Spatial variability of 2,4-dichlorophenoxyacetic acid (2,4-D) mineralisation potential at a millimetre scale in soil

International audienc

Opportunities and limits in imaging microorganisms and their activities in soil microhabitats

International audienceThe soil microhabitat is a heterogeneous and complex environment where local variations can modulate phenomena observed at the plot scale. Most of the current methods used to describe soil functioning are bulk soil analyses which do not account for fine-scale spatial variability and cannot fully account for the processes that occur under the influence of the 3D organisation of soil. A good representation of spatial heterogeneities is necessary for the parametrisation of new models, which aim to represent pore-scale processes that affect microbial activity. The visualization of soil at the scale of the microhabitat can be used to extract descriptors and reveal the nature of the relationships between the fine-scale organisation of soil's constituent parts and soil functioning. However, soil imaging techniques tend to be under-used, possibly due to a lack of awareness of the methods or due to a lack of access to the relevant instruments. In recent years, new methods have been developed, and continuously improved, offering new possibilities to decipher and describe soil physical, chemical and biological features of the soil microhabitat in evermore exquisite detail. 1 This review is structured into several parts in which first imaging methods that are useful for describing the distribution of microorganisms and microbial activities, followed by methods for characterising the physical organisation of the microhabitat and, finally, methods for characterising the distribution of soil chemical features, including soil organic matter, are described. Special attention is given to the preparation steps that are required for the proper use of the methods, either alone or in combination

Spatial and temporal evolution of detritusphere hotspots at different soil moistures

International audienceAs a result of the heterogeneous spatial distribution of microorganisms and substrates in soil and their restricted accessibility, biodegradation occurs mainly in hotspots, such as in the detritusphere, induced by decomposing plant residues. Knowing the characteristics of the volume of soil involved in biodegradation of a given organic substrate will facilitate the understanding and prediction of biodegradation. Our objectives were (i) to identify the volume of soil involved in the biodegradation of plant residues and (ii) to determine to what extent this volume is affected by soil moisture under diffusive conditions by monitoring the mineralization and spatio-temporal evolution of residue C and microorganisms in soil at the microbial habitat scale. We incubated repacked soil cores with a central layer of C-13-labelled maize residues at three different matric potentials (0.0031, 0.031 and 0.31 MPa). We monitored C-13 and total C mineralization, and at different dates over 45 days of incubation, we separated soil slices with increasing distances from the residues and analysed C-13 from the residues and the microbial community structure and its activity by PLFA and C-13-PLFA processing. Residue mineralization increased with increasing soil moisture. A detritusphere a few mm thick was rapidly established, with a decreasing gradient of C-13 and total PLFAs and C-13-PLFAs away from the residue layer. Most C-13 from the residues was located in the first 2 mm of the detritusphere and was not dependent on the matric potential. Residue mineralization seemed to take place mainly on the residues themselves, but increasing residue C was transferred to the surrounding soil with increasing soil moisture. Dry conditions slowed residue C transfer and favoured fungi, but residue mineralization was carried out by both bacteria and fungi

Modelling the impacts of maize decomposition on glyphosate dynamics in mulch

International audienceThe retention of crop residues as mulch on the soil surface in conservation agriculture systems greatly influences the fate of pesticides, as most of the applied pesticide is intercepted by mulch before moving to the soil. This work was conducted in order to model the effect of maize decomposition on glyphosate degradation in mulch and soil. Labelled 14C-glyphosate degradation was monitored for 49 days in three treatments with the same soils but with maize residues at different stages of decomposition (0, 20 and 49 days). Fresh residues of maize (0 days) exhibited an evolution of their biochemical fractions to a greater extent than decomposed residues. Glyphosate mineralization was faster in the 0-day treatment in mulch residues and in the soil layer below the mulch. However, a greater formation of non-extractable residues (NERs) was observed in mulch residues and soils in the 20- and 49-day treatments than in the 0-day treatment. Modelling maize mulch decomposition with the COP-soil model indicated that microbial activity was different in the three treatments and depended on the initial composition of maize residues. Glyphosate mineralization in mulch and soil can be simulated with an assumption of co-metabolism by coupling the modules of pesticide degradation and mulch carbon decomposition. Glyphosate and its metabolites, including soluble and adsorbed fractions, were simulated with the same adsorption coefficients for all treatments. The simulation of NER formation, however, suggested that more than one microbial population may be involved in the degradation process and could be added in the future development of the model

Effect of multiple stresses, organic amendment and compaction, on the fate and impact of isoproturon in soil

International audienceOrganic matter decline and compaction are two major processes of soil degradation. Organic amendment is a current practice to compensate the loss of organic matter, which could in addition contribute to increase soil aggregate stability and limit compaction. Therefore, the objective of this work was to study the effect of multiple physico-chemical stresses, organic amendment (compost of sewage sludge and green waste) addition and soil compaction, on the fate and impact (measured through the urease enzyme activity) of isoproturon. Compost addition and compaction did not significantly affect the fate and impact of isoproturon. The lack of effect of compost can be due to the delay between soil sampling and soil amendment. Compaction had no effect probably because the porosity reduction does not affect the habitable pore space accessible to degrading microorganisms. Nevertheless, isoproturon significantly increased the urease enzyme activity in compacted and not compacted unamended soils contrary to the amended ones. It seems that the organic amendment could act as a buffer with regards to the impact of isoproturon. The results obtained in this work suggest that, in general, the fate and impact of isoproturon in soils will not change following compaction and/or organic amendment addition, neither the corresponding risks for the environment

Trace contaminants in the environmental assessment of organic waste recycling in agriculture: Gaps between methods and knowledge

International audienceAgricultural recycling of organic waste (OW) derived from urban, agricultural and agroindustrial sources is an essential sustainable development strategy. Yet repeated application of nutrient-laden OW in crop fields can also drastically boost contaminant levels in soil. This review focuses on the consideration of three categories of OW-borne contaminants, namely trace elements, organic contaminants and pathogens (including antibiotic resistance), in environmental assessments, chiefly involving life cycle assessment (LCA) and risk assessment (RA). The in-depth discussion also focuses on gaps between empirical knowledge and the models underlying these frameworks. Potential improvements to fill the identified gaps are proposed, including novel approaches and uses of existing approaches, while also featuring various levels of “readiness.” Finally, a comprehensive theoretical framework to assess OW recycling scenarios, combining complementary approaches and models, is proposed and exemplified

Evaluation des services écosystémiques fournis par les sols de micro-fermes urbaines : Méthodologie et retours d'expériences

International audienceUrban micro-farms are rapidly developping driven by NGOs and supported by citizen and local politics. This new object of research wasdefined in two different contexts: urban and peri-urban/rural environments (Daniel 2019; Morel et al. 2017). In urban environments micro-farmare small scale farms (less than 1.5EFT per ha), involving often volunteers and generating a diversity of activities. The different functionsexpected from them (food provisioning, teaching support, social and cultural activity, water retention, valorization of wastes and landscape...)are directly linked to a diversity of ecosystem services that they can deliver. Nevertheless, only a few studies focus on micro-farms and donot allow a sufficient understanding of the ecosystem services that they can deliver. In parallel, the involvement of farmers in the evaluation ofecosystem services seems to be a great opportunity, if not a necessary condition. Based on this observation, the research project SEMOIRS(2018 – 2020), funded by ADEME, aims to evaluate ecosystem services delivered by urban micro-farms. The aim of this article is to presentand discuss the methodology of the project. Seven micro-farms were studied during two years: 4 were located at the soil level and 3 at rooftoplevel [see table 1]. Five ecosystem services were studied: food production (quantity and quality), biodiversity supporting (flora, fauna, andmicroorganisms), regulation (water catchment and quality, local climate regulation, use of urban waste and carbon storage) as well as socialand cultural services. A set of indicators was set up [see table 3] based on three types of data : (i) participative measurements achieved bythe farmers themselves, (ii) field measurements (including a questionnaire for micro-farm users) performed by researchers and (iii) externaldata, based on laboratory measurements, data bases, literature etc. The study has shown that all of the seven soils of micro-farms are dee-ply disrupted, from removed topsoil to isolatic Technosol (IUSS Working Group WRB, 2014). The methodology used raised key questionsregarding ecosystem services evaluation. (i) First, there is a trade-off to be dealt within the participative approach, between the targetedprecision of data needed on the one hand and the time spent by stakeholders for the measurement on the other hand. Despite these limits,the participative approach allows for a better understanding of the ecosystem services drivers by the farmers as well as collecting data notaccessible otherwise. (ii) Second, the choice of references to which we compare the ecosystem services delivered by urban micro-farmsis crucial. In the project, three reference were considered: professional vegetables farms, other urban agriculture forms and other type ofurban green spaces. We identified a serious lack of reference data on the considered ecosystem services that limits the comparison and thediscussion. (iii) Finally, our work raised the question of the scale at which to study different ecosystem services, as the different ecosystemservices operate at different spatial scales. Four scale were used here: soil, farm, neighborhood and the territory (here, the city; see figure 3).Encompassing different scales raises however the questions of a common study scale and that of the methodology needed to change fromone scale to another. However, despite those limits and perspectives the proposed methodology, relying partly on a participatory approach,appears promising to evaluate the ecosystem services provided by urban micro-farmsObjet récent de recherche, les micro-fermes urbaines sont aujourd'hui en plein essor, portées par un mouvement associatif, citoyen, entrepreneurial et politique relativement important. Ces fermes sont définies par leurs petites surfaces (moins d'1,5 ha paractif), l'implication de bénévoles et une diversité d'activités. Différentes fonctions sont ainsi associées à ces lieux (éducatives, productions alimentaires, loisirs etc.), faisant de ces micro-fermes urbaines des nouveaux types d'espaces végétalisés en ville susceptibles de fournir de multiples services écosystémiques. Néanmoins, le manque de connaissances actuelles sur le sujet ne permet pas d'appréhender dans le détail l'importance des services rendus. Par ailleurs, l'implication des parties prenantes dans une telle évaluation parait être un enjeu essentiel afin de comprendre les déterminants et facteurs d'influence de ces fermes ; les porteurs de projet étant eux-mêmes, par exemple, créateurs de savoirs et de pratiques agronomiques originales en s'adaptant aux contraintes du milieu urbain. L'objectif de cet article est de décrire et de discuter une méthodologie d'étude des services écosystémiques rendus par les micro-fermes urbaines, mise en oeuvre dans le cadre du projet SEMOIRS (Evaluation des Services Ecosystémiques rendus par les MicrO-fermes urbaInes et leurs Sols ; 2018 –2020) financé par l'ADEME. Au sein de ce projet, 7 micro-fermes et leurs sols à Paris et en petite couronne ont été étudiés durant deux ans. Ces fermes situées en toiture (3) et de plain-pied (4) sont illustratives de la diversité de cette forme d'agriculture urbaine. Des indicateurs ont été identifiés et sélectionnés pour étudier les différents services : (i) service d'approvisionnement alimentaire (rendement et qualité de la production), (ii) des services de régulation (rétention d'eau, qualité de l'eau de percolation, recyclage de résidus urbains, stockage de carbone dans le sol, fourniture de nutriments), (iii) service de support de biodiversité et (iv) des services culturels (impact sur l'apprentissage, le bien-être, l'esthétique) et paysagers. Trois types de méthodes ont été mobilisés pour acquérir les données : (1) participatives, (2) relevés, observations et enquêtes de terrain ainsi que (3) des mesures dites externes (en laboratoire ou à partir de bases de données). Après avoir présenté les indicateurs et méthodes utilisés par service, l'article discute de cette méthodologie et des perspectives pour l'étude de projets en milieu urbain. Cette approche participative couplée à une démarche d'acquisition de données « externes » paraît pertinente et génératrice de savoirs tant sur les services rendus que sur leurs facteurs d'influence