Differeces In Physical And Chemical Properties And Organic Carbon Stability Of Black Soil Between Cropping Systems In Northeast China

There are about 1100 million ha black soils in Northeast China, of which about 8.15 million ha are cultivated land, accounting for 32.5% of the total area of cultivated land in Northeast China. Grain production in this area accounts for 44.4% of Northeast China's total grain output. The black soil belt is the most important area of Northeast Corn Belt, and corn production and exports account for 1/3 and 1/2 of the whole country. However, the reduction of soil organic matter (SOM) and soil erosion in this region is exacerbating following long-term cultivation. Furthermore, the plowlayer thickness and soil fertility are declining rapidly. Therefore, investigating the effect of long-term different cropping systems on soil physical and chemical properties and the quatity and quality of soil organic carbon (SOC) has important implications for optimizing the current cropping systems and achieving the sustainable use of black soil cropland in Northeast China.There is a long-term black soil fertility experiment located in the Jilin Academy of Agricultural Sciences, which began in 1990. Four treatments were selected in the present study:fallow, long-term continuous maize cropping, maize-maize-soybean rotation, continuous soybean cropping. We determined the effects of different cropping systems on soil bulk density, aggregate distribution and stability, soil pH and nutrient concentrations. Differences in the topsoil organic carbon quality under long-term different cropping systems were investigated by SOC chemical fractionation with the aim to explore the differences of physical and chemical properties and stability of soil organic carbon under long-term different cropping systems in a black soil of Northeast China. Our objetive was to investigate the changing mechanism underlying the stability of black topsoil aggregates, the quality of SOC, and the nutrient characteristics of soil profiles. The main conclusions are as follows:Compared with the fallow treatment, long-term cropping significantly reduced the mean weight diameter (MWD) and geometric mean diameter (GMD) of black topsoil. Long-term crop cultivation increased topsoil bulk density, and significantly reduced the proportion of soil water stability aggregates (>2 mm and 2～0.25 mm), which may reduce soil aggregate stability and soil resistance to erosion. In comprision with the continuous corn cropping and continuous soybean cropping treatments, maize-maize-soybean rotation significantly increased the GMD value of soil aggregate, and the MWD value of the soil aggregate tended to increase, indicating that crop rotation could improve soil structure and increase the stability of soil aggregates.In comprision with the fallow, owing to manure plus inorganic fertilizer application, long-term continuous maize cropping and maize-maize-soybean rotation significantly increased the concentrations of SOC, total nitrogen (TN), and available nitrogen. Relative to the fallow, long-term cropping increased the concentration of available phosphorus in the plowlayer, and aggravated the loss of available potassium in both surface and subsurface layers. Long-term continuous soybean cropping significantly reduced the pH in the plowlayer that showed a declining trend as well in the maize-maize-soybean rotation treatment.The carbon proportion of soil microaggregates (0.25～0.053 mm) and silt and clay (<0.053 mm) was higher in the long-term continuous maize cropping, maize-maize-soybean rotation and continuous soybean cropping treatments than in the long-term fallow treatment. Shifts in SOC to the microaggregates and silt and clay were beneficial to the protection and stability of SOC. Soil microbial activity was significantly lower in the maize-maize-soybean rotation and continuous soybean cropping treatments than in the long-term continuous corn cropping and fallow treatments.The chemical lability index of SOC and the proportion of active carbon pool were significantly lower in the maize-maize-soybean rotation treatment than in he long-term continuous maize cropping, continuous soybean cropping and fallow treatments. Furthermore, the chemical activity of the plowlayer SOC was the lowest in the maize-maize-soybean rotation treatment, suggesting that crop rotation significantly increased the chemical stability of SOC in the plowlayer.