Abstract: To address soil degradation caused by unsustainable tillage practices under continuous maize monoculture in the Northeast Sanjiang Plain, we conducted a field experiment in 2019 to evaluate the effects of four conservation tillage and straw returning treatments: plow tillage with straw returning (PT), rotary tillage with straw mixing (RT), strip tillage with straw returning (ST), no-till with straw mulching (NT), and conventional rotary tillage without straw returning (CK). After five years of treatments, we analyzed the effects of straw returning under conservation tillage on maize yield, soil physicochemical properties, and soil bacterial community composition and function. The results showed that all treatments outperformed the control (CK) in maize yield, with the order of superiority being ST, PT>RT>NT, CK. Among them, the RT treatment exhibited the highest yield stability (coefficient of variation: 4.4%), while the NT treatment showed the largest yield fluctuation (coefficient of variation: 10.4%). In the 0-5 cm surface soil, all treatments significantly increased ammonium nitrogen, nitrate nitrogen, available phosphorus, available potassium, organic matter content, and soil moisture compared to CK, with the NT treatment having the stongest effect. All treatments except PT reduced soil pH. In the 5-10 cm layer, the ST treatment resulted in significantly higher available phosphorus content than other treatments (increase of 13.1%-83.7%), while the PT treatment showed the highest available potassium content. All treatments significantly reduced soil pH in this layer. Under conservation tillage, straw returning increased the Shannon index of bacteria in the 0-5 cm soil layer by 4.9%-9.0%. NT treatment promoted enrichment of Actinomycetes, whereas the ST treatment enhanced the abundance of Proteobacteria. Results of Mantel analysis, FAPROTAX functional prediction, and qPCR verification of functional genes showed that tillage and straw returning methods regulated microbial community composition by influencing soil physicochemical properties such as nitrate and available phosphorus, leading to distinct carbon-nitrogen coupling metabolic mechanisms. This resulted in an increase in nitrogen fixation function in both the surface and subsurface layers by 0.3%-22.6% and 69.3%-108.0%, respectively, with a significant increase in the abundance of nifH gene in the subsurface layer, which was involved in nitrogen fixation, by 78.7%-161.1%. In the Sanjiang Plain, the ST mode achieved the best overall performance in terms of maize yield and soil ecological benefits, while the NT mode showed advantages in surface soil improvement.

Key words: conservation tillage, straw returning, maize yield, soil nutrient, bacterial community, bacterial function prediction