Dynamics of dissolved and extractable organic nitrogen upon soil amendment with crop residues

Dissolved organic nitrogen (DON) is increasingly recognized as a pivotal pool in the soil nitrogen (N) cycle. Numerous devices and sampling procedures have been used to estimate its size, varying from in situ collection of soil solution to extraction of dried soil with salt solutions. Extractable organic N (EON) not only consists of DON but contains also compounds released from soil biomass and desorbed organic matter. There is no consensus whether DON or EON primarily regulates N mineralisation in soil, and their contribution to N mineralisation has not been quantified simultaneously. We evaluated three sampling procedures on their ability to determine the dynamic of dissolved organic N pools. The three procedures were the determination of DON in 1) soil solution collected by centrifugation, and the determination of EON in 2) a 0.01 M CaCl2 extract of field moist or 3) dried soil. We added unlabeled leek and N-15-labeled ryegrass residues to a loamy sandy soil to create a temporarily increase in DON and EON, to stimulate microbial activity, and to test whether the source and dynamics of the three pools differ. We also tested whether the flow of N through DON or EON was associated with the production of inorganic N using N-15 isotope tracing. Sampling procedures significantly affected the amount, but not the dynamics and origin of the three organic N pools. DON and EON (determined on field-moist and dried soils) showed all a significant increase upon crop amendment and returned to their background concentrations within 10 to 30 days. The fraction of DON and EON originating from the crop residue slightly decreased over 138 days and was not different for DON and EON. Field moist extraction of a loamy sandy soil with 0.01 M CaCl2 gave a reliable estimate of the concentration of in situ dissolved organic N. In contrast, extraction of dried soil significantly increased EON compared to DON. The agreement in dynamics, N-15 enrichment and C-to-N ratio's indicate that dissolved and extracted organic N have a similar role in N mineralisation. Our results also suggest that they make a minor contribution to N mineralisation; changes in the turnover rate of EON were not associated with changes in the net N mineralisation rate. (C) 2010 Elsevier Ltd. All rights reserved.