Turnover and fate of N-15-labelled cattle slurry ammonium-N applied in the autumn to winter wheat

The fate of (NH4)-N-15-N labelled cattle slurry applied before sowing in September of a winter wheat crop was studied on a loamy sand soil. The aim was to quantify immobilization of slurry NH4-N into microbial biomass, the speed at which nitrate derived from the slurry NH4-N was transported down the soil profile, and the utilization of slurry NH4-N by the winter wheat crop. Cattle slurry was applied at a rate corresponding to 75 kg NH4-N ha(-1), with very little loss by volatilization (<4%) due to rapid incorporation by ploughing. The slurry amendment resulted in a doubling of soil surface CO, flux, an index of microbial activity, over non-amended soil within the first c. 2 weeks, but ceased again after c. 4 weeks, due to depletion of the easily degradable substances, e.g, volatile fatty acids, in the slurry. Nitrification of the applied NH4-N was fast and complete by 3 weeks from application, and at this time, the maximum immobilization of slurry NH4-N into the microbial biomass (23% of applied (NH4)-N-15-N) was also observed, although no significant increase in total microbial biomass was observed. Rapid turnover of the microbial biomass quickly diluted the assimilated N-15, With only 6% of applied (NH4)-N-15-N remaining in the microbial biomass by next spring. Downwards transport of nitrate was rapid in spite of lower than normal precipitation, and slurry-derived (NO3)-N-15-N appeared in ceramic suction cups installed at 60 cm depth already 2 months after slurry application. Due to the unusually low winter precipitation in the experimental year, wheat yields were high, and the recovery of N in above-ground plant biomass derived from slurry NH4-N at harvest reached 32%. An additional 45% of the applied slurry NH4-N could be found in the soil to a depth of 100 cm (mostly in organic form in the plough layer), indicating that 23% had been lost by leaching or in gaseous form. It was concluded that although significant immobilization of slurry NH4-N did occur, this was not sufficient to prevent leaching of slurry-derived N over the winter and that the relatively high recovery of slurry-derived N in the wheat crop was due partly to lower than normal winter percolation and partly to a relatively high rooting depth on this particular site. (C) 2000 Elsevier Science B.V. All rights reserved.