Utilization of nitrogen accumulated by a clover containing ley following cultivation

The efficient use of biologically fixed N in agriculture is important in organic farming end when N fertilizers are either expensive or unavailable. The aim of the study was to determine the effects of cultivation and sowing dates on the efficiency of use of biologically fixed N built up during a period of grass/clover ley by subsequently sown ryegrass. Dates of cultivation in two field experiments conducted in consecutive years (1994/95 and 1995/96) ranged from August to October and sowing was carried out either immediately after cultivation or after a delay of one month. Nitrate-N losses through leaching, herbage yields and N offtake by ryegrass were measured from 1994 to 1996. A laboratory experiment was carried out to assess net N mineralization and nitrification in the soil of the field experiment under different conditions. The utilization of mineralized N ranged from 30 to 100 kg ha-1 in both field experiments. Herbage yield and recovery of mineralized N by ryegrass was greater when sowing was carried out immediately after cultivation than when it was delayed for one month. Cultivation in late October was inferior to cultivation in August or September but the optimum date (August to September) differed between the two years (1994/95 and 1995/96) due to differences in weather, especially rainfall, affecting N leaching during the germination and early growth phases. Indeed the main differences in the efficiency of utilization of mineralized N between treatments could be attributed to differences in NO3(-)N leaching losses during the autumn establishment period. Decreases in soil temperature during autumn resulted in a modest decrease in net N mineralization but a much more substantial decrease in the rate of nitrification, which has implications for NO3(-)N leaching. The optimum time for cultivation cannot be refined to a particular calendar date but reasonably accurate long-term rainfall forecasts in conjunction with data on soil moisture deficit would provide the means to set an optimum cultivation date.