Key recent developments in litter decomposition research are reviewed. Long-term inter-site experiments indicate that temperature and moisture influence early rates of litter decomposition primarily by determining the plants present, suggesting that climate change effects will be small unless they alter the plant forms present. Thresholds may exist at which single factors control decay rate. Litter decomposes faster where the litter type naturally occurs. Elevated CO2 concentrations have little effect on litter decomposition rates. Plant tissues are not decay-resistant; it is microbial and biochemical transformations of materials into novel recalcitrant compounds rather than selective preservation of recalcitrant compounds that creates stable organic matter. Altering single characteristics of litter will not substantially alter decomposition rates. Nitrogen addition frequently leads to greater stabilization into humus through a combination of chemical reactions and enzyme inhibition. To sequester more C in soil, we need to consider not how to slow decomposition, but rather how to divert more litter into humus through microbial and chemical reactions rather than allowing it to decompose. The optimal strategy is to have litter transformed into humic substances and then chemically or physically protected in mineral soil. Adding N through fertilization and N-fixing plants is a feasible means of stimulating humification.

Litter decomposition: what controls it and how can we alter it to sequester more carbon in forest soils?

Elemental stoichiometry constitutes an inherent link between biogeochemistry and the structure and processes within food webs, and thus is at the core of ecosystem functioning. Stoichiometry allows for spanning different levels of biological organization, from cellular metabolism to ecosystem structure and nutrient cycling, and is therefore particularly useful for establishing links between different ecosystem compartments. We review elemental carbon : nitrogen : phosphorus (C:N:P) ratios in terrestrial ecosystems (from vegetation, leaf litter, woody debris, and dead roots, to soil microbes and organic matter). While the stoichiometry of the plant, litter, and soil compartments of ecosystems is well understood, heterotrophic microbial communities, which dominate the soil food web and drive nutrient cycling, have received increasing interest in recent years. This review highlights the effects of resource stoichiometry on soil microorganisms and decomposition, specifically on the structure and function of h...

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The Application of ecological stoichiometry to plant-microbial-soil organic matter transformations

Biodiversity and carbon (C) cycling have been the focus of much research in recent decades, partly because both change as a result of anthropogenic activities that are likely to continue. Soils are extremely species-rich and store approximately 80% of global terrestrial C. Soil organisms play a key role in C dynamics and a loss of species through global changes could influence global C dynamics. Here, we synthesize findings from published studies that have manipulated soil species richness and measured the response in terms of ecosystem functions related to C cycling (such as decomposition, respiration and the abundance or biomass of decomposer biota) to evaluate the impact of biodiversity loss on C dynamics. We grouped studies where one or more biotic groups had been manipulated to include a richness of 10 species in order to reflect 'low' and 'high' extents of diversity manipulations. There was a positive relationship between species richness and C cycling in 77-100% of low-diversity experiments, even when the richness of just one biotic group was manipulated, whereas positive relationships occurred less frequently in studies with greater richness (35-64%). Moreover, when positive relationships were observed, these often indicated functional redundancy at low extents of diversity or that community composition had a stronger influence on C cycling than did species richness. Initial reductions in soil species richness resulting from global changes are unlikely to alter C dynamics significantly unless particularly influential species are lost. However, changes in community composition, and the loss of species with an ability to facilitate specialized soil processes related to C cycling, as a result of global changes, may have larger impacts on C dynamics.

Soil biodiversity and carbon cycling: A review and synthesis of studies examining diversity-function relationships

Climate and litter quality have been identified as major drivers of litter decomposition at large spatial scales. However, the role played by soil fauna remains largely unknown, despite its importance for litter fragmentation and microbial activity. We synthesised litterbag studies to quantify the effect sizes of soil fauna on litter decomposition rates at the global and biome scales, and to assess how climate, litter quality and soil fauna interact to determine such rates. Soil fauna consistently enhanced litter decomposition at both global and biome scales (average increment ~ 37%). [corrected]. However, climate and litter quality differently modulated the effects of soil fauna on decomposition rates between biomes, from climate-driven biomes to those where climate effects were mediated by changes in litter quality. Our results advocate for the inclusion of biome-specific soil fauna effects on litter decomposition as a mean to reduce the unexplained variation in large-scale decomposition models.

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Climate and litter quality differently modulate the effects of soil fauna on litter decomposition across biomes

Tropical rain forests play a dominant role in global biosphere-atmosphere CO2 exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (< 1000 m), a regression tree analysis revealed that foliar and soil-based measurements of phosphorus (P) were the only variables that explained a significant proportion of the variation in ANPP, although the relationships were weak. However, foliar P, foliar nitrogen (N), litter decomposition rate (k), soil N and soil respiration were all directly related with total surface (0‐10 cm) soil P concentrations. Our analysis provides some evidence that P availability regulates NPP and other ecosystem processes in lowland tropical forests, but more importantly, underscores the need for a series of large-scale nutrient manipulations ‐ especially in lowland forests ‐ to elucidate the most important nutrient interactions and controls.

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Relationships among net primary productivity, nutrients and climate in tropical rain forest: a pan‐tropical analysis

Summary 1. Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. 2. We used a short-term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760‐5797 mm). At each site, two standard substrates ( Raphia farinifera and Laurus nobilis ) were decomposed in fine- and coarse-mesh litterbags both above and below ground for approximately 1 year.

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Decomposition in tropical forests: a pan‐tropical study of the effects of litter type, litter placement and mesofaunal exclusion across a precipitation gradient

Rhizophora mangle L. trees of Biscayne National Park (Florida, USA) have two distinct growth forms: tall trees (5-10 m) growing along the coast and dwarf trees (1 m or less) growing in the adjacent inland zone. Sharp decreases in salinity and thus increases in soil water potential from surface soil to about a depth of 1 m were found at the dwarf mangrove site but not at the tall mangrove site. Consistent with our prediction, hydraulic redistribution detected by reverse sap flow in shallow prop roots was observed during nighttime, early morning and late afternoon in dwarf trees, but not in tall trees. In addition, hydraulic redistribution was observed throughout the 24-h period during a low temperature spell. Dwarf trees had significantly lower sapwood-specific hydraulic conductivity, smaller stem vessel diameter, lower leaf area to sapwood area ratio (LA/SA), smaller leaf size and higher leaf mass per area. Leaves of dwarf trees had lower CO(2) assimilation rate and lower stomatal conductance compared to tall trees. Leaf water potentials at midday were more negative in tall trees that are consistent with their substantially higher stomatal conductance and LA/SA. The substantially lower water transport efficiency and the more conservative water use of dwarf trees may be due to a combination of factors such as high salinity in the surface soil, particularly during dry periods, and substantial reverse sap flow in shallow roots that make upper soil layers with high salinity a competing sink of water to the transpiring leaves. There may also be a benefit for the dwarf trees in having hydraulic redistribution because the reverse flow and the release of water to upper soil layers should lead to dilution of the high salinity in the rhizosphere and thus relieve its potential harm to dwarf R. mangle trees.

Hydraulic redistribution in dwarf Rhizophora mangle trees driven by interstitial soil water salinity gradients: impacts on hydraulic architecture and gas exchange

Eric Manzane

Papers

Decomposition in tropical forests: a pan‐tropical study of the effects of litter type, litter placement and mesofaunal exclusion across a precipitation gradient

Hydraulic redistribution in dwarf Rhizophora mangle trees driven by interstitial soil water salinity gradients: impacts on hydraulic architecture and gas exchange