Is Plant Biomass Input Driving Soil Organic Matter Formation Processes in Grassland Soil Under Contrasting Management?

Grassland management practices vary in intensity (stocking rates, fertilization) and plant removal strategies (grazing versus mowing). They influence organic matter 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 organic matter (SOM) formation processes along a plant biomass input gradient. We used a thirteen-year experiment in Central France under contrasting management (unmanaged, grazing with two intensities, mowing, bare fallow), established a plant biomass input gradient based on biomass leftover after harvest and investigated microbial functioning, necromass accumulation, organic matter degradation and SOM accumulation processes. Our experimental approach included soil analyses for amino sugars, microbial biomass C and N, basal respiration, seven enzyme activities and microbial growth kinetics as indicators of microbial functioning and degradation processes.Response of the parameters to plant biomass input along the gradient could be arranged into four groups. Amino sugar content and microbial C/N ratio showed a linear response. SOC content and SOC-dependent parameters (enzyme activities, active microbial biomass) showed a bell-like response, most likely reflecting input diversity. Microbial activity showed a ripple-like response to decreasing biomass input indicating its dependence on physiochemical conditions and root biomass, while GluN/GalN and qCO2 were not influenced by grassland harvesting regimes. These results indicate that grassland harvesting management influences microbial activity and functioning not only by changing plant biomass input, but also through its effects on soil (microbial) functioning possibly related to changing physiochemical soil properties. We conclude that management controls SOM formation through its indirect impacts on belowground (microbial) processes in addition to modifying plant biomass inputs.