How microbial belowground communities cope with contrasted trophic resources across three land use type: a stoichiometry approach

Litters are the main trophic resources for soil microorganisms and play a key role in soil nutrient recycling. The amount of inorganic nutrients released into the ecosystem depends on the relative carbon (C) to nutrient (nitrogen and phosphorus, i.e. N and P) demand of the microorganisms, as well as the chemical composition of litters. The aim of this study was to determine how soil microbes regulate their C/N/P homeostasis to adapt to different litter resources and how this in turn affects the dynamics of organic matter. The hypothesis were a) litter mass losses are greater in “natural lands” because of wider functional dissimilarity and b) variation in soil microbial C/N/P ratio during the decomposition process reflects the difference of litter quality and is mainly caused by the shifts of community structure and function (enzyme excretion). In order to characterize the stoichiometric C/N/P ratio dynamics in added litters and soil microbial communities and the interaction between them, an in situ litter bag experiment was established using a reciprocal transplant design at the Mauguio INRA experimental station (Southern France). To explore the functional dissimilarity of soil microorganisms in different land use plots, a gradient going from little disturbed to highly disturbed soils was used. Low disturbed soil corresponded to a 25-year old walnut and black locust plantation with no fertilization and pesticides since its establishment. The second land use was an agricultural plot with organic practices since 2009 while the third land use corresponded to a plot with the same soil type but with conventional practices. Three different litters (wheat straw, pea and walnut leaves) were buried at 8cm depth into the three land use plots. Changes in litter quality, C/N/P ratio of the microbial biomass and of enzymes, DOC, mineral N and available P were monitored at different stages of decomposition.