Gross and Net Nitrogen Mineralization and Their Relationships with Microbial Biomass in a Volcanic Ash Soil and Alluvial Soil after Application of Crop Residues

Abstract

ABSTRACT Nitrogen (N) release from crop residues is an important process which sustains the pool of available N in soils. Effects of application of the residues of sugar beet (Beta vulgaris L.) and wheat (Triticum aestivum L.) on gross and net N-mineralization and microbial biomass were investigated during incubation. The aim was to clarify how slower N release from crop residues relates to microbial biomass in Brown Andosols (BA; U.S. Soil Taxonomy: Andisols) compared to Brown Lowland soils (BL; U.S. Soil Taxonomy: Inceptisols). When microbial biomass N was increasing, gross N-mineralization rate was high and much inorganic N was actively immobilized. After reaching a maximum, the microbial biomass N was stable in BA, whereas microbial biomass N in BL treated with sugar beet residue decreased and inorganic N accumulated after 1 week incubation. In both soils treated with wheat residues, the microbial biomass N increased slowly for a long period due to lack of labile carbon (C), especially in BA which was able to hold a large microbial biomass. These findings suggest that factors other than low microorganism activity just after application of crop residues, such as the stability of synthesized biomass are important for the slow release of N from crop residues in volcanic ash soils.