Nitrogen response index as a guide to fertilizer management

The efficiency with which fertilizer-nitrogen (N) is transferred to grain-N in cereals is usually less than 50% and averages 33% worldwide. Two long-term N fertility experiments were evaluated to examine temporal changes in nitrogen use efficiency (NUE) and their causes. Averaged over 30-yr, non-irrigated winter wheat NUE was 49% at a 22.4 kg N ha(-1) yr(-1) rate and decreased to 34% at a rate of 112 kg N ha(-1) yr(-1). The average NUE for a 15-yr irrigated corn experiment was 30.6% at the lowest N input (90 kg N ha(-1)) and decreased to 18.3% at a rate of 270 kg N ha(-1) yr(-1). Low NUE values are a result of excess N present in the soil-plant system. The extent to which N is present in excess is determined by the potential yield and how much of that yield will be supported by non-fertilizer N, presumably mineralized from soil organic matter. For both experiments there was greater temporal variability in non-fertilized (check-plot) yields (CV of 31.4 for wheat and 34.3 for corn) than in plots where maximum yields were obtained from N-fertilizer (CV of 20.1 for wheat and 15.7 for corn). The yield of unfertilized plots was not related to the maximum yield of fertilizer plots over time. A response index (RI) was calculated by dividing the maximum yield of fertilized plots by the yield of unfertilized plots to determine the extent of fertilizer need and response for a given year. For both wheat and corn the RI was unpredictably variable (CVs approximate to 35) over time, and ranged from lows of about 1.1 to a high of 4.1 for wheat and 3.5 for corn. In general, low RI values resulted when unfertilized yields were high, which even occurred after 10 to 30 yr without N fertilization. Low RI values may be more common in farmer's fields where N is applied annually. Except at the lowest N rates, WE increases for each rate as RI increases because N inputs are less likely to be excessive. Nitrogen management strategies that increase NUE may only be possible to evaluate for site-years when RI is substantially greater than 1 (e.g. > 1.5). Since response to N fertilizer is strongly dependant on supply of non-fertilizer N in a given year, any N management strategy that includes a reliable in-season predictor of RI should dramatically improve NUE in cereal production.