[Effects of Different Agricultural Practices on Soil Carbon Pool in North China Plain]

North China Plain is an important region of grain production.Soil fertility and grain production in this region are significantly influenced by the levels of soil carbon and nitrogen.In order to explore the effects of agricultural practices on the levels of soil carbon and nitrogen,a long-term field experiment was started in 1999 in Quzhou County,Hebei Province.Four treatments,including following nature (F),tillage without straw (N),no tillage with crushed straw incorporation (S),and tillage with crushed straw incorporation (TS),were chosen to collect soil samples at the layers of 0-20 cm and 20-40 cm in 2013.Soil organic carbon (SOC),soil inorganic carbon (SIC),total carbon (TC),total nitrogen (TN),δ¹³C_SOC,δ¹³C_SIC and δ¹⁵ N were analyzed.The results indicated that compared with F,SOC stocks of N,S and TS decreased by 21.6%,12.3% and 3.4% in the 0-20 cm soil layer,but the changes of SIC stocks were not significant.In the 20-40 cm soil layer,the changes of the SOC stocks were not significant,but the SIC stocks increased by 4.1%(N),7.3%(S) and 5.0%(TS) compared to F,respectively.Major contribution of SIC increase was the pedogenic inorganic carbonate (PIC),which increased by 97%-261% in the farmland soil.In the soil layer of 0-20 cm,the values of δ¹⁵N,δ¹³C_SIC and δ¹³C_SOC in the farmland treatments were higher than those of F,meanwhile,the values of δ¹³ C_SOC were significantly higher than that of F.In the soil layer of 20-40 cm,the values of δ¹⁵N and δ¹³C_SIC were lower than those of F,but the value of δ¹³C_SOC showed the opposite trend.In North China Plain,lithogenic inorganic carbonate (LIC) of farmland soil decomposed and PIC increased by the soil-crop system,which provided CO₂ for the formation of PIC,and straw returning was an effective agricultural practice to restore the soil carbon decreased by tillage.There should be more long-term monitoring and studies for the impacts of crop straw incorporation and tillage on SOC and SIC,especially for soil in deeper layers.