Returning stover to the field can significantly improve the physicochemical properties of the soil, which is a main way to utilize stover resources and is of great significance for conserving black soil in Northeast China. Addressing the problems of high carbon and nitrogen ratio, low water content, and slow decomposition of the stover returned to the field, the returned stover to the field by mixing biogas slurry was put forward in this paper by making full use of biogas slurry rich in water, ammonia nitrogen, and microorganisms, as well as combining the characteristics of late fall and early spring in cold regions where there is a significant difference in temperature between day and night and the stover is frequently in the process of freezing and thawing. The decomposition characteristics of stover mixed with biogas slurry under freezing-thawing conditions, as well as their effects on the nutrients and microbial communities of black soil, were systematically studied under two conditions: fixed freeze-thaw temperature and natural outdoor freeze-thaw temperature. The results showed that the mixed biogas slurry significantly increased the decomposition rate of the returned stover. The decomposition rate of the stover showed a tendency to increase and then decrease with the increase of the biogas slurry. The freezing and thawing temperatures, along with the number of freeze-thaw cycles during the initial freezing-thawing phase, as well as the temperature during the middle and later stages of continuous decomposition, significantly influenced the decomposition rate of the returned stover. Both biogas slurry and maize stover contributed to replenishing organic carbon and essential nutrients such as nitrogen, phosphorus, and potassium in black soil. The enhancement effect was more pronounced when biogas slurry and maize stover were applied together. When the amounts of biogas slurry and corn stover were 96 mL and 7.7 g, respectively, corresponding to a mass ratio of biogas slurry to stover of 12.5 g/g, the decomposition rates of stover after 108 days reached 64.73%. Meanwhile, comparing with the pure corn stover blank control without biogas slurry, the organic carbon, total nitrogen, available phosphorus, and available potassium contents in each soil layer within the 0-20 cm effective plow layer increased by 11.42%-23.75%, 4.14%-21.95%, 13.06%-21.46%, and 21.33%-42.41%, respectively. The microbial communities in the black soil after returning the stover with mixing biogas slurry shifted from “oligotrophic dominance” to “eutrophic dominance”, mainly included the phyla of Proteobacteria, Acidobacteriota, Bacteroidota, and Firmicutes, as well as the genera of Brevundimonas, Pedobacter, and RB41, etc., with decomposition of organic matter, heterotrophic nitrification, and nitrogen fixation. The findings of this study provide a foundation for promoting stover return technology and the effective utilization of biogas slurry in northern cold regions.

A large amount of biogas slurry rich in nutrients such as nitrogen and phosphorus is generated as a by-product with the rapid development of large-scale biogas projects and the biogas power generation industry, which will cause secondary pollution to the environment if not effectively treated. Microalgae production featuring strong environmental adaptability, rapid growth, and high photosynthetic efficiency is one of the synergistic ways to deal with energy crises, organic wastewater purification, and carbon sequestration and emission reduction. However, relatively few studies have been conducted on the interaction between multiple factors in the process of culturing microalgae using biogas slurry. Meanwhile, the culture temperature is generally high, requiring heat preservation and warming measures, and the wastewater addition ratio is generally low. Two types of biogas slurry from the anaerobic digestion of pig dung and cow dung were used in this study and the interactive effects of biogas slurry addition ratio, light intensity, CO₂ concentration, and culture temperature on the growth and nutrient removal of Chlorella sp. (FACHB-1554) and Kirchneriella obesa (FACHB-2104) were systematically studied based on a 4-factor, 5-level quadratic orthogonal rotating center combined experimental design to optimize the process conditions. The results showed that the dry mass of FACHB-1554 and FACHB-2104 increased with the increase of the culture duration, and first increased and then decreased with the increase of the biogas slurry addition ratio. FACHB-1554 was relatively more tolerant to pig manure biogas slurry with higher NH₃-N content than FACHB-2104. The growths of two kinds of microalgae exposed to CO₂ were significantly higher than those exposed to air under the same conditions. With the increase in CO₂ concentration, the dry mass of FACHB-1554 first increased and then decreased, while the dry mass of FACHB-2104 continued to increase. For the growth of two kinds of microalgae, the effect of the biogas slurry addition ratio was stronger than that of CO₂ concentration, while the effects of the light intensity and culture temperature were relatively weak. For NH₃-N and TP removal rates, the effect of the culture temperature was stronger than that of the biogas slurry addition ratio, and the effects of the light intensity and CO₂ concentration were relatively weak. The optimal process conditions of FACHB-1554 culture in pig dung biogas slurry and FACHB-2104culture in cow dung biogas slurry are as follows: biogas slurry addition ratios of 20% and 10%, light intensity of 7 000 lx, CO₂ concentrations of 10% and 12.50%, and culture temperatures of 22 ℃ and 24 ℃. Under the optimal conditions, the maximum dry mass of FACHB-1554 and FACHB-2104 reached 0.832 g/L and 0.648 g/L. Meanwhile, the removal rates of NH₃-N were 86.71% and 77.14% and the removal rates of TP were 89.31% and 78.23%, respectively. The dry mass of FACHB-1554 were 0.685 g/L and 0.796 g/L, respectively, under the pig dung biogas slurry addition ratio of 30% and the culture temperature of 18 ℃, and their nutrient removal rates were 80.97% and 78.13% for NH₃-N and 82.11% and 82.34% for TP, respectively. In comparison, the dry mass of FACHB-2104 were 0.338 g/L and 0.613 g/L, respectively, under the cow dung biogas slurry addition ratio of 25% and the culture temperature of 18 ℃, and the nutrient removal rates were 70.92% and 67.72% for NH₃-N and 71.65% and 71.92% for TP, respectively. Therefore, it was feasible to increase the addition ratio of biogas slurry and decrease the culture temperature, which provided the basic theoretical basis for further research on microalgae culture using biogas slurry and promoted the high-value utilization of biogas slurry and the low-cost development of microalgae industry.

Large-scale biogas has been widely used in recent years. A secondary source of pollution can be found in the disposal of the biogas slurry rich in nitrogen and phosphorus and the exhaust gas with carbon dioxide (CO₂) from biogas power generation. Alternatively, microalgae can be cultivated using the exhaust gas and biogas slurry. Nitrogen and phosphorus can be effectively removed from the biogas slurry. The CO₂ can be concurrently captured in the exhaust gas. The purification can be simultaneously realized on the biogas slurry and exhaust from the power generation. However, the microalgae culture is required for heat preservation and warming measures, thereby increasing the energy consumption and operating costs of microalgae production in northern regions. Therefore, this study aims to explore the growth and pollutant removal of cultivated microalgae using low-temperature exhaust from biogas power generation and biogas slurry. The study area was taken as the Heilongjiang Province in Northern China. Chlorella sp. and Kirchneriella obesa were used as the experimental microalgal species. A graded cooling was used for the microalgae domestication. The procedure was started at 25 ℃ and decreased by 5 degrees each time to the minimum of 10 ℃. The temperature of the microalgae culture was also selected, according to the actual temperatures in winter greenhouses in daylight. A systematic investigation was made on the microalgae growth and nutrient removal from the biogas slurry. Some parameters were optimized, such as the biogas slurry concentration, aeration, CO₂ concentration, and aeration flow rate. The tolerance of microalgae was improved on the biogas slurry and CO₂ exhaust under low-temperature environments. The results showed that it was practically feasible for the microalgae culture in the biogas slurry at low temperatures. Both Chlorella sp. and Kirchneriella obesa grew well under the temperature of 10 ℃. Chlorella sp. was more tolerant to the biogas slurry than Kirchneriella obesa. There was an increase and then a decrease tendency with the increasing biogas concentration. The optimal performance was achieved in the biogas slurry concentration of 10%. The aeration effect of nano aerated disc stone was significantly better than those of the aeration stone, aeration column, and glass straight tube. The glass straight tube was the worst. The removal of ammonia nitrogen (TAN) increased with the increasing aeration flow rate. There was the first increase and then a decrease in the Chlorella sp. biomass and protein concentration, the total nitrogen (TN), TAN, total phosphorus (TP), and chemical oxygen demand (COD), with the increasing CO₂ concentration and aeration flow rate. Chlorella sp. shared a continuous increase in the percentage contents of chlorophyll a and b, with the increasing CO₂ concentration. While the percentage of carotenoids shared a constant decrease. Chlorella sp. grew better under 10% biogas slurry concentration, aeration with a nano gas disc stone, 8% CO₂ concentration, and 1.2 L/min aeration flow rate. The dry weight of Chlorella sp. biomass production and protein concentration reached 2.43 and 1.46 g/L, respectively, at the end of 20 d for optimal cultivation. Meanwhile, the removal rates of TN, TAN, TP, and COD reached 83.73%, 93.00%, 86.02% and 93.83%, respectively. The finding can provide a strong reference for the low-temperature and low-cost cultivation of microalgae in biogas slurry combined with biogas exhaust from power generation. Some implications can also be given to promote the rapid development and application of microalgae and biogas industries in cold regions.