Nitrate Leaching in Continuous Winter Wheat: Use of a Soil-Plant Buffering Concept to Account for Fertilizer Nitrogen

Abstract

Food and fiber production depends heavily on fertilizer N inputs to sustain high yields. Various biological processes in soils and plants have generally restricted grain crops from obtaining N use efficiencies in excess of 70%. Nitrogen that could not be accounted for in N balance studies has often been assumed to have been lost to the atmosphere by denitrification or leached from the system. We evaluated wheat (Triticum aestivum L.) grain yield and soil profile inorganic N (ammonium N + nitrate N) accumulation in four long-term Oklahoma experiments where fertilizer N was applied annually (at a wide range of N rates) for more than 23 yr. Results showed that soil profile (0-8 ft) inorganic N accumulation did not increase until N rates exceeded that required for maximum yield. We suggest that the soil-plant system is able to buffer against soil accumulation of inorganic N. Major buffering mechanisms include increased plant protein, increased plant N volatilization, and denitrification in soil when N rates exceed those required for maximum yield. Annual N fertilization rates that increased soil profile inorganic N accumulation exceeded those required for maximum yields by more than 20 lb N/acre in all experiments. This suggests the presence of a fertilization safety zone with regard to fertilizer N recommendations. Applying more fertilizer N than that required for maximum grain yield did not immediately pose a risk to groundwater quality. The same processes that prevent 100% use of fertilizer N by crops also prevent effective soil accumulation of inorganic N and risk of subsequent leaching to groundwater.