Spatial-temporal Changes In Carbon Storage Of Cultivated Mollisols In Hailun City

As one of the world’s most precious black soil zone, Mollisols area of Northeast China is an important grain production base in China. Mollisols are rich in organic carbon and nutrient contents. With the increase in soil disturbance, croplands could be changed from a carbon sink to a carbon source. The aims of our study were to clarify the storage and change of soil organic carbon(SOC) of cultivated Mollisols over the past 30 years and to reveal the key controlling factors and driving mechanism for the SOC changes. In this study, SOC databases obtained from field survey in 1981 and 2011 were used to estimate the SOC storage(0-20 cm) and its change in croplands and to clarify the temporal and spatial variations of SOC and its influencing factors in Hailun City of Heilongjiang Province, Northeast China. A laboratory incubation experiment was performed and the main factors of carbon emissions of were analyzed at the same time. The results showed that:Soil organic carbon content of cropland in Hailun showed a decreasing trend over the initial 20 years from 1981 and a slow increasing trend over the latter 10 years. Over the last 30 years, however, the total SOC storage still decreased, and the distribution of SOC change showed a great spatial variation. The SOC storage of croplands were estimated to be 22.5×106 and 18.2×106 Mg in 1981 and 2011, respectively, with an SOC change rate of-0.49 Mg C ha/yr. The total SOC loss was 19.1% of the SOC in 1981. Based on the area, the ratio of percentages of SOC gain to loss was 3%:84% and 13% of the cropland area had no difference in the SOC density(SOCD) in 2011 compared with 1981.We found significantly negative correlations between the initial SOCD of the soil groups and SOCD change(P<0.001). Soil groups with a lower initial SOCD had lower SOC storage changes compared with 1981, whereas those with a higher initial SOCD had higher changes. Soil C:N ratio decreased significantly over the last 30 years(P<0.001), which might have promoted soil carbon mineralization and thus decreased SOC storage. Along the elevation gradient from the northeastern to the southwestern area in Hailun City, the SOCD change increased profoundly, with the largest SOCD change(loss) occurring in the southwestern and central agricultural areas(P<0.05).Soil organic mineralization was closely related to temperature, initial SOC content, and external inputs of organic matter. Generally, higher temperature and higher SOC content can promote SOC mineralization; the external organic matter with higher carbon availability had higher mineralization rates. However, the effects of temperature and external carbon inputs on SOC mineralization depended on the initial SOC content. To decrease soil CO2 emissions and sequester carbon into soils, therefore, appropriate measurements of carbon sequestration should be adopted according to the specific study area and condition.