experiments at 2000 m (Chapter 2 and 4) and along an altitudinal gradient from 1000 to 2000 to 3000 m (Chapter 3) in a tropical montane rainforest in Southern Ecuador. Chapter 2 investigates the effect of leaf litter diversity and identity on microbial functions and microarthropod abundance. The results suggest that decomposition and microbial parameters in litter vary with litter diversity as well as litter identity, while microarthropods respond only to litter identity. The results show that higher levels of diversity detrimentally affect soil microbial biomass and result in a decline in litter decomposition. Further, the results indicate that the differential response of soil biota was mostly due to differences in the initial chemical composition of litter species. However, the results also highlight the importance of leaf litter physical traits, particularly on the abundance of decomposer invertebrates. Overall, the results indicate that litter species identity functions as major driver of the abundance and activity of soil organisms and thereby exerts distinct effects on ecosystem processes such as decomposition and nutrient mobilization. Chapter 3 investigates the contribution of soil microbes and decomposer microarthropods to the decomposition of leaf and root litter along an altitudinal gradient of the studied tropical rainforests. The results suggest that the decomposition of both leaf and root litter in montane rainforests is mainly due to microorganisms, whereas the effect of microarthropods is minor along the altitudinal gradient. However, at higher altitudes soil microarthropods accelerate the decomposition of low-quality litter, such as root litter. Further, the study suggests that the abundance of microorganisms as food is of minor importance in structuring decomposer microarthropod communities, underscoring the role of litter quality. Overall, our findings highlight that resource quality or local interspecific variation in litter quality has stronger effects on decomposer organisms regardless climatic variations associated to altitude, at least during early stages of decomposition. Chapter 4 investigates the response of arbuscular mycorrhizal (AM) fungi, microorganisms and microarthropods to the rotation of hyphal-ingrowth cores, defaunation and nitrogen addition. The results suggest that in the study site AM fungi are closely associated with living roots and do not form extensive extraradical hyphae that can be cut by rotation of the cores. Nonetheless, the results suggest that on top of the litter layer, AM fungi likely compete with saprotrophic microorganisms for litter-derived resources, with mycorrhizal fungi suppressing the activity of saprotrophic microorganisms. While in the soil layer interactions of mycorrhizal fungi with other soil biota are restricted to the close vicinity of roots. Nitrogen addition increased the quality of litter material produced by plants and beneficially affected microbial activity, highlighting that decomposition processes in the studied montane rainforests are strongly limited by nutrient availability and microorganisms in these forests even respond to moderate increase in nitrogen. The results also document a restricted recovery of microorganisms and microarthropods after defaunation of the rotated cores, highlighting the importance of root-derived resources for fueling soil food webs. Chapter 5 presents a discussion and conclusions on the contribution of the research chapters to the overall state of knowledge. Generally, the results of this thesis suggest that during early stages of decomposition the abundance, diversity and activity of soil organisms are strongly associated with the quality and availability of the litter resources. Overall, the results suggest that decomposition processes in montane rainforests at early stages are mainly driven by microorganisms, whereas the contribution of microarthropods is of minor importance. Further, the results also highlight the importance of root-derived resources for fueling soil microarthropod abundance during early stages of decomposition. In addition, the results point to AM fungi as an important player for determining the abundance and activity of microbial communities during early stages of decomposition in tropical montane rainforests.

The canopies in evergreen coniferous plantations are often composed of various-aged needles. Plantation management, such as thinning, produced abundant harvest residue, including needles with different needle ages. However, little attention was paid to the effect of needle age on decomposition, although the needle chemical properties varied substantially with leaf ages. A field experiment was conducted for 3 years to investigate the decomposition of harvest residue needles at different needle ages, and determine the main controlling factors in different stages of decomposition in a Chinese fir plantation. We found that the initial decomposition rate varied 5-fold among different needle ages in a Chinese fir plantation. Litter quality controlled the overall litter decomposition rate, especially the initial decomposition rate. Needle nitrogen content was positively correlated to decomposition rate during the early stage of decomposition. However, it was negatively correlated to decomposition rate during the later stage of decomposition. The contents of needle tannins increased the asymptotic mass remaining (A, proportion of mass remaining at which decomposition approaches zero, i.e., the fraction of slowly decomposing litter). We also found that the initial litter decomposition rates in soil fauna presence were significantly higher than those in soil fauna absence across different needle ages. Moreover, the effect of soil fauna on initial litter decomposition is independent of needle quality. Our results suggest that needle age and plant secondary metabolites should be considered to understand the response of litter decomposition and nutrient cycling to management practices, such as thinning, in conifer plantations.

Litter decomposition contributes largely to global carbon (C) and nitrogen (N) cycling, and it is strongly determined by litter quality and microbial community composition in ways that are poorly understood. Here, we conducted a 2‐year field litter decomposition experiment by collecting leaf and root litter of crops (from cropland), shrubs (from shrubland) and wood (from woodland) and placing samples for decomposition in woodland soil in central China to investigate the effects of litter quality and microbial community composition on C and N losses of leaf and root litter of three species under different decomposition stages. Our results showed that the leaf litter C and N losses of shrubs were significantly higher than those of crops and wood, whereas the root litter C and N losses of crops were significantly higher than those of shrubs and wood. Generally, the leaf litter C and N losses of the three species were higher on average than those of fine root litter during the whole decomposition period. For the C loss of the three species, litter lignin and phosphorus as well as initial litter quality were predominant drivers of root litter decomposition, while litter lignin, cellulose and hemicellulose concentrations were dominant for leaf litter decomposition. For N loss, litter stoichiometry and litter quality directly governed leaf and root litter N loss, and the initial litter quality largely regulated N loss at the late decomposition stage. Unexpectedly, the effect of microbial community composition on litter C and N losses was relatively weak and only exhibited an effect on litter C and N losses during the early stage of decomposition. Thus, our results revealed the huge disparity in C and N loss of plant species and litter types at different decomposition stages, which should be considered jointly when evaluating their roles in plant–soil feedbacks under global land use change. A free Plain Language Summary can be found within the Supporting Information of this article.

Species‐rich forests can produce litter of varying carbon (C) and nitrogen (N) composition (i.e. quality), which can affect decomposition and play a central role in long‐term soil organic carbon (SOC) accumulation. However, how differences in litter quality affect SOC decomposition and formation remains unclear over the full litter decomposition trajectory. We followed the in situ complete decomposition of added 13C‐labelled high‐ (low C:N) and low‐quality (high C:N) leaf‐litter and its effect on particulate organic matter (POM) and mineral‐associated organic matter (MAOM) fractions over 2 years in a natural subtropical forest. We found that during early stages of decomposition, low‐quality litter inputs decreased SOC via a positive priming effect (i.e. new C inputs favoured decomposition of native SOC), but these SOC losses were offset by SOC gains observed via a negative priming effect during decomposition of high‐quality litter. In contrast, this pattern reversed during late stages of decomposition—SOC losses via a positive priming effect induced by high‐quality litter were offset by SOC gains via a negative priming effect induced by low‐quality litter. Over the full decomposition of litter, both high‐ and low‐quality litter stimulated microbial breakdown of SOC tied to POM, but also replenished more persistent SOC that associated with soil minerals (MAOM). Altogether, we observed that low‐quality litter formed twice as much new SOC as high‐quality litter (24% vs. 12% of added litter‐C). We extend the notion of the priming effect from primarily a negative role promoting losses of native SOC, to a functional role that can replenish persistent SOC. Synthesis. Our measurements raise the possibility that, in species‐rich forests, high‐ and low‐quality litter decomposition play opposite but dynamically complementary roles in renewing POM—both by inducing its decomposition and formation—while exclusively favouring MAOM formation, which can help explain how differences in litter quality favour SOC accumulation and persistence. Global change factors that shift plant community composition may ultimately affect the fate of soil C, as changes in litter quality may force soil transitions from sinks to sources or sources to sinks of atmospheric CO2.

We assayed the effects of simulated acid rain on the mass loss, CO2 evolution, dehydrogenase activity, and microbial biomass-C of decomposing Sorbus alnifolia leaf litter at the microcosm. The dilute sulfuric acid solution composed the simulated acid rain, and the microcosm decomposition experiment was performed at 23°C and 40% humidity. During the early decomposition stage, decomposition rate of S. alnifolia leaf litter, and microbial biomass, CO2 evolution and dehydrogenase activity were inhibited at a lower pH; however, during the late decomposition stage, these characteristics were not affected by pH level. The fungal component of the microbial community was conspicuous at lower pH levels and at the late decomposition stage. Conversely, the bacterial community was most evident during the initial decomposition phase and was especially dominant at higher pH levels. These changes in microbial community structure resulting from changes in microcosm acidity suggest that pH is an important aspect in the maintenance of the decomposition process. Litter decomposition exhibited a positive, linear relationship with both microbial respiration and microbial biomass. Fungal biomass exhibited a significant, positive relationship with CO2 evolution from the decaying litter. Acid rain had a significant effect on microbial biomass and microbial community structure according to acid tolerance of each microbial species. Fungal biomass and decomposition activities were not only more important at a low pH than at a high pH but also fungal activity, such as CO2 evolution, was closely related with litter decomposition rate.

Aims Litter constitutes the major source of organic matter entering the soil. Different litter layers reflect different phases of decomposition. The litter originated from different plant materials and decomposition phases may have a significant impact on cellulolytic enzyme activities. Our objective was to explore the effects of vegetation types and decomposition phases on cellulolytic enzyme activities during litter decomposition process in an alpine timberline ecotone at the end of snow melting. Methods The activities of three cellulolytic enzymes(β-1,4-endoglucanase, β-1,4-exoglucanase and β-1,4- glucosidase) and litter qualities(C, N, P and cellulose content) were measured in the fresh litter and fermentation layer(LF) and the humus layer(H) in alpine meadow, alpine shrub, and coniferous forest in the alpine timberline ecotone in western Sichuan. Two-way ANOVA was used for testing the main effects of vegetation types, decomposition phase and their interactions on cellulolytic enzyme activities and litter qualities. We used Spearman correlations to explore the relationships between cellulolytic enzyme activities and litter qualities of two decomposition phases. Important findings Cellulolytic enzyme activities and cellulose contents in the LF layer were significantly higher than in the H layer across all vegetation types. Two-way ANOVA results showed that decomposition phase had a more significant impact on cellulolytic enzyme activities and cellulose contents than vegetation types. Cellulolytic enzyme activities were under the control of different factors between the two decomposition stages. In the early decomposition stage, the activities of β-1,4-exoglucanase and β-1,4-glucosidase appeared to be limited by N and P contents of the substrate, while β-1,4-endoglucanase activity was mainly controlled by the cellulose content of litter. In the late decomposition stage, the activities of β-1,4-endoglucanase and β-1,4-glucosidase were mainly limited by C and N contents. According to the prediction of ecological stoichiometry theory, microbial growth is considered to be nutrient-limited on substrates with C:N 27 or C:P 186. Overall, litter C:N and C:P were greater than 27 and 186, respectively, in the study area, indicating that cellulolytic enzyme activities werelimited by litter N and P contents. In particular, the microbial biomass was limited more significantly by N and P contents in the early decomposition stage in the alpine meadow, indicating that litter quality indirectly regulates cellulolytic enzyme activities of litter decomposition process in this alpine timberline ecotone.

Atmospheric nitrogen (N) deposition affects litter decomposition. However, how endogenous litter quality and exogenous resource supply alter the N deposition effect on litter decomposition and deposited N immobilized by microbes remains unclear. We conducted a laboratory experiment to examine how the N deposition effect on litter decomposition varies with endogenous litter quality (needle litter with higher C/nutrients, low quality litter versus leaf litter with low C/nutrients, high quality litter) and exogenous resource supply (five treatments: N addition alone; N plus non-N nutrient and/or carbon addition; control) using a 15N tracing method. Nitrogen deposition increased the % mass and % N remaining across the decomposition process. Adding non-N nutrients increased the N deposition effect on % mass and % N remaining in the decomposing high quality litter but not in the low quality litter. Moreover, the % P remaining was increased in the low quality litter but was decreased in the high quality litter under N deposition. However, adding N and non-N nutrients together increased the % P remaining in both decomposing litters. The immobilized exogenous 15N abundance (IEN) was much higher in the decomposing low quality litter than high quality litter. For low quality litter, resource addition treatments affected IEN, but their effects depended on decomposition stages. For high quality litter, carbon addition alone generally increased IEN across the 720 days. Nitrogen deposition effect on litter decomposition could be altered by exogenous resource supply, but the pattern ultimately depended on endogenous litter quality. Nitrogen deposition generally suppressed the litter decomposition and non-N nutrients addition enhanced the inhibition effects of N deposition on litter decomposition, especially of high quality litter, while lower quality litter tended to immobilize more exogenous deposited N. Thus, the magnitude of both non-N nutrient availability and litter quality needs to be taken into consideration when assessing the effects of N deposition on litter decomposition.

Soil C:N:P stoichiometry of typical coniferous (Cunninghamia lanceolata) and/or evergreen broadleaved (Phoebe bournei) plantations in south China

We examined the dynamics and community structure of soil meso-microarthropods during litter decomposition in tropical rain forests of Xishuangbanna, SW China between May, 2000 and April, 2001. The experiment was carried out in three plots of tropical seasonal rain forest located within a distance of 15 km. Mixed-species litterbags were constructed and placed in the field for one year. Soil meso-microarthropods were extracted from the litterbags by the Tullgren method each month during litter decomposition . The densities of soil meso-microarthropod groups and individuals were calculated per gram of dry litter (relative density). The data showed that Collembola and Acari were the most abundant groups of arthropods in the tropical seasonal rain forests ( above 30% ). Diversity indices, numbers of groups and number of individuals of soil meso-microarthropod were all higher in the middle stage of decomposition than in the early and end stages of decomposition. Variation of soil meso-microarthropod communities, including abundance of some groups, was correlated with litter quantity and quality. Relative density of soil meso-microarthropod reflects the dynamic relationship between litter quality and number of groups and individuals of soil meso-microarthropods during the decomposition process. The differences of diversity and abundance of soil meso-microarthropods among three different plots were higher in the end stage of decomposition than in the early stage of decomposition, but litter weight loss did not differ among the three different study site plots.

Dynamics of litter decomposition rate and soil organic carbon sequestration following vegetation succession on the Loess Plateau, China

Summary Increasing atmospheric CO2 concentrations alongside more frequent and severe droughts are key global change factors impacting litter decomposition, and global carbon and nitrogen cycles. Yet, we have a poor understanding of how these perturbations impact interactions between litter chemistry and decomposer food webs. We tested how drought and elevated CO2 concentrations modify litter decomposition via litter chemistry and the decomposer communities using two separate, long-term manipulative drought or elevated CO2 field experiments in mature oak woodlands. Litter bags were deployed in a reciprocal transplant design whereby control and treated litter was incubated in control and treated plots. We measured litter mass loss, quality and stoichiometry, moisture content, and microbial and animal community composition. Elevated CO2 and drought affected litter stoichiometry and quality which in turn affected decomposition: litter in droughted plots decomposing slower than controls, while litter from elevated CO2 plots decomposed slower over the first three harvests. In addition, we found that litter mass loss and decomposers were related both directly and indirectly to litter C:N. Drought and elevated CO2 have distinct pathways of influence on litter quality, decomposers and decomposition which challenges our ability to predict how combinations of factors influence litter decomposition. Plain Language Summary Global change factors like drought and elevated atmospheric CO2 concentrations can impact litter decompositions, an important ecosystem process, via changes to litter properties and the decomposer community. Examining links between litter properties, decomposers and decomposition is therefore critical to understand how both drought and elevated CO2 will affect nutrient release and cycling of belowground environments.

Decomposition of Different Litter Fractions in a Subtropical Bamboo Ecosystem as Affected by Experimental Nitrogen Deposition