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Soil-Plant Buffering of Inorganic Nitrogen in Continuous Winter Wheat

Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078

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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 NH₄-N, NO₃-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 stablished for soil profile inorganic N accumulation (sum of NH₄-N + NO₃-N, converted to kg ha^–1) on annual N applied. Maximum yields were ohserved 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.

Received for publication August 20, 1994.

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