Changes of soil structure and function after 16-year conservation tillage in black soil

Abstract: Black soil rich in organic matter is one of the most fertile soils in Northeast China, one of the four largest black soil regions in the world. However, the structure and function of black soil have seriously deteriorated after long-term cultivation in recent years. Much effort has been made to ensure the sustainable development of black soil. However, most previous studies were focused on the soil structure and nutrients in the year of the experiment. In this study, a systematic investigation was performed on the soil structure and function during the whole growth period of crops, thereby elaborating the dynamic evolution after the long-term conservation tillage. A field test was also conducted in the Hailun Monitoring and Research Station of Soil and Water Conservation, Chinese Academy of Sciences (47°26′N, 126°38′E), located at the Hailun City, Heilongjiang Province, the center of the typical Mollisols zone in Northeast China. Taking the corn-soybean rotation black soil as the research object, four tillage treatments were set (conservation tillage: NT (No-tillage with straw returning), RT (Reduced-tillage); traditional tillage: MT (Moldboard-tillage), Rot (Rotary-tillage)) during the different growth periods of soybean (SS (Seedling stage), FlS (Flowering stage), FiS (Filling stage), and MS (Maturity stage)). Some parameters were measured, including the soil bulk density, porosity, water status, and water-stable aggregate content in response to tillage. Once the soybean matured, the soil samples were collected to quantitatively characterize the soil organic carbon in the vertical direction of different tillage treatments, and the soil moisture infiltration rate in the plow layer. The results showed that: 1) The NT improved to stabilize the soil structure. The soil bulk density under the NT system was more stable than the other three treatments. The soil surface field capacity under the NT treatment was higher than that of conventional tillage during the whole growth period of soybean. The NT treatment increased the content of large aggregates on the soil surface and the Average Weight Diameter (MWD) was higher than the rest. The NT also improved the soil water infiltration, where the initial infiltration rate was 1.26 to 1.63 times that of the rest treatments. 2) The NT significantly increased the accumulation of soil surface organic carbon. Specifically, the organic carbon content of soil surface under the NT treatment was 32.59%, 30.28%, and 25.79% higher than MT, RT, and Rot treatments, respectively. 3) The NT maintained the soil productivity. In short, the long-term no-tillage with the straw returning significantly determined the soil structure and organic carbon accumulation. The NT treatment performed better the structure and organic carbon accumulation in the lower layer soil, thereby promoting the sustainable development of black soil. This finding can provide a strong theoretical basis to explore the long-term effects of conservation tillage in the black soil area of Northeast China.