Evaluation in soil carbon and nitrogen characteristics under long-term cropping regimes in black soil region of Northeast China

Abstract: Soil carbon and nitrogen are of importance to evaluate soil fertility. To clarify the comprehensive effects of cropland management measures on soil carbon and nitrogen dynamic will provide theoretical and technical support to soil fertility cultivation and land sustainable utilization. Commercial grain production in Northeast China plays a crucial role in Chinese food security. However, long-term intensive maize cropping with chemical fertilizer application has resulted in great decrease of soil organic carbon and rapid decline of soil fertility in this region. During the past decades, many efforts have been paid on the impacts of plant patterns on soil organic carbon and soil chemical conditions in order to improve soil fertility, but little is known about the integrated impacts on soil aggregation and soil organic carbon situation, soil nitrogen and their relationship. Therefore, based on the long-term field experiment started in 1990 in Gongzhuling, Jilin Province, China, an investigation was performed to compare the differences in soil carbon, soil nitrogen, and their relationship among the different cropping regimes (continuous maize cropping, maize-soybean rotation and continuous soybean cropping, the three were with organic manure application, fallow). Soil samples were taken at 20 cm intervals from 0 to 100 cm soil depth after maize was harvested in October 2010 for analysis. The results showed that, within the 0-100 cm soil profile, soil organic carbon, total nitrogen, and available nitrogen decreased significantly with soil depth under all treatments. There were significant correlations between soil organic carbon and total nitrogen, and available nitrogen (P<0.01), suggesting that higher soil organic carbon could promote soil nitrogen concentration. In the topsoil layer (0-20 cm), soil organic carbon, total nitrogen and available nitrogen under continuous maize cropping, maize-soybean rotation and continuous soybean cropping treatments were higher than those under fallow treatment by 23.5%-46.8%, 4.9%-64.3% and 35.4%-121.9%, respectively. Furthermore, the highest values in soil organic carbon, total nitrogen and available nitrogen occurred in the continuous maize cropping treatment (P<0.05). Soil microbial biomass carbon and soil microbial biomass nitrogen under continuous maize cropping treatment in topsoil layer were significantly higher than those under maize-soybean rotation and continuous soybean cropping treatments by 23.1%-41.4% and 37.8%-135.3%, respectively (P<0.05). Continuous maize cropping achieved the greatest organic carbon associated by soil aggregate in the >2 000, ≥250-2 000, ≥53-250 and <53 μm aggregate fractions (P<0.05). However, high labile organic carbon under continuous maize cropping treatment was significantly higher than those under fallow, maize-soybean rotation and continuous soybean cropping treatments (P<0.05), and the chemical activity index was significantly higher than that under maize-soybean rotation treatment as well (P<0.05), resulting that soil organic carbon is more sensitive to the external environment changes under continuous maize cropping treatment. Present study indicates that long-term continuous maize cropping with organic manure amendment can increase soil organic carbon and nitrogen availability. However, more efforts should be paid to adopt reasonable cultivation measures to reduce the risk of soil organic carbon decomposition under continuous maize cropping.