EFFECTS OF A RYEGRASS CATCH CROP ON MICROBIAL BIOMASS AND MINERAL NITROGEN IN AN ARABLE SOIL DURING WINTER

The effects of an Italian ryegrass (Lolium multiflorum Lam.) catch cro p on microbial C and N, mineral N, plant N and N mineralization in an arable soil, were studied during the winters of 1988/89 and 1990/91. M icrobial C and N were estimated with the fumigation incubation techniq ue. On average, a smaller amount of microbial N, 110 kg N ha(-1), was found in the plot with an Italian ryegrass catch crop than in the plot without, 130 kg N ha(-1). However, no effect of the presence of ryegr ass was found on microbial C, which averaged 770 kg C ha(-1) in both t reatments. Only minor amounts of nitrate were found in the topsoil and subsoil of plots with a catch crop, and in 1988/89 and 1990/91 leachi ng of N was decreased by 26 and 21 kg ha(-1) respectively, by the pres ence of ryegrass. The catch crop did not affect net N mineralization, which was estimated to be about 50 kg N ha(-1) in both the ryegrass pl ot and the plot without ryegrass from September 1988 to April 1989. Co rresponding values obtained in 1990/91 were 21 and 30 kg N ha(-1), res pectively.