Irrigation practices may affect denitrification more than nitrogen mineralization in warm climatic conditions

Predicting the impact of irrigation practices on soil N mineralization and N balance is an important issue to optimize N fertilization and reduce the N losses towards the environment. The effect of summer irrigation on N dynamics was investigated in two arable fields in Southern France. Net N mineralization was assessed by combining frequent measurements of water and mineral N contents in soil and the use of a calculation model (LIXIM). It was first calculated assuming that denitrification was negligible. This hypothesis led to inconsistent results, apparent net N mineralized being smaller under irrigated than non-irrigated conditions and net mineralization kinetics being erratic. The occurrence of denitrification was confirmed by the use of (NO3)-N-15 tracing in an experiment carried out in summer, including three irrigated treatments. The average N-15 recovery varied from 45% to 85% and was smallest in the most frequently irrigated treatment. Over the 8-week experiment, the N losses varied from 30 to 38 kg ha(-1) in the irrigated treatments. They were satisfactorily simulated by a simple denitrification model (NEMIS). Combining the LIXIM model and the simulated or calculated denitrification allowed to predict satisfactorily the evolution of soil mineral N accounting for the effects of temperature and moisture. The net N mineralized for 8 weeks varied from 34 kg N ha(-1) in the un-irrigated to 46 kg N ha(-1) in the irrigated treatments. The drying-rewetting cycles did not induce a flush of N mineralization. Our results suggest that denitrification has to be accounted for in irrigated systems, particularly in warm conditions and when the topsoil contains high nitrate contents.