The Influence of N 2 O Emission to Water and Nitrogen Coupling Mechanism in Black Soil under Drip Irrigation Mode

This experiment is aimed at revealing the main characteristics of N 2 O emission from soybean in black soil and clarifying the relationship between N 2 O emission and environmental factors; in this experiment, static chamber-gas chromatography ﬁ eld observation method was used; meanwhile, three di ﬀ erent drip irrigation amounts (300mm, 350mm, and 400 mm) and ﬁ ve di ﬀ erent nitrogen application amounts (0, 63, 72, 81, and 90 kg/hm 2 ) were set up to study the e ﬀ ects of di ﬀ erent coupling modes on soybean N 2 O emission and the relationship between N 2 O emission and environmental factors. The results showed that the peak value of N 2 O emission ﬂ ux of soybean mainly occurred in July and August, and the amount of N 2 O emission ﬂ ux in each growth period was ﬂ owering-podding period > branching period > seedling period > grain ﬁ lling period. N 2 O emission increased with the increase of drip irrigation amount and nitrogen application amount. The e ﬀ ects of drip irrigation and nitrogen application on N 2 O emission were signi ﬁ cantly di ﬀ erent. Under the three drip irrigation modes, the N 2 O emission ﬂ uxes of most treatments were signi ﬁ cantly correlated with the temperature in that day, and the soil temperature in di ﬀ erent soil depths was signi ﬁ cantly correlated with the N 2 O emission ﬂ uxes. The correlation relationship between the temperature of each soil layer under drip irrigation 300mm and N 2 O emission ﬂ uxes was less than that under drip irrigation 350 mm and drip irrigation 400 mm. There was no signi ﬁ cant correlation between N 2 O emission ﬂ ux and NH 4+ -N and NO 3 N, and there was no signi ﬁ cant correlation between N 2 O cumulative emission and dry matter quality in the aboveground part of soybean. It shows that the emission of N 2 O itself is a ﬀ ected by multiple environmental factors and shows certain randomness. The research results can provide important theoretical basis and technical support for realizing nitrogen reduction and e ﬃ cient utilization of soybean water and fertilizer in the black soil region of northeastern part of China.


Introduction
Irrigation and nitrogen application are two important factors affecting soil N 2 O emission. Due to frequent spring droughts and excessive application of nitrogen fertilizer in the western part of Northeast China, N 2 O emissions showed a significant increase trend [1,2]. In recent years, the soybean planting area in the black soil area of northeast China has grown rapidly, and the soybean planting area in Heilongjiang Province alone has reached more than 60 million mu [3]. As an efficient water-saving irrigation technology, drip irrigation has the advantages of small irrigation quota, high utilization efficiency of irrigation water, and easy automatic control. It has been widely used in many countries in the world (especially in arid and semiarid areas). As for the environmental effects of drip irrigation, most of the previous studies focused on soil salt migration, water distribution, and coupling effect of water and fertilizer [4,5], and less attention was paid to its impact on greenhouse gas emission. With the increasing application of drip irrigation technology, it is necessary to carry out in-depth research on the impact of drip irrigation technology on N 2 O emission. At the same time, due to traditional planting habits, most farmers are also used to applying chemical nitrogen fertilizer in large quantities. Excessive application of nitrogen fertilizer not only increases costs but also causes environmental pollution and sharply increases N 2 O emissions [6].
Drip irrigation has a significant impact on N 2 O emission in farmland [7][8][9][10][11][12], and the reason is that drip irrigation changes the transformation process of soil nitrogen. However, the research results on N 2 O emission have increased and decreased, and the research conclusions are not consistent. There are also different views on the mechanism of water influencing N 2 O emission. Whether these differences are related to specific regional climate conditions and farmland ecological environment needs further experimental verification.
Increased application of nitrogen fertilizer can promote soil N 2 O emissions [13][14][15][16][17][18], but the quantitative relationship between emissions and nitrogen application rate varies greatly among regions. To further explore the response mechanism of different climate, soil, and crops to soybean N 2 O emission and nitrogen application rate, and to coordinate the contradiction between soybean nitrogen application and N 2 O emission, has become a research hotspot at home and abroad.
Aiming at the problems of waste of water and fertilizer resources, low nitrogen fertilizer utilization rate, and increased greenhouse gas N 2 O emission in the production of black soil soybean in China, this project studies the N 2 O emission effect and nitrogen fertilizer utilization efficiency improvement mechanism under the condition of reducing nitrogen fertilizer application under the drip irrigation mode. The research results can provide application basic theory and technical support for realizing the efficient utilization of soybean water and fertilizer resources and reducing greenhouse gas N 2 O emission in the black soil area. It is of great significance to ensure national food security, regional water security, and ecological environment.
Therefore, based on northeastern black soil soybeans, this study studied different drip irrigation amount and reduced nitrogen fertilizer application to control N 2 O emission and improve nitrogen use efficiency, which is of great significance for scientific development of water and nitrogen resource management strategies compatible with water saving, yield increasing (stable yield), and emission reduction of black soil soybeans. It can also play an important role in providing technical support to the realization of the strategic goals of "one control, two reduction, and three solving" which means as follows: firstly, the total amount of water used in agriculture industry should be controlled; secondly, the amount of fertilizer and pesticides should be reduced; and thirdly, the pollution caused by livestock, poultry, plastic film mulching, and waste straw burning should be solved by recycling.

Test
Site and Basic Information. The experiment was conducted in Heshan Farm Science and Technology Park, Heihe City, Heilongjiang Province (48°43 ′~4 9°03 ′ N, 124°56 ′~1 26°21 ′ E). The average annual temperature in the test area was ≥10°C, the annual effective accumulated temperature was 2000°C~2300°C, and the frost-free period was 115-120 d, belonging to the cold temperate continental climate, and the rainy season was mostly concentrated in summer. The local soil type is mainly black soil, and the cultivated land is weakly acidic. The basic physical and chemical properties of 0-20 cm soil layer in the test site are as follows: the soil bulk density was 1.21 g/cm 3 , alkalihydrolyzable nitrogen was 137.8 mg/kg, available phosphorus was 20.35 mg/kg, available potassium was 180.16 mg/kg, organic matter was 22.1 g/kg, and pH was 6.26.

Experimental Design
2.2.1. Layout of the Test Area. The plant crop is soybean, and the tested variety is Heihe 43, the main local soybean variety (provided by Beidahuang Seed Company). The tested fertilizers were urea (N = 46%), potassium (K 2 O = 60%), and diamine (P 2 O 5 = 16%). The planting method was "one ridge, single tube, and double row," the ridge height was 0.2 m, the ridge width was 0.4 m, the ditch bottom width was 0.4 m, the ridge spacing was 1 m, two rows of soybean were planted on the ridge, and the seedling number was 360,000 plants/hm 2 . Drip irrigation belt was laid in the middle of the ridge, the flow rate of the drip head was 1.38 L/h, the distance between the drip heads was 0.3 m, and the area of each test plot was 10 m × 10 m = 100 m 2 . The test layout is shown in Figure 1.

Field Plot Experiment
Design. Under the same conditions of soybean varieties, densities, pesticide application, and other technologies, three different drip irrigation amounts were set up in the experiment, 300 mm, 350 mm, 400 mm, respectively, and no irrigation was taken as the control. The drip irrigation amount ratio in each growth period was seedling period: branching period: floweringpodding period: grain filling period = 1:5 : 1:5 : 5 : 2. Five different nitrogen application rates were set: conventional nitrogen application level of 90 kg/hm 2 , nitrogen reduction level of 10% 81 kg/hm 2 , nitrogen reduction level of 20% 72 kg/hm 2 , nitrogen reduction level of 30% 63 kg/hm 2 , and zero nitrogen application level, with fertilization ratio of base fertilizer: top fertilizer (applied with water drops at flowering and grain filling period) = 2 : 1. Potassium fertilizer (30 kg/ hm 2 ) and phosphate fertilizer (150 kg/hm 2 ) were applied once as base fertilizer. A total of 16 treatments were used in the comprehensive experimental design method. Each treatment was repeated for 3 times, and a total of 48 test cells were randomly arranged. The experimental design is shown in Table 1. Wireless Communications and Mobile Computing method. The static chamber sampling system is shown in Figure 2. It is an airtight bottomless box made of organic glass, with a volume of 50 × 50 × 100 cm 3 . The depth of the ground box is about 15 cm. The upper part of the base is equipped with a 5 cm deep water tank, which is sealed with water before gas sample collection. The gas collection time is from 8 : 00 to 10 : 00. The micro electric fan is started before gas extraction, and the gas sample in the box is extracted with a 50 mL syringe at 0, 10, 20, and 30 min after the gas in the box is uniform. Then, the gas sample is brought back to the laboratory and detected and analyzed by Shimadzu GC-14B gas chromatograph. Gas samples were collected twice at each growth period, with an interval of 7 days (additional measurements were required in case of rainfall). The N 2 O emission flux soybean was calculated by using internationally common formula: In which, F is N 2 O emission flux, and the unit is μg/ m 2 ·h; ρ is the N 2 O density in standard state, and ρN 2 O is equal to 1.964 g/cm 3 ; H is the effective height of chamber, and the unit is m; dC/dt is the change rate of gas concentration in the sampling chamber during the sampling process, mL/m 3 ·h; t stands for average temperature in the sampling chamber, and the unit is°C; P is air pressure in sampling chamber, and the unit is kPa; P 0 is the standard atmospheric pressure, and the unit is kPa. (This test area is located in the plain region, and the impact of pressure is relatively small. P was considered equal to the standard atmospheric pressure.)

Determination of Nitrate and Ammonium Nitrogen.
The collected fresh soil samples were sipped 5 mm and weighed 24.00 g into a 200 mL plastic bottle. 100 mL KCL with a concentration of 1 mol/L was added for extraction, oscillation, filtration, and filtrate using Seal Analytical GmbH (Germany, AA3), with sensitivity 0.001 AUFS.

Observation of Meteorological Factors.
The meteorological data are recorded automatically by DZZ2 automatic weather station (Tianjin Meteorological Instrument Factory) of the test station. At the same time of gas sampling, soil surface temperature, soil temperature at 5 cm layer, and soil temperature at 10 cm layer were measured with a thermometer.

Dry Matter Determination.
Three representative plants were selected from each plot at each growth period for dry matter determination in an oven (105°C).

Data
Processing. Microsoft Excel 2010 and Surfer software were used for data processing, and SPSS was used for variance analysis.   Figure 3. The variation trend of N 2 O emission fluxes of all treatments is consistent. The peak values of emission fluxes of all treatments are concentrated in July and August, which is due to the increased rainfall in July and August of this year and the increase of soil moisture after rainfall, which improves the activities of nitrifying and denitrifying bacteria in the soil, promotes the production rate of nitrate and ammonium nitrogen, and thus significantly increases the production rate of N 2 O. The maximum N 2 O emission flux of each treatment in this experiment was 48.63 μg·m -2 ·h -1 on July 21.

Effects of Different Treatments on N 2 O Emission of
Soybean. The changes of N 2 O emission in different treatments and growth periods of soybean during the growth period are shown in Figure 4. The total N 2 O emission in each growth period was flowering-podding period > branching period > seedling period > seed filling period. W3N4 treatment, namely, drip irrigation amount of 400 mm, nitrogen application rate of 90 kg/hm 2 , and N 2 O emission, reached the maximum, which was 50.4 kg/hm 2 . In W1N0 treatment, when the drip irrigation amount was 300 mm and the nitrogen application rate was 0 kg/hm 2 , the cumulative emission of N 2 O reached the minimum, which was 22 kg/hm 2 . Under the same nitrogen application level, the greater the drip irrigation amount, the greater the N 2 O emission. Compared with the 300 mm drip irrigation amount, the N 2 O emission increased by 5.46% and 11.64% with the increase of drip irrigation amount and nitrogen application amount, respectively, when the drip irrigation amount was 350 mm and 400 mm. Under the same drip irrigation level, the higher the nitrogen application rate, the higher the N 2 O emission. Compared with the nitrogen application rate of 90 kg/hm 2 , the N 2 O emission of other nitrogen reduction levels decreased by 14.59%, 28.78%, 53.7%, and 109.79%, respectively. It can be seen that the N 2 O emission increased with the increase of drip irrigation rate and nitrogen application rate during the growth period of soybean. Table 2 shows the results of two-factor variance analysis of effects of different drip irrigation levels and nitrogen application levels on N 2 O emission of soybean. The results show that different drip irrigation levels and nitrogen application levels have extremely significant differences on N 2 O emission (P < 0:01), while the interaction between drip irrigation levels and nitrogen application levels has no significant difference on N 2 O emission (P > 0:05).

Relationship between Meteorological Factors and N 2 O
Emission Flux of Soybean. Correlation analysis was made between N 2 O emission fluxes of each treatment and the minimum, maximum, and average temperatures on the sampling day, and the results are shown in Table 3. N 2 O emission fluxes were positively correlated with maximum temperature, minimum temperature, and average temperature.
In W1 (300 mm) drip irrigation mode, there was no significant correlation between N 2 O emission fluxes and the maximum and average air temperature. In WIN2 (300 mm drip irrigation, 72 kg/hm 2 nitrogen application) and W1N0 (300 mm drip irrigation, 0 kg/hm 2 nitrogen application), there was a significant correlation between N 2 O emission fluxes and the minimum air temperature at the 0.05 level. W1N1 (drip irrigation 300 mm, nitrogen application 63 kg/ hm 2 ) was significantly correlated with the minimum temperature at 0.01 level, while the other treatments had no significant correlation with the minimum temperature. Under W2 (350 mm) drip irrigation mode, there was no significant relationship between W2N0 (drip irrigation volume 350 mm, nitrogen application rate 0 kg/hm 2 ) and the maximum temperature, and there was a significant relationship between the other treatments and the maximum temperature, minimum temperature, and average temperature at the level of 0.05 or 0.01. Under W3 (400 mm) drip irrigation mode, there was a significant correlation between each treatment and the minimum temperature and average  Wireless Communications and Mobile Computing temperature at the level of 0.05 or 0.01, but there was no significant correlation with the maximum temperature. By comparing the correlation coefficients, it was found that under W1 (300 mm) drip irrigation mode, the correlation coefficient of high fertilization treatment was slightly smaller than that of low fertilization treatment, indicating that nitrogen application weakened the influence of temperature on N 2 O emission flux.

Correlation between Soybean Soil Factors and N 2 O
Emission Fluxes. Table 4 shows the correlation analysis between N 2 O emission fluxes of different treatments and soil ammonium and nitrate nitrogen contents in soybean growing season. W1 (300 mm) drip irrigation treatment W1N1 (300 mm drip irrigation, nitrogen application rate 63 kg/ hm 2 ) had a significant positive correlation with ammonium nitrogen. W2 (350 mm) drip irrigation had no significant correlation with the contents of ammonium and nitrate nitrogen in soil. Under the W3 (400 mm) drip irrigation mode, W3N4 (400 mm drip irrigation, 90 kg/hm 2 nitrogen application rate), W3N2 (300 mm drip irrigation, 72 kg/ hm 2 nitrogen application rate) and W3N0 (400 mm drip irrigation, 0 kg/hm 2 nitrogen application rate) had a significant negative correlation with nitrate nitrogen (P < 0:05). Through the experiment, it was found that when the drip irrigation amount was small, there were more alternate times of drying and wetting in the soil, the physical and chemical environment of the soil was changed more frequently, and the N 2 O emission flux was affected by many factors. Therefore, the correlation between the treatments in this study and the soil NO 3 -and NH 4 + -N contents was poor. However, when the drip irrigation amount is large, the soil is humid for a long time and the soil environment is relatively stable. Therefore, the N 2 O emission flux of some treatments in W3 (400 mm) is significantly correlated with nitrate nitrogen.

Relationship between N 2 O Flux and Soil Temperature.
Different drip irrigation as well as the straw returned to change paddy soil physical and chemical conditions, leading to soil factors affecting N 2 O emissions in the change, different drip irrigation mode changed soil dry and wet state, different fertilizer rate changed the state of the growth of soybean plant, and these factors will lead to changes in soil temperature of different processing. The temperature of different soil layer is also one of the important factors affecting nitrification and denitrification in rice field. The relationship between soil surface temperature, soil temperature at 5 cm layer, and soil temperature at 10 cm layer and N 2 O emission flux under three drip irrigation quantities is shown in Figures 5-13. The correlation analysis results of soil temperature and N 2 O emission flux under different drip irrigation amounts are shown in Table 5.
The N 2 O emission fluxes under the three drip irrigation modes increased with the increase of soil temperature at different depths. The slope of the fitting equation of N 2 O emission flux and soil temperature shows a trend of gradual increase with the increase of soil depth, indicating that the soil depth under the three drip irrigation amounts is larger and the increase of soil temperature has a greater promoting effect on N 2 O emission flux. The increasing rate of N 2 O emission flux with the temperature of 5 cm soil layer and 10 cm soil layer was greater than that of drip irrigation of 350 mm and 400 mm, and the N 2 O emission flux with the increase of soil surface temperature was the largest under drip irrigation of 350 mm. By comparing R 2 of the fitting equation between N 2 O emission flux and soil temperature under three drip irrigation amounts, it can be seen that the linear fitting effect of soil temperature and N 2 O emission flux under drip irrigation amount of 400 mm is the best, followed by drip irrigation amount of 350 mm, and the worst is drip irrigation amount of 300 mm. The fitting effect of 300 mm drip irrigation and 400 mm drip irrigation equations gradually became better with the increase of soil depth, and when the drip irrigation amount was 350 mm, the fitting effect at 5 cm was the best, followed by the soil surface, and the worst at 10 cm. The correlation between temperature at different soil depths and N 2 O emission flux under three types of drip irrigation was extremely significant (P < 0:01). By comparing the correlation coefficient between temperature at different soil depths and N 2 O emission flux under three types of irrigation and three drip irrigation modes (Table 5), it can be seen that the correlation coefficient is the highest when drip irrigation is 400 mm under different soil depths. Drip irrigation of 350 mm followed, and drip irrigation of 300 mm was the smallest, indicating that soil temperature had a strong correlation with N 2 O emission flux when drip irrigation was large. When drip irrigation was small, the correlation coefficients increased gradually with the increase of soil depth, while when drip irrigation was 350 mm, the correlation coefficients increased first and then decreased with the increase of soil depth, and the correlation coefficients reached the maximum at 5 cm.

Relationship between Dry Matter Quality of Soybean
Overground and N 2 O Cumulative Emission. As shown in Figure 14, the correlation analysis between soybean overground dry matter quality and N 2 O cumulative emissions shows that there is a positive correlation between N 2 O cumulative emissions and soybean overground dry matter quality (R 2 = 0:0049), but the was no significant correlation. Although correlation analysis showed that N 2 O cumulative emissions increased with the increase of soybean dry matter mass, the slope of regression equation was very small and the correlation was low. The aboveground part of soybean can provide strong transport conditions for N 2 O emission through plant aerenchyma, but N 2 O emission is also affected by other environmental factors, and the process and influencing factors of N 2 O emission are complicated and greatly uncertain.

Discussion
N 2 O emissions are affected by nitrification and denitrification reactions occurring in soil, which will change due to the changes in soil physical and chemical properties caused by different irrigation and fertilization methods [19]. At present, there are different conclusions about the effect of irrigation amount on N 2 O emission. Studies have shown that due to the large amount of irrigation, the soil pore water content is higher, which will produce an environment that inhibits denitrification reaction and reduce the N 2 O emission compared with drip irrigation [20]. Other studies have shown that the effective reduction of N 2 O emission is due to the dominant role of nitrification reaction [21], while denitrification reaction mainly occurs under the condition of abundant water, thus promoting N 2 O emission. This study shows that under the condition of same n application rate, the smaller the drip irrigation, and effectively reduces the amount of N 2 O emissions, and in the whole soybean growth season, peak emissions are mainly concentrated in July and August, the results of the study and Liu et al. [22]. The results are consistent; this may be because the soil moisture content with low denitrification reaction is suppressed, thus reducing N 2 O emissions, and emission peak that appeared mainly in July and August may be because the Heilongjiang Province in the western region is a dry area; annual  precipitation mainly concentrated in July and August, with soil surface moisture after precipitation; soil moisture content increased significantly, microbial activity accelerated the mineralization of soil organic matter, and NH 4 + as nitration reaction substrate, NO 3 -is the reaction substrate of denitrification reaction [23], and the high temperature weather in Heilongjiang region concentrated in July and August throughout the year, and the high temperature and high humidity soil environment promote denitrification reaction, thus increasing the emission of N 2 O.    This study pointed out that the three kinds of drip irrigation mode most of the processing amount of N 2 O emission flux there is significant correlation with the temperature, soil temperature and air temperature is one of the important factors, further analysis showed that the three kinds of drip irrigation quantity model under different soil depth of Mark Twain and the relationship between N 2 O emission flux exists significant correlation, which is similar to previous research results. Ly et al. [24] also reached a similar conclusion in the study of grassland N 2 O emission. A large number of experiments have shown that both nitrification and denitrification are positively correlated with soil temperature [25]. In the soybean planting environment, the increase of temperature in different soil layers can promote microbial activity, thus promoting the emission of N 2 O. In this study, the correlation between soil temperature and N 2 O emission flux under 300 mm drip irrigation is less than that under 350 mm drip irrigation and 400 mm drip irrigation, which may be because frequent changes of soil moisture content under 300 mm drip irrigation weaken the influence of soil temperature on soil moisture content, while when the drip irrigation is larger, the variation range of soil moisture content is smaller. In a stable soil environment, temperature may be the main factor affecting N 2 O emission from paddy fields. Temperature not only affects the production of N 2 O but also affects the emission of N 2 O from soybean by affecting the diffusion of N 2 O in soil and the transport route of N 2 O. This is consistent with the research results of Yalan et al. [26] and Jinsai et al. [27].
Although the production of N 2 O is related to the transformation of different forms of nitrogen in the soil, it was found in this study that the correlation between N 2 O emission flux and NH 4 + -N and NO 3 -N concentrations was poor, which may be due to the fact that only soil samples at different growth periods were taken in this study, which was relatively small in number and not representative enough. The emission of N 2 O itself is affected by multiple environmental factors and shows certain randomness. Therefore, there is no significant correlation between N 2 O emission flux and NH 4 + -N and NO 3 -N in most treatments in this experiment. In the next step, we will increase the number and frequency of samples, to establish the correlation model between water, fertilizer, and N 2 O emission from soybean, verify the simulation results with the analysis results, and study the N 2 O emission of black soil soybean by different water and fertilizer from a deeper mechanism.

Conclusion
(1) The variation trend of N 2 O emission fluxes of soybean under different treatments was consistent during the whole growth period. The peak values of emission fluxes of all treatments were concentrated in July and August, and the overall N 2 O emission in each growth period is shown as follows: pod period > branching period > seedling period > bulking period. N 2 O emission increased with the increase of drip irrigation amount and nitrogen application amount. The effects of drip irrigation amount and nitrogen application rate on N 2 O emission were significantly different (P < 0:01), while the interaction between drip irrigation amount and nitrogen application level had no significant difference on N 2 O emission (P > 0:05) 10 Wireless Communications and Mobile Computing multiple environmental factors. There was no significant correlation between N 2 O cumulative emission and soybean overground dry matter (4) Based on experimental analysis, this paper controls N 2 O emission and improves nitrogen use efficiency by controlling the amount of water and fertilizer application, which is of great significance for scientifically formulating water and nitrogen resource management strategies compatible with water saving, yield increasing (stable yield), and emission reduction of black soil soybean. It can also play an important technical support role for the realization of the strategic goal of "one control, two reductions, and three basics" proposed by the Ministry of Agriculture and Rural Areas of China

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors do not have any possible conflicts of interest.