NONBIOMASS 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 (N-org) extracted by chem ical methods and used as N availability indices and renders some N-org mineralizable. My aim was to define the sources of these N flushes an d to evaluate the relevance of soil drying-rewetting as a driving forc e for N mineralization. The results of three laboratory experiments, s upplemented by a field study, are presented. The N-org extracted by Ca Cl2 after drying (40 degrees C) originated predominately from a non-bi omass soil organic N-pool. The same source of substrate was found to b ecome mineralizable by soil drying and rewetting. There was a signific ant relationship between the N-org extracted from dried soil samples a nd the magnitude of N mineralization flushes following soil drying-rew etting (r(2) = 0.861**). No such correlation was found with total soi l 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 c m of soil during the hot summer period, to render a substantial amount of N-org extractable. The possibility is discussed that the N-org ren dered extractable upon soil drying may represent a part of a labile no n-biomass soil organic N-pool and thus may provide an N availability i ndex, even for soils under continuously moist conditions. (C) 1998 Els evier Science Ltd. All rights reserved.