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 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 CO2 grown litter was decomposed in soils exposed to elevated CO2. Significantly greater levels of microbial biomass (p < 0.05, paired t-test) were detected in soils exposed to elevated CO2 (1174.1 compared to 878.9 μg N g−1 dry soil for ambient CO2 exposed soils). This corresponded with a significant increase (p < 0.005, paired t-test) in in situ soil respiration from the elevated CO2 acclimatised soils (28.7 compared to 20.4 μmol CO2 m2 h−1 from soils exposed to ambient CO2). However, when soil respiration was calculated per unit of microbial biomass, no differences in activity per unit biomass were detected (approx. 0.02 μmol CO2 m2 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 CO2 acclimatised soils (p < 0.05, ANOVA), even though the initial nutrient ratios of the litter were not significantly different.