Saline water irrigation stimulate N₂O emission from a drip-irrigated cotton field

ABSTRACT

Farmers in arid regions often use saline irrigation water because fresh water supplies are limited. A field study was conducted to evaluate the effects of irrigation water salinity and N application rate on soil properties and N₂O emission in a drip-irrigated cotton field. The experiment included three levels of irrigation water salinity (0.35, 4.61, or 8.04 dS m⁻¹) and two N application rates (0 or 360 kg N ha⁻¹). The results showed that soil moisture, salinity, and concentrations increased significantly as water salinity increased, whereas soil pH and concentrations decreased. N₂O emissions on the first day, the first two days, and the first three days in the fertilized plots accounted for 19.5–26.3%, 68.1–70.5%, and 80.6–89.7% of total N₂O emissions over the six days, respectively, while the corresponding proportions in the unfertilized plots were 13.5–23.2%, 40.5–51.7%, and 59.5–70.7%, respectively. In the unfertilized plots, N₂O emission flux and cumulative N₂O emission both increased as water salinity increased. In the fertilized plots, N₂O emission rate and cumulative N₂O emission were both highest in the 4.61 dS m⁻¹ treatment and lowest in the 0.35 dS m⁻¹ treatment. Correlation analysis showed that soil moisture, , and were positively correlated with N₂O emission rate. In contrast, soil salinity and pH had no significant correlation with N₂O emission rate. Our study demonstrates that irrigation with brackish or saline water (SW) significantly increased N₂O emission. To reduce the adverse effects of brackish or SW irrigation on soil N₂O emission, farmers should evaluate management practices such as additional irrigation before cultivation in spring, or the alternate use of fresh and SW during irrigation.

KEYWORDS:

• Water salinity
• N fertigation
• N₂O emission
• concentration
• concentration

Introduction

Nitrous oxide is one of the most important greenhouse gases, having up to 310 times the global warming potential of CO₂ over a 100-year time frame (IPCC 2007). Nitrous oxide also contributes to the destruction of stratospheric ozone (Montzka et al. 2003). Agricultural soils contribute approximately 50% of atmospheric N₂O emissions (IPCC 2001). This amount is equivalent to a global warming potential of 1.0 Pg C yr⁻¹ (Robertson et al. 2004).

Nitrification and denitrification are the two processes responsible for the vast majority of N₂O emissions from soil (Robertson & Groffman 2007). As substrates for nitrification and denitrification, ammonium-nitrogen and nitrate-nitrogen have important effects on N₂O emission rates (Ellis et al. 1998). Field management practices (e.g. irrigation, fertilization, and crop type), climate conditions, and soil characteristics are also reported to affect N₂O emissions from agricultural soils (Bond-Lamberty & Thomson 2010; Peng et al. 2011; Scheer et al. 2014).

Soil N₂O emissions vary greatly depending on the agricultural system. For example, N₂O emissions ranged from 0.2% to 0.3% of fertilizer N in rainfed maize fields in northeast China (Ni et al. 2012). In comparison, N₂O emissions accounted for 0.4–2.6% of fertilizer N from irrigated cotton (Gossypium hirsutum L.) fields in Australia (Scheer et al. 2008). Irrigation has significant influence on N₂O emissions. Irrigation management, which affects both soil water dynamics and microbial C and N turnover, has the potential to affect the exchange of greenhouse gasses between the biosphere and the atmosphere (Holst et al. 2008). Scheer et al. (2012) reported that N₂O fluxes were, respectively, 72% and 67% less in medium and low irrigation treatments than in a high irrigation treatment in a wheat field. Jiao et al. (2006) reported that intermittent irrigation increased total growing-season N₂O emissions by 24% compared with continuous flood irrigation in a rice system.

Fertilizer N is essential for increasing crop yields, and it can have significant impact on N₂O emission (Bouwman et al. 2002). Generally, there is a linear relationship between N₂O emission and N application rate in agricultural soils (Hoogendoorn et al. 2008). However, McSwiney and Robertson (2005) observed nonlinear increases in N₂O emission across a nine-point fertilizer gradient (0, 34, 67, 101, 134, 168, 202, 246, and 291 kg N ha⁻¹) in Michigan over a three-year period for continuous corn (Zea mays L.). The authors observed that N₂O emission increased as N fertilizer increased from 0 to 101 kg N ha⁻¹, declined somewhat at rates of 168 and 202 kg N ha⁻¹, and then increased again at rates of 246 and 291 kg N ha⁻¹. The relationship between N fertilizer rate and N₂O emission is complicated by factors that affect soil N saturation such as C availability and soil C/N ratio (Matzner & Borken 2008).

Farmers often use poor quality water for irrigation in Xinjiang Province in northwest China. In this region, saline water (SW) is increasingly used for drip irrigation of cotton, a highly salt tolerant plant (Chen et al. 2010). Drip irrigation is highly efficient because the application of water and N fertilizer can be controlled to meet the requirements of the crop at each physiological growth stage (Mohammad 2004). The concentrations of Na⁺ and Cl⁻ in SW are high, making it different than ordinary fresh water (FW). SW irrigation may cause an accumulation of soluble salts in agricultural soils (Zartman & Gichuru 1984). The electrical conductivity (EC) and the water-holding capacity of soil increase with the increasing salinity of irrigation water (Tedeschi & Dell’Aquila 2005; Huang et al. 2011). Accumulation of soluble salts negatively affects the mineralization and nitrification processes of the N cycle and may also affect the production of N₂O in soils (Adviento-Borbe et al. 2006; Reddy & Crohn 2014). There are conflicting reports about the effect of salt concentrations on N dynamics in relation to N₂O emission (Dalal et al. 2003). Inubushi et al. (1999) reported that salt concentration had no significant effect on N₂O emission. However, other studies showed that increases in salt concentration significantly stimulated N₂O emission (Tsuneda et al. 2005; Reddy & Crohn 2014; Kong 2015). Other researchers observed that the effect of salt on N₂O emission varies depending on salt concentration. For example, Shang et al. (2009) reported no significant change in N₂O production as water salinity increased from 2.5 to 7.5 g L⁻¹; however, there was a large increase in N₂O production when water salinity increased from 7.5 to 10 g L⁻¹. Some researchers (Azam & Müller 2003; Adviento-Borbe et al. 2006; Zeng et al. 2013) have reported that environmental conditions affect the relationship between salt concentration and N₂O emission. For example, Azam and Müller (2003) demonstrated that N₂O emissions decreased as salt concentrations increased under dry soil conditions (20%, w/w), but the opposite pattern was observed under moist conditions (40%, w/w).

The purposes of this study were (i) to investigate the effect of irrigation water salinity and N application rate on soil physicochemical properties and N₂O emissions, and (ii) to determine the interactive effects of SW irrigation and N fertigation on N₂O emissions in a drip-irrigated cotton production system.

Materials and methods

Study site

The field experiment was conducted during the 2014 growing season at the Shihezi University Experiment Station, Xinjiang, China (44°18′N, 86°02′E). The region is classified as a temperate arid zone with a continental climate. Average daily temperatures during the observation period (June–August 2014) ranged from 12.8°C to 31.7°C and total precipitation was 40.1 mm. The mean soil temperature (5 cm depth) ranged from 17.9°C to 28.7°C, with an average of 23.3°C. The soil in the study area is classified as a Calcaric Fluvisol (i.e. alluvial, gray desert soil). The experimental field used for this study has been irrigated with SW since 2009. The experimental design included two saline irrigation treatments (EC of water: 4.61 and 8.04 dS m⁻¹) plus a non-salinized control (EC of water: 0.35 dS m⁻¹). The crop was continuous cotton (Gossypium hirsutum L.). At the beginning of the 2009 season, soil samples taken at 0–0.2 m soil depth. Physical and chemical properties of the soil were as follows: pH, 7.9; organic matter, 16.8 g kg⁻¹; total N, 1.1 g kg⁻¹; available P, 25.9 mg kg⁻¹; available K, 253 mg kg⁻¹.

Experimental design and treatments

The experimental design was a 3 × 2 factorial design with three salinity levels of irrigation water and two N application rates. The irrigation water salinity and nitrogen fertilizer application rate were the same as in the previous experiment in all plots both years. The irrigation water salinities were 0.35, 4.61, or 8.04 dS m⁻¹ (referred to as FW, brackish water (BW) or SW, respectively). The N fertigation rates were either 0 or 360 kg N ha⁻¹ (abbreviated N0 and N360, respectively). The 360 kg N ha⁻¹ rate is commonly used by local farmers. The saline irrigation water was prepared using equal amounts of NaCl and CaCl₂.

The treatments were arranged in completely randomized block design with three replications for each treatment. In each plot, there were two 1.2 m × 16 m, with a 0.6-m separation distance (Figure 1). Four rows of cotton plants were planted in each bed, with a row space of 0.3 m (inner-row) or 0.6 m (inter-row) and the soil surface was covered with plastic film. The irrigation drip line was installed in inner-row under the plastic film. Cotton (Gossypium hirsutum L., cv. Xinluzao 48) was planted on 29 April 2014. The cotton plants were sown at 0.1-m intervals within each row to obtain a population of 22,000 plants ha⁻¹. Water was applied by drip irrigation at a rate of 2.7 L/h per emitter and the drippers were 0.4 m apart. The plots were drip-irrigated with 30 mm FW after sowing to improve germination and seedling establishment. During the cotton growing season, 450 mm water was applied to all treatments through the drip-irrigation system. The plots were irrigated eight times and each irrigation event lasted two days. The irrigation amount and frequency were consistent with the schedule of local farmers used for FW-irrigated cotton fields in the area (Table 1). The plots were all irrigated on the same dates and with equal watering depths. The amount of water applied to each plot was controlled using a water meter. Triple super phosphate (105 kg P₂O₅ ha⁻¹) and potassium sulfate (60 kg K₂O ha⁻¹) were applied at sowing. The N fertilizer was applied with irrigation through the drip-irrigation system. The fertilizer was applied in five equal amounts on 22 June, 30 June, 9 July, 17 July and 2 August (Table 1). The N fertilizer (urea) solution was kept in a 15-L plastic container and pumped into the irrigation system.

Figure 1. Layout of the experimental plot (inner-row and inter-row) and location of the chambers.

Table 1. The amounts of irrigation water, N fertilizer application rates, and dates of N₂O sampling under three water salinity treatments in field experiment (2014).

Date^a	Cotton growth stage	Irrigation amount (mm)	N rate (kg ha^−1)^b		Dates of N2O sampling
			N0	N360
29–30 April	Sowing	30	–	–
15–16 June	Squaring period	45	–	–
22–23 June	Early flowering period	60	–	72	24 June, 25 June, 26 June, 27 June, 28 June, 29 June
30 June–1 July		60	–	72	3 July
9–10 July	Full flowering period	60	–	72	11 July
17–18 July	Boll setting period	60	–	72	19 July
24–25 July		60	–	–	27 July
2–3 August	Peak bolling period	60	–	72	5 August
10–11 August		45	–	–	13 August

^aDates of irrigation and N fertigation under three water salinity treatments in field experiment.

^bN0 and N360 representing 0 and 360 kg ha⁻¹ of N application rates, respectively.

N₂O sampling and analysis

The N₂O gas samples were collected using a static closed chamber technique (Mosier & Hutchinson 1981). The volume of the chambers was 0.11 m³ (0.53 m length × 0.23 m width × 0.90 m height). The chambers were covered with tinfoil to slow the temperature rise and equipped with two internal battery-operated fans to mix the air. The chambers were also equipped with a small, silicone-sealed vent for sampling and another vent for measuring chamber temperature. The chambers were placed synchronously in both inner-row and inter-row for each experimental plot (Figure 1). The average N₂O flux emitted from the inner-row and inter-row was considered as the statistical value for each plot. A stainless steel base was permanently inserted to a depth of 5 cm into the soil before sampling. On each sampling date, a stainless steel sampling chamber was put into the groove in each chamber base, and the groove was sealed with distilled water.

Soil N₂O sampling was conducted at one day intervals in the first six days (24–29 June) after N fertigation (the N fertilizer was applied with irrigation through the drip-irrigation system), and then at eight day intervals in the other periods (30 June–13 August) (Li et al. 2015). The dates of N₂O sampling are shown in Table 1. Gas sampling was carried out from 09:00 to 11:00 every day. All measurements lasted for 30 min, and gas samples were extracted from each chamber 0, 10, 20, and 30 min after capping. About 200 mL of gas was extracted from the chamber with a syringe at each sampling time. The gas samples were then transferred to polyethylene-coated aluminum gas bags (Guangming Research & Design Institute of Chemical Industry, Dalian, China). The N₂O concentration in the gas samples was measured in a laboratory using an HP5890 gas chromatograph (Agilent Technologies, Palo Alto, America) fitted with an electron capture detector. The N₂O emission rate and the cumulative N₂O emission were calculated according to the methods of Guo et al. (2013).

Soil physical and chemical analyses

Soils samples (0–0.2 m depth) were collected with a soil auger on the same dates that we collected N₂O samples. Soil mineral N, soil pH, soil salinity, and soil moisture were measured daily in the first six days after N fertigation (24–29 June), and then at eight day intervals in the other periods (30 June–13 August). The soil moisture (i.e. gravimetric water content) was measured by drying the soil at 105°C for 24 h. The EC of the soil was measured using a conductivity meter with 1:5 soil:water extracts (EC_1:5). Soil pH was measured using a pH meter with 1:2.5 soil:water extracts. Mineral N concentrations were determined by extracting the fresh soil samples for 1 h with 2 M KCl (1:10) and then filtering the extracts through a 0.45 mm membrane. The  concentrations in the extracts were measured using a Smart Chem 140 discreet auto-analyzer (Analytik Jena AG, Jena, Germany).

Data analyses

Statistical analyses were carried out using SPSS statistical software (version 11.5, SPSS Inc, 2003) with two-way analysis of variance (ANOVA) at a significance level of 0.05. Water salinity and N were the independent variables. A Duncan multiple range test was carried out to determine if significant (p < .05) differences occurred between individual treatments.

Results

Soil moisture

Variations in soil moisture content (0–0.2 m depth) in the first six days after N fertigation (24–29 June) are shown in Figure 2. Averaged across the six days, soil moisture contents were significantly lower in the N0 plots (FW, 17.1%; BW, 17.8%; SW, 18.2%) than in the N360 plots (FW, 18.2%; BW, 18.7%; SW, 18.8%). The variability in soil moisture was greater between the N0 and N360 plots (0.6–1.1%) than between the salinity treatments at each fertilization level (1.1% at N0 and 0.6% at N360). There appears to be a compounded effect of N treatment and salinity. Soil moisture content did not increase as much in the N360 plots as in the N0 plots.

Figure 2. Changes in soil moisture during one irrigation cycle from 24 June to 29 June (irrigation and N fertigation on 22–23 June). Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD.

The effects of water salinity and N rate on soil moisture across the whole observation period (i.e. 18 June–13 August) are summarized in Figure 3. In the N0 plots, the average soil moisture content increased from 17.9% in FW to 18.5% in BW and 18.9% in SW. In the N360 plots, the average soil moisture content increased from 18.6% in FW to 18.9% in BW and 19.3% in SW.

Figure 3. Soil moisture content across the growing season (18 June–13 August) as affected by irrigation and N fertigation. Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD. Solid arrows indicate fertigation events. Dashed arrows indicate irrigation events.

Soil salinity

Variations in soil salinity (0–0.2 m salinity) in the first six days after N fertigation (24–29 June) are shown in Figure 4. Averaged across both N treatments, soil salinities were 232% greater in BW and 320% greater in SW than in FW. Averaged across all three water salinities, soil salinity was 10% greater in the N360 plots than in the N0 plots.

Figure 4. Changes in soil salinity (EC) during one irrigation cycle from 24 June to 29 June (irrigation and N fertigation on 22–23 June). Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD.

The effects of water salinity and N rate on soil salinity across the whole observation period (18 June–13 August) are summarized in Figure 5. Soil salinity generally increased with time in both BW and SW. In contrast, there was little change in soil salinity in FW. The average soil salinity was greatest in SW, being 30% higher than that in BW and 465% higher than that in FW. Averaged across all three water salinities, soil salinity was significantly greater by 5.2% in the N360 plots than in the N0 plots (p < .05).

Figure 5. Soil salinity (EC) across the growing season (18 June to 13 August) as affected by irrigation and N fertigation. Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD. Solid arrows indicate fertigation events. Dashed arrows indicate irrigation events.

Soil pH

Soil pH ranged from 7.60 to 8.52 in the first six days (24–29 June) after N fertigation (Figure 6). In general, soil pH was lowest in SW, being 3.0% less than FW and 2.9% less than BW. Averaged across all three water salinities, soil pH was 1.9% lower in the N360 plots than in the N0 plots, but there was no significant difference between the N0 and N360 treatments (p > .05).

Figure 6. Changes in soil pH during one irrigation cycle from 24 June to 29 June (irrigation and N fertigation on 22–23 June). Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD.

Soil pH varied from 7.72 to 8.36 across the entire observation period (Figure 7). In general, soil pH was highest in FW and lowest in SW. The N application rate had no significant effect on soil pH.

Figure 7. Soil pH across the growing season (18 June to 13 August) as affected by irrigation and N fertigation. Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD. Solid arrows indicate fertigation events. Dashed arrows indicate irrigation events.

Soil and

Soil  concentrations in the N0 plots remained stable during the first six days after N fertigation (24–29 June). In contrast,  concentrations in the N360 plots decreased rapidly (Figure 8). Averaged across all three water salinities,  concentrations were 140% greater in the N360 plots than in the N0 plots. Soil  concentrations were also significantly affected by irrigation water salinity in both unfertilized and fertilized plots. Averaged across N0 and N360,  concentrations in FW were 8.9% less than in BW and 19.1% less than in SW.

Figure 8. Changes in soil and  concentrations during one irrigation cycle from 24 June to 29 June (irrigation and N fertigation on 22–23 June). Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD.

The changes in soil concentrations in the first six days after N fertigation (24–29 June) were similar to those described above for , except that soil concentrations in the N360 plots reached a maximum on day 2 of the irrigation cycle and then decreased rapidly with time. Soil concentrations were significantly higher by 192% in the N360 plots than in the N0 plots. Irrigation water salinity also significantly affected concentrations. In the N0 plots, concentrations were 20.3% greater in FW than in SW. There was no significant difference in concentration between BW and FW. In the N360 plots, concentration decreased significantly as irrigation water salinity increased. The concentration in FW was 10.5% higher than in BW and 25.6% higher than in SW.

Variations in soil and concentrations across the growing season are shown in Figure 9. Nitrogen application significantly increased and concentrations. Averaged across all three water salinity treatments, concentrations were 124% greater in the N360 plots than in the N0 plots. Soil concentrations were 209% higher in the N360 plots than in the N0 plots. Irrigation water salinity had a significant effect on both and concentration. In the N0 plots, no significant differences in and concentrations were observed among the three irrigation water salinity treatments. In contrast, concentration in the N360 plots increased significantly in the order FW < BW < SW. Soil concentrations increased in the order BW, SW < FW.

Figure 9. Soil and concentrations across the growing season (18 June to 13 August) as affected by irrigation and N fertigation. Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD. Solid arrows indicate fertigation events. Dashed arrows indicate irrigation events.

N₂O emission

Similar temporal trends in soil N₂O emission rates for the N360 plots and N0 plots were observed for each treatment during the first six days (24–29 June) after N fertigation (Figure 10). For different N application rates in the present study, N₂O emissions on the first day, the first two days, and the first three days in the N360 plots accounted for 19.5–26.3%, 68.1–70.5%, and 80.6–89.7% of total N₂O emissions over the six days, respectively, while the corresponding proportions in the N0 plots were 13.5–23.2%, 40.5–51.7%, and 59.5–70.7%, respectively. Soil N₂O emission rates were also significantly affected by water salinity, N0: SW > BW > FW and N360: BW > FW > SW.

Figure 10. Changes in soil N₂O emission flux during one irrigation cycle from 24 June to 29 June (irrigation and N fertigation on 22–23 June). Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD.

The effects of irrigation water salinity and N rate on soil N₂O emission rates across the growing season are shown in Figure 11. The N₂O emission rates increased significantly as N application rate increased. On average, N₂O emission rates were 169% higher in the N360 plots than in the N0 plots. Irrigation water salinity had significant effects on N₂O emission. In the N0 plots, N₂O emission rates increased as irrigation water salinity increased. In the N360 plots, N₂O emission rates were greatest in BW and least in FW.

Figure 11. Soil N₂O emission flux across the growing season (18 June to 13 August) as affected by irrigation and N fertigation. Abbreviations: FW (0.35 dS m⁻¹), BW (4.61 dS m⁻¹), SW (8.04 dS m⁻¹), N0 (unfertilized), and N360 (360 kg N ha⁻¹). Values are the mean of three replicates. Error bars represent SD. Solid arrows indicate fertigation events. Dashed arrows indicate irrigation events.

Correlation analyses were conducted to determine the relationship between the soil properties and N₂O emission rate (Table 2). Soil salinity were positively correlated with soil moisture, concentration, and concentration, but negatively correlated with soil pH. Soil N₂O emission rate had a significant positive relationship with soil moisture, concentration, and concentration. Soil salinity and pH (7.5–8.5) had no significant relationship with N₂O emission rate.

Table 2. Correlation between soil N₂O emission rate and selected soil properties (n = 72).

	Salinity	Moisture	pH			N2O
Salinity	1
Moisture	0.21	1
pH	−0.61**	0.26	1
	0.23	−0.08	−0.32*	1
	0.04	0.11	−0.26	0.12	1
N2O	0.11	0.31*	−0.23	0.65**	0.62**	1

*Significant at the .05 probability level.

**Significant at the .01 probability level.

Cumulative N₂O emission

The cumulative N₂O emission during the observation period in different treatments ranged from 0.13 to 0.41 kg ha⁻¹ (Figure 12). In general, cumulative N₂O emission increased sharply as the N application rate increased. Averaged across all three water salinities, the cumulative N₂O emission was 173% greater in the N360 plots than in the N0 plots. Cumulative N₂O emission was significantly affected by water salinity, N rate, and salinity × N interaction (p < .001). In the N0 plots, cumulative N₂O emission increased as water salinity increased; however, the differences between BW and SW were not significant. In the N360 plots, the cumulative N₂O emission was highest in BW and lowest in FW.

Figure 12. Effects of irrigation water salinity and N rate on cumulative N₂O emission. Error bars indicate the standard error of the mean (n = 3). Bars with different letters are significantly different at p < .05 (Duncan's Least significant difference test).

Discussion

Soil properties in the first few centimeters nearest the soil surface significantly affect gas emissions by influencing gas production and diffusion (Ball et al. 1997). Among them, salinity is an important factor affecting gas production by influencing soil microbial activity (Tsuneda et al. 2005; Moussa et al. 2006). Many researchers have reported on the interaction between salt concentration and soil microbial activity. Investigations have shown that saline conditions broadly suppress soil microbial communities and their biochemical activities (Rietz & Haynes 2003; Yuan et al. 2007). Decreases in abundance and diversity of nitrifier and denitrifier with increasing salt concentrations are well documented (Yoshie et al. 2004; Bernhard et al. 2005).

Salinity not only affects the activity of the soil microbial activity, but also changes the soil moisture. In our study, soil moisture contents tended to increase as irrigation water salinity increased from 0.35 to 8.04 dS m⁻¹ (Figures 2 and 3). Similarly, Min et al. (2014) observed that soil water content increased as the salinity of the irrigation water increased. This suggests that both brackish and SW irrigation can increase soil moisture in the root zone, mainly because evapotranspiration rates are lower for SW than for FW (Malash et al. 2008). Water salinity and N rate affects soil moisture as well as N₂O emission. One study showed N₂O emission can be significantly affected by small changes in soil moisture (Smith et al. 2003). Increases in soil moisture reduce O₂ concentrations, thus inducing nitrifier denitrification and denitrification (Silva et al. 2008). Our correlation analysis indicated that soil moisture was significantly and positively correlated with N₂O emission rate (Table 2). This is similar to the observation of Wang and Cai (2008), who reported that N₂O production in soil increased slightly with an increase in soil moisture from 40% to 70% of saturated water content. Furthermore, Kiese and Butterbach-Bahl (2002) also demonstrated that N₂O emissions were positively correlated with soil moisture content.

Soil pH affects N₂O emissions by altering N transformations and microbial communities (Bäckman et al. 2003; Nicol et al. 2008). In a recent study about the effect of pH on N₂O emission, Law et al. (2011) observed that the N₂O emission rate of an enriched ammonia-oxidizing bacteria culture was pH dependent. This was most likely due to the effect of pH on the ammonia oxidation rate. This is important for N₂O emissions because ammonia oxidation is the rate-limiting step, providing substrate for denitrification. In this study, soil pH (7.5–8.5) decreased as both irrigation water salinity and N application rate increased (Figures 6 and 7). Our results agree with observations that soil pH decreased as soil salt concentrations increased, mainly due to the accumulation of strong acidic ions (e.g. and Cl⁻) in salinized soil (Fan et al. 2009). However, there was no significant correlation between soil pH and N₂O emission (Table 2). This suggests that N₂O emission is not sensitive to soil pH within the range of 7.5–8.5. This is important because nitrifying bacteria are known to have a narrow optimal range between pH 7.5 and 8.6. A previous study also showed that the optimum pH for long-term denitrification was between 6.6 and 8.3 (Šimek et al. 2002).

Soil mineral N ( and ) is the most important factor influencing N₂O emission. Salinity affects mineral N concentration as well as N₂O emission. Inubushi et al. (1999) reported that at high salt levels and 45% soil water-holding capacity, concentration tended to increase or remain the same across time. In contrast, concentrations tended to decrease. This indicated that nitrification was inhibited at high salt concentration. Total N₂O emission was positively correlated with nitrification rate as calculated by the increase in soil concentration. Our results indicated that soil concentrations increased as irrigation water salinity increased (Figures 8 and 9). This result supports the idea that nitrification is inhibited by high water salinity, allowing to accumulate in soil (Akhtar et al. 2012). Our observations are similar to those of Irshad et al. (2005) who reported significantly greater concentrations in salt-affected soil than in normal soil. In contrast, irrigation water salinity significantly decreased concentrations (Figures 8 and 9). Lodhi et al. (2009) also observed that the amount of decreased as soil salinity increased under different temperature and moisture regimes. One explanation is that organic matter decomposition was inhibited by increasing soil salinity, causing a substantial decline in potentially mineralizable N (Rietz & Haynes 2003). Soil and concentrations were significantly and positively correlated with N₂O emission when soil moisture content was 50–60% of field capacity (Table 2). This suggests that in this study, nitrification and denitrification had equally important roles in N₂O emission. Pihlatie et al. (2004) also observed that nitrification and denitrification made similar contributions to N₂O emission in loamy sand soils with 60–100% water-filled pore space.

Our results also showed that irrigation with brackish and SW can cause salt accumulation in the soil surface, soil salinity increased as irrigation water salinity increased (Figures 4 and 5). This is in agreement with earlier findings that brackish or SW irrigation could increase soil salinity (Ahmed et al. 2012; Min et al. 2014). Soil salinity increased may also lead to the change of N₂O emission. In this study, both N₂O emission flux and cumulative N₂O emission were highest in BW and lowest in FW (Figures 10, 11 and 12). A trend of increased N₂O emission with increasing soil salinity was similar to that observed by Ruiz-Romero et al. (2009) who reported elevated N₂O emission in soil with EC_e 56 dS m⁻¹ compared to 12 dS m⁻¹. In addition, Craft et al. (2009) demonstrated that potential denitrification rates decreased as one moved from tidal freshwater marshes (102 ng g⁻¹ h⁻¹) to salt marshes (21 ng g⁻¹ h⁻¹) in southeastern Georgia. In grassland soil, amendment with 0.2 mol L⁻¹ KCl increased cumulative N₂O emission to 1442 μg N kg⁻¹ compared with 40 μg N kg⁻¹ in an unamended control (Cheng et al. 2013). However, amendment with 0.4 mol L⁻¹ KCl reduced cumulative N₂O emission to 40 μg N kg⁻¹ or less. Salinity affects normal N transformations in soil by retarding several biological or microbial processes responsible for maintaining the NH₄/NO₃ balance (Lodhi et al. 2009). Soil microbes responsible for transforming to are negatively affected by elevated salinity, reducing the conversion of to (Irshad et al. 2005; Kumar et al. 2007). Salt also inhibits nitrification, leading to incomplete nitrification and the accumulation of NO₂ in soil. The accumulation of NO₂ in soil leads to an increase in N₂O production. On the other hand, N₂O reductase (NOS) is very sensitive to the environment. When salinity is high, nitrite toxicity can inhibit NOS activity, resulting in increased N₂O production (Shang et al. 2009).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by the National Natural Science Foundation of China [grant number 31360504] and the Innovative Research Foundation for Excellent Young Scientists of Xinjiang Production and Construction Corps, China [grant number 2014CD002].