Non-biomass soil organic N — the substrate for N mineralization flushes following soil drying–rewetting and for organic N rendered CaCl2-extractable upon soil drying

Soil drying enhances the amount of organic N (Norg) extracted by chemical methods and used as N availability indices and renders some Norg mineralizable. My aim was to define the sources of these N flushes and to evaluate the relevance of soil drying–rewetting as a driving force for N mineralization. The results of three laboratory experiments, supplemented by a field study, are presented. The Norg extracted by CaCl2 after drying (40°C) originated predominately from a non-biomass soil organic N-pool. The same source of substrate was found to become mineralizable by soil drying and rewetting. There was a significant relationship between the Norg extracted from dried soil samples and the magnitude of N mineralization flushes following soil drying–rewetting (r2=0.861***). No such correlation was found with total soil N. These results suggest that the substrate rendered extractable and that becoming mineralizable were derived mainly from the same source. A field study showed that deep loess soils under the moderate climate of central Europe did not dry out sufficiently, even in the upper 2 cm of soil during the hot summer period, to render a substantial amount of Norg extractable. The possibility is discussed that the Norg rendered extractable upon soil drying may represent a part of a labile non-biomass soil organic N-pool and thus may provide an N availability index, even for soils under continuously moist conditions.