Tropentag, October 5 - 7, 2011 in Bonn "Development on the margin"

Fate of Particulate and Dissolved Organic Matter in Soil N Mineralisation

Wageningen University and Research Centre, Soil Science, The Netherlands

Abstract

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 mineralisation. However, it is unknown how POM fractions are related to DOM pools, and how these particulate and dissolved pools are quantitatively involved in N mineralisation. To understand and quantify the fate of these pools in N mineralisation, we conducted an incubation experiment with ¹⁵N labeled radish residues to trace the flow of ¹⁵N through the different N pools. POM was fractionated with density fractionation as light and heavy fraction (LF and HF). The dissolved organic matter fraction was collected using a centrifugal drainage technique and subsequently separated into a bioavailable and recalcitrant fraction. The enrichment of ¹⁵N in POM, DON and inorganic N along with their concentrations were examined to identify their role in N mineralisation. Our analysis showed that neither DON 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. Moreover, the mineralisation rate of crop residue increased with incubation time but the ¹⁵N enrichment of DON showed gradual decrease in time. The gradual decrease of ¹⁵N enrichment in DON indicates that there was no change in their inputs. Hence these results suggested that a change in the turnover rate of DON was not associated with the N mineralisation rate of crop residue. From these results,we concluded that the main flow of decomposed N from residue did not pass or may not necessary to pass via the DON pool. Further research should focus on bioavailable fraction analysis.

Keywords: ¹⁵N enrichment, bioavailable fraction, dissolved organic carbon, dissolved organic matter, dissolved organic nitrogen, particulate organic matter

Contact Address: Bandhu Raj Baral, Wageningen University and Research Centre, Soil Quality, Bornsesteeg 1, Wageningen, 6708GA, Wageningen, The Netherlands, e-mail: bandhu_baralyahoo.com