DENITRIFICATION IN THE TOP AND SUB SOIL OF GRASSLAND ON PEAT SOILS

Denitrification is an important process in the nitrogen (N) balance of intensively managed grassland, especially on poorly drained peat soil s. Aim of this study was to quantify the N loss through denitrificatio n in the top and sub soil of grassland on peat soils. Sampling took pl ace at 2 sites with both control (0 N) and N fertilised (+ N) treatmen ts. Main difference between the sites was the ground water level. Deni trification was measured on a weekly basis for 2 years with a soil cor e incubation technique using acetylene (C2H2) inhibition. Soil cores w ere taken from the top soil (0-20 cm depth) and the sub soil (20-40 cm depth) and incubated in containers or 24 hours. The denitrification r ate was calculated from the nitrous oxide production between 4 and 24 hours of incubation. Denitrification capacities of the soils and the s oil layers were also determined. The top soil was the major layer for denitrification with losses ranging from 9 to 26 kg N ha(-1) yr(-1) fr om the 0 N treatment. Losses from the top soil of the + N treatment ra nged from 13 to 49 kg N ha(-1) yr(-1). The sub soil contributed, on av erage, 20% of the total denitrification losses from the 0-40 layer. Lo sses from the 0-40 cm layer were 2 times higher on the +N treatment th an on the 0 N treatment and totalled up to 70 kg N ha(-1) yr(-1). Sign ificant correlation coefficients were found between denitrification ac tivity on the one hand, and ground water level, water filled pore spac e and nitrate content on the other, in the top soil but not in the sub soil. The denitrification capacity experiment showed that the availab ility of easily decomposable organic carbon was an important limiting factor for the denitrification activity in the sub soil of these pear soils.