Combining rotary and deep tillage increases crop yields by improving the soil physical structure and accumulating organic carbon of subsoil

Continuous rotary tillage has resulted in several issues, including a thin tillage layer with low soil organic carbon (SOC) and soil compaction, impeding crop root development and resulting in low crop yields, especially in clay soils. Although deep tillage can increase crop yields by loosening the soil structure and expanding the tillage layer it is rarely applied in soils with high clay contents (such as lime concretion black soil) because of its high energy consumption and low economic benefit. This study aimed at investigating the modified tillage practice with lower energy consumption (combining rotary and deep tillage to return crop straw into different depths among different years) in the higher crop yield on a clay soil. We conducted a 5-year (2017-2021) field experiment in a lime concretion black soil with high clay content. The experiment included four treatments: conventional tillage (CT) to return crop straw into the 15-cm layer without and with fertilizer addition, modified tillage (MT) to return crop straw into different depths (i.e., 35 cm in 2017, 20 cm in 2018, 10 cm in 2019, and 20 cm in 2020) with fertilizer addition, and MT combined with fertilizer and activator addition. We investigated the crop yields, soil physicochemical properties, and microbial communities at the topsoil (0-15 cm) and subsoil (15-30 cm) layers. Compared with CT, MT increased maize (Zea mays Linn.) and wheat (Triticum aestivum L.) yields by 9.8 % and 11.4 %, respectively, by enhancing the SOC content and improving the soil physical properties of the subsoil (i.e., aggregate stability, macroaggregate proportion, soil porosity, and the proportion of large and small pores). We suggest a scientific tillage practice for future attempts to increase SOC sequestration and promote crop productivity in agricultural soils, especially those with a high clay content.