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 °C for 200 days under aerobic conditions. Cumulative N release curves were split into N released from fresh materials (Nfast) and N released from the larger pool of stabilized, older OM (Nslow rates, 0.06–0.77 μg N g−1 soil d−1, or 0.7–49 μg N g−1 OM). Correlating the Nslow rates with total N contents of soils yielded no satisfactory relationships while their relationship with C to N ratios was very close (negative exponential, R2=0.88). Low rates of N release (Nslow) 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.