SOIL DRYING AND REWETTING AND THE TURNOVER OF C-14-LABELED PLANT RESIDUES - 1ST ORDER DECAY-RATES OF BIOMASS AND NON-BIOMASS C-14

The effects of drying and rewetting on soil organic C derived from added plant material were determined. Three soils, one silty-loam and two loamy sands, were incubated with C-14-labelled plant material for 27 days. Each soil was then subjected (1) to drying at 40-degrees-C for 3 days, remoistening and incubation at 25-degrees-C for 10 days, and (2) to storage at 4-degrees-C for 3 days and incubation. The silty-loam soil was also treated after 7 days of incubation with C-14-labelled plant material. Residual C-14 concentrations were determined at the beginning and at the end of the 10 day incubation. During drying or storage and subsequent incubation of soils, biomass C and C-14 were measured, and also the amounts of CO2 and (CO2)-C-14 released during the 10 day incubation. Residual C-14 concentrations at the end of the incubation were not significantly affected by soil desiccation and remoistening, but the percentages of residual C-14 due to biomass C-14 were greatly reduced in dried, rewetted and incubated soils. The effect was largest for the soil which had been incubated for the shortest time with C-14-labelled plant material. Average first-order gross decay rates were calculated for biomass C-14 and non-biomass C-14, and for different efficiencies of substrate utilization (viz. 20, 40 and 60%). Two time intervals were chosen: with and without inclusion of the drying period. Drying and rewetting of soils enhanced first-order gross decay rates of the two carbon pools. The relative increases were larger for decay rates of biomass C-14 than for those of non-biomass C-14. When decay rates were averaged over a time interval that did not include the drying period, observed effects of soil desiccation and remoistening were less pronounced, but still apparent. This suggests that a previous drying-rewetting cycle has an appreciable influence on decomposition processes during later incubation of soils. It was concluded that soil drying and rewetting promoted the turnover of C derived from added plant material, and that this increase in C cycling was mainly due to enhanced turnover of microbial products. This may finally result in a change of quality of the organic C pool coming from added plant residues.