The significance of incorporation date of a catch crop on the nitrogen supply for the subsequent crop, the N effect (N(eff)), was examined. Winter rye was grown as a catch crop for 3 years during the autumn, and incorporated on five dates, two in the autumn and three in the spring. Two of the winters had high precipitation, and the N(eff) was small at the early autumn incorporation date, but increased when incorporation was delayed into late autumn and further increased by early spring incorporation. In the third winter, which was very dry, the N(eff) was negative at all incorporation dates, with the negative effect gradually increasing in value the later the incorporation date. In all 3 years the N(eff) was reduced when incorporation was delayed from early spring until later in the spring. The main processes determining this pattern were found to be (1) the risk of leaching of N mineralized after catch crop incorporation, which can reduce the N(eff) at early incorporation under wet conditions, (2) pre-emptive competition which can reduce the N(eff) when incorporation is delayed until later in the spring, and in dry conditions is already apparent during the autumn, and (3) catch crop growth leading to carbon gain and increased C/N ratio which decreases mineralization and thus the N(eff) after delayed incorporation in the spring. Lack of time for catch crop N uptake prior to early incorporation, or lack of time for N mineralization after late incorporation which might also reduce the N(eff) did not appear to be important in our experiment. The results show that catch crops grown in high rainfall areas on sandy soils should be incorporated later than those in low rainfall areas on nitrate retentive soils.
