NITROGEN COMPOUND EMISSIONS FROM FERTILIZED SOILS IN A MAIZE FIELD PINE TREE FOREST AGROSYSTEM IN THE SOUTHWEST OF FRANCE

The use of nitrogen fertilizers in agriculture is a significant source of reactive nitrogen compounds for the atmosphere. Emissions of NO, N H3, and N2O were measured on irrigated maize fields, on acidic podzols , set up in a large pine tree forest system in the southwestern part o f France. Measurements were carried out over a yearly period: before, during, and after fertilization. Trace gas emissions appear to be stro ngly linked to soil characteristics and to the type of fertilizer and application procedures used. All emissions are strongly enhanced after fertilizer application, although NO and N2O emissions are observed al l year long. No emission was observed on forest soils which only const itute an atmospheric NO2 sink. The imbalance of the nitrogen budget (u p to 100 kg N ha-1) might be explained by gaseous emissions into the a tmosphere, watertable pollution by nitrate and ammonium ions remaining very low. This is one of the peculiarities of this agrosystem.