Microbial, physical, and chemical factors causing higher gross rate and lower net rate of nitrogen mineralization in volcanic ash soils than in alluvial soils in the Tokachi District

Abstract

To test the hypothesis that a lower net rate of nitrogen (N) mineralization is due to a smaller ratio of N mineralization rate to immobilization rate in Brown Andosols (BA, Typic Low-humic Andosls, U.S. Soil Taxonomy: Typic Hapludands, FAO / Unesco: Haplic Andosols) which have a higher gross N mineralization rate caused by a larger microbial biomass compared with Brown Lowland soils (BLs, Coarse-textured Haplic Brown Lowland soils, Typic Udifluvents, Dystric Fluvisols), the gross and net rates of N mineralization and the microbial biomass in BA and BLs in the Tokachi District of Hokkaido in northern Japan were determined and the relationships among these and other physical and chemical properties of the soils were investigated. The soil microbial biomass carbon (C) and N were larger in BA than BLs. The gross rates of N mineralization in BA were 2.9–8.3 times higher than those in BLs, while not all the net rates of N mineralization were higher in BA than in BLs. Expressing the N mineralization rate as the rate per unit of microbial biomass N, the net rates were lower in BA than BLs. The soil microbial biomass N and the gross rate of N mineralization showed significant (p < 0.05) correlations with the maximum water holding capacity (MWHC), silt content, sand content (negative), the phosphate absorption coefficient, the amount of organic matter extracted with 0.5 M K2SO4, the gross rate of N mineralization, and the amount of soil microbial biomass N. In contrast, the net rate of N mineralization did not show significant (p < 0.05) correlations with any factors except pH. These correlations suggest that the presence of more microorganisms, more pore space, and more organic substrate in BA results in a higher gross rate of N mineralization. A large microbial biomass can be held in BA due to the high availability of substrate and the larger volume of mesopores and micropores. The C / N ratios of the organic extract with 0.5 M K2SO4 were higher than the C / N ratios of microbial biomass in BA, while both C / N ratios were similar in BLs. An organic substrate with a higher C / N ratio for microorganisms in BA probably caused a lower net rate of N mineralization. The present study which confirmed that BA have a higher gross rate of N mineralization due to a larger microbial biomass, suggests that a higher C / N ratio of substrate for microorganisms may result in a lower net rate of N mineralization in BA compared with BLs in the Tokachi District.