TURNOVER OF RESIDUAL N-15-LABELED FERTILIZER-N IN SOIL FOLLOWING HARVEST OF OILSEED-RAPE (BRASSICA-NAPUS L)

We studied the fate of N-15-labelled fertilizer nitrogen in a sandy lo am soil after harvest of winter oilseed rape (Brassica napus L. cv. Ce res) given 100 or 200 kg N ha(-1) in spring, with or without irrigatio n. Our main objective was to quantify the temporal variations of the s oil mineral N, the extractable soil organic N and soil microbial bioma ss N, and fertilizer derived N in these pools during autumn and winter . Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irr igated treatment. However, only in the latter treatment did we find si gnificantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the fir st sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested p lants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0-5 cm depth, but from the 5-15 cm depth a marked propo rtion of N derived from fertilizer was lost, probably by leaching. Neg ligible amounts of fertilizer derived extractable soil organic and min eral N (< 1 kg N ha(-1), 0-15 cm) were found in all treatments after t he first sampling. Soil microbial biomass N was not significantly affe cted by treatments and showed only small temporal variability (+/- 11% of the mean 76 kg N ha(-1), 0-15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was signif icantly affected by the treatments, with the extremes being 5.5 and 3. 1 kg N ha(-1) in the 200N, non-irrigated and 100N, irrigated treatment s, respectively. Also, the estimated exponential decay rate of microbi al biomass N derived from fertilizer, differed greatly (2 fold) betwee n these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In st udies utilising N-15 labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, becaus e it may question the assumption that turnover rates are not affected by the insertion of the label.