Active organic matter fractions and microbial biomass as predictors of N mineralization

Abstract

It was tested whether light, intermediate and heavy macro-organic N (> 150 µm) fractions and active and total microbial biomass are good predictors of N mineralization when soils differing in organic matter input (grassland vs arable) and soil texture were compared. We found that the relative difference between arable and grassland soils decreased in the order light, intermediate and heavy macro-organic N, active microbial biomass, total microbial biomass and total soil organic N. The relative changes in heavy macro-organic N and active microbial biomass were similar to the relative change in N mineralization.The percentage of soil N that was mineralized in grassland soils was lower in fine-textured soils than in coarse-textured soils. The same difference was found when the amount of N that was mineralized per unit of microbial biomass was considered. It is assumed that this difference in mineralization was caused by the greater physical protection of organic matter and microorganisms in the finetextured soils. However, both the size of the light macro-organic matter fraction and the active part of the microbial biomass correlated very well with the rate of N mineralization when grassland soils of different textures were compared. We assume that the relationship between the light fraction and the active microbial biomass and N mineralization was the same for fine- and coarse-textured soils, because the light fraction and the active microbial biomass are not physically protected in the soil.The incorporation of the active fractions obtained by physical fractionation and the degree of physical protection of organic matter pools into organic matter models might help in the analysis of N mineralization in different agricultural soils.