Effect of Organic matter application on the fate of 15N-labeled ammonium fertilizer in an upland soil

Abstract

The effect of the application of organic matter on the fate of 15N-labeled ammonium was investigated in a field. The organic materials incorporated into the experimental plots consisted of wheat straw, rape, pig compost, cow compost, plant manure. In May 2000, 10 g N m−2 of 15N-labeled ammonium was applied to the field together with the organic materials, and maize and winter wheat were consecutively cultivated. The recovery of applied 15N in soils and plants was determined after the harvest of each crop. Although only about 10% of the applied 15N-labeled fertilizer remained in the 0-30 cm layer of the Control Plot and the Plant Manure Plot, more than 25% of the applied 15N remained in the Pig Compost Plot. Amount and proportion of the immobilized 15N to those of total N or microbial biomass N in soils were determined for the topsoil samples (0–10 cm layer). The amounts of both microbial biomass N and total immobilized 15N in soil were highest in the Pig Compost Plot. Although the amount of microbial biomass N was comparable to the amount of immobilized 15N-labeled fertilizer in soil, the amounts of 15N-labeled fertilizer contained in the microbial biumass accounted for less than 10% of the amount of total immobilized 15N in soil. The ratio of the amount of 15N-labeled fertilizer contained in biomass N to the total amount of biomass N was one order of magnitude higher than the ratio of the amount of immobilized 15N-labeled fertilizer to the amount of total N in soil. No conspicuous changes in the amount of immobilized 15N in soil were observed during the cultivation of winter wheat except for the Pig Compost Plot. No significant correlation was recognized between the amount of 15N-labeled fertilizer contained in microbial biomass before wheat cultivation and that of 15N-labeled fertilizer absorbed by wheat, indicating that microbial N immobilized during the growth period of the former crop (maize) was not a significant source of N for the latter crop (wheat).