Global soil carbon: understanding and managing the largest terrestrial carbon pool

The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6-0.8 m depth) stimulated the microbial mineralization of 2,567 +/- 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.

Soil respiration, R(S), the flux of microbially and plant-respired carbon dioxide (CO(2)) from the soil surface to the atmosphere, is the second-largest terrestrial carbon flux. However, the dynamics of R(S) are not well understood and the global flux remains poorly constrained. Ecosystem warming experiments, modelling analyses and fundamental biokinetics all suggest that R(S) should change with climate. This has been difficult to confirm observationally because of the high spatial variability of R(S), inaccessibility of the soil medium and the inability of remote-sensing instruments to measure R(S) on large scales. Despite these constraints, it may be possible to discern climate-driven changes in regional or global R(S) values in the extant four-decade record of R(S) chamber measurements. Here we construct a database of worldwide R(S) observations matched with high-resolution historical climate data and find a previously unknown temporal trend in the R(S) record after accounting for mean annual climate, leaf area, nitrogen deposition and changes in CO(2) measurement technique. We find that the air temperature anomaly (the deviation from the 1961-1990 mean) is significantly and positively correlated with changes in R(S). We estimate that the global R(S) in 2008 (that is, the flux integrated over the Earth's land surface over 2008) was 98 +/- 12 Pg C and that it increased by 0.1 Pg C yr(-1) between 1989 and 2008, implying a global R(S) response to air temperature (Q(10)) of 1.5. An increasing global R(S) value does not necessarily constitute a positive feedback to the atmosphere, as it could be driven by higher carbon inputs to soil rather than by mobilization of stored older carbon. The available data are, however, consistent with an acceleration of the terrestrial carbon cycle in response to global climate change.