Soil fauna precipitate the convergence of organic matter quality during decomposition

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.Output Status: Forthcoming/Available Onlin

Applying SMT Solvers to the Test Template Framework

The Test Template Framework (TTF) is a model-based testing method for the Z notation. In the TTF, test cases are generated from test specifications, which are predicates written in Z. In turn, the Z notation is based on first-order logic with equality and Zermelo-Fraenkel set theory. In this way, a test case is a witness satisfying a formula in that theory. Satisfiability Modulo Theory (SMT) solvers are software tools that decide the satisfiability of arbitrary formulas in a large number of built-in logical theories and their combination. In this paper, we present the first results of applying two SMT solvers, Yices and CVC3, as the engines to find test cases from TTF's test specifications. In doing so, shallow embeddings of a significant portion of the Z notation into the input languages of Yices and CVC3 are provided, given that they do not directly support Zermelo-Fraenkel set theory as defined in Z. Finally, the results of applying these embeddings to a number of test specifications of eight cases studies are analysed.Comment: In Proceedings MBT 2012, arXiv:1202.582

Detritivore conversion of litter into faeces accelerates organic matter turnover

Litter-feeding soil animals are notoriously neglected in conceptual and mechanistic biogeochemical models. Yet, they may be a dominant factor in decomposition by converting large amounts of plant litter into faeces. Here, we assess how the chemical and physical changes occurring when litter is converted into faeces alter their fate during further decomposition with an experimental test including 36 combinations of phylogenetically distant detritivores and leaf litter of contrasting physicochemical characteristics. We show that, across litter and detritivore species, litter conversion into detritivore faeces enhanced organic matter lability and thereby accelerated carbon cycling. Notably, the positive conversion effect on faeces quality and decomposition increased with decreasing quality and decomposition of intact litter. This general pattern was consistent across detritivores as different as snails and woodlice, and reduced differences in quality and decomposition amongst litter species. Our data show that litter conversion into detritivore faeces has far-reaching consequences for the understanding and modelling of the terrestrial carbon cycle

La diversité chimique des litières et son rôle dans le fonctionnement des écosystèmes forestiers

La décomposition des litières de feuilles mortes et le recyclage des nutriments sont des processus complexes essentiels au fonctionnement des écosystèmes. L'objectif principal de cette thèse est de mieux comprendre les déterminants de la décomposition dans une forêt tropicale humide de Guyane Française. Je me suis plus particulièrement intéressé au rôle de la diversité chimique des litières et aux interactions entre les plantes vivantes et les litières. Grâce à une caractérisation précise des tannins présents dans les litières de 16 espèces communes d'arbres tropicaux, j'ai pu montrer que la variation interspécifique des concentrations en tannins constitue le principal déterminant de la décomposition des litières dans l'écosystème étudié. Cet effet des tannins semble principalement résulter d'une consommation préférentielle des litières pauvres en tannins par la macrofaune détritivore. Parmi ces 16 espèces de litières, certaines ont été mélangées dans une expérience en serre incluant également des plantules de ces espèces. J'ai ainsi mis en évidence expérimentalement un effet de la diversité des litières sur le processus de décomposition et sur la dynamique des nutriments. Les effets non-additifs observés sont principalement négatifs, c'est-à-dire que la décomposition a été moins rapide par rapport aux valeurs attendues à partir de la décomposition des espèces prises individuellement. L'identité des espèces de litière a fortement contribué à ces effets négatifs, et la présence de plantules a modulé ces effets de la diversité. J'ai aussi montré que la décomposition des mélanges de litières a également un impact sur la croissance, l'allocation de la biomasse et les traits foliaires des plantules. Ces rétroactions étaient à nouveau liées à la présence de quelques espèces particulières dans les litières. Mes résultats suggèrent que la diversité des litières ayant des compositions chimiques contrastées joue un rôle central dans le fonctionnement des forêts tropicales. La décomposition des mélanges de litières est au cœur de rétroactions entre les compartiments aériens et souterrains de ces écosystèmesLitter decomposition and nutrient recycling are key processes for ecosystem functioning. The objectives of this thesis were to improve our understanding of the determinants of decomposition in a tropical rainforest in French Guiana, with a particular focus on the role of litter chemical diversity and live plant litter interactions. With a detailed characterization of condensed tannins in leaf litter of 16 common tropical tree species, I showed that inter-specific variation in litter tannins are the main driver of decomposition in the studied nutrient-poor Amazonian rainforest. Experimental evidence suggests that the most important mechanism of tannin control is selective feeding on tannin-poor litter by soil macrofauna. The mixing of leaf litter of a subset of the 16 tree species in a green house experiment including seedlings of the different tree species showed litter diversity effects on decomposition and nutrient release. These non-additive effects were predominantly negative, resulting in decreased decomposition rates compared to expected rates based on individual litter species decomposition. The identity of litter species contributed strongly to these negative effects, and the presence of tree seedlings modified the observed litter diversity effects. Decomposing litter mixtures showed also a feedback effect on tree seedling growth, biomass allocation and leaf traits, which again was mainly related to the identity of a few litter species within mixtures. My results suggests that the diversity of chemically contrasted litter plays a major role in the functioning of tropical forets. The decomposition of these mixtures results in functional feedbacks between the aboveground and belowground compartments of these ecosystemsMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

The fate of condensed tannins during litter consumption by soil animals

International audienceCondensed tannins (CT) can strongly affect litter decomposition, but their fate during the decomposition process, in particular as influenced by detritivore consumption, is not well understood. We tested the hypothesis that litter CT are reduced by the gut passage of two functionally distinct detritivores of Mediterranean forests, the millipede Glomeris marginata, and the land snail Pomatias elegans, as a fixed proportionof initial litter CT, but more so in Pomatias since snails are known to have a more efficient enzymatic capacity.Contrary to our hypothesis, both detritivore species reduced litter CT to near zero in their faecal pellets irrespective of the wide range in initial leaf litter CT concentrations of 9–188 mg g1 d m among three Mediterranean tree species (Pistacia terebinthus, Quercus ilex, Alnus glutinosa) and different decompositionstages of their litter. The almost complete disappearance of CT even from some litter types highly concentrated in CT, due to either degradation by gut microorganism or complexation of CT into insoluble high molecular weight structures, suggests a high ‘‘de-tanning’’ efficiency across functionally distinct detritivore species. The transformation of CT-rich litter into virtually CT-free faecal pellets by detritivores might be highlyrelevant for the subsequent decomposition process in ecosystems with a high macrofauna abundance and CT-rich plant species such as Mediterranean forests

coevolution

Python 2 code to smile;ulate the coevolutionary dynamics of symbiotic capacity and tannin production

spatial implicit

Python 2 code to reproduce Fig. 3. The prediction for tannin evolution in the spatial implicit approach, as a function of turnover rate and soil fertility

Data from: Positive plant-soil feedbacks trigger tannin evolution by niche construction: a spatial stoichiometric model

1. Among plant traits, plant secondary metabolites such as tannins mediate plant herbivore interactions but also have after-life effects on litter decomposition and nutrient cycling. We propose that niche construction mechanisms based on positive plant-soil feedbacks (PSF) could influence the evolution of tannin production. 2. By modeling the flow of nitrogen (N) and carbon (C) through plants and soil in a spatially explicit context, we explored the relative contribution of herbivory and positive PSF as drivers of tannin evolution. We assumed soil N to be contained in labile and recalcitrant compartments, the latter made of tannin-protein complexes from which plants can absorb nutrients via associations with mycorrhizal fungi. 3. In infertile environments and for plants with low biomass turnover rates, we show that when tannins modify soil properties locally, positive PSF alone can drive their evolution. We further predict the existence of positive coevolutionary feedbacks between associations with mycorrhizal fungi with a decaying ability and tannins, possibly triggered by the evolution of the latter as protection against herbivores. In line with our theoretical results, empirical evidence suggest that tannins are mostly present in plants with low tissue turnover, associated with mycorrhizal fungi able to decay organic matter and inhabiting infertile environments. 4. Synthesis. Our model proposes that the evolution of tannin production can be triggered by positive PSF, provided that tannins promote the local N retention and that mycorrhizal fungi associated with plants are able to absorb N from tannin-protein complexes. In our model, tannin production evolves only in infertile ecosystems, in agreement with field observations. Our findings highlight that the strength of niche construction depends on the ecological context, hence that global ecological properties constrain local eco-evolutionary dynamics

cellular_automaton

Python 2 code of the cellular automaton, used to generate Fig. 2 and Fig. 4

Quels enjeux sont associés à la biodiversité des sols ?

Ce numéro est constitué d’articles issus du colloque «De la connaissance de la biologie des sols et de ses fonctions, à son pilotage » organisé en partenariat avec l’Ademe et l’AFB, le 18 octobre 2018 à Dijon.Les organismes du sol, caractérisés par une diversité phénoménale mais mal répertoriée, organisée enréseaux d’interactions complexes, sont traditionnellement traités comme une boîte noire dans lacompréhension conceptuelle et mécanistique du fonctionnement de l’écosystème. Dans le contexted’une érosion globale de la biodiversité, il est important de mieux connaître cette diversité et d’encomprendre le rôle fonctionnel afin de prédire les conséquences de la disparition des espèces sur lefonctionnement des écosystèmes naturels et agricoles. En utilisant quelques exemples issus de larecherche récente, nous montrons ici qu’une caractérisation relativement simple de la biodiversité sur labase de quelques traits fonctionnels pourrait permettre de quantifier son impact sur des processus cléscomme le recyclage de la matière organique. Nous mettons en lumière l’importance de prendre encompte les interactions entre niveaux trophiques, et en particulier le rôle de la diversité fonctionnelledes résidus végétaux dans ces interactions. Une recherche qui vise à définir des traits fonctionnelspertinents pour les processus du sol et les services écosystémiques associés, comme la production dematière végétale, le maintien de la fertilité des sols ou le stockage du carbone, devrait permettre deprendre en compte la diversité fonctionnelle des organismes du sol dans les décisions de gestion et lespratiques agricoles