Aiming to address a series of problems caused by inefficient nitrogen fixation in soybean within the maize–soybean rotation system under cold-region conditions in Heilongjiang Province, China—such as reduced crop yields, declining soil fertility, and increased dependence on chemical fertilisers—this study investigated the partial substitution of chemical nitrogen fertilisers with bio-organic fertilisers at replacement rates of 10%, 20%, and 30% during soybean cultivation. The treatments included bio-organic fertilisers (OB1, OB2, OB3), inactivated bio-organic fertilisers (O1, O2, O3), Bacillus subtilis (B1, B2, B3), and a control (CK) with the conventional application of chemical fertilisers. In the rotational maize cropping phase, a 50% nitrogen reduction was applied. The results showed that replacing 20% of soybean nitrogen fertiliser with bio-organic fertiliser (OB2 treatment) yielded the most significant increase in productivity and economic return. Compared with CK, the OB2 treatment increased soybean yield by 26.56%, maize yield by 26.69%, and nitrogen fertiliser use efficiency by 3–5%. According to the GRA-TOPSIS model, the OB2 treatment demonstrated the greatest capacity to improve quality and efficiency in the maize–soybean rotation system. At the soybean maturity stage, the OB2 treatment increased soil total organic carbon, available phosphorus, and soil protease activity by 25.36%, 22.20%, and 87.50%, respectively, compared with CK. At maize maturity, soil ammonium nitrogen and soil protease activity increased by 80.24% and 62.47%, respectively. Bio-organic fertilisers combine the benefits of organic fertiliser substrates with those of functional microorganisms. Correlation, cluster, and interaction analyses revealed that the synergistic mechanisms between maize–soybean rotation and bio-organic fertilisers in cold regions are primarily reflected in improved soil quality, enhanced nutrient cycling efficiency, increased nitrogen fixation in soybean root nodules, stimulated microbial activity, and greater resilience to environmental stress. Sustainable agricultural production in cold regions can be achieved through the integrated functioning of these system components. This study provides a theoretical basis for enhancing yield and efficiency in maize–soybean rotation systems under cold climatic conditions.

In this study, we collected soybean inter-root soil (clay soil) from the cold region of Heilongjiang Province, China, screening for cold-tolerant phosphorus- and potassium-solubilizing bacteria by the gradient-cooling-directed screening and design of mixed bacterial agents. This study screened cold-tolerant phosphorus-solubilizing and potassium-solubilizing bacteria by gradient-cooling-directed screening and constructed mixed bacterial agents. We analyzed the strain’s phosphorus/potassium solubilizing capacity, as well as its organic acid secretion ability, to reveal its mechanism of detoxification of phosphorus and potassium. Clay soil from the cold region of Heilongjiang, China, was collected for soybean potting trials. The soybean pot experiment was used to investigate the growth-promoting effects of cold-tolerant strains applied to soybeans and to analyze the effects of changes in soil bacterial activity on soybean growth. A dominant phosphorus-solubilizing bacteria strain P-5 (Pantoea ananatis) was screened, and the amount of phosphorus-solubilizing bacteria was 34.94 mg/L with a PSI (Phosphorus Solubilization Index) of 1.80 for 7 days of incubation at 15 °C. The OD₆₀₀ value (optical density of 600 nm) on the 10th day was 0.61. The dominant potassium-solubilizing bacteria, K-1 (Rahnella aquatilis), solubilized 23.02 mg/L of potassium under the same culture conditions, with a KSI (Potassium Solubilization) of 1.51 and an OD₆₀₀ value of 0.59 on day 10. The dominant mixed bacterial agents H-5 (25% P-5 and 75% K-1), designed with the above two, detoxified phosphorus at 62.58 mg/L and potassium at 37.59 mg/L under the same culture conditions. It was shown that P-2 secretes gluconic acid and P-5 secretes lactic acid to degrade insoluble phosphorus at low temperatures and that K-1 secretes citric acid to degrade insoluble potassium. The soybean pot experiment study showed that the application of phosphorus-solubilizing bacteria (PSB), potassium-solubilizing bacteria (KSB), and mixed bacterial agents (MSB) alone could enhance the relative abundance of Pseudomonas, Rhizobium, and other bacterial genera in the soybean planting soil, compared with the control treatment without bacterial agents (BLK). Indirectly, they can enhance the accumulation of phosphorus and potassium in various organs of soybean, promote the development of soybean leaves and stems, and achieve the effect of promoting production and increasing income. This study has important theoretical value for the replacement of chemical fertilizer application by microbial agents in the cold region of China, and it has important practical and strategic significance for the promotion of income green safety in China’s main grain-producing areas.

Global atmospheric N₂O concentrations have risen to 335 ppb, with agricultural soils serving as a major emission source and rice paddies accounting for approximately 11% of agricultural N₂O emissions. Rice–crab co-culture has been widely adopted because of its potential to increase and stabilize crop yields; however, the underlying mechanisms of N₂O mitigation and the synergistic effects of crab bioturbation with water and nitrogen management remain unclear. Therefore, in this study, we conducted a two-year field experiment in Zhaodong, Heilongjiang Province, China, to elucidate the N₂O mitigation effects of rice–crab co-culture under water and nitrogen regulation and the associated driving mechanisms. The results showed that rice–crab co-culture significantly reduced N₂O emissions. Specifically, the N₂O flux decreased by 19.9%, while cumulative N₂O emissions decreased by 19.8%. Under the combined regulation of water and nitrogen management, the mitigation effect on N₂O emissions was further enhanced, with a reduction of up to 30.8%. Regarding environmental factors, crab activity combined with shallow wet irrigation reduced soil water content and increased surface temperature. These changes promoted the transformation of nitrogen from inorganic forms to microbially assimilable forms, increasing the microbial nitrogen content by approximately 29.5%. Meanwhile, soil enzyme activities changed significantly: the activities of urease, sucrase, and protease increased, whereas nitrate reductase activity decreased. Structural equation modeling showed that the indirect effect of management practices was much greater than the direct effect, accounting for 63% of the total effect. Nitrogen transformation was the core mitigation pathway, characterized by the conversion of inorganic nitrogen into microbial biomass nitrogen, which reduced substrate availability for nitrification and denitrification. Enzyme activity regulation served as a secondary pathway, mainly through the inhibition of nitrate reductase activity. Overall, the rice–crab system achieved sustained N₂O reduction by improving soil aeration and jointly regulating substrate limitation and weakening nitrogen transformation capacity.