Is plant biomass input driving soil organic matter formation processes in grassland soil under contrasting management?

Grassland management practices vary in stocking rates and plant removal strategies (grazing versus mowing). They influence organic matter (OM) inputs, which were postulated as main controls of soil organic carbon (SOC) sequestration and might therefore control SOC stabilization. The aim of this study was to test this hypothesis by investigating the impacts of grassland harvesting regimes on parameters related to soil microbial functioning and soil organic matter (SOM) formation processes. We used a thirteen-year experiment in Central France under contrasting management (unmanaged, grazing with two intensities, mowing, bare fallow) to establish a carbon input gradient based on biomass leftovers after harvest. We investigated microbial biomass, basal respiration and enzyme activities as indicators of microbial functioning, and amino sugar content and composition as indicator of persistent SOM formation and origin through necromass accumulation. Responses of these parameters to carbon input along the gradient were contrasting and in most cases unrelated. Only the microbial C/N ratio and amino sugar contents showed a linear response indicating that they are influenced by plant-derived OM input. Other parameters were most probably more influenced by root activity, presence of herbivores, and/or physicochemical changes following management activities impacting soil microbial functioning. Grassland harvesting strategies influence SOC sequestration not only by changing carbon input quantity, but also through their effects on belowground processes possibly related to changing carbon input types and physiochemical soil properties.

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