Effects of long-term conservation tillage on black soil organic carbon components in sloping farmland

Sloping farmland has been seriously eroded and degraded in black soil regions in Northeast China, particularly harmful to sustainable food production. Conservation tillage can be an efficient measure to reduce soil and water loss for better control of soil degradation. Soil organic carbon content (SOC) can also be expected to increase the SOC fractions. This study aims to explore soil quality restoration under long-term conservation tillage. A continuous 15a conservation tillage field experiment was conducted in the typical black soil sloping farmland in Hailun country, Suihua City, Heilongjiang Province of north China. A systematic investigation was implemented to clarify the effect of conservation tillage on the SOC fractions of degraded black soil by erosion. Two field experiments were carried out in the Hailun Soil and Water Conservation Monitoring Research Station, Chinese Academy of Sciences, including the conservation tillage (NT), and conventional tillage (CT) with corn-soybean rotation mode year by year. The NT treatment was no tillage with the mulch of all straw returning to the field. The soil was sampled in 0～5, >5～10, >10～15, >15～20, >20～40, >40～60, >60～80, and >80～100 cm soil layers in the top, middle, and bottom of the experiment field. Some parameters were measured, including the total SOC, total nitrogen (TN), particulate organic carbon (POC), mineral-bounded organic carbon (MOC), light fraction organic carbon (LFOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) contents. A comparison was also made on the differentiation between NT and CT modes. The vertical distribution and changing characteristics of SOC were obtained for the different carbon fractions along the soil profile. The results showed that: (1) The NT increased the SOC content in the 0-5 and 5-10 cm soil layers by 29.54% and 22.38%, respectively ( P <0.05), compared with the CT. The SOC storage increased by 31.11% and 27.34%, respectively ( P <0.05), and the TN content increased by 53.74% and 37.60%, respectively ( P <0.05). The SOC and TN content in the surface soil increased significantly, whereas, there was no significant change in the SOC and TN content in the deep soil. (2) The NT mainly increased the POC content of 5-10 cm soil layers by 69.85%, respectively ( P <0.05), and the LFOC content of 0-5 cm soil layers by 130.81%, respectively ( P <0.05), and the MBC content of 0-5 and 5-10cm soil layers by 85.59% and 59.53%, respectively ( P <0.05), indicating a certain positive impact on the organic carbon composition of deep soil. The POC, MOC, LFOC, and MBC contents decreased with the increase of the soil layer, while the DOC contents increased. In addition, the ratio of POC/SOC, LFOC/SOC, and MBC/SOC decreased with the increase of soil layers, while the ratio of DOC/SOC increased. (3) The NT shared a significant impact on the soil aggregate stability indicators, in terms of tillage effect. The soil aggregate stability indicators were dominated to improve the SOC content. The NT mainly increased the content of active organic carbon after 15a continuous conservation tillage, compared with the CT. Long-term conservation tillage posed a positive effect on the restoration of black soil quality. Both the topsoil and subsoil organic carbon were dependent on the long-term conservation tillage. The finding can provide a strong reference for the soil carbon sequestration in sloping farmland under long-term conservation tillage.