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Plant litter constitutes the dominant resource to soil food webs, which gradually decompose litter and transform it into soil organic matter. A central paradigm of this transformation posits that differences in quality between distinct litter types disappear during decomposition, as litter types converge towards similar physicochemical characteristics. Yet, this paradigm is debated and not based on clear metrics. It is also largely derived from microbial decomposition studies, while the effect of litter‐feeding soil animals, by transforming large quantities of litter into faeces, remains poorly documented. We addressed this knowledge gap by quantifying the variability in physicochemical characteristics amongst leaf litter of six tree species of contrasting quality, before and after conversion into faeces by six soil animal species. We found that litter conversion into faeces by diverse soil animals largely reduced the variability in physical and chemical characteristics between contrasting litter types. We also evaluated the consequences of this animal‐driven convergence on further microbial‐driven convergence during decomposition, by decomposing intact litter and soil animal faeces and comparing the chemical characteristics of decomposed materials. Chemical variability amongst uningested litter and amongst soil animal faeces converged at similar rates. This indicates that animal‐ and microbial‐driven convergence are additive, and that soil animals precipitate organic matter quality convergence during decomposition. We propose here a new framework and an associated metric to study changes in organic matter quality variability during decomposition, which we argue are essential for an improved understanding and modelling of litter decomposition and soil organic matter formation. [ABSTRACT FROM AUTHOR]

In many terrestrial ecosystems, earthworms operate at the interface between plants and soil. As ecosystem engineers, they affect key ecosystem functions such as decomposition, nutrient cycling and bioturbation. Their incidence and abundance depends on several soil properties, yet simultaneously they also impact soil properties themselves. The existence of a positive feedback loop in which earthworm activity maintains their own niche—by promoting turnover rate in the forest floor, thereby increasing topsoil pH and creating suitable living conditions for themselves—has been suggested before, yet lacks supporting evidence.Here we assessed how tree species litter traits relate to such below‐ground interactions in forests across Europe. Specifically, we hypothesized a below‐ground feedback loop between burrowing earthworm biomass, humus form and pH, affected by litter quality. We tested this hypothesis by means of structural equation modelling.Our results demonstrate that litter nutrient concentrations affect both burrowing and litter dwelling earthworm biomass, which in turn directly impacts humus form and indirectly soil pH. At a continental scale, that is, including all edaphic conditions, soil pH did not feed into earthworm biomass nor could we link leaf structural recalcitrance (e.g. lignin) or functional diversity to below‐ground interactions.However, in forests where moisture is not limiting, soil acidity proved an important factor determining the context of below‐ground interactions. Therefore, we were able to confirm the hypothesized feedback loop for forest ecosystems with soil pH ≤ 5. In calcareous and/or periodically dry forests, other factors than soil chemistry and litter quality became determinant for earthworm biomass.The activity of burrowing earthworms is pivotal in below‐ground ecosystem functioning of mesic forest soils, impacting litter accumulation and forest floor conditions above‐ground, the pH and nutrient status below‐ground and ultimately their own living conditions. This highlights earthworm bioturbation as a key mechanism for understanding plant–soil interactions in forests. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Increasing climate variability is one of the dominant components of climate change, resulting particularly in altered rainfall patterns. Yet, the consequences of rainfall variability on biogeochemical processes that contribute to greenhouse gas emissions has received far less attention than have changes in long-term mean rainfall. In particular, it remains unclear how leaf litter decomposition responds to changes in rainfall frequency compared to changes in cumulative rainfall quantity, and if changes in rainfall patterns will differentially affect organisms in the decomposer food web (e.g., microbial decomposers that break down leaf litter through saprotrophic processes versus detritivores that directly ingest leaf litter). To address this knowledge gap, we disentangled the relative importance of cumulative rainfall quantity and rainfall frequency on both microbial- and detritivore-driven litter decomposition, using the isopod Armadillidium vulgare as a model macro-detritivore species and simulating rainfall in a full-factorial microcosm experiment. We found that microbially-driven decomposition was positively related to cumulative rainfall quantity, but tended to saturate with increasing cumulative rainfall quantity when rainfall events were large and infrequent. This saturation appeared to result from two mechanisms. First, at high level of cumulative rainfall quantity, large and infrequent rainfall events induce lower litter moisture compared to smaller but more frequent ones. Second, microbial activity saturated with increasing litter moisture, suggesting that water was no longer limiting. In contrast, isopod-driven decomposition was unaffected by cumulative rainfall quantity, but was strongly controlled by the rainfall frequency, with higher isopod-driven decomposition at low rainfall frequency. We found that isopod-driven decomposition responded positively to an increase in the weekly range of soil moisture and not to mean soil or litter moisture, suggesting that an alternation of dry and moist conditions enhances detritivore activity. Collectively, our results suggest that A. vulgare morphological and behavioral characteristics may reduce its sensitivity to varying moisture conditions relative to microbial decomposers. We conclude that the activity of microorganisms and isopods are controlled by distinct aspects of rainfall patterns. Consequently, altered rainfall patterns may change the relative contribution of microbial decomposers and detritivores to litter decomposition. • We disentangled the effect of rainfall quantity from frequency on decomposition. • We compared the responses of microbially- and isopod-driven litter decomposition. • Microbially-driven decomposition increased with increasing rainfall quantity. • Isopod-driven decomposition increased with decreasing rainfall frequency. [ABSTRACT FROM AUTHOR]

In many terrestrial ecosystems, detritivorous soil organisms ingest large amounts of leaf litter returning most of it to the soil as faeces. Such conversion of leaf litter into faeces may stimulate decomposition by increasing the surface area available for microbial colonisation. Yet, experimental support for either the outcome or the mechanism of these conversion effects is lacking.Based on the hypothesis that the identity of plant species from which leaf litter is transformed into faeces has a critical role in how faeces decomposition proceeds, we collected faeces of the widely abundant millipede Glomeris marginata fed with leaf litter from seven distinct tree species. We compared the physical and chemical characteristics and the rates of carbon (C) and nitrogen (N) loss between litter and faeces.We found that after 100 days of exposure under controlled conditions, C loss was on average higher in faeces (40.0%) than that in litter (26.6%), with a significant increase for six of the seven species. Concurrently, N dynamics switched from a net immobilisation (7.7%) in litter to a net release (14.6%) in faeces, with a significant increase for five of the seven species.Litter conversion into faeces generally homogenised differences in physical and chemical characteristics among species. Despite such homogenisation, variability in rates of faeces C and N loss among species was similar compared to leaf litter, but correlated with a different set of traits. Specifically, faecal pellet C loss was positively related to compaction (decreased specific area and increased density of faecal pellets), and both C and N loss from faecal pellets were positively related to fragmentation (increased specific area and perimeter of particles within faecal pellets).We conclude that litter fragmentation and compaction into detritivore faecal pellets lead to substantially enhanced decomposition, with a particularly strong impact on N dynamics that changed from immobilisation to net release depending on litter species. Moreover, litter quality control on decomposition is reshuffled by litter conversion into faeces. In ecosystems with high detritivore abundance, this so far largely overlooked pathway of organic matter turnover may strongly affect ecosystem C and N cycling. A plain language summary is available for this article. Plain Language Summary [ABSTRACT FROM AUTHOR]

Leaching of water-soluble compounds is a dominant process during the first stages of litter decomposition, providing the microorganisms in the underlying soil with an important source of labile carbon and nutrients. Leachate composition (quantity and quality) can vary considerably among different plant species, but its consequences for soil microbially-driven processes remains largely unexplored. Here, we evaluated the differences in leachate quantity and quality from freshly fallen leaf litter of widely distributed coniferous and deciduous broadleaf tree species of European temperate forests, and their effects on soil microbial responses in a microcosm experiment under controlled conditions. Leachates of broadleaf litter contained higher amounts of carbon and nitrogen available for microbes, but with substantially higher aromaticity than leachates from coniferous litter. A one-time leachate addition to soils immediately increased soil microbial respiration with longer lasting effects of deciduous broadleaf compared to coniferous litter leachates leading to a microbial community with an apparently more efficient use of carbon. When leachates of different species were mixed, the observed microbial responses differed in some cases from that expected based on soils to which leachates from single species were added. These non-additive effects were partly explained by the functional dissimilarity of leachate traits, suggesting complementary resources for microorganisms when leachates of different species are available. Our data show that species-specific litter-derived leachates of varying quantity and quality and their mixtures distinctly affect soil microorganisms. In forest ecosystems with recurrent leaf litter inputs from the same species, such leachate effects may determine soil processes also in the longer term, controlling biogeochemical cycling to an important degree. [ABSTRACT FROM AUTHOR]

In biodiverse ecosystems, leaf litter of different plant species decomposes in mixtures, for which decomposition rates notoriously deviate from that expected from monospecific treatments. Despite important research efforts in past decades, these litter diversity effects remain difficult to predict. We hypothesized that this is due to a focus on bulk litter decomposition, while different carbon fractions constituting the litter may respond differently to litter diversity, thereby blurring the overall response. To test this hypothesis, we determined how the decomposition of (i) soluble compounds, (ii) cellulose, and (iii) lignin responded to litter mixing in a 3.5-year field experiment in an alpine forest. We found that the decomposition of soluble compounds and cellulose in mixtures was faster than expected from monospecific treatments, while that of lignin was slower. These deviations from expected decomposition rates of each litter carbon fraction were driven by different aspects of the litter functional diversity. This suggests that different mechanisms operating on distinct litter fractions lead to synergistic and antagonistic interactions that simultaneously affect bulk litter decomposition. Furthermore, the magnitude of these fraction-specific deviations from expected decomposition rates consistently decreased throughout decomposition. Considering the response of litter fractions and their temporality, rather than focusing on bulk litter thus seems critical to evaluate the response of decomposition to plant diversity and identify underlying mechanisms. • We disentangled the response of different litter fractions to litter diversity. • Litter functional diversity increased labile compound decomposition. • Litter functional diversity decreased recalcitrant compound decomposition. • These effects were driven by different aspects of litter functional diversity. • The strength of litter diversity effects faded throughout decomposition. [ABSTRACT FROM AUTHOR]