Effects of deep tillage on yield of maize based on national and regional scales: A meta-analysis

To understand the effects of deep tillage on the yield and yield composition of maize on national and regional scales, we collected data from 1998 to 2023 of published papers in China and abroad. We conducted a meta-analysis, and quantified the overall and regional impacts of deep tillage on maize yield and yield composition, with conventional tillage (e.g. plow tillage, rotary tillage or harrow tillage with the depth less than 18 cm) as the control group and deep tillage (e.g. subsoiling, deep ploughing or deep mixing with the depth more than 25 cm) as the treatment. We further quantitatively analyzed the effect of annual average temperature, annual average precipita-tion, soil texture, pH, soil organic carbon (SOC) content, total nitrogen (TN) content, planting method, cropping system, straw returning, experimental duration, and fertilizer application on maize yield of deep tillage. Results showed that deep tillage significantly increased maize yield by 8.1% on the national level. Responses of yield to deep tillage in different regions were highly variable. Deep tillage significantly increased maize yield by 9.2%, 8.1%, and 7.8% in Northwestern, Northeastern, and Northern China, respectively. There was no significant difference for yield effect of maize in Southeastern China and Southwestern China. The significant increase in maize yield through deep tillage was attributed to the combined improvement in effective number of spikes, the grains per spike and hundred grains weight. Random forest analysis showed that experimental duration had greatest impact on the relative change rate of maize yield, accounting for 13.3%. Deep tillage could improve maize yield in the Northwes-tern China under one crop per annum and continuous cropping with the annual average temperature, annual precipitation, SOC and TN content was less than 10 ℃, 400 mm, 10 g·kg^-1 and 1 g·kg^-1, respectively. In climate zones with an average annual temperature of less than 10 ℃ and an average annual precipitation of 400-800 mm, as well as neutral (pH 6.5-8) soils with moderate soil nutrient content (SOC of 10-15 g·kg^-1 and TN of 1-1.5 g·kg^-1), deep tillage could increase maize yield in Northeast China during continuous cropping of one crop per year. In the neutral soil with an average annual temperature of 10-15 ℃, an average annual precipitation of 400-800 mm, SOC content＜10 g·kg^-1 and TN content of 1-1.5 g·kg^-1, where two crop rotation was used in the Northern China region, deep tillage significantly increased maize yield. The higher mean annual temperature may be the main reason for the insignificant yield increase of deep tillage maize in Southern China. The average yield increasing rate decreased with the extension of deep tillage duration. Yield effect reached the maximum when deep tillage lasted for 1-3 years. Straw returning and reasonable fertilization were the best methods increasing maize yield under deep tillage. Therefore, deep tillage would benefit maize yield in Northern China. The duration of continuous deep tillage should not exceed three years in combination with straw returning and suitable fertilization.