The decomposition of Lolium perenne in soils exposed to elevated CO 2: comparisons of mass loss of litter with soil respiration and soil microbial biomass

Abstract

Two key questions regarding the effects of elevated atmospheric CO 2 on soil microbial biomass are, (a) will future levels of elevated CO 2 affect the amount of microbial biomass in soil? and (b) how will any observed changes impact on C-flux from soils? These questions were addressed by examining soil microbial biomass, and in situ estimations of soil respiration in grassland soils exposed to free air carbon dioxide enrichment (60 Pa). Corresponding measurements of plant litter mass loss were taken using litter bags, ensuring that ambient litter was decomposed in ambient soil, and elevated CO 2 grown litter was decomposed in soils exposed to elevated CO 2. Significantly greater levels of microbial biomass ( p < 0.05, paired t-test) were detected in soils exposed to elevated CO 2 (1174.1 compared to 878.9 μg N g −1 dry soil for ambient CO 2 exposed soils). This corresponded with a significant increase ( p < 0.005, paired t-test) in in situ soil respiration from the elevated CO 2 acclimatised soils (28.7 compared to 20.4 μmol CO 2 m 2 h −1 from soils exposed to ambient CO 2). However, when soil respiration was calculated per unit of microbial biomass, no differences in activity per unit biomass were detected (approx. 0.02 μmol CO 2 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 litter was greater in the elevated CO 2 acclimatised soils ( p < 0.05, ANOVA), even though the initial nutrient ratios of the litter were not significantly different.