Global climatic changes have been world widely concerned, particularly the continuous rise of atmospheric temperature, mainly due to the increase in CO₂  concentration in the atmosphere. The release of soil organic C during the past century (e.g. due to deforest) has been considered as one of the main source of atmospheric CO₂ . It is predicted that, assuming that the global temperature rise by 0.03℃ annually by 2050, about 7% of soil organic C (equivalent to 61×10¹⁵g C accumulated in world soil) would release as CO₂  into atmosphere. However, information on the responses of soil organic C to climatic changes in different regions shows a lack in many parts of the world, although such information are important knowledge in maintaining soil organic C and fertility in the regions.
This paper reports a study of the present trend of organic C in cultivated soils in the subtropical (with warm and moist conditions) and semi-arid (the Loess Plateau) regions in China and the response to global climatic changes, based on predictions using a model (SCNC) recently developed for simulating the turnover of organic matter and nitrogen in cultivated soils in China. The model was tested by data from long-term field experiments in these regions and Rothamsted, UK. Soil management to remedy the detrimental effects of climatic changes on the accumulation of organic C was also discussed.
The model predicted that, when the amounts of fresh organic material input and soil clay content are the same, the stock of soil organic C at the equilibrium was 50% smaller in the subtropical region than that in moist temperate zones (using Rothamsted in UK as the representative), whist the amount in the semi-arid region was slightly larger. Because multi-cropping systems in the subtropical region have resulted in the high input rates of fresh organic materials (e.g. crop residues), the amounts of organic C accumulated can consequently maintain generally larger than that in the Loess Plateau region where the input rates of fresh organic materials are generally low, due to the low productivity of crops. Assuming atmospheric temperature rises at the gradients of 1.5 and 3℃ by 2050 (equivalent to annually mean rises of 0.03℃ and 0.06℃, respectively), and fresh organic material input maintains steady, the amounts of soil organic C would decrease by 5.6%~10.9% in the subtropical region and 3.6% ~ 9.4% in the semi-arid region. The percentages of decreases were dependent on the present amounts of soil organic C, and were slightly smaller than that predicted for moist temperate zones e.g.  5.1%~10.3% for Rothamsted soils). When 40% ~ 60% of crop straw produced incorporates into the soils, the amount of organic C in these regions can increase by over 40% by 2050. Thus, it is proposed that increasing the incorporation of crop residues and manure is essential for maintaining the pool of soil organic C in the regions.