Soil carbon dynamics would be influenced by fertilization management in the agro-ecosystem. In this study, we analyze carbon inputs and soil organic carbon (SOC) dynamics under wheat-corn double cropping system based on four long-term experimental sites in different climate zones of China. We examine soil carbon responses to various carbon inputs by using linear (S = aA - b) and non-linear () equations. The term S is the SOC change rate; a, the proportion of C inputs incorporated into soil; b, minimum change rate of SOC; S (M) , the asymptotic maximum value at SOC change rate approaching infinity (Mg C ha(-1) year(-1)); S (L) , the decomposition rate of SOC substrates, and K (S) , a constant value (or 'half-saturation constant'). The S value is fitted using linear equation with SOC data over the duration of the experiment. The annual C input (A) is estimated by measured crop biomass and C input from manure. Different amounts of balanced fertilization show little impact on the C inputs derived by plants, reaching to similar to.5 Mg C ha(-1) year(-1). The SOC change rate is much higher under the manure application than treatments with chemical fertilizers only. Statistical analysis shows that the linear and non-linear equations perform equally well (p < 0.01) within the experimental data interval. But the non-linear equation is more suitable for specific purpose. Using the non-linear equation, we can predict that minimum C input to maintain the current SOC level would be 0.33-1.32 Mg C ha(-1) year(-1) at the most sites but only 0.03 Mg C ha(-1) year(-1) at the Changping site. The chemical nitrogen and phosphate fertilization yield sufficient carbon biomass inputs to maintain the current SOC levels. However, to increase SOC at 1 Mg C ha(-1) year(-1), soils need over 10 Mg C ha(-1) year(-1) at most sites. Our results suggest that the increment of SOC stocks would be mainly related to the additional carbon inputs for the long-term perspectives.
