The Soil Carbon Stock of Northern China's Grasslands
Yang, Y., Fang, J., Ma, W., Smith, P., Mohammat, A., Wang, S. and Wang, W. 2010. Soil carbon stock and its changes in northern China's grasslands from 1980s to 2000s. Global Change Biology 16: 3036-3047.
In an effort to answer this question, Yang et al. "conducted five consecutive regional soil surveys in China's grasslands during 2001-2005 and sampled 981 soil profiles from 327 sites across the northern part of the country," after which they compared their results "with data of 275 soil profiles derived from China's National Soil Inventory during the 1980s."
Results indicated, according to the seven scientists, that the organic carbon stock in the upper 30 cm of soil in northern China's grasslands "did not show significant association with mean annual temperature, but was positively correlated with mean annual precipitation," reaching a plateau when soil moisture was above 30%. And, most importantly, they found that "grassland soil organic carbon stock did not change significantly over the past two decades, with a change of 0.08 kg carbon per m2, ranging from -0.30 to +0.46 kg carbon per m2 at 95% confidence interval."
Yang et al. write that "it has been often asserted that soil will act as a carbon source because of its sensitivity to global environmental change (e.g., Melillo et al., 2002; Bellamy et al., 2005; Schipper et al., 2007)," but that "in contrast to these previous reports, our results indicate that soil organic carbon stock in northern China's grasslands has not experienced significant changes during the past two decades, despite measureable climate change," i.e., global warming. Hence, there is good reason (i.e., real-world data) to not believe that "climate warming is likely to accelerate the decomposition of soil organic carbon which could lead to increased carbon release from soils, providing a positive feedback to climate change," as some have suggested based on primarily theoretical considerations.
Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, R.M. and Kirk, G.J.D. 2005. Carbon losses from all soils across England and Wales 1978-2003. Nature 437: 245-248.
Davidson, E.A. and Janssens, I.A. 2006. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440: 165-173.
Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173-2176.
Schipper, L.A., Baisden, T., Parfitt, R.L., Ross, C. and Claydon, J.J. 2007. Large losses of soil C and N from soil profiles under pasture in New Zealand during the past 20 years. Global Change Biology 13: 1138-1144.
Schlesinger, W.H. (Ed.) 1997. Biogeochemistry: An Analysis of Global Change. Academic press, San Diego, California, USA.