Compound microbial fertilizer is a new type of environmentally friendly slow-release fertilizer that can effectively improve the physical and chemical properties of the soil, significantly improve the ecological environment, and promote the sustainable development of agriculture. In this study, we conducted a field experiment to evaluate the impact of different applications of chemical fertilizer combined with composite microbial fertilizer on soil physical properties and soybean yields at Heshan Farm, Heilongjiang Province, China, during 2021–2022. Soybean varieties “Jinyuan 55” and “Keshan 1” were treated with three treatments implemented as follows: T1 (conventional fertilization), T2 (50% N fertilizer + compound microbial fertilizer), and T3 (0 N fertilizer + compound microbial fertilizer). Compared to conventional fertilization (T1 treatment), the application of composite microbial fertilizers (T2 and T3) resulted in a decrease in soil bulk density and an increase in porosity. Notably, we observed that moderate application of the composite microbial fertilizer (T3) led to a decrease in the volume fraction of clay particles and an increase in the volume fraction of sand particles. Furthermore, all treatments exhibited high content of agglomerates larger than 5 mm at 0–20 cm. The application of composite microbial fertilizers (T2 and T3) promoted the formation of large soil agglomerates and reduced the presence of micro-agglomerates smaller than 0.25 mm. In 2021–2022, The soybean yield increased by 13.02% in the T2 treatment compared with the T1 treatment and decreased by 9.34% in the T3 treatment. We concluded that the appropriate application of compound microbial fertilizer can help protect black soil, enhance the self-repair capability of black soil, and improve soybean quality in abnormal precipitation years. These results provide an actionable basis for constructing and developing green fertilizer systems for the soybean industry.

Melatonin can directly or indirectly eliminate reactive oxygen species, regulate hormone levels, and improve drought-stressed soybean crop resistance, yield, and quality. The nutrient contents of soybeans grown under normal conditions (WW), drought stress (D), and drought stress with exogenous melatonin (D + M) ( ) were compared. The differences in the quality of natto from the three groups of soybeans were also analyzed. The results showed that compared with soybeans under normal conditions, those grown under drought stress had reduced yield and carbohydrate, protein, essential amino acid, soybean isoflavone, and other nutrient contents. Besides, natto presented low nattokinase levels (674 U/mL), natto drawing ability was weak, the surface was dull, the taste was poor, and the sensory score was 12 points. Exogenous melatonin increased the carbohydrate content (starch, sucrose, glucose, and fructose) and improved the yield and quality of soybeans under drought. The natto produced by soybeans under drought stress with exogenous melatonin had high nattokinase content (756 U/mL) and long wire drawing. The surface of the product was rich in mucus and had a natto aroma. Its comprehensive sensory score was 20 points. Natto from soybeans under drought stress that were treated with exogenous melatonin showed significantly higher yield, nutrient content, and quality, than those under drought stress without treatment. This study provides theoretical data that can facilitate the development of new methods to improve the quality of soybeans grown under drought conditions.

Enhancing irrigation and nitrogen fertilizer application has become a vital strategy for ensuring food security in the face of population growth and resource scarcity. A 2-year experiment was conducted to determine to investigate the effects of different irrigation lower limits and nitrogen fertilizer application amounts on millet growth, yield, water use efficiency (WUE), N utilization, and inorganic nitrogen accumulation in the soil in 2021 and 2022. The experiment was designed with four irrigation lower limits, corresponding to 50%, 60%, 70%, and 80% of the field capacity (FC), referred to as I₅₀, I₆₀, I₇₀, and I₈₀. Four nitrogen fertilizer application were also included: 0, 50, 100, and 150 kg·hm⁻² (designated as F₀₀, F₅₀, F₁₀₀, and F₁₅₀), resulting in a total of 16 treatments. Binary quadratic regression equations were established to optimize the irrigation and nitrogen application. The results demonstrated that the plant height, stem diameter, leaf area index, aboveground biomass, yield, spike diameter, spike length, spike weight, WUE, and nitrogen agronomic efficiency for millet initially increased before subsequently decreasing as the irrigation lower limit and nitrogen fertilizer application increased. Their maximum values were observed in the I₇₀F₁₀₀. However, the nitrogen partial factor productivity (PFPN) exhibited a gradual decline with increasing nitrogen application, reaching its peak at F₅₀. Additionally, PFPN displayed a pattern of initial increase followed by a decrease with rising irrigation lower limits. The accumulation of NO₃⁻-N and NH₄⁺-N in the 0~60 cm soil layer increased with the increase of nitrogen fertilizer application in both years, while they tended to decrease as the irrigation lower limit increased. An optimal irrigation lower limit of 64% FC to 74% FC and nitrogen fertilizer application of 80 to 100 kg ha⁻¹ was recommended for millet based on the regression equation. The findings of this study offer a theoretical foundation and technical guidance for developing a drip irrigation and fertilizer application for millet cultivation in Northeast China.

The grain-filling stage is the most complex stage of carbon and nitrogen metabolism. Drought stress inevitably inhibits the assimilation, distribution, and transition of carbon and nitrogen at grain-filling stage in soybean, resulting in less soybean yield. The objective of this study was to investigate the effects of exogenous melatonin on the carbon and nitrogen metabolism genes and pathways under drought stress in soybean. Transcriptome analysis showed that, compared with drought stress treatment, 37 and 493 genes were jointly up-regulated and down-regulated in soybean leaves treated with normal water supply and treated with exogenous melatonin under drought stress, respectively. The up-regulated genes included functional genes directly and indirectly involved in carbon and nitrogen metabolism, such as the key genes involved in the cysteine synthesis pathway, photosynthesis, carbohydrate metabolism, and glucose metabolism. Metabolomic analysis revealed that, compared with drought stress treatment, 17 and 43 metabolites were jointly up-regulated and down-regulated in soybean leaves treated with normal water supply and treated with exogenous melatonin under drought stress, respectively. Most (14/17) of up-regulated metabolites were amino acids, lipids, organic acids, and carbohydrates, which further indicated that exogenous melatonin could improve soybean carbon and nitrogen metabolism and drought resistance in soybean. Combined with transcriptome and metabolomic profile, melatonin promoted the relative expression level of β-D-Glucosidase gene due to regulate the pathway of amino acid metabolism and starch and sucrose metabolism, improved the contents of L-Asparagine and D-glucose-6P metabolites, and ultimately improves the ability of drought resistance in soybean.

This field experiment investigated the combined effects of straw and biochar on Black Soil physicochemical properties and soybean growth in cold regions, with four treatments: total straw return (TS), high straw + low biochar (PS), low straw + high biochar (PB), and total biochar (TB). Results showed biochar amendments improved soil physical structure: TB had the lowest bulk density (1.230 g/cm ³ ) and highest porosity (52.72%), with >0.25 mm macroaggregates increased by 28.4% relative to TS. Nutrient supply differed by treatment: TB and PB enhanced nitrogen and phosphorus availability throughout growth, while TS and PS favored potassium and dissolved organic carbon. TB and PB promoted better soybean growth indices (plant height, stem diameter, LAI). PB treatment obtained the highest soybean yield value among all groups, but no statistically significant difference in grain yield was detected among the four straw-biochar combined treatments. Straw–biochar co-application synergistically improved soil structure and regulated nutrients for early promotion and late stability. The PB ratio (13 t/hm ² straw + 6 t/hm ² biochar) performed best, effectively improving Black Soil fertility and increasing soybean yield.