Summary 1. The leaf economics spectrum (LES) provides a useful framework for examining species strategies as shaped by their evolutionary history. However, that spectrum, as originally described, involved only two key resources (carbon and nutrients) and one of three economically important plant organs. Herein, I evaluate whether the economics spectrum idea can be broadly extended to water – the third key resource –stems, roots and entire plants and to individual, community and ecosystem scales. My overarching hypothesis is that strong selection along trait trade-off axes, in tandem with biophysical constraints, results in convergence for any taxon on a uniformly fast, medium or slow strategy (i.e. rates of resource acquisition and processing) for all organs and all resources. 2. Evidence for economic trait spectra exists for stems and roots as well as leaves, and for traits related to water as well as carbon and nutrients. These apply generally within and across scales (within and across communities, climate zones, biomes and lineages). 3. There are linkages across organs and coupling among resources, resulting in an integrated whole-plant economics spectrum. Species capable of moving water rapidly have low tissue density, short tissue life span and high rates of resource acquisition and flux at organ and individual scales. The reverse is true for species with the slow strategy. Different traits may be important in different conditions, but as being fast in one respect generally requires being fast in others, being fast or slow is a general feature of species. 4. Economic traits influence performance and fitness consistent with trait-based theory about underlying adaptive mechanisms. Traits help explain differences in growth and survival across resource gradients and thus help explain the distribution of species and the assembly of communities across light, water and nutrient gradients. Traits scale up – fast traits are associated with faster rates of ecosystem processes such as decomposition or primary productivity, and slow traits with slow process rates. 5. Synthesis. Traits matter. A single ‘fast–slow’ plant economics spectrum that integrates across leaves, stems and roots is a key feature of the plant universe and helps to explain individual ecological strategies, community assembly processes and the functioning of ecosystems.

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The world‐wide ‘fast–slow’ plant economics spectrum: a traits manifesto

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

In a review published over two decades ago I asserted that, along soil fertility gradients, plant traits change in ways that reinforce patterns of soil fertility and net primary productivity (NPP). I reevaluate this assertion in light of recent research, focusing on feedbacks to NPP operating through litter decomposition. I conclude that mechanisms emerging since my previous review might weaken these positive feedbacks, such as negative effects of nitrogen on decomposition, while others might strengthen them, such as slower decomposition of roots compared to leaf litter. I further conclude that predictive understanding of plant species effects on nutrient cycling will require developing new frameworks that are broadened beyond litter decomposition to consider the full litter–soil organic matter (SOM) continuum.

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Plant species effects on nutrient cycling: revisiting litter feedbacks

The ecology of forest soils is an important field of research due to the role of forests as carbon sinks. Consequently, a significant amount of information has been accumulated concerning their ecology, especially for temperate and boreal forests. Although most studies have focused on fungi, forest soil bacteria also play important roles in this environment. In forest soils, bacteria inhabit multiple habitats with specific properties, including bulk soil, rhizosphere, litter, and deadwood habitats, where their communities are shaped by nutrient availability and biotic interactions. Bacteria contribute to a range of essential soil processes involved in the cycling of carbon, nitrogen, and phosphorus. They take part in the decomposition of dead plant biomass and are highly important for the decomposition of dead fungal mycelia. In rhizospheres of forest trees, bacteria interact with plant roots and mycorrhizal fungi as commensalists or mycorrhiza helpers. Bacteria also mediate multiple critical steps in the nitrogen cycle, including N fixation. Bacterial communities in forest soils respond to the effects of global change, such as climate warming, increased levels of carbon dioxide, or anthropogenic nitrogen deposition. This response, however, often reflects the specificities of each studied forest ecosystem, and it is still impossible to fully incorporate bacteria into predictive models. The understanding of bacterial ecology in forest soils has advanced dramatically in recent years, but it is still incomplete. The exact extent of the contribution of bacteria to forest ecosystem processes will be recognized only in the future, when the activities of all soil community members are studied simultaneously.

Forest Soil Bacteria: Diversity, Involvement in Ecosystem Processes, and Response to Global Change

The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium’s first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and ‘ecosystem engineers’ (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm–μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.

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Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review

Conceptual frameworks relating plant traits to ecosystem processes such as organic matter dynamics are progressively moving from a leaf-centred to a whole-plant perspective. Through the use of meta-analysis and global literature data, we quantified the relative roles of litters from above- and below-ground plant organs in ecosystem labile organic matter dynamics. We found that decomposition rates of leaves, fine roots and fine stems were coordinated across species worldwide although less strongly within ecosystems. We also show that fine roots and stems had lower decomposition rates relative to leaves, with large differences between woody and herbaceous species. Further, we estimated that on average below-ground litter represents approximately 33 and 48% of annual litter inputs in grasslands and forests, respectively. These results suggest a major role for below-ground litter as a driver of ecosystem organic matter dynamics. We also suggest that, given that fine stem and fine root litters decompose approximately 1.5 and 2.8 times slower, respectively, than leaf litter derived from the same species, cycling of labile organic matter is likely to be much slower than predicted by data from leaf litter decomposition only. Synthesis. Our results provide evidence that within ecosystems, the relative inputs of above- versus below-ground litter strongly control the overall quality of the litter entering the decomposition system. This in turn determines soil labile organic matter dynamics and associated nutrient release in the ecosystem, which potentially feeds back to the mineral nutrition of plants and therefore plant trait values and plant community composition. © 2013 The Authors. Journal of Ecology © 2013 British Ecological Society.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12092

Linking litter decomposition of above‐ and below‐ground organs to plant–soil feedbacks worldwide

Summary 1. Conceptual frameworks relating plant traits to ecosystem processes such as organic matter dynamics are progressively moving from a leaf-centred to a whole-plant perspective. Through the use of meta-analysis and global literature data, we quantified the relative roles of litters from above- and below-ground plant organs in ecosystem labile organic matter dynamics. 2. We found that decomposition rates of leaves, fine roots and fine stems were coordinated across species worldwide although less strongly within ecosystems. We also show that fine roots and stems had lower decomposition rates relative to leaves, with large differences between woody and herbaceous species. Further, we estimated that on average below-ground litter represents approximately 33 and 48% of annual litter inputs in grasslands and forests, respectively. 3. These results suggest a major role for below-ground litter as a driver of ecosystem organic matter dynamics. We also suggest that, given that fine stem and fine root litters decompose approximately 1.5 and 2.8 times slower, respectively, than leaf litter derived from the same species, cycling of labile organic matter is likely to be much slower than predicted by data from leaf litter decomposition only. 4. Synthesis. Our results provide evidence that within ecosystems, the relative inputs of above- versus belowground litter strongly control the overall quality of the litter entering the decomposition system. This in turn determines soil labile organic matter dynamics and associated nutrient release in the ecosystem, which potentially feeds back to the mineral nutrition of plants and therefore plant trait values and plant community composition.

Linking litter decomposition of above- and below-ground organs to plant-soil feedbacks

Soil pH varies by several units among ecosystems. While soil pH is known to be a key driver of plant species composition, we still have a poor understanding of how it affects carbon cycling processes. For instance, soil pH, or its associated chemistry in terms of base cations and organic acids, may affect decomposition rates of dead matter directly, by controlling decomposer composition and activity and, indirectly, by controlling the traits of the plant species and thereby the afterlife effects of those traits on litter decomposition. Leaf and litter pH may play a role in this control. Based on the very limited empirical data available, we hypothesized that variation in species traits including leaf (litter) pH, within and between ecosystems contrasting in soil pH, would have stronger effects on leaf litter decomposition rates than variation in soil chemistry would. We tested this hypothesis by carrying out a ‘common garden’ litterbed experiment in subtropical SW China, in which leaf litters of the 30 predominant plant species from mid‐successional forest on acidic sandstone (soil pH around 4.0) and calcareous soil (pH around 7.5), respectively, were incubated and their decomposition rates measured over two harvests in 14 months, both in soil plus litter matrix from their ‘home’ forest and in those from the ‘away’ forest. We found that leaf (litter) trait variation among species and plant functional types, headed by species' dry matter content but also including tissue pH, was the strongest driver of variation in leaf litter decomposition rates. Surprisingly, however, while these effects of interspecific trait variation were very strong among species from the same site, there was no overall difference in litter decomposability between the species from the acidic versus calcareous site. Equally surprising was that this strong difference in pH of soil substrate plus litter matrix from an acidic sandstone site versus a calcareous karst site did not directly affect leaf litter decomposition rates across a given species set. This first attempt to disentangle the multiple potential direct and indirect ways in which soil and leaf (litter) acidity might be related to litter decomposition rates has important implications for our understanding of soil–plant feedbacks. Based on our forest‐based study, we predict that soil–plant feedbacks via acidity are unlikely to be strong in ecosystems with wide‐ranging species in terms of their leaf functional traits, including leaf pH. A free Plain Language Summary can be found within the Supporting Information of this article.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2435.13413

Wendan Liu

Papers

Linking litter decomposition of above‐ and below‐ground organs to plant–soil feedbacks worldwide

Linking litter decomposition of above- and below-ground organs to plant-soil feedbacks

Traits including leaf dry matter content and leaf pH dominate over forest soil pH as drivers of litter decomposition among 60 species