Fertilizer nitrogen recovery in long-term continuous winter wheat

Fertilizer N recovery in crop production systems seldom exceeds 50%. Two lo ng-term winter wheat (Triticum aestivum L.) experiments were selected for u se of N-15 to evaluate fertilizer recovery in the grain, straw, and soil. E ach experiment included variable nitrogen rates that had been incorporated preplant each year for >20 yr. In 1988, ammonium nitrate enriched with 11.8 88 atom % N-15 ((NH4NO3)-N-15-N-15) was applied to microplots within the ma in plots of these long-term experiments. Conventional ammonium nitrate (0.3 66% N-15) was added to the microplots the following 2 yr to allow measureme nt of residual effects of the enriched fertilizer. Three years after the N- 15 microplots were established, soil cores (0.025 m in diam.) mere taken to a depth of 1.20 m and partitioned into 0 to 0.15, 0.15 to 0.30, 0.30 to 0. 45, 0.45 to 0.60, 0.60 to 0.90, and 0.90 to 1.20 m. From all microplots, pe rcentage N-15 recovered in the grain and straw at harvest and in the soil w as determined and expressed as atom % excess corrected for background abund ance. Total fertilizer N recovery (N-15 removed in the grain and straw plus that remaining in the soil) decreased with increasing N application at bot h locations. Results from this study combined with previously published fin dings from these locations suggest that fertilizer N recovery was greater w here there was: (i) no evidence of priming (increased net mineralization of organic N pools when low rates of fertilizer N are applied); and (ii) redu ced soil-plant buffering (N that can be applied in excess of the amount nee ded for maximum yield without resulting in increased soil profile inorganic N accumulation).