Abstract
In order to use more organic materials without creating environmental pollution in farmers' fields, a better understanding of the nitrogen (N) mineralization of organic materials in soil is required. We conducted a field experiment for N release and formation and the decay of microbial biomass from ^<15>N-labeled sugar beet (Beta vulgaris L.), sweet corn (Zea mays L.) and spring wheat (Triticum aestivum L.) residues over 5 y. The amounts of residue-N absorbed by crop from sugar beet and corn residues were largest in the first crop growing season after an incorporation of the residues. Thereafter, the amounts decreased and became very small in the fifth crop growing season. In the case of wheat residue, the amount of N derived from it in the first successive crop was similar to that in the second and third successive crops, and the amount of residue-N absorbed by crop became very small in the fourth and fifth crop growing seasons. Total amount of residue-N absorbed by crops and the recovery rate of the residue-N over 5 y were, respectively, 2.9-4.6 g N m^<-2> and 26-34% from sugar beet residue, 1.7-3.4 g N m^<-2> and 17-36% from corn residue and 1.0-1.6 g N m^<-2> and 18-29% from wheat residue. The recovery rate was on the whole higher as the C/N ratio of residue was lower. Additionally, the recovery rate in Brown Lowland soil (BLs) (FAO/Unesco : Fluvisols) was highest, and that in Gleyic Cumulic Andosol (GCA) (Fluvisols) was higher than in Brown Andosol (BA) (Andosols). Of the residue-N, 38-76% remained in the soils at 52 months after incorporation. Incorporated crop residue-N plays an important role for the preservation of organic N in soil. The ratio of remaining residue-N to incorporated residue-N was larger in the case of wheat residue than other residues. The ratio was smaller in the BLs than the two Andosols. The maximum amount of N derived from crop residues in the soil microbial biomass was 2.7-9.7% of the residue-N incorporated at the time the experiment began. The values were larger than the ratio of biomass-N to soil organic N, which indicated microbial biomass plays an important role in the dynamics of crop residue-N in soil. The amount of N derived from crop residues in soil microbial biomass decreased slowly after the end of the first successive crop growing season, and the net half-life was calculated to be 490-980 d. However, a distinct property of soil microbial biomass according to the type of crop residue and soil was not detected, because there were irregular changes in the analysis data of microbial biomass during the first successive crop growing season. This was due to the heterogeneous distribution of microorganisms caused by fresh crop residues.
