EFFECTS OF REPEATED STRAW INCORPORATION ON CROP FERTILIZER NITROGEN REQUIREMENTS, SOIL MINERAL NITROGEN AND NITRATE LEACHING LOSSES

The effects of straw disposal by burning and incorporation on soil and crop nitrogen (N) supply were investigated on two light textured soil s in central (ADAS Gleadthorpe) and eastern England (Morley Research C entre) over the period 1984 to 1995. Nitrogen balance calculations sho wed that after 11 years of contrasting straw incorporation versus burn treatments, the cumulative N returns in straw were c, 570 kg/ha at Gl eadthorpe and c. 330 kg/ha at Morley. However, these N returns via str aw incorporation were not reflected in increased total soil N levels i n autumn 1994. There were no differences (P > 0.05) between straw disp osal treatments in autumn soil mineral N supply, readily mineralizable N or organic carbon. Similarly, there were no consistent differences between the treatments in terms of crop yield, crop N uptake or optimu m fertilizer N rates. Fertilizer N applications of 200 kg N/ha/y incre ased topsoil organic carbon from 1.18 to 1.28% and total N content fro m 0,091 to 0.102% on the loamy sand textured soil at ADAS Gleadthorpe, but not at Morley. Precious fertilizer N applications increased the q uantity of nitrate-N leached in drainage water by c. 20 kg/ha at Glead thorpe and c. GO kg/ha at Morley overwinter 1994/95, and by 10-20 kg/h a at both sites overwinter 1995/96. There was some indication overwint er 1994/95 that straw incorporation reduced nitrate-N leaching by 10-2 5 kg/ha, but there were no differences between treatments overwinter 1 995/96.