Soil carbon storage in response to chronic warming in a temperate forest

Background/Question/Methods

Climate warming is occurring on a global scale with unknown long-term effects on soil microbial communities and the biogeochemical processes they perform. While most current carbon-climate models predict increases in decomposition rates and thus a positive feedback to warming, field studies have shown varying results on C turnover times in response to long-term warming. As a result, it is not clear what the fate of soil carbon stocks under chronically warmed conditions will be. The main objective of this study was to examine the response of soil C pools to chronic warming. Samples were collected from a long-term soil warming experiment at the Harvard Forest Long-Term Ecological Research (LTER) site where soils have been continuously warmed to 5°C above ambient for five years. Soils were sampled in October 2011 from warmed and ambient plots. Organic horizon subsamples were collected and then mineral cores were taken at incremental 10 cm depths to a maximum of 50 cm. Samples were analyzed for total C&N, labile C, soil organic matter chemistry, and microbial biomass and community structure.

Results/Conclusions

Seventy percent of the total organic C and N stock at this site is found in the top 20 cm of the soil profile.  Five years of soil warming to 5°C above ambient has resulted in a 30% decline in total organic C and N stored in the forest floor (O-horizon) and a 12-14% decline across the soil profile to a depth of 20 cm.  These results are consistent with our earlier findings at this site that in situ soil respiration has increased by greater than 40% with warming. Chronic soil warming has also resulted in a decline in soil microbial biomass and a shift in microbial community structure and function.