Nutrient scarcity strengthens soil fauna control over leaf litter decomposition in tropical rainforests

Prescribed fire alters nematode communities in an old‐field grassland

Fire is a common disturbance in many biomes, with both beneficial and detrimental effects on soil biology, which largely depend on fire intensity. However, little is known about the impact of fire on soil nematode communities in terrestrial ecosystem. In the present study, we investigated the effects of short‐term prescribed fire on soil nematode communities and soil properties in an old‐field grassland in Northern China. The results showed that burning significantly increased soil nematode abundance by 77% and genus richness by 49% compared to the control. Burning also decreased taxon dominance by 45% (Simpson's D) and increased nematode diversity by 31% (Shannon‐Weaver H'). However, burning increased plant parasites (particularly genera Cephalenchus and Pratylenchus) and shifted community to more bacterial‐feeding genera (i.e., decreased Channel Index). Generally, burning increased soil bio‐available nitrogen (NH₄⁺–N and NO₃⁻–N) content, which would be the main drivers causing nematode community to flourish via a “bottom‐up” effect. These results suggest that prescribed fire increases nematode diversity and alters community composition toward more plant parasites and bacterial feeders. Our findings highlight the importance of prescribed fire management in shaping short‐term nematode community structure and function, but the long‐term effects and impacts of these changes on soil nutrient and carbon cycling remain unknown.

Litter accumulation and fire risks show direct and indirect climate-dependence at continental scale

Litter decomposition / accumulation are rate limiting steps in soil formation, carbon sequestration, nutrient cycling and fire risk in temperate forests, highlighting the importance of robust predictive models at all geographic scales. Using a data set for the Australian continent, we show that among a range of models, >60% of the variance in litter mass over a 40-year time span can be accounted for by a parsimonious model with elapsed time, and indices of aridity and litter quality, as independent drivers. Aridity is an important driver of variation across large geographic and climatic ranges while litter quality shows emergent properties of climate-dependence. Up to 90% of variance in litter mass for individual forest types can be explained using models of identical structure. Results provide guidance for future decomposition studies. Algorithms reported here can significantly improve accuracy and reliability of predictions of carbon and nutrient dynamics and fire risk.

Litter quality and stream physicochemical properties drive global invertebrate effects on instream litter decomposition

Plant litter is the major source of energy and nutrients in stream ecosystems and its decomposition is vital for ecosystem nutrient cycling and functioning. Invertebrates are key contributors to instream litter decomposition, yet quantification of their effects and drivers at the global scale remains lacking. Here, we systematically synthesized data comprising 2707 observations from 141 studies of stream litter decomposition to assess the contribution and drivers of invertebrates to the decomposition process across the globe. We found that (1) the presence of invertebrates enhanced instream litter decomposition globally by an average of 74%; (2) initial litter quality and stream water physicochemical properties were equal drivers of invertebrate effects on litter decomposition, while invertebrate effects on litter decomposition were not affected by climatic region, mesh size of coarse-mesh bags or mycorrhizal association of plants providing leaf litter; and (3) the contribution of invertebrates to litter decomposition was greatest during the early stages of litter mass loss (0-20%). Our results, besides quantitatively synthesizing the global pattern of invertebrate contribution to instream litter decomposition, highlight the most significant effects of invertebrates on litter decomposition at early rather than middle or late decomposition stages, providing support for the inclusion of invertebrates in global dynamic models of litter decomposition in streams to explore mechanisms and impacts of terrestrial, aquatic, and atmospheric carbon fluxes.

Decay of similarity across tropical forest communities: integrating spatial distance with soil nutrients

Understanding the mechanisms that drive the change of biotic assemblages over space and time is the main quest of community ecology. Assessing the relative importance of dispersal and environmental species selection in a range of organismic sizes and motilities has been a fruitful strategy. A consensus for whether spatial and environmental distances operate similarly across spatial scales and taxa, however, has yet to emerge. We used censuses of four major groups of organisms (soil bacteria, fungi, ground insects, and trees) at two observation scales (1-m² sampling point vs. 2,500-m² plots) in a topographically standardized sampling design replicated in two tropical rainforests with contrasting relationships between spatial distance and nutrient availability. We modeled the decay of assemblage similarity for each taxon set and site to assess the relative contributions of spatial distance and nutrient availability distance. Then, we evaluated the potentially structuring effect of tree composition over all other taxa. The similarity of nutrient content in the litter and topsoil had a stronger and more consistent selective effect than did dispersal limitation, particularly for bacteria, fungi, and trees at the plot level. Ground insects, the only group assessed with the capacity of active dispersal, had the highest species turnover and the flattest nonsignificant distance-decay relationship, suggesting that neither dispersal limitation nor nutrient availability were fundamental drivers of their community assembly at this scale of analysis. Only the fungal communities at one of our study sites were clearly coordinated with tree composition. The spatial distance at the smallest scale was more important than nutrient selection for the bacteria, fungi, and insects. The lower initial similarity and the moderate variation in composition identified by these distance-decay models, however, suggested that the effects of stochastic sampling were important at this smaller spatial scale. Our results highlight the importance of nutrients as one of the main environmental drivers of rainforest communities irrespective of organismic or propagule size and how the overriding effect of the analytical scale influences the interpretation, leading to the perception of greater importance of dispersal limitation and ecological drift over selection associated with environmental niches at decreasing observation scales.

Litter decomposition by soil fauna: effect of land use in agroecosystems

Soil fauna plays a key role in organic matter decomposition. Litter decomposition depends on the relationships of soil fauna and microorganisms as well as climate and litter quality. The decomposer community is sensitive to land use. Thus, physical-chemical disturbances, like soil tillage, can exercise important control on the soil fauna. In order to study the effect of land use and its impact on litter decomposition by soil fauna, a litter-bag experiment was conducted in the Pampa Serrana region, Azul district, Argentina. Litter-bags were made in three different mesh-sizes, allowing the access of micro, micro + meso and micro + meso + macrofauna. Four different treatments were defined: naturalized grassland and three agricultural agroecosystems under different tillage systems, i.e., conservation tillage, conventional-conservation tillage and conventional tillage. Decomposition rate and remaining litter were measured across three different seasons. We found that naturalized grassland obtained the highest decomposition rates and the least remaining litter compared to conservation and conventional tillage systems. No difference in litter decomposition was identified among agricultural agroecosystems. Micro + meso + macrofauna presented the highest decomposition rate and the lowest remaining litter of soil fauna groups, in all agroecosystems. In contrast, microfauna decomposition rate was the lowest and produced the highest remaining litter. Micro + mesofauna presented values of decomposition rate and remaining litter that differed significantly from the rest of the groups in some seasons. These results highlight the importance of soil fauna in litter decomposition and the negative effects of different land use systems on litter decomposition by soil fauna.

Nutrient scarcity strengthens soil fauna control over leaf litter decomposition in tropical rainforests

Soil fauna is a key control of the decomposition rate of leaf litter, yet its interactions with litter quality and the soil environment remain elusive. We conducted a litter decomposition experiment across different topographic levels within the landscape replicated in two rainforest sites providing natural gradients in soil fertility to test the hypothesis that low nutrient availability in litter and soil increases the strength of fauna control over litter decomposition. We crossed these data with a large dataset of 44 variables characterizing the biotic and abiotic microenvironment of each sampling point and found that microbe-driven carbon (C) and nitrogen (N) losses from leaf litter were 10.1 and 17.9% lower, respectively, in the nutrient-poorest site, but this among-site difference was equalized when meso- and macrofauna had access to the litterbags. Further, on average, soil fauna enhanced the rate of litter decomposition by 22.6%, and this contribution consistently increased as nutrient availability in the microenvironment declined. Our results indicate that nutrient scarcity increases the importance of soil fauna on C and N cycling in tropical rainforests. Further, soil fauna is able to equalize differences in microbial decomposition potential, thus buffering to a remarkable extent nutrient shortages at an ecosystem level.