Soil‐Plant Buffering of Inorganic Nitrogen in Continuous Winter Wheat

The soil‐plant system can limit soil profile inorganic N accumulation when N fertilizers are applied at rates greater than needed for maximum yield. Nitrogen rates that maximized grain yield and increased soil profile inorganic N accumulation under continuous dryland winter wheat ( Triticum aestivum L.) were evaluated in four long‐term experiments. Soil cores (0‐210 cm) were taken in 1988 and again in 1993 from N rate treatments where wheat had been grown for more than 23 yr. Soil cores were split into 15‐ to 30‐cm increments and analyzed for NH 4 ‐N, NO 3 ‐N, total N, and organic C. Critical N fertilization rates were determined from linear‐plateau models of wheat grain yield on annual N applied. Plateau‐linear models were established for soil profile inorganic N accumulation (sum of NH 4 ‐N + NO 3 ‐N, converted to kg ha −1 ) on annual N applied. Maximum yields were observed at N rates less than that required to increase soil profile inorganic N accumulation. Annual N fertilization rates that increased inorganic N accumulation exceeded the N requirement for maximum yields by more than 23.3 kg N ha −1 in all experiments. Increased plant N volatilization and grain N uptake have been found when N rates exceed yield maximums. High N rates can also increase straw yield and straw N, subsequently increasing surface soil organic C and N and the potential for denitrification when wheat straw residues are incorporated. In dryland production systems, soil‐plant buffering (considering the processes discussed) implies that the system buffers against (resist) soil accumulation of inorganic N, even when N rates exceed that required for maximum grain yield.