Bulk soil C to N ratio as a simple measure of net N mineralization from stabilized soil organic matter in sandy arable soils

Investigations of 23 northwestern German sandy Ap horizons (mean clay content 35 g kg(-1)), that had higher organic matter (OM) levels than expected for sands, showed that the bulk soil C to N ratio reliably indicated the release of N from stabilized OM. Soils were incubated at 35 degreesC for 200 days under aerobic conditions. Cumulative N release curves were split into N released from fresh materials (N-fast) and N released from the larger pool of stabilized, older OM (N-slow rates, 0.06-0.77 mug N g(-1) soil d(-1), or 0.7-49 mug N g(-1) OM). Correlating the N-slow rates with total N contents of soils yielded no satisfactory relationships while their relationship with C to N ratios was very close (negative exponential, R-2 = 0.88). Low rates of N release (N-slow) per unit of OM occurred if C to N exceeded 15. This was associated with historical factors like podzolization, calluna heathland, plaggen fertilization or a combination of these. (C) 2003 Elsevier Science Ltd. All rights reserved.