Variations in cropland soil organic carbon fractions in the black soil region of China

The use of three discrete soil organic carbon (SOC) pools proposed by the CENTURY model to study the SOC dynamics helps to understand the changes in cropland SOC pool and its stability. This study focuses on a typical black soil (mollisol) region in Northeast China. Based on historical soil data from samples that were collected in the 1980s, we selected 44 sampling points and collected 88 soil samples from the surface (0-20 cm) and subsurface (20-40 cm) layers in 2010. A 100-day laboratory incubation for each sample was conducted to measure the decomposition rates of SOC at different times, and data from the incubation experiment were fitted to a three-pool first-order model that divided the total SOC into active (Ca), slow (Cs) and resistant (Cr) SOC fractions. A method for predicting the concentrations of the three SOC fractions was developed and used to obtain the concentrations of SOC fractions in the 1980s at each sampling point. The results showed that a power function model (Dsoct = a × tb) could be used as the universal SOC decomposition curve model. Using the universal model, the predictive method accurately estimated the concentrations of Ca, Cs and Cr of upland soils in this black soil region, which effectively solved the problem of a lack of soil samples and SOC fractions data in previous studies of the spatial-temporal variations of SOC fractions in regional soils. From 1980 to 2010, the estimated variations of the Ca, Cs and Cr in the surface upland soil were +0.37, −5.53 and −6.32 g kg−1, and the corresponding contributions to the loss of the total SOC were -3.2%, +48.2% and +55.1%, respectively, while the variations of the Ca, Cs and Cr in the subsurface soil were +0.19, −0.43 and −2.45 g kg−1, and the corresponding contributions to the loss of the total SOC were −7.1%, +16.0% and +91.1%, respectively. The decrease in the total SOC is primarily attributed to the decrease of the Cs and Cr fractions in the region, although the Ca fraction has significantly increased. The overall variations in the SOC fractions suggest a declining stability of the SOC pool in the black soil region of Northeast China.