Delayed and asymmetric responses of soil C pools and N fluxes to grassland/cropland conversions
Résumé
Agricultural lands are subject to changes in use for a variety of environmental and economic reasons. Among these changes, shifts between crops and grasslands are particularly common. Understanding the effects of these changes on soil functioning is crucial to anticipating crop productivity and environmental impacts and yet has rarely been studied. The purpose of this study was to investigate the in situ temporal dynamics of soil N cycling and C pools after grassland/cropland conversions and risks of N losses from such systems. The experiment was conducted in western France at a long-term experimental site with a temperate oceanic climate. We followed soil N cycling and C pools for four experimental treatments over 36 months: (i) continuous grassland, (ii) continuous rotation of annual crops, (iii) conversion from grassland to a rotation of annual crops, and (iv) conversion from a rotation of annual crops to grassland. Gross N mineralization, potential N immobilization and potential nitrification were estimated using the 15N dilution/enrichment technique and FLUAZ model, based on samples from the 0–10 cm soil layer (0, 0.75, 3, 12, 24 and 36 months after change in land use) and 10–30 cm soil layer (after 24 and 36 months) in all 4 treatments. Nitrogen pools (NH4+, NO3−, and organic N), total carbon (C), dissolved organic C, microbial biomass C, and C mineralization were also determined on all soil samples