Managing Fertigation Frequency and Level to Mitigate N₂O and CO₂ Emissions and NH₃ Volatilization from Subsurface Drip-Fertigated Field in a Greenhouse

Abstract

Agricultural practices such as water and N management can contribute to greenhouse gas (GHG) emissions. Fertigation frequency and level are the two most important factors of irrigation scheduling. Proper irrigation management can establish moderate moist conditions throughout the crop growth period in the root zone and reduce GHG emissions and NH₃ volatilization. The main objective was to evaluate the possibility of reducing soil N₂O and CO₂ emissions and NH₃ volatilization without crop yield reduction by manipulating the subsurface-drip fertigation (SDF) frequency and level. An experiment was carried out adopting three SDF frequencies, High-Frequency (7-day, HF), Medium-Frequency (8-day, MF), and Low-Frequency (10-day fertigation intervals, LF), and two irrigation levels, 80% (I₈₀) and 70% (I₇₀) of amount in farmer’s common practice (1500 m³ ha⁻¹). Urea, N > 46.2% at the rate of 90% of traditional fertilization level (270 Kg N ha⁻¹) was injected with irrigation water. Results indicated that soil N₂O, CO₂, NO₃⁻-N, NO₂⁻-N, and water-filled pore space increased with fertigation frequency and an opposite pattern for NH₄⁺-N and NH₃. HF significantly (p < 0.05) increased crop yield by 45.1% and 49.2% compared to LF, under I₈₀ and I₇₀ levels, respectively. At the same irrigation level, HF was the optimum management practice. Person correlation analysis showed significant correlations between NO₂⁻-N and N₂O, CO₂ and soil temperature, and NH₄⁺-N and NH₃. The study suggests that HF of SDF with emitters buried at 0.15 m depth helps to keep high Chinese cabbage yield increases GHG emissions, but is not significant, and decreases NH₃ volatilization.

1. Introduction

Water resources are an important factor in ensuring food security. China’s water resources are strained by increasing demand from the rapid development of intensive agriculture [1]. The excessive use of irrigation water has been practiced and caused several environmental problems, such as nitrate leaching (NO₃⁻-N), the reduction of water and nitrogen utilization efficiency (WUE, NUE), and the comprehensive greenhouse effect [2,3]. Therefore, it is particularly important to adopt reasonable irrigation techniques and increase the utilization coefficient of irrigation water.

Subsurface drip irrigation (SDI), a water-saving irrigation technology, is widely used worldwide. It offers many advantages, such as: saving water, reducing deep percolation and evaporation, controlling soil water status precisely within the crop root zone, decreasing greenhouse gas (GHG) emissions, and enhancing water and fertilizer use efficiencies [4,5,6]. For example, Kallenbach et al. [7] compared the effects of SDI with flood irrigation (FI) on N₂O emissions using a flux chamber method and concluded that SDI reduced N₂O emissions by half compared to FI. Maris et al. [5] compared the effects of SDI with drip irrigation (DI) on N₂O emissions and reported that SDI decreased N₂O emissions by 69.7% and 94.5% in 2011 and 2012, respectively, compared with DI. Meanwhile, nitrogen (N) is another important factor affecting soil nitrogen distribution and thus crop growth, NUE, and GHG emissions. Several previous studies have studied the impacts of different N managements on soil GHG emissions and concluded that higher N application rates generally promote N₂O emissions. For instance, Shcherbak et al. [8] studied the response of soil N₂O emissions to N fertilizer application and found that higher N application rates generally result in greater N₂O emissions. With the development of the economy and the increase of the population, China has become the world’s largest fertilizer user during the past few years, and producers habitually use excessive fertilization to achieve high yields [9]. Therefore, combinations of high-efficiency irrigation technology and fertilization management practices are of great importance to mitigate N₂O emissions from the agricultural system while producing high yields.

Irrigation frequency, a key factor of irrigation scheduling, significantly affects soil nitrogen form and distribution, crop growth, GHG emissions, and NH₃ volatilization. Generally, the desired irrigation frequency depends on factors such as rainfall, irrigation, crop type, and developmental stage [10,11]. For instance, Beshir [12] reported that irrigation scheduling is a vital management input in the growing crop field to provide optimum soil moisture conditions for proper plant growth and development and optimum yield, water efficiency, and economic benefit. Chafutsa et al. [13] evaluated the yield response of cabbage to various fertilizers under different irrigation frequencies and reported that the irrigation frequency of cabbage varies between 3 and 12 days. Irrigation frequencies of 3 to 7 days’ intervals are common, particularly in the early growth stages, but it might extend up to 7 to 12 days’ intervals at later growth stages. Moreover, optimizing irrigation water application rate could help achieve maximum yield and crop WUE, reduce N₂O emissions, and reduce nutrients leaching below the root zone to prevent soil and groundwater pollution and mitigate NH₃ volatilization [14]. The amount of irrigation has been shown to influence microbial metabolic processes such as nitrification and denitrification, which are responsible for the release of N₂O [14]. Chen et al. [15] investigated the effects of different irrigation amounts on soil CO₂, N₂O, and CH₄ in two growing seasons and concluded that soil N₂O emissions increased with increasing irrigation amounts. Holcomb III et al. [16] studied the effect of irrigation rate on NH₃ volatilization and revealed that the peak of NH₃ volatilization was measured in the lower irrigation rate treatments compared with the higher irrigation rates. Generally, the frequency and intensity of irrigation water application are relatively easy to control, which may provide a chance to improve yield and mitigate N₂O emissions simultaneously. However, limited studies have been conducted to understand the effects of irrigation management techniques such as fertigation frequency and level under SDI on GHG emissions under cabbage cultivation. There is a dire need to evaluate the extensive study of fertigation frequency and level with saving water irrigation systems such as SDF for reducing N₂O emissions without yield reduction.

Thus, we hypothesized that higher fertigation frequency and level under SDF would result in higher N₂O and CO₂ emissions and NH₃ volatilization due to higher soil moisture content and substrate availability to the soil microbes. The objectives of this study were: (1) to quantify the N₂O and CO₂ emissions and NH₃ volatilization from a subsurface drip-fertigated Chinese cabbage; (2) to evaluate the possibility of reducing these emissions by manipulating the subsurface-drip fertigation frequency and level; and (3) to assess the emissions as a function of the crop to make general recommendations for farmers and policymakers.

2. Materials and Methods

2.1. Descriptions of Study Area

The experiment was carried out in a greenhouse under natural light conditions on Chinese cabbage at Water-Saving Park of Hohai University, Nanjing (31°95′ N, 118°83′ E), Jiangsu province, China, in a suburb of Nanjing at a downstream area of the Yangtze River basin with an average elevation of 15 m above the sea level. The greenhouse was covered by glass and has semi control system supported by shading system on the roof and cooling system in order to reduce heat when the temperature is extreme. The climate of the study area is characterized as humid under the influence of the East Asia Monsoon. The experimental plots were prepared through well-conventional tillage (manually) to a depth of 0.30 m. The soil of the experimental field is clay loam. The soil’s chemical and physical properties are shown in our previous work [17].

2.2. Experimental Design

The experiment was conducted comprising two experimental factors, three SDF frequencies: High-frequency (7-day fertigation intervals, HF), Medium-frequency (8-day fertigation intervals, MF), and Low-frequency (10-day fertigation intervals, LF), and two irrigation levels: 80% (I₈₀) (1200 m³ ha⁻¹) and 70% (I₇₀) (1050 m³ ha⁻¹) of farmer’s practice (1500 m³ ha⁻¹). Waterproof material with a thickness of 8 mm and a height of 0.50 m was inserted on the ridge of two adjacent plots to reduce the water and N transport between treatments and increase the accuracy and reliability of the research results. A completely randomized block design with six treatments; LF + I₈₀, LF + I₇₀, MF + I₈₀, MF + I₇₀, HF + I₈₀, and HF + I₇₀. The size of each plot area was 0.8 m² (L × W = 1 m × 0.8 m) approximately. Each treatment had to replicate three plots and be distributed randomly to minimize any effects from the differences between plots, for a total of 18 plots. Chamber bases were mounted in each plot at a depth of approximately 0.05 m and at different horizontal distances of 0.07, 0.14, and 0.20 m (1, 2, and 3) from the emitters (Figure 1). For NH₃ measurement, we installed PVC collectors in the center part of each plot to measure NH₃ volatilization. Each harboring six lateral lines with a lateral spacing of 0.20 m. Each lateral line has three emitters, spaced 0.40 m apart (Figure 1). Irrigation water was delivered to the plots via a gravity drip system. At the upper end of each plot, a tank in which to store irrigation water was installed at the height of 2 m. Urea-based N-fertilization (N 46.%), Jiangsu Huachang Chemical Co Ltd., Zhangjiagang City, China was incorporated with irrigation water (fertigation) in the water tank, each treatment having a separate drip line.

2.3. Agronomic Practices

The land was prepared, tilled, and leveled using a hand shovel. The soil was thoroughly mixed to raise the beds manually. Two weeks old Chinese cabbage seedlings were transplanted into the experimental plots in 0.2 m line spacing and 0.4 m plant spacing on 10 May 2021 and harvested on 29 June 2021. Surface drip fertigation with emitters buried at 0.15 m depth was applied. Urea-based N-fertilization (N > 46.2%, Jiangsu Huachang Chemical Co Ltd., Zhangjiagang, China) at application rate (270 kg N ha⁻¹) was applied with irrigation water. At the end of the experiment, Chinese cabbage was weighed in the field immediately after cutting the above-ground part of the plants to determine the crop yield. Weeds and insects were controlled whenever necessary.

2.4. Gas Sampling and Analyses

Before running the experiment, three rectangular stainless-steel chamber bases were mounted in each plot at a depth of approximately 0.05 m and different horizontal distances from the emitters. The bases served to mitigate lateral gas diffusion during gas sampling and remained open throughout the crop growing period except during gas sampling. The chambers (length × width × height = 0.20 × 0.10 × 0.20 m) were made of polyvinyl chloride and fitted with thermometers (Figure 1). The chambers were closed by placing them into rings made of stainless steel, which were sunk into the soil. Gas samples were obtained using 10-mL syringes linked to the chambers by a pipe and valve system. During the growing season, the bases of the chambers were left to prevent soil disruption. The samples of gas (N₂O and CO₂) were obtained between 10:00 and 10:30 a.m. Three gas samples were collected from each chamber at 15-min intervals. During each sampling, air temperatures inside the chambers were recorded using thermometers. A gas chromatograph (Agilent 7890A, Santa Clara, CA, USA) was used to analyze the N₂O using an electron capture detector (ECD). While CO₂ concentration was measured using a thermal conductivity detector TCD. The average daily emission of each treatment is calculated according to the proportion of wetted body area and the emission weighting of the corresponding distance. Then the cumulative emissions of gases were calculated by integrating daily values along the sampling period [18].

2.5. Ammonia Volatilization Measurement and Analyses

NH₃ volatilization loss from each plot was measured via the ventilation method using 0.20 m high PVC collectors (diameter = 0.16 m) located in the central part of each plot, with a sponge moistened with a phospho-glycerol solution as an absorbent [19]. The samples collected in the phospho-glycerol-soaked sponge were immediately immersed in 0.3 dm³ of 1.0 mol dm⁻³ KCl solution in 0.5 dm³ containers, sealed, and then shaken for one hour. NH₃ quantities in the KCl extracted solution were analyzed using colorimetry (λ = 630 nm) using a UV–VIS spectrophotometer (Unico, Shanghai, China). The NH₃ volatilization rate was calculated using Equation (1) [20].

$${\mathrm{R}}_{\mathrm{AV}}=\frac{\mathrm{M}}{\mathrm{D}·\mathrm{A}}\times {10}^{-2}$$

(1)

where: R_AV is the ammonia volatilization rate (kg N ha⁻¹d⁻¹), M is the amount of ammonia N collected by the PVC collector (mg), A is the cross-sectional area of the PVC collector (m²), and D is the interval for AV sample collection (days).

2.6. Soil Analyses

In each treatment plot, 20–30 g soil samples were collected using a 0.05 m diameter hand auger from varying depths (0.05, 0.10, 0.15, 0.20, and 0.25 m) and at three horizontal distances from the emitter 0.07, 0.14, and 0.20 m named as point1, point2, and point3, respectively. The samples were immediately taken to the laboratory to determine soil moisture content (θ), determined by the traditional drying method (105 °C, 8 h). Water-filled pore space (WFPS) was then determined by dividing the volume of gravimetric water by the soil’s total porosity ($\varphi$), itself determined by the calculating soil’s bulk density by the following relationship:

$$\varphi =1-\frac{{\rho }_b}{{\rho }_p}$$

(2)

where: ${\rho }_b$ and ${\rho }_p$ are the bulk density and particle density, respectively. The ${\rho }_p$ was estimated to be 2.65 g cm⁻³ [21]. A further 10 g of soil was extracted with 1 M KCl solution shaken at 300 rpm for 1 h, and filtered through filter paper (Whatman No. 42). Using the colorimetric method [20,22], the contents of soil ammonium (NH₄⁺-N), nitrate (NO₃⁻-N), and nitrite (NO₂⁻-N) in the filtrate were measured by colorimetric (λ = 630 nm) using an UV–VIS spectrophotometer (Unico, Shanghai, China). The details are as follows: first, we weighed 10 g of the soil sample, put it into a 100 mL conical flask, then measured 50 mL of 1 M KCl solution, sealed the mouth of the conical flask, and then used a 300 rpm shaking box for 1 h (H). Then left for 4 H at 4 °C. The supernatant was extracted and analyzed by using a flow injection apparatus. For NO₃⁻-N and NO₂⁻-N, we prepared 3 buffer solutions:

• E. buffer solution (dilute imidazole in 800 mL of distilled water. Adjust pH to 8.2 ± 0.1 with hydrochloric acid (32%). Add distilled water to 1 L, add Brij35 and mix
• F. Buffer solution (dilute ammonium chloride in 800 mL distilled water. Adjust pH to 8.2 ± 0.1 with NH₃. Add distilled water to 1 L, add Brij35 and mix)
• G. Color developing solution (dissolve phosphoric acid in 700 mL distilled water, adding sulfonamide and α- Naphthyl ethylene diamine, add distilled water to 1 L and dissolve). For NH₃ nitrogen, we have prepared 4 kinds of buffer solutions:
• A. buffer solution (in distilled water, dissolve potassium sodium tartrate and sodium citrate, set the volume to 1000 mL, add Brij 35 (30%), and thoroughly mix)
• B. Sodium salicylate solution
• C. Sodium nitroprusside solution
• D. Sodium dichloroisocyanurate solution.

In addition, considering the instability of nitrite nitrogen, it is easily oxidized. In order to minimize the error of nitrite, we analyze the concentration of nitrite nitrogen in the extracted water samples within 2 h, and then analyze nitrate nitrogen and ammonium nitrogen.

2.7. Statistical Analysis

Statistical analyses of the data were performed using the IBM-SPSS (IBM-SPSS, 19, Chicago, IL, USA) analytical software package. ANOVA was carried out to test whether the differences between all the treatments were significant at the 0.05 probability level. When the F-test was statistically significant (p < 0.05), mean comparisons were made using the least significant difference (LSD) at the 0.05 probability level. The correlation between the cumulative N₂O emissions and NO₂⁻-N concentrations, CO₂ emissions, and soil temperature, and NH₃ volatilization and NH₄⁺-N concentration was analyzed using the Pearson correlation coefficients at the 0.05 probability level.

3. Results

3.1. Soil Temperature, Relative Humidity, and WFPS

Over the whole experimental period, the average daily soil temperature and relative humidity inside the greenhouse were 32.6 °C and 35.3%, respectively (Figure 2a). The highest WFPS were 61.2% and 60.7%, 59.8% and 59.8%, and 61.5% and 60.4% for LF, MF, and HF treatments under I₈₀ and I₇₀, respectively. The mean soil WFPS of 0–0.25 m soil depth was significantly different between treatments under both irrigation levels (p < 0.05), and the WFPS from HF treatment was 2.6% and 1.8%, and 3.1% and 2.1% higher than those from LF and MF treatments under I₈₀ and I₇₀ irrigation levels, respectively (Figure 2b,c).

3.2. Soil Inorganic Nitrogen

As shown in Figure 3, the highest NH₄⁺-N concentrations from the LF treatment were significantly higher than those from MF and HF treatments under both irrigation levels (p < 0.05). Generally, NH₄⁺-N concentration was maintained at a high level after the first fertigation, with the largest peaks of 91.8 and 80.6 mg NH₄⁺-N kg⁻¹, 67.0 and 53.7 mg NH₄⁺-N kg⁻¹, and 46.2 and 38.2 mg NH₄⁺-N kg⁻¹ for LF, MF and HF, under I₈₀ and I₇₀, respectively, then the values decreased gradually until to the following fertigation event (Figure 3). Among different fertigation frequencies, the order of mean NH₄⁺-N (0–0.25 m) concentration was: LF > MF > HF, and the values for LF and MF treatments were enhanced by 21.9% and 11.7%%, and 23.9% and 8.5% when compared to HF, under I₈₀ and I₇₀, respectively.

NO₃⁻-N showed a similar trend to that of NH₄⁺-N. The pulse NO₃⁻-N concentrations were measured immediately after fertigation events in each treatment; then, the values decreased gradually to the next fertigation event (Figure 3). The largest peaks were observed after the first fertigation with values of 641.0, 596.9, and 573.12 mg NO₃⁻-N kg⁻¹ for LF, MF, and HF under I₈₀ level; and 551.4, 513.4, and 506.2 mg NO₃⁻-N kg⁻¹ under I₇₀ level. Over the whole experimental period, the mean NO₃⁻-N concentrations under the I₈₀ level were 346.6, 349.0, and 365.0 mg NO₃⁻-N kg⁻¹ for LF, MF, and HF, while under the I₇₀ level were significantly lower, and the sequence as follows: LF < HF < MF. Compared to HF, the concentrations from LF and MF treatments were decreased by 5.3% and 4.6%, and 11.5% and 3.3% under I₈₀ and I₇₀, respectively.

The result indicated a significant difference between fertigation frequencies on soil NO₂⁻-N concentrations, and the mean soil NO₂⁻-N concentrations at 0–0.25 m soil depth increased with the increase of fertigation frequency (p < 0.05), and the order is as follows: LF < MF < HF. Compared to HF, the values from LF and MF treatments were decreased by 20.7% and 9.4%, and 23.1% and 3.2%, under I₈₀ and I₇₀, respectively. While no significant difference was found in NO₂⁻-N concentrations between irrigation levels, the high NO₂⁻-N was found within each treatment after the fertigation event and then decreased to the next fertigation event (Figure 3).

3.3. N₂O and CO₂ Fluxes

Several N₂O peaks were observed after fertigation application (Figure 4). The highest N₂O peaks (101.3, 84.3, and 76.5 μg N₂O m⁻² h⁻¹) and (77.1, 65.1, and 60.1 μg N₂O m⁻² h⁻¹) were measured after the first fertigation for the LF, MF, and HF under I₈₀ and I₇₀, respectively. It is clear that N₂O increased after each fertigation within a short period and then decreased the following fertigation (Figure 4). The largest mean pulses from HF treatment were always observed for distance 1 (0.07 m from the emitter), with the emissions from this zone being significantly higher than those from distances 2 (0.14 m) and 3 (0.20 m) to the emitter (

Table 1. Mean N₂O and CO₂ fluxes under high and low irrigation levels (I₈₀ and I₇₀) on three fertigation frequencies (LF, MF, and HF) at three different distances (1, 2, and 3) from the emitter.

GHG	Horizontal Distances from Emitter	I80			I70
		LF	MF	HF	LF	MF	HF
N2O	1	63.4 ± 4.4 a	60.7 ± 4.2 a	69.5 ± 4.9 a	48.2 ± 3.4 a	48.4 ± 3.4 a	56.4 ± 3.9 a
	2	33.5 ± 2.3 c	36.1 ± 2.5 c	45.3 ± 3.2 c	28.7 ± 2.3 c	32.6 ± 2.3 c	39.0 ± 2.7 c
	3	39.6 ± 2.8 b	42.2 ± 2.9 b	52.8 ± 3.7 b	32.8 ± 2.3 b	37.3 ± 2.6 b	43.9 ± 3.1 b
CO2	1	298.1 ± 20.8 a	293.8 ± 20.5 a	336.6 ± 23.5 a	247.4 ± 17.3 a	260.9 ± 18.2 a	302.7 ± 21.2 a
	2	215.4 ± 15.1 c	209.4 ±14.6 c	246.3 ± 17.2 c	179.6 ± 12.6 c	198.0 ± 13.8 c	239.2 ± 16.7 b
	3	247.5 ± 17.3 b	243.7 ± 17 b	278.2 ± 19.4 b	220.1 ± 15.4 b	236.6 ± 16.5 b	282.1 ± 19.7 a

Note: LF, MF, and HF represent low, medium, and high fertigation frequencies, respectively. I₈₀ and I₇₀ denote 80% and 70% irrigation levels of farmer’s practice. Different letters within columns indicate significant differences by applying the Least Significant Difference (LSD) test at p < 0.05.

). Under the I₈₀ level, the mean N₂O emissions increased by 1.1% and 1.4% for MF and HF, respectively, compared to LF. In addition, under I₇₀ level followed the same trend with values of 3.1% and 3.5%. Over the whole growing period, the cumulative N₂O emissions were (p >0.05) higher for HF treatment under both I₈₀ (57.3 mg N₂O m⁻²) and I₇₀ (46.7 mg N₂O m⁻²) levels (

Table 2. Cumulative N₂O and CO₂ emissions and NH₃ volatilization over the whole experimental period.

Irrigation Level (I)	Fertigation Frequency (F)	N2O	CO2	NH3
		(mg N2O m−2)	(g CO2 m−2)	(mg NH3 m−2)
I80	LF	56.48 ± 2.38 a	309.22 ± 13.02 a	34.76 ± 1.46 a
	MF	57.11 ± 2.40 a	310.96 ± 13.09 a	31.86 ± 1.34 b
	HF	57.29 ± 2.41 a	318.39 ± 13.41 a	24.99 ± 1.05 e
I70	LF	45.14 ± 1.90 b	273.05 ± 11.50 b	30.19 ± 1.27 c
	MF	46.53 ± 1.96 b	301.51 ± 12.70 a	27.79 ± 1.18 d
	HF	46.71 ± 1.97 b	308.05 ± 12.97 a	23.34 ± 0.98 f
F		ns	ns	*
I		*	*	*
F × I		ns	ns	ns

Note: LF, MF, and HF represent low, medium, and high fertigation frequencies, respectively. I₈₀ and I₇₀ denote 80% and 70% irrigation levels of farmer’s practice. F: frequency, I: Irrigation. Different letters within columns indicate significant differences by applying the Least Significant Difference (LSD) test at p < 0.05. * 0.05 probability level. ns: no significant difference.

).

Fertigation frequencies did not significantly affect CO₂ emissions, while with the increase in fertigation frequency, the CO₂ emissions increased (Table 2). Similarly, the irrigation level also did not significantly affect CO₂ emissions, but the values under I₈₀ were higher than I₇₀. As shown in Figure 5, the highest CO₂ peaks were 469.0, 421.4, and 428.9 mg CO₂ m⁻² h⁻¹ for the LF, MF, and HF treatments under I₈₀, respectively. The same trend was noticed under the I₇₀ irrigation level. The result demonstrated that the highest CO₂ peak occurred after each fertigation and then decreased with moisture to the next fertigation event (Figure 5). Under I₈₀, the emissions from a distance 1 were significantly higher than those from distances 2 and 3 (Table 1). The cumulative CO₂ emissions were (p > 0.05)higher for HF treatment, under both I₈₀ (318.39g CO₂ m⁻²) and I₇₀ (308.05g CO₂ m⁻²) irrigation levels, in comparison with MF and LF treatments (Table 2).

3.4. NH₃ Volatilization

As shown in Figure 6, the peak NH₃ from all treatments generally occurred after each fertigation, then decreased until the following fertigation event. Over the whole experimental period, the highest NH₃ peaks (65.3, 51.46, and 47.33 μg NH₃ m⁻² h⁻¹) and (53.1, 45.01, and 39.51 μg NH₃ m⁻² h⁻¹) were observed immediately after the first fertigation for the LF, MF, and HF under I₈₀ and I₇₀, respectively. The cumulative NH₃ emissions from HF treatment were significantly lower than those from LF and MF treatments (p < 0.05), but no significant differences were observed between LF and MF treatments (Table 2).

3.5. Crop Yield and Yield-Scaled Global Warming Potential

HF treatment significantly increased Chinese cabbage yield by 31.0% and 20.6% for I₈₀ and 41.5%, and 20.8% for I₇₀ irrigation levels. As expressed on a CO₂-equivalent basis, global warming potential (GWP) incorporates GHGs to exhibit a production system’s overall global warming impact and each GHG’s contribution to the global warming process. The current study showed that HF reduced GWP/Y by 1.9% and 6.1% for I₈₀ and I₇₀, respectively, relative to LF (

Table 3. Crop yield and yield-scaled Global Warming Potential (GWP/yield).

Irrigation Level	Fertigation Frequencies	Yield (kg m−2)	GWP/Y
			(mg CO2 eq kg−1)
I80	LF	1.71 ± 0.12 b	3.12 ± 0.22 a
	MF	1.97 ± 0.14 b	2.68 ± 0.19 b
	HF	2.48 ± 0.17 a	2.63 ± 0.18 b
I70	LF	1.55 ± 0.07 c	2.67 ± 0.19 a
	MF	1.83 ± 0.08 b	2.31 ± 0.16 b
	HF	2.31 ± 0.10 a	2.17 ± 0.15 b
F		*	*
I		*	*
F × I		ns	ns

Note: LF, MF, and HF represent low, medium, and high fertigation frequencies, respectively. I₈₀ and I₇₀ denote 80% and 70% irrigation levels of farmer’s practice. F: frequency, I: Irrigation. Different letters within columns indicate significant differences by applying the Least Significant Difference (LSD) test at p < 0.05. *: Significant difference at the probability of 0.05 (p < 0.05). ns: no significant difference.

).

4. Discussion

4.1. Effect of Subsurface Drip Fertigation Frequency and Irrigation Level on Soil Inorganic N

Both irrigation frequency and level significantly affected inorganic N concentration [23,24,25]. In the current research, the LF and MF treatments increased NH₄⁺-N concentration by 21.9% and 11.7%, and 23.9% and 8.5% under I₈₀ and I₇₀ levels, respectively, compared to HF. That might be explained by the fact inadequate moisture content limited the transformation of NH₄⁺-N to NO₃⁻-N, resulting in an availability of NH₄⁺-N concentration in the soil under LF and MF treatments compared to HF. The result is supported by Liu et al. [26], who found that insufficient soil water limited the downward movement of soil NH₄⁺-N, and therefore more NH₄⁺-N ions are readily emissions. Meanwhile, the current results showed that the mean soil NO₃⁻-N concentration increased with fertigation frequency. This agrees well with Chang et al. [23], who showed that soil NO₃⁻-N concentration increased with increases in irrigation frequency by studying the effects of irrigation levels and irrigation frequencies on soil NO₃⁻-N, tomato growth, and yield. The largest effect of NO₃⁻-N leaching was achieved by improved water management, which was significantly different from the other strategies [27]. In the current study, subsurface drip fertigation with emitters buried at of 0.15 m was used, and the average soil moisture under the I₈₀ level is mainly maintained at 32.2~61.5% WFPS; Under the I₇₀ level, the soil moisture was mainly maintained at 31.8–61.2% WFPS which is aerobic condition it is confirmed by the existing literature that in the range of 30–90% WFPS, for example [28] found nitrification an important source of N₂O released from the soil at low soil water contents (<62% WFPS). The microbial activity increases with the increase of soil moisture, which is more conducive to the nitrification process, that will transform NH₄⁺ to NO₃⁻, thus producing and releasing N₂O into the atmosphere. For example Hosono et al. [29] reported that N₂O was produced at WFPS values ranging from 30% to 90% but reached a peak near 60% WFPS. Since the WFPS of our study were generally less than 70% WFPS, we assume the majority of N₂O measured was produced from aerobic nitrification.

4.2. Effect of Subsurface Drip Fertigation Frequency and Irrigation Level on N₂O and CO₂ Emissions and NH₃ Volatilization

Soil moisture is a key factor affecting N₂O emissions in agricultural soils [30,31,32]; It can promote the movement of the soil matrix and create a good micro-environment for microorganisms activities that mediate N₂O production [21]. The studies indicate that less frequent irrigation events lead to lower N₂O emissions, though the amount is dependent upon local climate. A likely mechanism for this trend is that less frequent water application allows more time for oxygen to penetrate into the soil matrix between irrigation events, which would favor microbial nitrification; when the soil water content is low enough, these factors lead to a suppression of all microbial activity in the soil and hence an overall decrease in N₂O emission [29]. Moreover, in the current study, soil N₂O emissions from HF treatment were higher than those from LF and MF treatments under both irrigation levels. That is because HF fertigation increases the number of dry-wet alternations, resulting in more O₂ entering the soil and promoting soil nitrification. For example, Han et al. [33] studied the effects of different irrigation regimes on N₂O emissions in greenhouse tomato fields. They found that irrigation frequency was associated with higher irrigation amount, which would induce more frequent dry-wet cycles and cause the greatest N₂O emissions. On the other hand, HF causes increases in soil moisture and reduces O₂ concentrations, thus inducing nitrifier denitrification and denitrification. Additionally, the results are supported by Schindlbacher et al. [34], who demonstrated that N₂O emission is positively related to soil moisture content. Generally, when the soil moisture is less than 80%, the soil is in aerobic condition [26]. However, under the condition of high water content, on the one hand, the relatively significant alternation of dry and wet may create a good microenvironment for microorganisms. On the other hand, the higher water content promotes the migration of soil nitrogen [35], which may promote the increase of microbial activity and its energy matrix source, thus leading to the increase in N₂O emissions.

Soil CO₂ emissions originate primarily from plant roots and microbial respiration [36]. Soil moisture content is one of the factors with a higher impact on CO₂ fluxes from soils [37]. Our study demonstrated that fertigation frequency showed a p > 0.05 effect on CO₂ emissions. The MF and HF irrigation treatments increased CO₂ emissions by 0.6% and 10.4%, and 3.0% and 12.9% under I₈₀ and I₇₀ levels, respectively, compared to LF. This may be because MF and HF fertigation resulted in more uniform soil moistening throughout the entire research period. Uniform humidity (without excesses and deficits of water) contributes to the balanced development of microorganisms that were constantly involved in the transformation of soil nitrogen and supplied in the form of fertigation. At the same time, the soil enzymatic activity of microorganisms resulted in higher CO₂ emissions. The peak WFPS of the current study is about 61.5%, close to the optimum 70% for stimulating microbial activity and enhancing root respiration, thereby increasing CO₂ emission. The result is in line with Abalos et al. [38] and Sapkota et al. [39], who reported that increased soil water content accelerates microbial respiration of the soil, which enhances CO₂ flux. The findings are also coherent with [38], who stated that high-frequency irrigation significantly increased CO₂ emissions. Furthermore, evidence is shown by the positive correlation between WFPS and CO₂ fluxes and significantly higher emissions measured from the zone close to the emitter point.

The main source of NH₃ volatilization worldwide comes from the agriculture system. It does not only reduce the utilization rate of nitrogen fertilizer but also leads to the waste of resources and environmental problems [40]. Two factors affect the NH₃ volatilization from the viewpoint of agricultural practices. One is the rate of N fertilizer used for each application, and the other is the water depth at the time of the application of N fertilizer [41]. The present study showed that NH₃ volatilization was significantly affected by fertigation frequency and level. Compared to LF, HF fertigation significantly decreased NH₃ volatilization by 28.1% and 22.7% under I₈₀ and I₇₀ levels (p < 0.05), respectively. This may be attributed to the low soil moisture under LF limited microorganisms activates to transform NH₄⁺-N to NO₂⁻-N, and thus enhanced NH₃ volatilization. The result is in line with Liu et al. [26], who concluded that the lack of adequate soil water content restricted downward transport of NH₄⁺-N in the soil and thus rendered applied NH₄⁺-N readily available for NH₃ volatilization. Increased irrigation decreased NH₃ losses from volatilization [16]. Further, the NH₃ volatilization rate is affected by NH₄⁺-N concentration. The Pearson correlation analysis demonstrated a strong and positive correlation between NH₃ volatilization rate and NH₄⁺-N concentration (R² = 0.75, p < 0.05) (Figure 7), which agreed well with other literature [22,42,43]. We speculate that insufficient soil water limits NH₄⁺-N movement to NO₃⁻-N, thus promoting NH₃ volatilization [42].

Similarly, a positive correlation was found between N₂O fluxes and NO₂⁻-N concentrations (R² = 0.8, p < 0.05) (Figure 7). This likely indicates that microbial processes are responsible for the N₂O produced by NO₂⁻-N. The result is consistent with the findings of Maharjan and Venterea [43] and Li et al. [22], who revealed a significant positive correlation between N₂O fluxes and NO₂⁻-N. Furthermore, the correlation analysis showed that CO₂ flux positively correlated with soil temperature. On the other hand, air temperature also increases CO₂ emissions (Figure 7). This is because the temperature is one of the most important factors affecting the activity of microorganisms and soil respiration. The study by Kamali et al. [44] investigated the impacts of wind erosion and seasonal changes on soil CO₂ emission and found a positive correlation between air temperature and CO₂ emission.

4.3. Influence of Subsurface Drip Fertigation Frequency and Irrigation Level on Chinese Cabbage Yield

Badr and El-Yazied [45] conducted the effects of N application rates and fertigation frequencies on tomato yield and concluded that high-frequency fertigation is a vital management variable for subsurface drip-fertigation tomatoes. El Naim and Ahmed [46] investigated the effect of irrigation intervals and inter-row spacing on the yield and WUE of the sunflower. They observed that irrigation every 7 days increased the head diameter of the sunflower. Chafutsa et al. [13] evaluated the yield response of cabbage to various fertilizers and irrigation frequencies and found that cabbage irrigation is carried out every 3 to 12 days, depending on the environment, crop development, and soil type. Similarly, fertigation intervals of 3 to 7 days are common, especially in the early growth stages, but they can extend up to 7 to 12 days. In the current research, the yield from HF fertigation treatment (7-day fertigation intervals) was 31.0% and 20.6%, and 41.5% and 20.8% higher than those from the LF and MF treatments, under I₈₀ and I₇₀ irrigation levels (p < 0.05), which is more consistent with the conclusion that fertigation with an interval of 7 days can increase crop yield [13]. This may be due to the fact that high-frequency fertigation provides suitable conditions for crop roots to uptake water and nutrients more easily. On the other hand, high-frequency fertigation could increase the soil dry-wet alternating process and promote the increase of soil O₂ concentration. Both reasons together may account for the increased crop yield in HF treatment. Silber et al. [47] found that high-frequency fertigation may improve crop performance due to increased nutrient and water availability by examining the effects of fertigation frequency and potassium application rate on bell pepper growth. Similarly, Wang et al. [48] studied the effect of irrigation frequency on potato root distribution and yield. They pointed out that increasing the irrigation frequency can help plant roots better absorb water in the soil, which is beneficial for improving the yield of crops. In addition, our study indicated that the crop yield under high irrigation level (I₈₀) were higher than those under low irrigation level (I₇₀); this indicates that I₈₀ creates favorable moisture conditions in the crop root zone, which enables crop roots to uptake water and nutrient to the plant. This is consistent with the findings by Chafutsa et al. [23] and Gu et al. [49], who indicated that a higher amount of irrigation could increase crop yield due to better soil moisture conditions. These results implied that a proper irrigation technique and scheduling should be recognized as keys to increasing crop production on a sustainable basis.

4.4. The Implications of Subsurface Drip-Fertigation Management

As one of the largest vegetable production countries globally, China’s annual planting area of Chinese cabbage is about 2.7 million ha, accounting for about 15% of the total planting area of vegetables in China. According to the current results, if SDF with fertigation at a 7-day interval (HF) is applied, when irrigation water is under 80% of the traditional irrigation level (1200 m³ ha⁻¹), the NH₃ volatilization will be reduced by 758.9 t, and the yield will increase by 1.2 × 109 t, compared to LF. These results imply that a reasonable irrigation schedule (fertigation frequency and irrigation level) of SDF can alleviate the environmental impacts under high N load and helps reduce the comprehensive greenhouse effect of GHG emissions, NH₃ volatilization, and realize cleaner production in greenhouse cabbage cultivation. However, there are still some difficulties that need to be solved in the process of transforming and popularizing these technical achievements. For example, the cost of drip irrigation pipes and fertilizer tanks needs to be controlled, which can be realized by developing corresponding integrated water and fertilizer devices. In addition, large-scale training and publicity are required for farmers to understand and accept the importance of this management measure as soon as possible. Training groups should be established to create conditions for the continuation of training. From the practical viewpoint of this research, the findings demonstrated that high-frequency fertigation treatment (7-day fertigation intervals) generally caused the lowest yield-scaled global warming potential (Y/GWPs) and NH₃ volatilization losses compared with MF and LF, regardless of irrigation levels. These results likely suggest that the management strategy of “small amount and several times” of SDF could increase yield-scaled global warming potential and reduce NH₃ volatilization and should be a promising strategy for farmers and decision-makers to achieve an acceptable yield while maintaining the environment. Moreover, the results are mainly based on a plot experiment with three subsurface drip-fertigation frequencies and two irrigation levels on GHG emissions, NH₃ volatilization, and yield. As climate conditions and soil characteristics will be largely varied among different regions, similar research in field experiments with more crops and water-fertilizer management is needed to investigate the most suitable irrigation-fertilization scheduling for different crops under the background of climate change, with the characteristics of low cost, increasing crop production, and reducing GHG emissions and NH₃ volatilization.

5. Conclusions

The experiment was conducted to evaluate the effects of fertigation frequency and level on N₂O and CO₂ emission, NH₃ volatilization, and Chinese cabbage yield. The results demonstrated that N₂O and CO₂ were increased with fertigation frequency and irrigation level. Soil NH₄⁺-N and NH₃ volatilization were decreased with fertigation frequency increase while soil NO₂⁻-N, NO₃⁻-N, and water-filled pore space increased with fertigation frequency. Chinese cabbage yield significantly increased in high fertigation frequency treatment under both irrigation levels. High irrigation level increased crop yield relative to low irrigation level. It is a challenge to balance crop requirements and emissions reduction simultaneously. This study provides a basis for the practical application of fertigation scheduling and irrigation levels that could reduce greenhouse gas emissions and improve Chinese cabbage yield in greenhouse cultivation. From a practical viewpoint, we recommend high-frequency fertigation with a 7-day interval under subsurface drip fertigation to help keep high Chinese cabbage yield with negligible GHG effect. In addition, we speculate that current fertigation scheduling can be applied for other crops besides Chinese cabbage. More studies are necessary to completely understand the influence of fertigation frequency on greenhouse gas emissions and NH₃ volatilization in vegetable cultivation.