N mineralization parameters of sandy arable soils

The objective of this study was to experimentally investigate net N mineralization in sandy arable soils and to derive adequate N mineralization parameters for simulation purposes. Long-term incubations at 35°C were done for at least 200 days with 147 sandy arable soils from Northwest Germany. To cumulative net N mineralization curves the simultaneous two-pool first-order kinetic equation was fitted in order to differentiate between N mineralization from an easily decomposable, fresh organic matter pool (N_fast) and from a slowly decomposable pool (N_slow) of more humified OM. North German loess soils served as a reference, since available model parameters were mainly derived from those soils. Although curve patterns in sandy soils often somewhat deviated from typical double-exponential patterns, the mineralization equation generally could be fitted. Two pools were clearly revealed, but a transfer of the standard parameters was found to be not appropriate - except maybe for the pool size of the fast decomposable N pool. The mean k _fast at 35°C (0.1263 d^-1) is about 46% higher than the known 'standard' loess value, indicating better conditions for decomposition of fresh residues at this temperature. The mean k_slow at 35°C (0.0023 d^-1), which is 60% lower than reported earlier from loess soils, and much lower mineralization rates of the slowly decomposable N pool give reason to the presence of generally more resistant organic material in these sandy soils. The relation between N_slow and total N was found to be not close enough to derive the pool size of slowly decomposable N just from total N as done for loess soils. Reducing the variability is necessary, promising approaches exist. The eight reference loess soils revealed - on an average - the known N mineralization parameters.