Nitrogen leaching and crop availability in manured catch crop systems in Sweden

Results are presented from five years (1990-1995) of a field leaching exper iment on a sandy soil in south-west Sweden. The aim was to study N leaching , change in soil organic N and N mineralization in cropping systems with co ntinuous use of liquid manure (two application rates) and catch crops. N le aching from drains, N uptake in crops and mineral N in the soil were measur ed. Simulation models were used to calculate the N budget and N mineralizat ion in the soil and to make predictions of improved fertilization strategie s in relation to manure applications and changing the time for incorporatio n of catch crops. In treatments without catch crops, a normal and a double application of manure increased average N leaching by 15 and 34%, respectiv ely, compared to treatment with commercial fertilizer. Catch crops reduced N leaching by, on average, 60% in treatments with a normal application of m anure and commercial fertilizer, but only by 35% in the treatment with doub le the normal application rate of manure. Incorporation of catch crops in s pring increased simulated net N mineralization during the crop vegetation p eriod, and also during early autumn. In conclusion, manured systems resulte d in larger N leaching than those receiving commercial fertilizer, mainly d ue to larger applications of mineral N in spring. More careful adaptation o f commercial N fertilization with respect to the amounts of NH4-N applied w ith manure could, according to the simulations, reduce N leaching. Under-so wn ryegrass catch crops effectively reduced N leaching in manured systems. Incorporating catch crop residues in late autumn instead of spring might be preferable with respect to N availability in the soil for the next crop, a nd would not increase N leaching.