Grain Nitrogen Concentration in Wheat Grown under Intensive Organic Manure Application on Andosols in Central Japan

Abstract

Grain nitrogen concentration (N%) is a major determinant of grain quality in winter wheat. The objective of this study was to compare the responses of wheat grain N% to organic manure with those to inorganic fertilizer in long-term experiments. We analyzed the grain N accumulation using soft wheat (Kinunonami) and hard wheat (Yumeshihou) cultivars grown with a high rate of organic manure application (OM; 80 t ha-¹ yr-¹ for >10 years and 30 t ha-¹ yr-¹ during the three years of the present study) and with standard (SF; 204–252 kg N ha-¹ yr-¹) or low (LF; 102–126 kg N ha-¹ yr-¹) rates of inorganic fertilizer for three years in Japan. The results agreed with previous research on the underlying mechanisms for grain N% under conventional fertilizer management: both sink and source regulation affected N accumulation in grains, and the accumulation of N in grains and of dry matter in grains are independent. Grain N% was significantly higher in the OM treatment than in the SF and LF treatments as a result of lower dry matter accumulation in the grains. High straw N% led to higher N accumulation in grains in the OM treatment during the late grain-filling period in Yumeshihou. Our results suggest that too much organic manure was applied, i.e., more than was required to optimize grain N%, when manure application was designed to produce a grain yield equivalent to that in conventional fertilizer management. We discuss ways to stabilize grain N% under intensive organic manure application.