Surface mulching with red clover in white cabbage production. Nitrogen uptake, ammonia losses and the residual fertility effect in ryegrass

Green manure crops are an important nitrogen source in organic production of vegetables. In field trials near Uppsala, Sweden, the release of nitrogen to the soil and crop from red clover mulch (Trifolium pratense L., cv. Vivi), and losses of ammonia to the atmosphere were investigated in a study with white cabbage (Brassica oleracea L. capitata L. var. alba DC.). The effect of time of post-harvest incorporation of white cabbage crop residues on the uptake of nitrogen in ryegrass (Lolium multiflorum var. westerwoldicum cv. Barspectra) was investigated in the following year. Nitrogen release during the growing season from red clover mulching corresponded to 26% of the nitrogen applied (243 kg ha(-1)). Nitrogen utilization efficiency was between 25 and 28% based on total uptake in the above ground parts of the white cabbage plants. The combined N losses via ammonia emissions during the sampling periods were 6-14 kg ha(-1), which corresponded to an average loss of 1.4-2.8 kg ha(-1) day(-1) or 2.7-5.6% of the total amount of nitrogen supplied by red clover mulching. Due to uncertainty as regards the general process of ammonia emissions from green manure mulches, estimation of emissions in the periods before, between and after the sampling periods was not attempted. Uptake of nitrogen in ryegrass was higher after autumn incorporation of crop residues than after spring incorporation, with an apparent nitrogen recovery of 27% for the former and 11% for the latter. The results of this study indicate that a cropping system with surface mulching is no less nitrogen-efficient than most other systems based on organic fertilizers.