Topsoil Organic Carbon Increases But Its Stability Declines After Five Years Of Reduced Throughfall

Climate change alters the regional precipitation pattern and causes seasonal droughts that affect forest growth and belowground carbon cycling. This is expected to create greater oscillations in soil moisture that affect the stability of the soil organic carbon (SOC) pool. This study examined the effects of the precipitation decline on the SOC dynamics by experimentally controlling the throughfall in a Pinus massoniana plantation in subtropical region of China. Soil physicochemical properties were analyzed in topsoil (0-5cm depth) during the dry-cool and the wet-warm season after five consecutive years of similar to 30% throughfall reduction. We also assessed the SOC stock and its stability, as indicated by the easily oxidizable carbon (EOC) pool, SOC mineralization rates, soil aggregate size distributions and fine root biomass in relation to soil microbial community structure. The throughfall reduction reduced the soil moisture by 15-20% between sampling dates, with an average 18% during the five-year experiment. With throughfall reduction, topsoil SOC stocks significantly increased by 0.18 kg C m(-2) or 10%, but fine root biomass, EOC content, and the micro-aggregate fraction were significantly higher, suggesting less stable SOC. The abundance of bacterial r-strategists (e.g. Bacteroidetes, Betaproteobacteria, and Gammaproteobacteria) increased significantly while K-strategists (e.g. Planctomycetes) decreased. Both SOC and EOC mineralization were positively correlated with the abundance of r-strategists, but negatively correlated with the K-strategists abundance. We posit that micro-aggregates, fine root biomass, and altered microbial strategist ratios are associated with the decline in SOC stability in forests with less throughfall.