Response of Microbial Recovery Rate to Straw Return after Calcium Cyanamide Soil Disinfection

: At present, returning vegetable straw in situ is an effective measure to solve environmental pollution and improve soil properties. However, the direct return of straw to the field can reduce the release rate of soil organic matter and cause serious soilborne diseases. The combined application of calcium cyanamide (CaCN 2 ) and straw can solve this problem. The objective of this study was to determine the effect of CaCN 2 combined with pepper straw return on cucumber yield, soil physicochemical properties, and soil microbial communities during 2020 to 2021 in Shandong Province, China. The treatments were designed as follows: (1) calcium cyanamide soil disinfection, CC; (2) fresh pepper straw return, LJ; (3) fresh pepper straw return combined with calcium cyanamide disinfection, LJ+CC; and (4) natural soil without straw return treatment, CK. Compared with CK, the LJ+CC treatment significantly improved cucumber production by 20%. The cultivable microbial community in the soil was temporarily inhibited during soil fumigation treatment, and the cultivable bacterial and actinomycete communities in the soil return to their initial levels after the film was removed (harvest period). The numbers of culturable bacteria and actinomycetes in the soil in the LJ+CC treatment were 4.68 × 10 7 CFU/g and 5.17 × 10 7 CFU/g, respectively, higher than those in the soil in the CC treatment. The contents of TN and OM in the LJ+CC treatment increased by 13.1% and 13.5%, respectively, compared with that in LJ. Therefore, the LJ+CC treatment enhanced soil fertility and cucumber yields. CaCN 2 can promote straw decomposition and straw can promote soil microbial recovery, and their combined application is considered a feasible and sustainable technique for utilizing vegetable residues in the greenhouse. The combination of returning pepper straw to the field and calcium cyanamide technology achieves a win-win situation of resource circulation and economic circulation by converting agricultural waste into fertilizer before being put into production. Based on this, it is recommended that the straw returning technology receives strong policy support, stimulates researchers to explore the feasibility of different vegetable straw returning to the field, promotes the implementation of this technology achievement, and leverages the environmental benefits of the application of straw returning technology.


Introduction
As the largest traditional agricultural country in the world, China produces approximately 1.04 billion tons of crop straw annually, accounting for over 30% of the world's production [1].In response to the issue of crop straw recycling, the Ministry of Agriculture of China has proposed methods for straw utilization, among which straw return to soil as fertilizer is the most widely used and popular method [2].Pepper, as one of the main vegetable varieties, has a planting area second only to tomatoes and cucumbers in China and is one of the four major protected vegetables in China.Peppers are harvested in autumn and removed from the fields, stacked on both sides of the roads or abandoned in ditches and rivers.This not only hinders the construction of beautiful rural areas, but also causes a large amount of waste of agricultural organic waste resources.Pepper plants are tall, with thick stems and large biomass, and are the main source of stem and leaf waste in the vegetable industry.Therefore, accelerating the research on the reuse of pepper straw in fields has both urgent and important practical significance.
At present, the treatment methods of plant straw include anaerobic digestion and composting, but due to the complexity of the environment and the requirements of equipment, this method has not been widely applied [3][4][5].Composting is considered a practical technology to help return straw to the field, but the release of (carbon dioxide) CO 2 and (ammonia) NH 3 during the composting process results in severe losses of (nitrogen) N and carbon (C) [6,7].Previous studies have proposed that crop straw return can enhance the physicochemical quality of soils (i.e., nitrogen, phosphorus, and potassium) to improve crop yield [8,9].Straw return can return nutrients into the soil and promote the growth of soil microorganisms (including bacteria, actinomycetes, and fungi), which may also be one of the indicators used to measure soil health [10,11].Returning straw to the field can also increase soil fertility, replace the application of some fertilizers, and avoid ecological imbalances and infection sources caused by indiscriminate littering of farmland [12,13].Returning diseased straw to the field can lead to the aggravation of airborne vegetable diseases [14].Xie et al. [14] found that returning diseased cucumber straw (Corynespora cassiicola) directly to the field can infect the second healthy cucumber leaves again.(Calcium cyanamide) CaCN 2 treatment can inhibit the spread of soil diseased straw pathogens to the air, effectively controlling the path of disease transmission.Therefore, the rational and effective utilization of crop straw is of great significance for the sustainable and green development of farmland.
CaCN 2 is not only a good environmentally friendly soil disinfectant in agriculture, but also a good alkaline fertilizer [15].Our previous research has shown that CaCN 2 enhanced soil fertility and tomato crop yields, reduced the presence of soil Fusarium, Verticillium, Phytophthora, etc., and increased the relative abundance of beneficial Trichoderma, Bacillus, Streptomyces, etc. [16].In addition, CaCN 2 was used as a pre-emergence herbicide.The positive effect of a CaCN 2 amendment included both ammonium toxicity on weed seed germination and a decrease in weed rates growth such as relative leaf expansion rate (RLAE) and relative growth rate (RGR) [17].CaCN 2 showed as far as 80% annual weed control [18].There is relatively little research on the impact of CaCN 2 as an agricultural fertilizer on soil quality.At present, there are few studies on the effects of vegetable straw return combined with CaCN 2 treatment on soil quality and microbial quantity [14].Do we assume that CaCN 2 has a decomposition and ripening effect on the straw itself while killing bacteria in the soil or straw?
The objectives of this study were: (1) to analyze the recovery rate of soil microorganisms after returning pepper straw to the field and disinfecting it with CaCN 2 and (2) to analyze the synergistic impact of pepper straw return and CaCN 2 treatment on soil quality (e.g., content of nitrogen, phosphorus, potassium, organic matter).Straw is rich in nutrients and organic matter, and its resource utilization can not only reduce environmental pollution caused by waste straw, but also supply soil nutrients.It has become one of the important ways to develop circular agriculture and reduce fertilizer application and increase efficiency, which can effectively promote the healthy and sustainable development of modern agriculture.

Experimental Location
The experiment was conducted from 2020 to 2021 in Zhaili village (N 37 • 11 ′ and E 118 • 48 ′ ) in Shouguang city, Shandong Province, China (Figure 1).The length and span of 3 of 13 the east-west greenhouse were 50 m and 10 m, respectively.The total amount of pepper straw in the experimental area was 170 kg.No fumigant had been used in the soil.The soil was silt loam with a mean particle size distribution of 35% sand, 61% silt, and 4% clay.Before the experimental treatment, we measured the physicochemical characteristics of the soil (0-20 cm soil layer) and the initial residual amount of vegetable straw (Table 1).

Experimental Location
The experiment was conducted from 2020 to 2021 in Zhaili village (N 37°11′ and 118°48′) in Shouguang city, Shandong Province, China (Figure 1).The length and span the east-west greenhouse were 50 m and 10 m, respectively.The total amount of pepp straw in the experimental area was 170 kg.No fumigant had been used in the soil.Th soil was silt loam with a mean particle size distribution of 35% sand, 61% silt, and 4% cla Before the experimental treatment, we measured the physicochemical characteristics the soil (0-20 cm soil layer) and the initial residual amount of vegetable straw (Table 1).

Experimental Design
At the experimental site, twelve trial plots (1 m × 3 m) were arranged in a complete randomized block design including four treatments with three replicates.The treatmen were designed as follows: (1) soil fumigation with CaCN2 (CC); (2) fresh pepper stra return (LJ); (3) fresh pepper straw return combined with CaCN2 fumigation (LJ+CC); an (4) natural soil without straw return (control, CK).In brief, the pepper straw return de sity was 3 plants/m 2 , approximately 7.5 kg/m 2 [12].We used a 75% alcohol disinfecte chopper to crush the residue of pepper vegetables into fragments smaller than 1-3 cm and manually dug it into the soil.Before soil disinfection, the soil humidity is require to be 60~80% (w/v), and the application rate of CaCN2 was 120 g/m 2 .Then, we tho oughly plowed the soil to a depth of 0-30 cm and immediately covered it with 0.2 m polyethylene plastic film to reduce the loss of disinfectant gas.After 20 days of so

Experimental Design
At the experimental site, twelve trial plots (1 m × 3 m) were arranged in a completely randomized block design including four treatments with three replicates.The treatments were designed as follows: (1) soil fumigation with CaCN 2 (CC); (2) fresh pepper straw return (LJ); (3) fresh pepper straw return combined with CaCN 2 fumigation (LJ+CC); and (4) natural soil without straw return (control, CK).In brief, the pepper straw return density was 3 plants/m 2 , approximately 7.5 kg/m 2 [12].We used a 75% alcohol disinfected chopper to crush the residue of pepper vegetables into fragments smaller than 1-3 cm, and manually dug it into the soil.Before soil disinfection, the soil humidity is required to be 60~80% (w/v), and the application rate of CaCN 2 was 120 g/m 2 .Then, we thoroughly plowed the soil to a depth of 0-30 cm and immediately covered it with 0.2 mm polyethylene plastic film to reduce the loss of disinfectant gas.After 20 days of soil disinfection, we removed the cling film and naturally ventilated and dried for 5 days before transplanting cucumber seedlings.Soil temperature and moisture were measured with a smart agricultural sensor (BRW100-2006A, Firstrate, Beijing, China) during soil disinfection.

Cucumber Planting
Cucumber seeds ("Cucumber Zhongnong No.6") were initially sown in greenhouse seedling trays and regularly watered daily to prevent dead seedlings.After 7 days of cultivation and growth of cucumber seedlings, they were manually transplanted into a greenhouse (two rows, 35 cm between rows, 30 cm between plants).Cucumber plants were staked and tied during the growing period and weeded and irrigated as needed.The ventilation openings of the greenhouse were equipped with insect prevention nets to prevent pest damage, and drip irrigation belts were laid underground in accordance with the normal operation and management methods of farmland.According to normal management in the field, drip irrigation is usually performed at 7:00 every day for 30 min.The watering time is adjusted appropriately based on soil moisture to ensure the uniformity of greenhouse drip irrigation.

Determination of Cultivable Microorganisms in Soil
We determined the density of cultivable microorganisms in soil through plate culture analysis [19].Fungi, bacteria, and actinomycetes were cultured in soil using Bengal Red medium, NA medium, and Gauss 1 medium, respectively, and then the colony forming unit (CFU) of microorganisms were cultivated.The soil was collected with a probe from each treatment and mixed as a sample, and each treatment contained five replicates.The detailed measurement methods and calculation formulas are provided in the references [19].

Soil Physicochemical Properties
Soil pH and electrical conductivity (EC) were measured in sample suspensions with a 1:2.5 soil to H 2 O ratio; the nitrate nitrogen (NO 3 -N), ammonium nitrogen (NH 4 -N), and phosphorus (P) concentrations were assessed using a Futura TM continuous flow analytical system (Alliance Instruments, France); organic matter (OM) and total nitrogen (TN) were determined on an element analyzer (Vario ISOTOPE Cube, Frankfurt, Germany), available potassium (K) was determined in 1N ammonium acetate (CH 3 COONH 4 ) (pH = 7.0) using a flame photometer.The soil was air dried at room temperature and analyzed after passing a 2.0 mm sieve.The soil chemical properties were determined following detailed steps according to the method described by Shen et al. [20,21].During soil fumigation, a temperature and moisture recorder was used to measure the temperature and moisture of the soil environment in real time.

Cucumber Yield
At the seedling stage of cucumber, steel tape measures were used to determine plant height and leaf area.Single leaf area was calculated by using the formula: single leaf area = leaf length × leaf width × 0.7007 (coefficient).The yield was measured during the cucumber harvest, and the average fruit weight of cucumber was determined and calculated.Average fruit weight was calculated by dividing total fruit weight to the total number of fruits harvested from the marked area.

Statistical Analysis
One-way analysis of variance (ANOVA) was used to determine the significance among treatments and Duncan's multiple range test was used to find differences among treatments (p < 0.05) using SPSS ver.19.0 statistical software (SPSS, Chicago, IL, USA).

Cucumber Yield
Plant height was higher in Lj+CC than all other treatments; CC and LJ showed intermediate values lower than LJ+CC but higher than CK.In addition, the cucumber yields were increased significantly (p < 0.05) in the LJ+CC treatment compared with the CK treatment.Therefore, the CC and LJ+CC treatments, especially the LJ+CC treatment, had the highest cucumber yield, which increased by 20% compared with CK (Table 2).

Culturable Microorganism Analysis
By measuring and analyzing the dynamic changes in the number of cultivable microorganisms in soil, the results showed that the total number of cultivable fungi in soil treated with CK showed an upward trend from 0 DAT to 80 DAT, while the number of bacteria and actinomycetes was the opposite (Figure 2).The bacterial, fungal, and actinomycete populations decreased sharply at 20 DAT in the CC and LJ+CC treatments.In the LJ treatment, there was no significant difference between bacteria and actinomycetes, and the number of fungi decreased slightly (Figure 2).The numbers of culturable bacteria and actinomycetes in the soil in LJ+CC were 4.68 × 10 7 CFU/g and 5.17 × 10 7 CFU/g, respectively.In the CC and LJ+CC treatments, particularly in the latter, the number of cultivable bacteria and actinomycetes in the soil significantly increased, while the number of fungal populations slowly increased (Figure 2), indicating that pepper straw return to the field can increase the growth rate of soil microorganisms after CaCN 2 disinfection.

Soil Physicochemical Analysis
Compared with the CK (7.23), the soil pH significantly increased to 7.74 and 7.77 at 80 DAT in the CC and LJ+CC treatments, respectively, while the pH decreased in the LJ treatment (Figure 3a).Conversely, the EC of the soil decreased in the CC and LJ+CC treatments and increased in the LJ treatment compared with the CK (Figure 3a).These results indicated that the CC and LJ+CC treatments, especially the LJ+CC treatment, effectively alleviated the soil continuous cropping obstacles (acidification and salinization) during the cucumber production process in facilities.The content of OM increased in the LJ+CC treatment (13.24 g/kg) and was higher than that in all treatments.

Soil Physicochemical Analysis
Compared with the CK (7.23), the soil pH significantly increased to 7.74 and 7.77 at 80 DAT in the CC and LJ+CC treatments, respectively, while the pH decreased in the LJ treatment (Figure 3a).Conversely, the EC of the soil decreased in the CC and LJ+CC treatments and increased in the LJ treatment compared with the CK (Figure 3a).These results indicated that the CC and LJ+CC treatments, especially the LJ+CC treatment, effectively alleviated the soil continuous cropping obstacles (acidification and salinization) during the cucumber production process in facilities.The content of OM increased in the LJ+CC treatment (13.24 g/kg) and was higher than that in all treatments.
The TN content in the LJ+CC treatment was 1.01 g/kg, which was higher than that in all treatments.The NH4-N and NO3-N contents in the CK treatment were 19.48 mg/kg and 35.92 mg/kg, respectively.In the LJ and LJ+CC treatments, the NH4-N of the soil was significantly increased compared with that in the CK.In the CC, LJ, and LJ+CC treatments, the NO3-N of the soil was significantly increased compared with that in the CK (Table 3).In terms of TP, AP, TK, and AK contents, the contents in the LJ treatment were significantly higher than those in the CK, while the contents in the LJ+CC treatment were relatively lower.Soil temperature and moisture were determined during soil fumigation.The The TN content in the LJ+CC treatment was 1.01 g/kg, which was higher than that in all treatments.The NH 4 -N and NO 3-N contents in the CK treatment were 19.48 mg/kg and 35.92 mg/kg, respectively.In the LJ and LJ+CC treatments, the NH 4 -N of the soil was significantly increased compared with that in the CK.In the CC, LJ, and LJ+CC treatments, the NO 3-N of the soil was significantly increased compared with that in the CK (Table 3).In terms of TP, AP, TK, and AK contents, the contents in the LJ treatment were significantly higher than those in the CK, while the contents in the LJ+CC treatment were relatively lower.Soil temperature and moisture were determined during soil fumigation.The soil temperature of the CC and LJ+CC treatments was higher than that of the other treatments and began to increase at 10:00 and slowly decreased at 17:00 (Figure 2b).The highest soil temperature in CC and LJ+CC during fumigation was ~40 • C, which was ~10 • C higher than the temperature in CK (Figure 3b).The highest soil moisture in CC and LJ+CC during fumigation was ~40%, which was ~10% higher than the temperature in CK (Figure 3c), indicating that combining pepper straw return with CaCN 2 for fumigation quickly increased the soil temperature and moisture.
soil temperature of the CC and LJ+CC treatments was higher than that of the other treat ments and began to increase at 10:00 and slowly decreased at 17:00 (Figure 2b).The high est soil temperature in CC and LJ+CC during fumigation was ~40 °C, which was ~10 °C higher than the temperature in CK (Figure 3b).The highest soil moisture in CC and LJ+CC during fumigation was ~40%, which was ~10% higher than the temperature in CK (Figur 3c), indicating that combining pepper straw return with CaCN2 for fumigation quickl increased the soil temperature and moisture.

Discussion
Reasonable utilization of plant straw can effectively reduce the use of chemical ferti lizers, save production costs, and avoid the pollution of ecological environment caused by burning straw [22].In addition, returning straw to the field can change the soil deteri oration structure, improve soil physical properties, increase the diversity and activity o potentially beneficial microbial communities in the soil, and alleviate soil continuou cropping obstacles [23].It is worth noting that as the area of vegetable facilities graduall increases, a large amount of residual straw is randomly placed or buried, leading to th waste of green resources.At present, straw returning technology is mainly focused on field crops, and it will become a popular research object in the field of vegetable straw recycling treatment.

Discussion
Reasonable utilization of plant straw can effectively reduce the use of chemical fertilizers, save production costs, and avoid the pollution of ecological environment caused by burning straw [22].In addition, returning straw to the field can change the soil deterioration structure, improve soil physical properties, increase the diversity and activity of potentially beneficial microbial communities in the soil, and alleviate soil continuous cropping obstacles [23].It is worth noting that as the area of vegetable facilities gradually increases, a large amount of residual straw is randomly placed or buried, leading to the waste of green resources.At present, straw returning technology is mainly focused on field crops, and it will become a popular research object in the field of vegetable straw recycling treatment.

Vegetable Straw Added to Soil Promotes the Microbial Recovery Rate after Soil Disinfection
Among all indicators for evaluating soil quality, soil microorganisms, as the most active and vital component of the environmental ecosystem, exhibit significant differences in community richness and composition structure after straw return treatment [24].Dilution plating method is a conventional method for estimating the number of cultivable microorganisms in soil by utilizing the differences in the growth habits of different microbial communities in the soil.Although it can only reflect a small part of the microorganisms in the sample, on the whole, it can reflect the differences in the number of soil microorganisms between different treatments [25].Soil bacteria are important indicators for evaluating soil health, soil fertility, and agricultural ecological functions.Soil bacteria are important drivers of soil nutrient cycling and play a decisive role in the decomposition of crop straw [26].Most studies have shown that straw returning treatment can significantly improve the abundance structure of soil bacterial genera, which may be due to the long-term selection of key species leading to competition in a favorable environment [27,28].The accelerated recovery rate of the bacterial population after the application of CaCN 2 in this study indicates that the effect of CaCN 2 on soil bacteria is less than that of fungi.During the process of their nutrient cells differentiating into hyphae and further producing spores, cell metabolism produces different secondary metabolites, which can synthesize specific antibacterial substances to control cell differentiation and cell cycle, thereby inhibiting the growth of pathogenic bacteria [29].At present, excessive application of fertilizers in China can lead to deterioration of soil properties, imbalance of soil bacterial community structure and diversity, and affect the normal growth of crops [30].Chen et al. [31] found that during the fallow period of lettuce, after one month of disinfection treatment with calcium cyanide combined with corn straw returning to the field, the number of cultivable microorganisms in the soil during the harvest period recovered and was higher than the initial level.This effect is significantly higher than that of a single CaCN 2 treatment.The recovery rate of culturable microorganisms in soil is slow after CaCN 2 disinfection.However, the number of soil bacteria and actinomycetes increased significantly after calcium cyanamide fumigation combined with pepper straw return to the field.This indicates that vegetable straw return combined with CaCN 2 can promote soil microbial growth due to the addition of energy and nutrients [10].

CaCN 2 Promotes the Decomposition Efficiency of Nutrients in Vegetable Straw
The phenomenon of soil acidification and salinization is relatively common in China, which is closely related to soil pH and conductivity.It is the most prominent soil obstacle problem in facility cultivation systems [32].The data from this study indicate that vegetable straw return decreased soil pH, which was partly due to the presence of weakly acidic substances in the straw itself, which produce a large amount of organic and inorganic acids through the decomposition of soil organic matter, which is consistent with the findings of Gao et al. [33].This study found that the vegetable straw combination with CaCN 2 increased the soil pH and decreased the EC value, which was due to the CaCN 2 combining with soil moisture to form calcium hydroxide.The results of this study indicate that soil fumigation treatment can change soil EC content and alleviate soil salinization, which is consistent with previous research results [34].
Returning straw to soil is an important way to improve soil physical and chemical properties, increase soil organic matter, and enhance soil nitrogen supply capacity [35].In this study, vegetable straw return significantly increased soil OM, P, and K (p < 0.05), which was consistent with reported results [36].Crop straw residues in the soil can effectively promote the conversion rate of soluble substances in the soil and promote the availability of effective nutrients in the soil [37].Interestingly, compared with the vegetable straw return treatment, the vegetable straw return combined with CaCN 2 treatment increased the N and OM contents, indicating that CaCN 2 can promote the decomposition and release of organic matter in the straw.This experiment showed that the vegetable straw combined with CaCN 2 treatment reduced the AP and AK contents, which may be due to the absorption of nutrients by plants.This result was consistent with those reported by Shan et al. [38], who found that more organic acids were released during straw decomposition and more organic P was activated, leading to an increase in soil P availability.Crop straw is rich in organic matter, nitrogen, phosphorus, potassium, and trace elements, serving as a carrier of matter, energy, and nutrients [39].Returning crop straw to the field is beneficial for soil carbon and nitrogen accumulation, improving soil temperature and humidity, and increasing soil nutrient content [40].In addition, vegetable straw return increased the soil AK content, which could also be attributed to the decreased soil pH.This might increase the content of available potassium formed by the acid hydrolysis reaction of mineral potassium in the soil [41], which is consistent with the findings of Wei et al. [12].
Returned straw increased cucumber seedling growth [42].We found that the cucumber yield did not increase with pepper straw return, which may be due to the lack of decomposition of straw nutrients that were rapidly released into the soil in a short time to promote plant growth.The combined application of biofertilizer and decomposing straw can increase crop yield [43,44].We found that pepper straw combined with CaCN 2 could increase cucumber yield and biomass.Some reports show that CaCN 2 application can increase yield in eggplant [45] and cucumber [16].Therefore, CaCN 2 fumigation combined with returning vegetable straw to the field can increase the yield of cucumber, which may be attributed to the fact that CaCN 2 promotes the decomposition of straw and the release of nutrients [46].

Analysis of the Interaction between Soil Microorganisms and Physicochemical Properties after Straw Return
Soil microbial community is one of the key indicators for measuring soil health and productivity.Returning straw to the field has changed the efficiency of material information transmission between soil microbial species, the complexity of community structure, and the stability of community structure.Returning straw to the field will release various nutrients (nitrogen, carbon, and organic matter) in the soil, activating the growth rate of soil microbial communities.Soil organic matter is an important driving factor for bacterial communities, making the structure of soil microbial communities more diverse and richer [47].This study found that the pepper straw combined with CaCN 2 treatment increased soil pH, OM, temperature, moisture, and bacterial abundance, and decreased soil fungal abundance.It is speculated that the straw is degraded by soil microorganisms after returning to the field, converting the decomposed soil lignocellulose and other components into organic matter through fermentation, decomposition, and other methods, thereby increasing the soil organic matter content.The decomposition of organic matter by soil microorganisms requires sufficient oxygen, which may lead to the risk of soil hypoxia [48].Therefore, in this study, after pepper straw and CaCN 2 treatment, uncovering the membrane for ventilation not only releases toxic gases but also replenishes soil oxygen, which is harmless to plants.
The increase in soil pH can inhibit the proliferation of soil pathogenic bacteria, effectively promoting the activity of potential beneficial microbial populations and providing antibacterial areas for the growth of healthy crops [49].The data indicate that increasing soil pH content can effectively promote the activity and growth of potentially beneficial microbial communities in the soil, thereby creating a favorable soil microecological environment and achieving the goal of inhibiting the activity of pathogenic microorganisms [50].When this shift occurs, soilborne diseases become more prevalent [51].Plant straw applied to the soil tillage layer increased the thermal radiation of the soil, increasing soil temperature and moisture [52,53], which is consistent with the findings of this study.
The soil bacterial population can convert into carbohydrate enzymes and combine with soil cellulose, hemicellulose, and chitin to obtain the carbon source required by plants, providing them with healthy growth [54,55].Bacteria have been confirmed to dominate and prefer C biomass sources [56] and degrade cellulose, lignin, and lignocellulose in vegetable straw by secreting hydrolases such as β-glucosidase and xylanases [57,58].Soil pH and temperature are conducive to carbon utilization efficiency and improve the decomposition of cellulose and chitin [59,60].Carbon sources can provide energy for microbial decomposition [61].The amount of carbohydrates in soil depends on the amount of animal and plant residues, and the higher the content of organic matter is, the faster the decomposition rate of straw [62].Therefore, CaCN 2 can promote the rapid decomposition of vegetable straw in soil.The application of the two is complementary and is a creative approach to the sustainable utilization and development of crop straw.

Conclusions
Straw is of great significance to circular agriculture.However, it is difficult to completely release straw nutrients into the soil for plants to uptake when the plant straw is directly returned to the field.CaCN 2 can promote the release rate of pepper straw nutrients in the soil.At the same time, returning pepper straw to the field can accelerate the recovery rate of soil microorganisms after CaCN 2 disinfection.Pepper straw combined with CaCN 2 increased soil OM, improved soil microbial abundance, temperature, moisture, and physicochemical balance, alleviated soil acidification and salinization, and promoted the growth and yield of cucumber plants.This combined treatment provides an effective way to improve the utilization rate of large amounts of straw in vegetable fields as well as the quality of vegetable soil.The combination of returning pepper straw to the field and calcium cyanamide technology achieves a win-win situation of resource circulation and economic circulation by converting agricultural waste into energy before being put into production.Based on this, it is recommended that the straw returning technology receives strong policy support, stimulates researchers to explore the feasibility of different vegetable straw returning to the field, promotes the implementation of this technology achievement, and leverages the environmental benefits of the application of straw returning technology.

Figure 2 .
Figure 2. Change in bacterial (a), fungal (b), and actinomycetes (c) culturable populations from 0 DAT to 80 DAT in the CC, LJ, LJ+CC, and CK treatments.DAT: days after treatment.Different letters indicate significant differences at the 0.05% level by Duncan's multiple range test.*: p < 0.05.

Figure 2 .
Figure 2. Change in bacterial (a), fungal (b), and actinomycetes (c) culturable populations from 0 DAT to 80 DAT in the CC, LJ, LJ+CC, and CK treatments.DAT: days after treatment.Different letters indicate significant differences at the 0.05% level by Duncan's multiple range test.*: p < 0.05.

Table 1 .
Soil and vegetable straw physicochemical characteristics.

Table 2 .
Effects of soil treatments on cucumber growth.
CC: calcium cyanamide treatment; LJ: fresh pepper straw return treatment; LJ+CC: fresh pepper straw return combined with calcium cyanamide treatment; CK: control.Different letters in columns indicate significant differences at the 0.05% level by Duncan's multiple range test.Data are given as the mean ± standard deviation.Stimulation effect (%) = (single fruit weight of different treatments-single fruit weight of CK)/CK × 100.*: p < 0.05.Same as below.

Table 3 .
Soil physical and chemical characteristics of the soil samples at 80 DAT in different soil treatments *.
Treatment TN (g/kg) NH 4-N (mg/kg) NO 3-N (mg/kg) TP (mg/g) AP (mg/kg) TK (mg/g) AK (mg/kg) * Data are given as the mean ± standard deviation.DAT: days after treatment.Different letters indicate significant differences at the 0.05% level by Duncan's multiple range test.