Ammonia Volatilization Reduced by Combined Application of Biogas Slurry and Chemical Fertilizer in Maize–Wheat Rotation System in North China Plain

: Digestate and biogas slurry (BS) are the byproduct of biogas engineering that could be used for elevating plant growth. However, the consequent emissions of ammonia from BS are considered a severe threat to the atmosphere. Herein, we conducted two consecutive ﬁeld experiments with wheat–maize rotations to ﬁnd out the optimum ratio of BS to combine with chemical fertilizer (CF) to reduce ammonia volatilization (AV) while keeping the stable crop yield. In maize season, 226.5 kg N / ha of CF was applied. In wheat season, 226.5 kg N / ha was applied at di ﬀ erent ratios (100%, 80%, and 50%) between BS and CF. Our results found that the maximum yield of 6250 kg / ha was produced by CF, and this yield could be obtained through a combined application of 38% BS mixed with CF. Highest AV produced of 16.08 kg / ha by CF. BS treatments signiﬁcantly reduced the emission from 18% to 32% in comparison to CF. The combined application of BS-CF produced the highest yield due to essential nutrients coming from both BS-CF. Subsequently, it reduced the AV depending on fertilizer type and fertilizer rate. An optimal ratio of 38% BS was recommended to produce the highest yield and lowest ammonia emissions. The application of BS together with di ﬀ erent ratios of CF could be an alternative agricultural strategy to obtain desired crop yield and reduce AV in North China Plain (NCP).


Introduction
China is one of the major agricultural countries and provides nearly one-fifth of calories to humans worldwide [1]. There is high application of chemical nitrogen (N) fertilizer in the agricultural field to achieve high crop yield. The two main agricultural production systems in China are winter wheat and summer maize, that are grown annually. NCP is a hotspot of N input and N loss in China [2]. CF inputs have kept on increasing and have been far higher than crop demand in the NCP [3,4]. Data show that the Nitrogen Use Efficiency (NUE) of NCP is less than 39.6%, which means more than 60-70% of CF applied is often misused [4]. Excessive use of CF has caused severe environmental degradation [5]. Large amounts of N fertilizer are lost to the environment through N runoff, leaching, 12 cells, each cell area was 14 m × 10 m = 140 m 2 . The crop planting method in the test area was summer maize-winter wheat rotation. Summer maize was planted on 22 June 2018 and harvested in week of October 2018. With every treatment, the total nitrogen application rate in the maize season included 226.5 kg N ha −1 which was split into two parts, one at the time of sowing as basal fertilizer at 174 kg N ha −1 where 69 kg N was from urea and 105 kg N from Diammonium Phosphate (DAP). The other application was conducted at the jointing stage named tillering, at 52.5 kg ha −1 . The total phosphorus application rate in the maize season was 322.5 kg P 2 O 5 ha −1 as basal fertilizer. The water input in the maize season depended on natural rainfall. Winter wheat was planted on 9 October 2018 and harvested on 3 June 2019. With every treatment, the total nitrogen application rate in the winter wheat season was 226.5 kg ha −1 , which was split into two parts. The first one at the sowing time was 68 kg ha −1 , or one-third of the total, used as basal fertilizer, and the rest 158 kg ha −1 included both BS and CF. The application amount is shown in Table 2. The BS (digestion substrate: Pig manure) was provided by Zhongyu Ecological Industrial Park near the field. The physical and chemical properties of BS are mentioned in Table 3. Table 1. Physical and chemical properties of the soil.

Layer
Organic Matter (g kg −1 ) Total N (g kg −1 ) Water was irrigated in the greening period (28 November) and springtime (1 March). BS was irrigated at the second time. BS mixed with water to prepare different ratios was irrigated, which is mentioned in Table 2. Sufficient water was irrigated during that time. Slurry and water irrigation rate was 2500 m 3 /ha. The total phosphorus application rate in the wheat season was 322.5 kg P 2 O 5 ha −1 and was used as basal fertilizer. The remaining management measures, such as weeding and pest control, are consistent with local farmland management methods.

Measurement of AV
The dynamic chamber was applied to measure NH 3 fluxes from summer maize and winter wheat field. The device consisted of a dynamic chamber, a vacuum pump, and an acid trap to collect NH 3 gas. A vent pipe was connected to the chemical trap bottle and another part connected to the plastic to form a confined space ( Figure 1). The cylindrical dynamic chamber was made of poly-methyl methacrylate with an inner diameter of 20 cm and a height of 5 cm. The vent pipe was held at a height of about 2.5 m above the ground surface. During the collecting time, a piece of sponge was put on the top of the vent pipe. Before putting sponge into the vent pipe, the sponge must be wetted by diluted sulfuric acid. Then the cover was placed on the base and the base was sealed with pure water to make it air tight. A chemical trap bottle filled with 60 mL of 0.01 M H 2 SO 4 connected to one hole was used to collect NH 3 gas from the top vent pipe and the other end was connected to the flowmeter. The air exchange rate through the chamber was set to 20 L min −1 . The rubber plug of the flowmeter without the valve was inserted into one end of the PVC tube, and the other end of the PVC tube was connected to the vacuum pump. The airtightness of each latex tube connection, the metal head of the flowmeter, and the vacuum pump was thoroughly checked. The three pumps were properly connected to the power supply separately. The main switch was turned on and the flow rate of the flowmeter was adjusted to 20 L min −1 . At the end of the hour, the pump switch was turned off. The acid absorption liquid bottle must be covered with red plug during transport to the field and back to the laboratory from the field. Immediately after the acid was returned to the laboratory, it was poured into a clean (purified water rinsed) plastic tube. The plastic tube must be numbered. The plastic bottles were stored at room temperature in the sampling order. NH 3 volatilization was measured twice per day during the whole week after fertilization and one day every two weeks during other times. The NH 3 volatilization rate was measured twice daily: Morning (9:00-11:00 a.m.) and afternoon (16:00-18:00 p.m.). The NH 4 + -N concentration in the acid trap was titrated with 0.01 mol L −1 standard diluted sulfuric acid solution [18,34]. The NH 3 emission flux from maize-wheat fields was calculated according to the following equation: where V Ni represents the NH 3 volatilization flux (Kg ha −1 d −1 ) at day i, C 1i and C 2i are the concentration of H 2 SO 4 (mol L −1 ) for the first and second time at day i, V 1i and V 2i are the consumption volume of H 2 SO 4 (mL) for the first and second time at day i, t is the duration of each collection (h), and R is the cross-sectional area of the chamber (m). hole was used to collect NH3 gas from the top vent pipe and the other end was connected to the flowmeter. The air exchange rate through the chamber was set to 20 L min −1 . The rubber plug of the flowmeter without the valve was inserted into one end of the PVC tube, and the other end of the PVC tube was connected to the vacuum pump. The airtightness of each latex tube connection, the metal head of the flowmeter, and the vacuum pump was thoroughly checked. The three pumps were properly connected to the power supply separately. The main switch was turned on and the flow rate of the flowmeter was adjusted to 20 L min -1 . At the end of the hour, the pump switch was turned off. The acid absorption liquid bottle must be covered with red plug during transport to the field and back to the laboratory from the field. Immediately after the acid was returned to the laboratory, it was poured into a clean (purified water rinsed) plastic tube. The plastic tube must be numbered. The plastic bottles were stored at room temperature in the sampling order. NH3 volatilization was measured twice per day during the whole week after fertilization and one day every two weeks during other times. The NH3 volatilization rate was measured twice daily: Morning (9:00-11:00 a.m.) and afternoon (16:00-18:00 p.m.). The NH4 + -N concentration in the acid trap was titrated with 0.01 mol L -1 standard diluted sulfuric acid solution [18,34]. The NH3 emission flux from maize-wheat fields was calculated according to the following equation: where VNi represents the NH3 volatilization flux (Kg ha -1 d -1 ) at day i, C1i and C2i are the concentration of H2SO4 (mol L -1 ) for the first and second time at day i, V1i and V2i are the consumption volume of H2SO4 (ml) for the first and second time at day i, t is the duration of each collection (h), and R is the cross-sectional area of the chamber (m).

Soil Samples and Plant Collection
During the collection of ammonia volatilization, the fresh soil samples were collected and stored frozen. The 50 g of surface soil samples were collected three times randomly near the ammonia volatilization sampling point in each plot. The water content of the collected fresh soil samples was measured, as well as the extracted NH4 + −N. Fresh soil samples were sieved with a 2 mm sieve. After that, 10 g of the soil sample was put into a 100 mL plastic bottle. Then 50 mL of 0.01mol L -1 CaCl2 solution was added and shaken for 1 h. The extractable soil NH4 + was measured using an autoanalyzer 3(AA3) [35,36]. The rest of the soil samples were air-dried at room temperature to measure soil pH. The pH value of the soil was determined using distilled water suspensions. The soil to solution ratio was 1 to 2.5 (w:v). After shaking the suspensions for five minutes at 300 rpm on an orbital shaker, the samples were kept for 30 min at room temperature for sedimentation. The pH was then measured in the liquid supernatant [37]. During the harvesting of summer maize-winter wheat season, three separate areas (each 4 m 2 ) were selected in the center of each plot and they were harvested to determine the fresh yield. After that, fresh yield was dried at natural sunlight and measured the actual yield of maize and wheat.

Soil Samples and Plant Collection
During the collection of ammonia volatilization, the fresh soil samples were collected and stored frozen. The 50 g of surface soil samples were collected three times randomly near the ammonia volatilization sampling point in each plot. The water content of the collected fresh soil samples was measured, as well as the extracted NH 4 + -N. Fresh soil samples were sieved with a 2 mm sieve. After that, 10 g of the soil sample was put into a 100 mL plastic bottle. Then 50 mL of 0.01mol L −1 CaCl 2 solution was added and shaken for 1 h. The extractable soil NH 4 + was measured using an auto-analyzer 3(AA3) [35,36]. The rest of the soil samples were air-dried at room temperature to measure soil pH. The pH value of the soil was determined using distilled water suspensions. The soil to solution ratio was 1 to 2.5 (w:v). After shaking the suspensions for five minutes at 300 rpm on an orbital shaker, the samples were kept for 30 min at room temperature for sedimentation. The pH was then measured in the liquid supernatant [37]. During the harvesting of summer maize-winter wheat season, three separate areas (each 4 m 2 ) were selected in the center of each plot and they were harvested to determine the fresh yield. After that, fresh yield was dried at natural sunlight and measured the actual yield of maize and wheat.

Environmental Indicator
A small weather station was set near the field to monitor the environmental indicators, such as air temperature, ground temperature, and wind speed. A rain gauge near our field was used to measure precipitation.

Statistical Analysis
Statistical analyses of the data were performed using the SPSS 22.0 analytical software package. A one-way ANOVA was carried out to test whether the differences in soil properties and NH 3 gas fluxes among all the treatments were significant at the Turkey HSD test 0.05 probability level. Microsoft Excel Professional Plus 2016 was used for graphic design.

Environment Parameters
The precipitation was 373 mm in the maize season, and 75% of rainfall occurred between July and August in the maize season ( Figure 2). The total amount of precipitation was very low (69 mm) in wheat season, of which 75% happened in April. The daily air temperature ranged from 12.9 • C to 31.6 • C and −9.1 • C to 28.7 • C with a mean of 25 • C and 7.4 • C in maize and wheat seasons, respectively ( Figure 1). The trend of daily ground soil temperature was consistent with the air temperature. The daily ground soil temperature ranged from 13.9 • C to 39.4 • C with a mean 13.9 • C in maize season and −7.2 • C to 36 • C with a mean 10 • C in wheat season ( Figure 2).

Environmental Indicator
A small weather station was set near the field to monitor the environmental indicators, such as air temperature, ground temperature, and wind speed. A rain gauge near our field was used to measure precipitation.

Statistical Analysis
Statistical analyses of the data were performed using the SPSS 22.0 analytical software package. A one-way ANOVA was carried out to test whether the differences in soil properties and NH3 gas fluxes among all the treatments were significant at the Turkey HSD test 0.05 probability level. Microsoft Excel Professional Plus 2016 was used for graphic design.

Environment Parameters
The precipitation was 373 mm in the maize season, and 75% of rainfall occurred between July and August in the maize season ( Figure 2). The total amount of precipitation was very low (69 mm) in wheat season, of which 75% happened in April. The daily air temperature ranged from 12.9 °C to 31.6 °C and −9.1 °C to 28.7 °C with a mean of 25 °C and 7.4 °C in maize and wheat seasons, respectively ( Figure 1). The trend of daily ground soil temperature was consistent with the air temperature. The daily ground soil temperature ranged from 13.9 °C to 39.4 °C with a mean 13.9 °C in maize season and −7.2 °C to 36 °C with a mean 10 °C in wheat season ( Figure 2). Temporal trends of soil NH4 + -N concentration showed a similar pattern for all the N treatments. The soil NH4 + -N concentration was significantly affected by N application, water irrigation and rainfall events, BS input, and air temperature in both maize and wheat season ( Figure 3). A high peak  The soil NH 4 + -N concentration was significantly affected by N application, water irrigation and rainfall events, BS input, and air temperature in both maize and wheat season ( Figure 3). A high peak value occurred on the second and fourth day after the basal and tillering fertilization application and it then sharply decreased. The maximum concentration, 1.66 mg/kg, appeared on second day after basal fertilizer in treatment CK and it was four times higher than the average concentration, in maize season. The maximum concentration, 3.87 mg/kg, appeared on first day after the basal fertilizer in treatment CK and it was 1.5 times higher than the average concentration, in wheat season. It is clear that CK produced the highest NH 4 + -N concentration compared to the other treatments. During the wheat season, among all treatments, CK produced the highest NH 4 + -N concentration after basal, water, and biogas slurry irrigation.
Sustainability 2020, 12, x FOR PEER REVIEW 6 of 15 value occurred on the second and fourth day after the basal and tillering fertilization application and it then sharply decreased. The maximum concentration, 1.66 mg/kg, appeared on second day after basal fertilizer in treatment CK and it was four times higher than the average concentration, in maize season. The maximum concentration, 3.87 mg/kg, appeared on first day after the basal fertilizer in treatment CK and it was 1.5 times higher than the average concentration, in wheat season. It is clear that CK produced the highest NH4 + -N concentration compared to the other treatments. During the wheat season, among all treatments, CK produced the highest NH4 + -N concentration after basal, water, and biogas slurry irrigation. The pH value ranges between 7.60 to 8.12 in the maize season and 7.41 to 8.25 in the wheat season ( Figure 4), but there was no significant difference among the treatments. The pH value ranges between 7.60 to 8.12 in the maize season and 7.41 to 8.25 in the wheat season ( Figure 4), but there was no significant difference among the treatments.

Flux of Ammonia Volatilization in Whole Season
During the maize season, all treatment showed a trend of increasing AV after fertilization followed by a trend of decrease ( Figure 5). On the third day, all four treatments were in peak position and then decreased gradually to flat until the tillering fertilizer application. Although the highest accumulation appeared in CK treatment, there was no was significant difference among the treatments. Maximum emissions happened after tillering fertilizer but very little after basal fertilizer. After basal fertilization, the cumulative ammonia volatilization of about 2.40 ± 0.42, 1.70 ± 0.05, 1.09 ± 0.03, and 1.85 ± 0.05, (kg/ha/d) was found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T6 > T1 > T3 ( Table 4). The AV rate peaked on the second day, continued in the next four days after tillering fertilizer, and then, gradually decreased until 24 July. On the second day, the highest value in CK was 14.10 kg/ha/d and the lowest value in T1 was 8.58 kg/ha/day. After tillering fertilization, the cumulative AVs were 26.19 ± 0.10, 14.34 ± 0.66, 22.87 ± 0.98, and 21.13 ± 0.90, (kg/ha/d in CK, T1, T3, and T6 treatments, respectively. There was no significant result found after tillering fertilizer, although significant results were observed in separate events on 20 July, 22, and 24 July. Total AV after tillering fertilizer showed this trend: CK > T3 > T6 > T1 (Table  4).
During the whole growing season of maize, the highest accumulation was shown in CK about 28.9 kg/ha/d and T1, T3, and T6 about 14.34 kg/ha/d, 22.87 kg/ha/d, and 21.13 kg/ha/d, respectively.

Flux of Ammonia Volatilization in Whole Season
During the maize season, all treatment showed a trend of increasing AV after fertilization followed by a trend of decrease ( Figure 5). On the third day, all four treatments were in peak position and then decreased gradually to flat until the tillering fertilizer application. Although the highest accumulation appeared in CK treatment, there was no was significant difference among the treatments. Maximum emissions happened after tillering fertilizer but very little after basal fertilizer. After basal fertilization, the cumulative ammonia volatilization of about 2.40 ± 0.42, 1.70 ± 0.05, 1.09 ± 0.03, and 1.85 ± 0.05, (kg/ha/d) was found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T6 > T1 > T3 ( Table 4). The AV rate peaked on the second day, continued in the next four days after tillering fertilizer, and then, gradually decreased until 24 July. On the second day, the highest value in CK was 14.10 kg/ha/d and the lowest value in T1 was 8.58 kg/ha/day. After tillering fertilization, the cumulative AVs were 26.19 ± 0.10, 14.34 ± 0.66, 22.87 ± 0.98, and 21.13 ± 0.90, (kg/ha/d in CK, T1, T3, and T6 treatments, respectively. There was no significant result found after tillering fertilizer, although significant results were observed in separate events on 20 July, 22, and 24 July. Total AV after tillering fertilizer showed this trend: CK > T3 > T6 > T1 (Table 4).
During the whole growing season of maize, the highest accumulation was shown in CK about 28.9 kg/ha/d and T1, T3, and T6 about 14.34 kg/ha/d, 22.87 kg/ha/d, and 21.13 kg/ha/d, respectively. After applying the base fertilizer, all treatments had a significant effect on AV, which showed a trend of increasing at first and then decreasing in the wheat season ( Figure 6). AV mainly occurred within seven days after fertilization. On the third day, all four treatments were in peak position after basal fertilization and then decreased gradually and became flat except a small peak was observed on 1 November. Another peak point was found on 5 December after water irrigation. The highest peak was found in CK treatment at 0.873 kg/ha/d. At the end of basal fertilization, in January and February, we recorded higher value in CK treatment compared to the other three treatments. After basal fertilization, the cumulative AVs of 7.79 ± 0.08, 5.88 ± 0.05, 5.96 ± 0.05, and 6.50 ± 0.06, (kg/ha/d) were found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T6 > T3 > T1 ( Table 4). The highest accumulation appeared in CK treatment and it is significantly different in comparison to the other treatments. Similar trends were observed after tillering fertilization. The AV rate peaked on the second to third day after fertilization. On the third day, the highest value in CK was 1.641 kg/ha/d and the lowest value in T1 was 0.559 kg/ha/d. After tillering fertilization, the cumulative AVs of 8.29 ± 0.10, 5.00 ± 0.04, 7.14 ± 0.07, and 6.58 ± 0.05, (kg/ha/d) were found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T3 > T6 > T1 ( Table 2). The highest accumulation appeared in CK treatment and it is only significantly different compared to the T1 treatment.
During the whole growing season of wheat, the highest accumulation was shown in CK at 16.08 kg/ha/d. The cumulative AV for wheat season showed a trend: CK > T3 > T6 > T1 (Table 4).  After applying the base fertilizer, all treatments had a significant effect on AV, which showed a trend of increasing at first and then decreasing in the wheat season ( Figure 6). AV mainly occurred within seven days after fertilization. On the third day, all four treatments were in peak position after basal fertilization and then decreased gradually and became flat except a small peak was observed on 1 November. Another peak point was found on 5 December after water irrigation. The highest peak was found in CK treatment at 0.873 kg/ha/d. At the end of basal fertilization, in January and February, we recorded higher value in CK treatment compared to the other three treatments. After basal fertilization, the cumulative AVs of 7.79 ± 0.08, 5.88 ± 0.05, 5.96 ± 0.05, and 6.50 ± 0.06, (kg/ha/d) were found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T6 > T3 > T1 ( Table 4). The highest accumulation appeared in CK treatment and it is significantly different in comparison to the other treatments. Similar trends were observed after tillering fertilization. The AV rate peaked on the second to third day after fertilization. On the third day, the highest value in CK was 1.641 kg/ha/d and the lowest value in T1 was 0.559 kg/ha/d. After tillering fertilization, the cumulative AVs of 8.29 ± Sustainability 2020, 12, 4400 9 of 15 0.10, 5.00 ± 0.04, 7.14 ± 0.07, and 6.58 ± 0.05, (kg/ha/d) were found in CK, T1, T3, and T6 treatments, respectively, and the overall performance was CK > T3 > T6 > T1 ( Table 2). The highest accumulation appeared in CK treatment and it is only significantly different compared to the T1 treatment.
During the whole growing season of wheat, the highest accumulation was shown in CK at 16.08 kg/ha/d. The cumulative AV for wheat season showed a trend: CK > T3 > T6 > T1 (Table 4).

Optimal Ratio of BS Substitution
In maize season, the grain yield from the four different treatments averaged 5.62 t ha -1 in a range of 4.63-6.96 t ha -1 . Treatment T3 produced the highest yield of 6.96 t ha -1 , where T1, T6, and CK produced 4.63, 5.00, and 5.88 t ha -1 , respectively (Table 5). There was no significant difference found among the treatments. In wheat season, the grain yield averaged 5 t ha -1 with a range of 3.33-6.25 t h--1 ( Table 5). Treatment CK produced the highest yield of 6.25 t ha -1 , where T1, T3, and T6 produced 3.33, 4.87, and 5.57 t ha -1 , respectively. T1 is significantly lower than other treatments.

Optimal Ratio of BS Substitution
In maize season, the grain yield from the four different treatments averaged 5.62 t ha −1 in a range of 4.63-6.96 t ha −1 . Treatment T3 produced the highest yield of 6.96 t ha −1 , where T1, T6, and CK produced 4.63, 5.00, and 5.88 t ha −1 , respectively (Table 5). There was no significant difference found among the treatments. In wheat season, the grain yield averaged 5 t ha −1 with a range of 3.33-6.25 t ha −1 (Table 5). Treatment CK produced the highest yield of 6.25 t ha −1 , where T1, T3, and T6 produced 3.33, 4.87, and 5.57 t ha −1 , respectively. T1 is significantly lower than other treatments. Note: The different lowercase letters in the same row indicate significant differences and same lowercase letters in the same row indicate no significant differences among the treatments according to Tukey HSD test (p < 0.05; n = 3), where BS = biogas slurry, and CF = Chemical fertilizer. Biogas slurry only applied in wheat season and only chemical fertilizer applied in maize season. Different numbers indicate different amounts of nitrogen per hectare.
During the maize season, we used only CF, while in the wheat season, we substituted part of the chemical fertilizer by biogas slurry. Different ratios of biogas slurry, such as 100%, 80%, and 50%, combined with chemical fertilizer were used (Figure 7). From the figure, it is clear that CK produced maximum yield of all. The yield decreased following the increasing ratio of BS. To simulate the optimal ratio of BS substitution, we firstly fitted a curve ( Figure 6) to find out that the ratio of 38% BS with 62% CF would give the equivalent yield with CK. And then, we verified under the ratio of 38% BS substitution, NH 3 flux was 13.92 kg/ha, which was 13.5% lower than NH 3 flux of CK. The optimal ratio of BS substitution was 38% based on this simulation (Figure 7).
Note: The different lowercase letters in the same row indicate significant differences and same lowercase letters in the same row indicate no significant differences among the treatments according to Tukey HSD test (p < 0.05; n = 3)., where BS = biogas slurry, and CF = Chemical fertilizer. Biogas slurry only applied in wheat season and only chemical fertilizer applied in maize season. Different numbers indicate different amounts of nitrogen per hectare.
During the maize season, we used only CF, while in the wheat season, we substituted part of the chemical fertilizer by biogas slurry. Different ratios of biogas slurry, such as 100%, 80%, and 50%, combined with chemical fertilizer were used (Figure 7). From the figure, it is clear that CK produced maximum yield of all. The yield decreased following the increasing ratio of BS. To simulate the optimal ratio of BS substitution, we firstly fitted a curve ( Figure 6) to find out that the ratio of 38% BS with 62% CF would give the equivalent yield with CK. And then, we verified under the ratio of 38% BS substitution, NH3 flux was 13.92 kg/ha, which was 13.5% lower than NH3 flux of CK. The optimal ratio of BS substitution was 38% based on this simulation (Figure 7).

Factors That Control the AV
AV was found to be the main way of nitrogen loss in the field, and the amount of loss was closely related to fertilization type and nitrogen application rate [38][39][40][41]. NH3 emission contributed to 19.4 ± 5.2% and 24.7 ± 5.6% of total N application rate in wheat and maize season, respectively, in NCP and typical pH of calcareous soil is 7.5 to 8.5 in NCP, where urea or ammonium bicarbonate are used, accounting for 80% of total fertilizers. Therefore, AV is usually considered the main N loss pathway in the maize season on NCP [3].
The factors that control the NH3 flux were soil NH4 + , water input (irrigation and precipitation), soil pH, air temperature, and wind speed [42][43][44][45]. After being applied to the soil, urea hydrolyzes firstly. The hydrolysis rate of urea is correlated with the availability of water, urease activity, pH, and air temperature [46][47][48]. Water input is important for urea hydrolysis, which is related to soil microbial and enzymatic activity [49,50]. Hydrolysis of urea will be delayed or inhibited if the soils are very dry and water content lower than the permanent wilting point. Normally, the rate of hydrolysis will increase if the water content of the soil increases [47,48].
In this study, we observed an increase of NH3 emission after water irrigation in the wheat season. [51] also proved that AV rate from soil irrigated immediately following fertilizer application is considerably higher than that from soil irrigated 5 h after fertilization. However, too much water input has the potential to carry urea or BS into the soil, minimizing loss of volatilization [7,38,45]. Previous studies found that rainfall increased infiltration of NH4 + into the soil, contributing to a

Factors That Control the AV
AV was found to be the main way of nitrogen loss in the field, and the amount of loss was closely related to fertilization type and nitrogen application rate [38][39][40][41]. NH 3 emission contributed to 19.4 ± 5.2% and 24.7 ± 5.6% of total N application rate in wheat and maize season, respectively, in NCP and typical pH of calcareous soil is 7.5 to 8.5 in NCP, where urea or ammonium bicarbonate are used, accounting for 80% of total fertilizers. Therefore, AV is usually considered the main N loss pathway in the maize season on NCP [3].
The factors that control the NH 3 flux were soil NH 4 + , water input (irrigation and precipitation), soil pH, air temperature, and wind speed [42][43][44][45]. After being applied to the soil, urea hydrolyzes firstly. The hydrolysis rate of urea is correlated with the availability of water, urease activity, pH, and air temperature [46][47][48]. Water input is important for urea hydrolysis, which is related to soil microbial and enzymatic activity [49,50]. Hydrolysis of urea will be delayed or inhibited if the soils are very dry and water content lower than the permanent wilting point. Normally, the rate of hydrolysis will increase if the water content of the soil increases [47,48].
In this study, we observed an increase of NH 3 emission after water irrigation in the wheat season [51] also proved that AV rate from soil irrigated immediately following fertilizer application is considerably higher than that from soil irrigated 5 h after fertilization. However, too much water input has the potential to carry urea or BS into the soil, minimizing loss of volatilization [7,38,45]. Previous studies found that rainfall increased infiltration of NH 4 + into the soil, contributing to a reduction in AV.
In this study, after rainfall on 23 and 24 July, AV decreased due to the loss of NH 4 + concentration. In soils where water content is higher, activity of urease increases with temperature [48]. Optimum pH for soil urease activity ranges between pH 8 and 9. Hydrolysis of urea can be delayed in low pH soils or after adding acidifying anions combined with urea [52].
Urea in solution is hydrolyzed to NH 4 ions and dissolved to NH 3 (aq). An equilibrium exists between these two forms, that is governed by the pH and air temperature. Based on the acid base equilibrium, we found that the NH 3 loss is low if pH < 7, but it increases noticeably when pH is > 7 [47]. Other than the formation of NH 3 (aq), air temperature controls equilibrium distribution between the NH 3 (g) and NH 3 (aq). On-site observations proved that high air temperature, which promoted the formation of NH 3 (aq) and accompanying NH 3 (g), significantly increased NH 3 emission [11,53]. In this study, we saw a raise of NH 3 fluxes after 1 May 2019 because the air temperature was higher than 20 • C. The roughness of surface soil contributed to the increase in turbulence, which resulted in an improvement in the friction speed. Consequently, it increases the exchange between the atmosphere and the surface of soil. Therefore, the top portion of NH 3 gas mixed perfectly and circulated in the atmosphere and increased the NH 3 emission. Accordingly, it increased the NH 3 emission from supplied fertilizers due to the increasing temperature and wind speed. In contrast, high ground radiation, along with increasing wind speed, will decrease ground and plant surrounding temperatures and, subsequently, the possible emission from the solution. Hence, several studies of AV from applied manure to farmland revealed that NH 3 emissions were not always associated with wind speed [42,47].
CF substituted by BS significantly reduced ammonia volatilization loss than urea in this study. Similar results were proved by previous studies in northern China for the wheat yield cropping system [51,54]. Urea is an alkaline fertilizer, it will hydrolyze and produce NH 4 + and OH − rate of hydrolysis depending on soil pH and temperature [47]. BS also contains a higher amount of NH 4 + -N concentrations and has higher slurry surface pH, which significantly affects ammonia emission [44]. However, through the same amount of N input, applications of different CF and BS ratio led to different soil NH 4 + , pH. We can see higher soil NH 4 + and pH for CK treatment after tillering fertilization both in maize and wheat seasons (Figures 5 and 6). That is the reason why NH 3 flux for BS treatment were lower than CK.

Effects of Fertilizer Type on Crop Yield
The farmers use CF to increase crop production. However, if only CFs are continuously applied, it causes soil acidification and compaction, reduced fertility, and severe decline of land productivity, which seriously threatens the production capacity of farmland and its self-repairing capability and endangers the sustainable agricultural growth [41,55]. As we know, BS is considered a good source of macro (N, P, K) and micronutrients (Zn, Mn, B) that are necessary for plant growth [41]. However, if only organic manure is added to the soil, the desired increase in crop yield cannot be achieved according to our results and the previous study [30].
Crop productivity can be significantly increased if the slurry is used in combination with CF [31,32]. Likewise, BS and CF combination increased peanut grain yield and biomass, due to the increase of soil N availability. A similar effect was observed in rice, where a 10% yield increase occurred with a BS-CF combination. Moreover, the combination of BS-CF improved the yields in rapeseed [56]. In line with recent studies, medium concentration of BS to replace CFs herein significantly increased the crop yields, and NUE in NCP [57]. Similarly, a moderate ratio of manure (<40%) substituted by CF could reduce NH 3 emissions and increase crop yields significantly were proved [5]. In this study, 38% of BS for substitution was proposed, which is not similar to previous studies. Previous studies only discussed BS and CF used separately. However, in our study, we used combined BS-CF and developed the ratio of 38-62%, which is a completely new idea.

Effects of Substituting Chemical Fertilizer by Biogas Slurry
CF substituted by BS significantly reduced ammonia emission in this study. Full substitution of CF by manure significantly decreased the AV in upland crops and rice fields [58]. However, full substitution of CF led to a yield penalty in both types of crops. Recent studies have suggested that a moderate ratio (<40%) of CF substitute by manure significantly increased NUE by 14.0% and 25.1% for upland crops and rice, respectively [5]. Studies also found that medium concentration of digestate biogas slurry (MDBS) increased the yield both in wheat and maize season. After that, no significant growth in yield was observed with a further increase in higher concentration of digestate biogas slurry (HDBS). The average nitrate leaching losses were 6% and 5% produced by CF and MDBS, respectively, in wheat growing season. In maize growing season, the average leaching losses were 10% and 8% contributed by CF and MDBS, respectively. In both growing season, CF produced the highest leaching loss compare to MDBS. Whereas higher concentration of digestate biogas slurry (HDBS) increased the leaching loss in comparison to MDBS. Therefore, MDBS increased the NUE with decreased leaching loss [57]. From the above two articles, it was clear that a moderate ratio of BS or partial substitution of CF by manure increased crop yield, NUE, and concurrently decreased AV and as well as N leaching. Similar results were observed in our study. We found that full substitution of CF by BS significantly decreased the AV in comparison to CF. However, full substitution of CF by BS led to a yield penalty. Different ratios of BS, such as 100%, 80%, and 50%, combined with CF, were used in our study. The yield was decreased following the increasing ratio of BS. From the above discussion, we come to this point that a moderate ratio of CF substituted by either BS or manure could increase the crop yield and synchronously reduced AV. Our findings, CF substituted by a ratio of 38% BS, significantly decreased the AV and increased the crop yield.

Conclusions
The main ambition of this study was to systematically evaluate and forecast NH 3 losses during maize and wheat season after biogas slurry was applied in the field, compared with the CF practice at NCP. For both crops, the highest AV was obtained by CK treatment. Different emissions were found in maize season, although the amount of fertilization was the same and only CF was applied at that time. A possible reason for this may be the effect of BS from last year. In wheat season, the highest yield, as well as ammonia emission, was produced by CF and this yield could be produced from the ratio of 38% BS mixed with 62% CF. The ratio of 38% BS substitution, NH 3 flux was 13.92 kg/ha, which was 13.5% lower than NH 3 flux of CK. The novelty of this research was the lowest emissions and highest crop yield were observed in the combined use of both BS-CF and the ratio of 38:62. We conclude that there are great potentials for substituting CF by BS, and the optimal ratio of 38% BS was suggested to produce the highest yield and lowest NH 3 emissions. Further research is required to find the optimum ratio of CF combine with BS could be an alternative agricultural strategy to obtain desired crop yield and simultaneously reduce the AV.