Relative contributions of fungi and bacteria to litter decomposition under low and high soil moisture in an Australian grassland

Abstract

Fungi and bacteria differ in their sensitivity to soil moisture, but their activity and contributions to soil carbon (C) cycling under different soil moisture conditions remain unclear. We manipulated soil fungi and bacteria with the fungicide captan and bactericide bronopol, respectively, and examined their effect on C dynamics at low and high moisture levels (40 and 80 % water holding capacity) in a grassland soil over a 21-day incubation period in the laboratory. 13C-labeled ryegrass was added with and without biocide treatments to separate ryegrass from soil-derived C in respiration, dissolved organic C (DOC) and soil C. We also measured activities of four C-degrading enzymes. Soil total- and ryegrass-respiration rates and microbial biomass C (MBC) decreased, while ryegrass-derived C in DOC and soil increased with decreased soil moisture, suggesting reduced microbial activity with decreased soil moisture. Initially, ryegrass respiration rates were suppressed by both biocides and both soil moisture levels. However, at low soil moisture, ryegrass respiration no longer was suppressed by bronopol after eight days of incubation, but remained suppressed with captan, suggesting that with low moisture fungi played a more important role in decomposing ryegrass litter than bacteria. On the other hand, at high soil moisture, biocide effects on ryegrass respiration mostly disappeared after eight days, suggesting that more labile components of ryegrass litter were no longer present, and that the surviving microorganisms adapted similarly to decomposing the more recalcitrant litter. Bacteria preferred to decompose labile substrates early on during the incubation, and fungi played a greater role in decomposing more recalcitrant ryegrass left over in soil at the end of the incubation. At low moisture with captan, a reduction in oxidative enzyme production contributed to the increase in the hydrolytic:oxidative enzyme ratio, further indicating the importance of fungi for decomposing more recalcitrant substances under low moisture conditions.