Influence of defoliation on CO2 efflux from soil and microbial activity in a Mediterranean grassland

Defoliation of grasses affects carbon (C) input from plants into the rhizosphere and so may affect C turnover in soil. We examined the effect of grassland's defoliation on root-derived CO2 efflux and microbial activity in Mediterranean Phaeozem. In situ partitioning of total CO2 efflux into root-derived and microbial-derived CO2 fluxes was performed by mesh-exclusion technique. Microbial basal respiration, N mineralization and the activity of enzymes involved in the cycling of C, N, P and S (also used to calculate microbial functional diversity) were measured in soils of defoliated and control plots. Cumulative CO2 efflux in defoliated plots was 18% lower in 2006 and equal to control plots in 2007. The contribution of microbial CO2 to total CO2 efflux from soil ranged from 71% to 86% without significant differences between defoliated and non-defoliated plots. The lack of correlation between root-derived CO2 and soil temperature after defoliation indicates that photoassimilate supply is the major determinant for root-derived CO2. Microbial-derived CO2 efflux was 20% lower in defoliated plots after accounting for temperature and humidity differences between the two treatments. Defoliation suppressed the activity of enzymes involved in the cycling of C, S and P and decreased basal respiration rates of soil microorganisms by 19%. In turn, defoliation stimulated activity of enzymes involved in the cycling of N, as indicated by the increase of potential nitrification rates and of leucine-aminopeptidase activity. Stimulation of N mineralization promotes a fast regeneration of defoliated plants. We confirm the presence of strong links between plant and microbial activity in a grassland community, as well as close coupling of aboveground photosynthetic activity with root-derived CO2.