Drying and rewetting of a loamy sand soil did not increase the turnover ofnative organic matter, but retarded the decomposition of added C-14-labelled plant material

Drying and subsequent rewetting of soils has been recognized as an importan t process for accelerating the decomposition of soil organic matter. This e ffect has been attributed to (1) increasing solubility of humic substances (molecular level) (2), microbial death during desiccation and due to osmore gulatory shock by rewetting and subsequent higher turnover through microbia l regrowth (biotic level), and (3) release of protected organic matter by d isruption of macroaggregates during rewetting due to 'slaking' (soil struct ural level). We amended a loamy sand with C-14-labelled Lolium perenne shoo t material in which the effects of slaking were unlikely to be of importanc e. After 4 d some amended and unamended soils were dried at the temperature used for incubation (14 degrees C) to a pressure potential of -3.8 MPa. Th e soil remained structurally intact during the drying-rewetting, as in a fi eld situation. This regime was chosen to ascertain that only the moisture c onditions were changed. The imposed drying-rewetting regime did not result in a measurable increase in the rate of soil organic matter decomposition, but it possibly induced microbial death. It affected decomposition of added plant residues negatively, as shown by a significantly higher amount of re sidual C-14 in the dried and rewetted soils after 100 d of incubation compa red to the continuously moist soils. In previous work the drying process wa s often accompanied by other physical changes, such as increasing temperatu re, and the drying-rewetting process was also accompanied by a manual pertu rbation of the soil fabric, such as spreading the soil in a thin layer befo re drying or homogenizing by sieving or mixing after rewetting. We performe d a second experiment to test the effect of heating and manual disruption b y sieving, on soil respiration and decomposition. This confirmed that the d ecomposition of recently added 14C labelled material was retarded, whereas drying-rewetting per se apparently did not increase the decomposition of na tive SOM. Heating consistently increased respiration of native SOM in all t reatments and combinations, whereas, as it could be expected in a sandy soi l, the sieving treatment did not result in a considerable increase. This in dicates that the effect of drying-rewetting per se on humus decomposition m ay be less than usually assumed. It is suggested that future work on this t opic should pay attention to the drying regime, and especially to possible unintended artifacts due to heating or manual disruptions of the soil fabri c. (C) 1999 Elsevier Science Ltd. All rights reserved.