EFFECT OF NITROGEN-FERTILIZER RESIDUES ON THE RESPONSE OF IRRIGATED BROMEGRASS TO FERTILIZER NITROGEN

Two nitrogen (N) fertilizer response trials were superimposed, in 2 consecutive years, on a set of large plots of irrigated bromegrass (Bromus inermis Leyss.) that had been fertilized with different rates of fertilizer N up to 200 kg ha(-1) for the previous 9 and 10 yr, respectively. During those years, forage dry matter responded in direct proportion to fertilizer N rate. In the subsequent two trials we determined the residual effects of the prior fertilizer treatments on the response of bromegrass to new applications of N fertilizer, and the N rate required to achieve maximum yields. The yield response of the bromegrass to the applied N was a function of prior fertilizer history and the moisture conditions. In the first trial, under good moisture conditions, the previously unfertilized plots had maximum yields at a N rate of 382 kg N ha(-1): yields declined at higher rates. Responses of previously fertilized plots to additional N were linear. The y-intercepts (where no N was applied) were higher for plots that had been fertilized at higher N rates in the initial 9-yr study while the slopes of the yield responses were less steep. In contrast, in the second trial, conducted in a year when irrigation water was restricted, all forage yield responses to N fertilizer were curvilinear. y-intercepts were again higher on plots that had been fertilized at higher N rates in previous years. In this case, however, the slopes of the N responses became progressively steeper with increasing N rate while increasingly larger quadratic coefficients resulted in maximum yields being attained at progressively lower N rates. Nevertheless, maximum yields were higher than those of the previously unfertilized plots. Changes in the response curves were attributed to alterations in the soil organic N and to a lesser extent, to changes in the capability of the bromegrass to respond to fertilizer N. Soil tests found no carry-over of fertilizer N as residual inorganic N but the initial potential rate of mineralization (N(0)k) reflected changes in the quality of soil organic matter influencing the response to N fertilizer applications. The results suggest the need for soil testing laboratories to take into account the prior fertilizer history of the grass stand when developing recommended N fertilizer rates for irrigated bromegrass.