The fate of nitrogen from incorporated cover crop and green manure residues

Nitrogen retention and release following the incorporation of cover crops and green manures were examined in field trials in NE Scotland. These treatments reduced the amounts of nitrate-N by between 10–20 kg ha-1 thereby lowering the potential for leaching and gaseous N losses. However, uptake of N by overwintering crops was low, reflecting the short day-lengths and low soil temperatures associated with this part of Britain. Vegetation that had regenerated naturally was as effective as sown cover crops at taking up N over winter and in returning N to the soil for the following crop. Incorporation of residues generally resulted in lower mineralisation rates and reduced N2O emissions than the cultivation of bare ground, indicating a temporary immobilisation of soil N following incorporation. Emissions from incorporated cover crops ranged from 23–44 g N2O-N ha-1 over 19 days, compared with 61 g N2O-N ha-1 emitted from bare ground. Emissions from incorporated green manures ranged from 409–580 g N2O-N ha-1 over 53 days with 462 g N2O-N ha-1 emitted from bare ground. Significant positive correlations between N2O and soil NO3- after incorporation (r=0.8–0.9; P<0.001 and r=0.1–0.4; P<0.05 for cover crops and green manures, respectively) suggest that this N2O was mainly produced during nitrification. There was no significant effect of either cover cropping or green manuring on the N content or yield of the subsequent oats crop, suggesting that N was not sufficiently limiting in this soil for any benefits to become apparent immediately. However, benefits of increased sustainability as a result of increased organic matter concentrations may be seen in long-term organic rotations, and such systems warrant investigation.