Impacts of Grassland/arable Crop Conversion on Soil C Pools, Gross N Fluxes and the Denitrifying Community.

Changes in land use and land management impact fertilizer and organic matter management, nutrient dynamics in soil, organic matter stabilization, soil microbial community structure and activities, and C and N fluxes in soil and to the atmosphere and hydrosphere. The present work examines the effects of shifting from an established grassland to a rotation of annual crops and vice et versa, at the long-term observatory (ORE) of the experimental INRA site in Lusignan (France).

Four treatments were studied: (i) Annual crop rotation, (ii) Establishment of grassland on a previously cropped soil, (iii) Permanent grassland, (iv) Establishment of annual crop rotation after ploughing grassland. Concurrently soils from the 4 treatments were sampled from the 0-10 cm top soil layers, at 6 times during 3 years after the conversion, and used for determination of microbial C, soluble C, nitrification and denitrification enzyme assays. Gross mineralization, nitrification and immobilization were calculated using the 15N isotope techniques and the FLUAZ model. The genetic structure of the denitrifying community was studied from extracted DNA by PCR-DGGE targeting the functional gene nirK. To quantify the size of the denitrifying community with the abundance of nirK copies, PCRq was performed on each soil samples.

The ploughing of grassland affected significantly DOC, microbial biomass, gross N fluxes, the denitrifying activity and the genetic structure of the denitrifying community. Conversely, the establishment of grassland on arable land had a significant effect only on the genetic structure of the denitrifying community. On the short term (< 3 years), the destruction of the grassland followed by the establishment of an annual crop rotation was a more intense disturbance than the establishment of grassland on arable soils.