Soil organic matter fractions and their role in soil nitrogen mineralization

The transformation of insoluble organic N via particulate and dissolved organic matter (POM and DOM) into inorganic N may represent the bottle neck mechanism in N mineralization. However, it is unknown how POM fractions are related to DOM pools, and how these particulate and dissolved pools are quantitatively involved in N mineralization. To understand and quantify the role of these pools in N mineralization, we conducted an incubation experiment with 15N labeled radish residues to trace the flow of 15N through the different N pools. The enrichment of 15N in POM, DON and inorganic N along with their concentrations were examined to identify their role in N mineralization. Our analysis showed that neither DOM nor POM function as a distinct N source fraction in soil. The collected DOM was predominantly recalcitrant (~80%), suggesting that the bioavailable DOM fraction cannot be measured with current sampling techniques. The concentration of DOM strongly increases upon incorporation of crop residues, but diminish sharply within a few days. Our results also suggest that the DOM fraction is heterogeneous in composition; the most bioavailable part is consumed within a few days whereas the remaining part is fairly constant.