Farmland application of BS can improve the physical and chemical properties of farmland soil [38,46,47] while effectively valorizing the BS [48]. This has a direct positive effect on increasing soil organic matter, improving soil structure and maintaining soil fertility [49,50,51]. The decrease in soil organic matter content is one of the reasons for the deterioration of soil structure and the reduction in soil productivity [52]. The application of BS rich in organic matter to farmland can increase the content of organic matter, especially dissolved organic matter in the soil, thereby improving soil structure [53,54,55]. For instance, the pig manure BS can increase the organic matter content of the topsoil to 3.0 kg/hm² [56]. After 5 years of BS irrigation, the soil organic carbon content increased significantly by 90.3% compared with the soil with chemical fertilizers [57]. However, some studies applying BS from chicken manure, pig manure, and cow manure on the soil in comparison to the control had no significant effect on soil organic matter content [58]. Also, BS rich in nitrogen promotes the consumption of organic carbon by non-autotrophic microorganisms [59], thereby offsetting the accumulation of organic matter present BS in the soil [60,61].

The effect of BS application on soil organic matter content is related to the application mode and the composition of the BS. The increase in soil organic matter content was proportional to the amount of BS applied [62]. The soil organic matter content of all the different fertilizers applied gradually decreased with the growth of corn, while the BS treatment was the opposite. At the mature stage, the organic matter content of all the treatments with BS was found to be significantly increased [63].

The consumption of BS on farmland can enhance soil permeability, water retention, and fertilizer retention capabilities, an advantage that chemical fertilizers do not have [64]. For instance, the 3-year application of BS (165.1 and 182.1 t/hm²) improved the nutrient content of yellow soil under rice–rape rotation and promoted the formation of soil aggregate structure [65]. With the increase in the amount of BS in the mixed solution, the soil stability indicators of dry-fed red soil aggregates (i.e., soil > 0.25 mm water-stable aggregate content, aggregate mean mass diameter, and geometric mean diameter) showed an upward trend, while the fractal dimension showed a downward trend. Similarly, after long-term BS irrigation, soil porosity, soil aggregate structure, and microorganisms in soil increased [66].

Similarly, the application of BS can effectively adjust the proportion of various nutrients in the soil, with the potential to enhance the nutrient absorption capacity of crops, increase crop resistance to diseases [37], increase soil organic matter and improve soil structure [36]. For example, soil ammonium nitrogen and soil nitrate increased by 47.8% and 19.0% when treated with BS compared with the control, respectively [48]. Also, after applying BS formulated fertilizer in the orchard, the soil organic matter content in each soil layer increased from 3.0% to 3.9%, total phosphorus increased from 5.6% to 18.6%, and the available potassium increased from 25.2% to 39.2% [67]. Also, compared with the control without any addition of BS, the BS from chicken manure, pig manure, and cow manure added to soil under equal nitrogen conditions improved inorganic nitrogen, total nitrogen, total phosphorus, available potassium, pH, and conductivity. It is worth noting that the increase in soil nitrate nitrogen is optimal after pig manure BS treatment, followed by chicken manure BS treatment [58,68].

Likewise, continuous application of BS can increase the content of total nitrogen, total potassium, and available nitrogen in farmland soil [69,70]. The content of soil organic matter, cation exchange capacity, electrical conductivity, soil total nitrogen, total phosphorus, total potassium, alkali-hydrolyzed nitrogen, available phosphorus, available potassium, and NH₄⁺-N content of a paddy field with continuous application of BS for four years were significantly higher than those without BS application [71]. Interestingly, a tea garden with extremely low soil fertility level reached high fertility level after continuous application of BS for 2 years. The soil indexes of the soil treated with BS for 4 years were significantly improved compared with the soil without BS treatment [72].

Achieving increase in crop yield is the primary objective soil fertilization such as using BS as fertilizer. Using meta-analysis method, the effect of BS application on crop yield under different conditions was quantitatively analyzed [73]. The results showed that the effect of BS application on wheat, corn, tomato, and rice yield were all improved. Moreover, the impact of BS on farmland could be influenced by soil type or prevailing climate conditions. For example, BS application in northwest and north China increased crop yield significantly compared with other regions such as southwest and east China regions.

The physicochemical properties, application period and concentration of BS plays an important role in formulating a safe and efficient use of BS [50,65,109]. Applying 50% BS instead of chemical fertilizer resulted in the same corn yield as using chemical fertilizer only [110]. Similarly, when applying BS derived from anaerobic fermentation of pig urine and feces, a significant increase in the yield of corn was obtained. When the concentration was controlled within the range of 60–90 t/hm², the maximum corn yield was obtained [81]. Likewise, many studies have shown that the application of BS was beneficial in rice cultivation to increase rice yield [61,74,77,111]. On the other hand, it was reported that the complete replacement of fertilizer with BS significantly reduces rice yield [40,112]. Hence, the application of appropriate BS is critical in improving crop yield than conventional fertilization [78].

Moreover, the application of BS in farmland can improve the nutritional quality of cultivated crops. This will turn enhance the commodity attributes and economic value of crops [82,111]. For instance, the increasing use of BS in the irrigation of rapeseed cultivation improved Fe, Mn, Cu, and Zn mineral content in rapeseed, while the content of oleic acid, Ca, and Mg in rapeseed increased first and then decreased. The optimal quality of rape was achieved when BS was applied in the range of 78.8–101.3 t/hm² [113]. Though the application of BS will increase the protein content of rice and improve the nutritional quality of rice [77,111], some studies have also shown the application of BS has little impact on rice nutritional quality [114,115]. Based on the 8–9-year data analysis of long-term BS application, an improvement in the rice yield, taste value of rice, and gel consistency of rice was obtained when compared with those obtained from chemical fertilizer treated soil [80]. The application of BS promoted the accumulation of vital components such as polysaccharides, carotenoids, flavonoids, and betaine in lycium barbarum fruit, thereby improving the nutritional quality of lycium barbarum and its efficacy [116]. Similarly, the application of nitrogen fertilizer and fermented pig manure in the cultivation of Chinese cabbage showed lower content of amino acids and soluble sugar when compared with the application of pig manure BS alone [117]. Likewise, in a related study, the effects of BS treatment with concentration of 25%, 50%, 75%, and original liquid on the quality of Capsicum spp. were studied. The results showed that with the increase in BS concentration, the chlorophyll content, vitamin C content, soluble sugar content, and organic acid content of Capsicum spp. were improved [118]. It was also found that the application of BS to replace chemical fertilizer could significantly increase the soluble sugar, soluble solids and sugar acid ratio of muskmelon [119].

Biogas slurry undergoes long-term anaerobic fermentation to produce a variety of biologically active substances, such as organic acids, vitamin B12, and gibberellin, which could inhibit the proliferation of soil bacteria, fungi, and viruses [12]. In addition, the high concentration of NH₄⁺-N in the BS has the potential of killing pests and pathogenic bacteria [120,121]. For instance, fresh BS from cattle farm has strong inhibition effect on botrytis cinerea, phytophthora capsici, alternaria solani, colletotrichum gloeosporioides, botrytis capsici, and botrytis cinerea of eggplant. However, when the BS storage time was increased, the inhibition rate of BS against phytophthora capsici and fusarium solani decreased significantly [122]. Similarly, concentrated BS remarkably inhibits the growth of cotton verticillium wilt mycelium (the inhibitory effect of 0.5% concentrated solution BS on cotton verticillium wilt disease was 64.9%). The BS also prevent spore production, conidial germination and microsclerotia germination [123]. Likewise, the application of BS had effective repellent effect on adult brown rice plant hopper [101]. The spraying of biogas slurry with 66.6% concentration had the best repellent effect [106].

Furthermore, the application of BS was effective in the prevention and control of root borne diseases of crops [124,125,126,127]. Although nitrogen input is considered to be the key factor to stimulate soil microbial biomass carbon [46], a large amount of ammonium nitrogen in BS may play a role in inhibiting microbial growth in the short term. The bacteria population in soil decreased after BS was applied in pot culture system [120,121]. For example, irrigation with high concentration of BS in broccoli field reduces soil fungi by 55.0% [128], thus significantly reducing the plant disease index. Similarly, when BS was applied to watermelon, a substantial inhibitory effect on Fusarium wilt was observed, and the disease index was lowered by 36.4% compared to the control treatment [125,126]. Further analysis showed that the inhibition of basidiomycota and mortierella growth was the reason for the decrease in the disease index [39]. Remarkably, root irrigation with BS effectively prevents and cure astragalus root rot. The same inhibitory effect was obtained when this was repeated many times [129]. Likewise the inhibitory effect of 1.3% BS concentrate on cotton verticillium wilt by root irrigation reached 78.0% [123].

Soil acidification is one of the main factors affecting agricultural productivity as well as negatively impacting the environment. Soil acidification will destroy the structure of biological cell membranes, reduce microbial activity of soil microorganisms, and consequently, crop health, growth, and productivity. Prevention and control of soil acidification is of great significance to maintaining sustainable agricultural development [130]. The use of BS in farmland can effectively adjust the proportion of various nutrients in the soil, enhance the ability of soil to buffer acidity and alkalinity changes, reduce the pH value of alkaline soil [91], and increase the pH value of acidic soil, thereby improving soil quality [131,132,133]. Studies have shown that irrigation with BS in coastal poplar forests and coastal saline-alkali rice–wheat rotation fields cause a pH reduction [134,135,136]. For instance, the soil pH of an alkaline paddy field treated with BS for 4 years was significantly lower than that of soil without BS application [71]. Compared with conventional fertilization treatment, BS application can effectively prevent further soil acidification caused by long-term application of chemical fertilizers [80]. Similarly, a 3-year field experiment carried out on the yellow soil under rice–rape rotation showed that the application of BS (165.1 and 182.1 t/hm²) could increase the soil pH [65]. Lower concentration of BS do not prevent soil acidification, while higher BS concentration inhibited the growth of acidobacteria, thereby reducing soil acidification [39]. Likewise, long-term application of BS, resulted in an increasing trend of the soil pH of cultivated Hongmeiren citrus. The soil pH after 4 years of BS application was significantly higher than that of the conventional fertilization [108]. Also, the application of BS in an economic fruit plantation such as a tea garden [137], grapefruit [55], apple [97], and citrus [107] showed similar results of an increasing soil pH value.

The role of soil organisms in underground ecological processes are vital to maintaining a healthy farmland fertility and productivity [138]. An important group of soil organisms are the microorganisms; as decomposers in the food web [139], they occupy more than 80% of food web biomass [68]. These microbes participate in the decomposition and synthesis of soil organic matter, the fixation and release of nutrients, as well as the degradation of pollutants. The impact of farmland application of BS on underground ecological processes will inevitably lead to changes in microbial community structure, metabolic characteristics, and functional diversity, which in turn can be used as important indicators for the evaluation of the health of farmland.

Soil microbial biomass C/N ratio reflects the composition of soil microbial flora. The lower the microbial biomass C/N ratio, the more bacteria in the soil. The application of BS can increase the culturable quantity of soil bacteria [140], fungi [38], and actinomycetes [141] to a certain extent. In a related study, after BS application, the ratio of soil microbial biomass C/N decreased by 25.2–48.0% [108]. The application of BS promoted the proliferation of soil bacteria, and the activity of soil bacteria increased significantly with long term application of BS on farmland [142]. The ratio of bacteria and fungi (B/F) in the soil is usually used to evaluate the soil microbial flora [143]. A high B/F value indicates that the soil is a “bacterial type” with high fertility and less damage to the soil, while a low B/F value indicates that the soil is a “fungal type” with low fertility and high damage to the soil. For instance, the treatment of BS mixed with chemical fertilizer reduced the B/F value. The increase in the concentration of BS application resulted in a B/F value that initially decreased and then increased, while the application of pure BS increased the B/F value considerably [144]. Hence, the B/F value of soil can be kept stable or even increased by using appropriate BS, and consequently, improving the soil fertility.

Nitrogen, phosphorus, potassium, organic matter, growth hormone, humic acid, cellulose, and other substances in BS can further promote the growth and enrichment of soil dominant bacteria [145] as well as promote microbial alpha diversity by improving soil structure and increasing organic matter [49,121,125,126,146,147]. For example, in paddy field with BS applied continuously for 6 years, campylobacter, proteus, and acidobacter were the dominant bacteria, which shows that BS can improve soil microbial structure and, consequently, soil quality and soil fertility [148]. Moreover, the increase in BS concentration resulted in the actinomycetes population; however, excessive BS application inhibits the growth of actinomycetes [39,65]. The Chao1 index and Shannon index of soil bacteria treated with 180 t/hm² BS were higher than those of control treatments; however, the Chao1 index of fungi was lower than that of chemical fertilizer (100 t/hm² and 220 t/hm² treatments). The concentration of BS at 180 t/hm² can improve the bacteria richness and diversity, while reducing the diversity of fungi [39].

In addition, the application of BS in farmland has a certain impact on the activities of soil organisms. When the concentration of BS increased from 0 to 300 m³/hm², the density of soil organisms increased by 94%, the number of the groups increased by about 2, and the dominance index increased by 9.4% (p < 0.05). When 66% of BS was used to replace chemical fertilizer, soil organism density, number of groups, and dominance index were at the highest. The principal component analysis of the application of BS alone or mixed with the chemical fertilizer, showed collembola, prestoma, and ortychia were the most sensitive groups, and they could be used as indicators of the response of small arthropods in the soil to decomposing BS.

The abundance nitrogen, phosphorus, potassium, various trace elements, growth hormones, and other substances in BS can be absorbed and utilized by seeds through seed soaking and infiltration. This can accelerate the metabolism of seeds during the dormant period, thereby promoting seed germination. Reports have shown that the proper concentration of BS and soaking time could improve the germination rate of seeds and promote the growth of seedlings. Soaking seeds with 50% BS for 5 h had the best comprehensive effect on the germination of marigold seeds and seedling growth [152]. For instance, the germination rate and seedling rate of watermelon seeds treated with 40% BS for 24 h was observed to give the best performance [86]. Similarly, soaking seeds with 25% BS for 5 h had a significant effect on seed germination and seedling growth of Astragalus mongholicus [153]. In addition, soaking seeds with BS can increase crop yield. The seed soaking treatment of wheat seeds with BS at optimal exposure time can increase the germination rate by about 13% compared with the water treatment. This resulted in the seeds emerging 3 days earlier, the leaf length increasing by 1.70 cm, the leaf width increasing by 0.10 cm, the dry weight of seedlings increasing by 0.70 g, the maturity period shortening by 2 days, and the yield per hectare increasing by 379.50 kg [154]. Hence, BS could be a potential fertilizer for improve agricultural productivity and sustainable green environment. The effect of soaking seeds with biogas slurry has been reported by many studies, but there is a lack of systematic understanding of the concentration, time, temperature, and operational precautions during the soaking process for different crop seeds. The corresponding mechanism of the soaking effect of biogas slurry still needs further research, and as one of the ways to utilize biogas slurry, its environmental and economic benefits need to be evaluated.

Using biogas slurry as a leaf fertilizer is one of the important ways for farmland to utilize BS. BS is often used as foliar fertilizer for it contains a variety of available nutrients and amino acids which can promote plant growth, increase yield, and improve crop quality [54,155]. BS has been directly used as foliar fertilizer to spray on fruit trees and vegetables, which significantly increased chlorophyll content and yield [87,156]. For instance, proper application of BS sprayed on the leaves improved the growth, yield, and quality of tomato plant [105]. Similarly, investigating the effect of BS application on walnut production, Bi T et al. [157] observed that the BS has an enhancing effect on the walnut quality and the control of pests as well as diseases. The authors pointed out that when BS is used as a leaf fertilizer and pest control, it should be diluted and sprayed on the back of leaves.

Likewise, foliar topdressing of BS can increase the yield of cucumber by 6% and tomato by 8% [158]. Adding humic acid and other nutrients to BS up to 10% of the original volume, and then compounding it to organic fertilizer with large, medium, and trace element, as a foliar fertilizer, significantly improve the yield and quality of Chinese cabbage. The yield of Chinese cabbage increased by 23.3%, and the content of vitamin C and soluble sugar increased by 68.5% and 43.1%, respectively. At the same time, soil fertility, enzyme activity, and soil nutrient increased significantly [159]. In addition, topdressing BS application increased Capsicum spp. yield [160], and increased the contents of vitamin C, soluble sugar, and protein in Capsicum spp., among which the vitamin content increases by 18.3% compared with the market foliar fertilizer [161].

Biogas slurry as basic fertilizer is the most traditional approach of BS application. Compared with chemical fertilizer, under the same treatment condition as BS as base fertilizer (52.5 t/hm²) and root irrigation twice (0.25 kg/root · time), the length, diameter, leaf area, and chlorophyll content of sweet melon vine were increased. Also, in the same study, the weight of melon and melon plant were increased [162]. Similarly, a study by Li and Jiang [163], showed that when the concentration of BS is between 10% and 20%, it is favorable for the growth of the container seedlings of Dendrobium candidum using water moss substrate, while BS between 10% and 30% was found to be favorable for the growth of disk seedlings of Dendrobium candidum using a sawdust pine bark substrate. Under the condition of total application of BS with 600 t/hm² (base fertilizer/top dressing = 1:1), the total panicle number of rice and the yield was increased, while the content of heavy metals in grains did not increase [75]. When the fermented BS of livestock manure is used as the base fertilizer of the tea garden in the autumn and the top dressing in the spring of the following year, the production and quality of spring tea was improved with the content of heavy metals in the soil and tea leaves maintained within a safe range. However, when the BS is applied alone, potassium depletion occurred. Therefore, it is necessary to pay attention to the supplement of potassium in practical application [92]. Practically, the treatment of BS should be carried out according to the ratio of base fertilizer to fruit expanding fertilizer of 1:1. When the application rate of BS was 70–110 t/hm², the plant height and stem diameter of melon were not significantly different from that of compound fertilizer of 600 kg/hm²; when the total application of BS is 180 t/hm² (base fertilizer 90 t/hm², fruit expansion fertilizer 90 t/hm²), it promoted melon plant growth, dry matter, and fruit quality [39].

The application of BS instead of chemical fertilizer for crop top dressing can increase the content of nitrogen and phosphorus in the soil, and could increase with the increase in BS concentration. For instance, the top dressing application of BS containing nitrogen of 396 kg/hm² has a rice yield greater than that treated with chemical fertilizer. Also remarkable is the utilization rate of nitrogen and phosphorus, which was higher in the BS treated soil [112]. Compared with the control (non-BS topdressing treatment), BS topdressing treatment increased the yield of angelica sinensis by 112.89 kg, an increase of 58.0%. Additionally, this significantly reduced the disease index of angelica hemp mouth disease with a preventive effect of 82.3% [164]. Moreover, using the nutrient balance method, it was found that there was no significant difference in the dry matter quality and nutrient content of root, stem, leaf, and fruit of muskmelon between the BS fertilizer integrated topdressing group and the chemical fertilizer group. Hence, the BS fertilizer integration could completely replace the chemical fertilizer [165].

Even though previous studies on the use of biogas slurry as a base fertilizer and topdressing mainly focusing on replacing some chemical fertilizers have been reported, there is a lack of research on the application methods, equipment, relevant engineering measures, parameters, and environmental benefits assessment of BS base and topdressing fertilization application.

Using BS to replace the inorganic nutrient solution of hydroponic cash crops for vegetable cultivation is one approach of resource utilization of BS. Through a biological floating bed process, celery was hydroponically cultured in different concentrations of BS. After 80 days of planting, the celery which was hydroponically cultured in 30–40-times-diluted BS achieved high environmental and economic benefits [166]. In the concentration range of 3–5%, the stepwise addition and one-time addition of chicken manure BS increased the chlorophyll content, biomass, and vitamin C content of water spinach in the solar greenhouse, while nitrite content was reduced [99]. Compared with ordinary soil cultivation treatment, BS soilless cultivation treatment can significantly increase the number of lateral roots and total yield of water spinach by 45.4% and 12.8%, respectively, as well as reduce nitrate nitrogen content in water spinach by 31.5% [167]. In another related study using BS as nutrient substitute for the second growth stage of lettuce, the replacement of nutrient solution with BS has a better effect on lettuce yield, photosynthetic characteristics, and quality. The replacement ratio of 40% BS has the best effect, and the yield is 67.0% higher than that of the control (lettuce hydroponics with nutrient solution prepared according to the original Yamasaki formula) [168]. After the BS deamination, pretreated and diluted by 5–10 times, lettuce was hydroponically cultured for 35 days. Then, compared with hydroponics in nutrient solution, the relative growth of lettuce increased by 60%, the leaf width became wider by 4–5 cm, the number of leaves increased by 2 pieces on the average, the carotenoids content increased by 20.4%, and the content of nitrate nitrogen improved from 2.1 top 4.0% compared to that of chemical nutrient solution group [169].

Biogas slurry hydroponic microalgae is a new type of resource treatment process with potential and stable operation. Meanwhile, it is also an effective way to achieve high-value utilization of BS. Compared with traditional biochemical methods, it can improve the nitrogen removal efficiency of BS by about 20% [109,170], and obtain higher-efficiency functional microalgae products. Different microalgae were cultured with pig manure-based BS, with the nitrogen removal ability and sugar accumulation potential investigated. Chlorella vulgaris ESP-6 showed the best sugar production capacity, with the maximum sugar content and average daily sugar production capacity of 61.5% and 395.73 g/L, respectively. The ammonia nitrogen removal rate and daily average removal concentration were 96.3% and 91.7 mg/L, respectively. Accumulating more carbohydrates in microalgae cells can be regarded as a new strategy for sugar production, which fully proves the value of BS hydroponic microalgae utilization and the regeneration potential of BS waste resources [171].

The use of BS as animal feed and feed additive is another environmentally friendly approach to comprehensively utilize BS for both ecological and economic benefits. Reports in the literature are mostly found in empirical research and attempts research. For instance, the number of heterotrophic bacteria in the sediments of fish ponds with BS or BS combined with feed was higher than that of cattle dung or BS combined with inorganic fertilizers. Similarly, the sediment–water interaction in fish ponds with BS was better than the conventional fertilized fish ponds [172]. Fish farming with BS can increase the yield and economic benefits of feeding and filter-feeding fish; however, attention should be paid to the amount, the frequency, and the timing of BS dosing [44].