N-MINERALIZATION IN SANDY SOILS FOR THE FUHRBERG WELL FIELD (HANNOVER) DURING WINTER

Net N mineralization was measured under field conditions during winter and spring 1991/92 in sandy arable soils (Gleyic Podzols, Mollic Gleysols, Gleyic Arenosols) of the 'Fuhrberg well field', a drinking water catchment north-east of Hannover. The aim was to assess leaching losses of nitrate from mineralization processes druing the winter on soils formerly used as grassland. Two field procedures were used: the incubation of soil material in polyethylene bags at its original location and rain sheltered fallow plots. Between 6 and 40 (100) kg N ha(-1) were mineralized during 73 days from Dec., 17th to March, 2nd. Mineralisation rates were closely correlated to the organic N and C contents of the soils (r(2) approximate to 0.9). In the uncovered soils, the NO3 was completely leached out. On five out of seven fields the process 'N-mineralization during winter' alone was sufficient to exceed the official limit for drinking water (50 mg/l(-1) No-3(-)) in the uppermost groundwater. it is concluded that even 15 years after converting grassland into arable and the N-org and C-org levels in the soils had not reached a new equilibrium.