Accumulation of Fertilizer Nitrogen Applied at Different Times into Grain and Uptake of Soil Nitrogen by Winter Wheat in Hokkaido

Abstract

The percentage uptake of a fertilizer nitrogen (N) by winter wheat (Triticurn aestivurn L. cv. Chihokukomugi) and the distribution of absorbed N into the grain were investigated by tracing ^<15>N in field experiments. The three-split application of a 12 g N m^<-2> was conducted and (^<15>NH_4) SO_4 was used as each split of fertilizer N. Also, the time course of uptake of soil N by winter wheat was compared with the estimated progress of N mineralization by kinetic analysis. The recovery rates of fertilizer N were 29-40% in the case of application at sowing time, 43% at mid-April, when the regrowth of wheat plants starts after snow melting away and 58-64% at the heading stage. At harvest, the proportion of labelled N absorbed in the grain was 52-63% in the case of application at sowing time, 59-66% at mid-April and 73-81% at the heading stage. At 10-13 d before harvest, 33-45% of absorbed fertilizer N applied at the heading stage was still found in the leaf and steam, and a part of it translocated to the grain during the ripening process. On the other hand, the amount of N absorbed from soil N was 46-60% of the total N in the grain. That indicated the necessity of estimating the progress of uptake of soil N by wheat plants to control the yield and quality of grain. The amount of N mineralized in the soil from sowing time to harvest was estimated at less than 40% of the amoun of soil N absorbed by winter wheat. The residual inorganic N in soil, especially in the sub-soil at sowing time, contributed considerably to N uptake by winter wheat.