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

Drying and subsequent rewetting of soils has been recognized as an important process for accelerating the decomposition of soil organic matter. This effect has been attributed to (1) increasing solubility of humic substances (molecular level) (2), microbial death during desiccation and due to osmoregulatory shock by rewetting and subsequent higher turnover through microbial regrowth (biotic level), and (3) release of protected organic matter by disruption of macroaggregates during rewetting due to ‘slaking' (soil structural level). We amended a loamy sand with 14C-labelled Lolium perenne shoot material in which the effects of slaking were unlikely to be of importance. After 4 d some amended and unamended soils were dried at the temperature used for incubation (14°C) to a pressure potential of −3.8 MPa. The soil remained structurally intact during the drying–rewetting, as in a field situation. This regime was chosen to ascertain that only the moisture conditions 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 residual 14C in the dried and rewetted soils after 100 d of incubation compared to the continuously moist soils. In previous work the drying process was often accompanied by other physical changes, such as increasing temperature, and the drying–rewetting process was also accompanied by a manual perturbation of the soil fabric, such as spreading the soil in a thin layer before drying or homogenizing by sieving or mixing after rewetting. We performed a second experiment to test the effect of heating and manual disruption by sieving, on soil respiration and decomposition. This confirmed that the decomposition of recently added 14C labelled material was retarded, whereas drying–rewetting per se apparently did not increase the decomposition of native SOM. Heating consistently increased respiration of native SOM in all treatments and combinations, whereas, as it could be expected in a sandy soil, the sieving treatment did not result in a considerable increase. This indicates that the effect of drying–rewetting per se on humus decomposition may be less than usually assumed. It is suggested that future work on this topic should pay attention to the drying regime, and especially to possible unintended artifacts due to heating or manual disruptions of the soil fabric