The decomposition of Lolium perenne in soils exposed to elevated CO2: comparisons of mass loss of litter with soil respiration and soil microbial biomass

Two key questions regarding the effects of elevated atmospheric CO2 on soil microbial biomass are, (a) will future levels of elevated CO2 affect the a mount of microbial biomass in soil? and (b) how will any observed changes i mpact on C-flux from soils? These questions were addressed by examining soi l microbial biomass, and in situ estimations of soil respiration in grassla nd soils exposed to free air carbon dioxide enrichment (60 Pa). Correspondi ng measurements of plant litter mass loss were taken using litter bags, ens uring that ambient litter was decomposed in ambient soil, and elevated CO2 grown litter was decomposed in soils exposed to elevated CO2. Significantly greater levels of microbial biomass (p < 0.05, paired t-test) were detecte d in soils exposed to elevated CO2 (1174.1 compared to 878.9 mu g N g(-1) d ry soil for ambient CO2 exposed soils). This corresponded with a significan t increase (p < 0.005, paired t-test) in in situ soil respiration from the elevated CO2 acclimatised soils (28.7 compared to 20.4 mu mol CO2 m(2) h(-1 ) from soils exposed to ambient CO2). However, when soil respiration was ca lculated per unit of microbial biomass, no differences in activity per unit biomass were detected (approx. 0.02 mu mol CO2 m(2) h(-1) unit biomass(-1) ), suggesting that increased soil microbial biomass, rather than increased activity was responsible for the observed differences. The mass loss of lit ter was greater in the elevated CO2 acclimatised soils (p < 0.05, ANOVA), e ven though the initial nutrient ratios of the litter were not significantly different. (C) 2000 Elsevier Science Ltd. All rights reserved.