Effects of Irrigation Amount and Salinity Levels on Maize (Zea mays L.) Growth, Water Productivity and Carbon Emissions in Arid Region of Northwest China

Abstract

To address the growing scarcity of freshwater resources, the use of saline water for agricultural irrigation is gaining increasing attention. This study presents findings from a two-year field experiment conducted during the 2023 and 2024 maize-growing seasons in northwestern China. The objective of the experiment was to evaluate the main and interactive effects of saline irrigation water on soil water–salt dynamics, maize growth, photosynthetic characteristics, water productivity, and carbon emissions. The experiment involved nine treatments with three irrigation amounts: 4500 m³ hm⁻² (W1), 5625 m³ hm⁻² (W2), and 6750 m³ hm⁻² (W3), combined with three water salinity levels: 0.85 g L⁻¹ (S1, freshwater), 3 g L⁻¹ (S2), and 5 g L⁻¹ (S3). Results indicated that both irrigation amount and salinity level significantly affected water–salt dynamics, with more soil accumulating in the 0–100 cm soil layer under saline irrigation water; however, this effect diminished with higher irrigation amounts. The maximum leaf area index and plant height were affected by both the irrigation amount and salinity level, as well as their interaction. Photosynthetic capacity declined with increasing salinity of the irrigation water, ultimately reducing grain yield and irrigation water use efficiency. Compared to freshwater (S1), the average maize grain yield under S2 and S3 treatments decreased by 6.28% and 15.43% in 2023 and by 7.82% and 17.48% in 2024, respectively. Additionally, for the same irrigation amount, higher salinity of the irrigation water (S2, S3) significantly reduced total soil CO₂ emissions, with reductions of 10.08% and 27.53% in 2023, and 11.97% and 28.01% in 2024, respectively. In summary, to optimize the utilization of saline water, enhance maize yield, and improve soil carbon sequestration, we recommend maintaining the salinity of irrigation water below 3 g L⁻¹, and using an irrigation amount of 6750 m³ hm⁻² (W3S2) for optimal outcomes in the study area.

1. Introduction

The shortage of freshwater resources, exacerbated by climate change, severely constrains the development of irrigated agriculture, placing the global food supply at risk [1,2]. This situation has promoted the agricultural sector to explore more efficient irrigation strategies to alleviate pressure on freshwater resources, including the use of saline or brackish water as an alternative for irrigation. However, this shift also poses risks, such as soil salinization and potential yield loss [3,4,5,6]. Therefore, understanding crop productivity and soil responses to saline irrigation water is crucial for optimizing irrigation regimes and addressing freshwater shortages.

A critical consideration in the adoption of saline water for irrigation is its impact on crop yield [7], which is largely determined by the specific salt tolerance of the crop. Numerous studies have demonstrated that crop yields under saline irrigation water can approach or even match those achieved with freshwater irrigation in certain salt-tolerant crops [8,9,10]. For instance, winter wheat yields were unaffected by saline irrigation water with salinities ranging from 2 to 5 g·L⁻¹ [11,12]. However, saline irrigation water typically leads to significant accumulation of soil salinity, altering the water–salt balance [13] and increasing osmotic potential in the root zone, which hinders water uptake and may reduce crop yields.

Maize (Zea mays L.), recognized as a crop with low salt tolerance [14], is particularly susceptible to saline conditions. Research has indicated that irrigation with a water salinity level of 3 g·L⁻¹ represents the upper limit for maintaining normal transpiration in maize [15]. A study in southern Italy found that maize grain yield decreased with increasing average soil salinity under saline water furrow irrigation [16]. Furthermore, Wang et al. [17] reported that for every 1 g·L⁻¹ increase in the salinity of irrigation water, maize yield decreased by 622 kg·ha⁻¹. Salt stress also disrupts chloroplast structure, reduces the activity of photosynthesis-related enzymes, and impairs leaf growth, collectively lowering photosynthetic rates, reducing leaf area, and ultimately decreasing maize grain yield [18,19,20]. Therefore, understanding the effects of saline irrigation water on grain development is essential for optimizing maize yield and improving irrigation water use efficiency.

As a key component of global carbon cycling, agricultural soils contribute between 19% and 29% of total carbon emissions [21,22]. Saline irrigation water has been shown to influence the carbon cycle by altering the soil environment [23]. However, the impact of salinity on carbon emissions remains variable. On one hand, irrigation with saline water containing high salt ion concentrations can reduce nutrient availability in the soil, negatively affecting the growth and activity of soil microbes [24]. Higher soil salinity generally reduces soil CO₂ emissions by lowering carbon inputs, decreasing soil permeability, and causing toxicity to microbes [25]. For instance, a previous study [25] reported that irrigation with 5 g·L⁻¹ saline water significantly reduced cumulative CO₂ emissions in a spring maize field. On the other hand, elevated salinity may enhance CO₂ emissions by increasing soil aggregate dispersion, promoting organic matter precipitation, and stimulating sulfate-utilizing anaerobic microbes [26]. Reducing carbon emissions while enhancing carbon sequestration in agricultural soils is critical for achieving sustainable agricultural development [27,28,29]. There is typically a synergistic relationship between yield improvements and carbon sequestration in agricultural systems [30]. However, few studies have concurrently examined the effects of saline irrigation water on both grain yield and carbon balance in maize production, which is essential for the sustainable development of agriculture.

The key to using saline water for irrigation lies in matching the appropriate irrigation amount with different water salinity levels to optimize crop yield and water use efficiency. As the world’s second-largest maize producer, China accounts for 23.5% of global maize production and 42.2% of total cereal production [31]. Additionally, China’s underground saline water resources are estimated at approximately 2 × 10¹⁰ m³, with an exploitable volume of 1.3 × 10¹⁰ m³, presenting significant potential for agricultural utilization [32]. Therefore, establishing an optimal irrigation regime with saline water for maize is of great scientific significance for ensuring food production capacity and conserving freshwater resources. The objectives of this study were: (1) to investigate the main and interactive impacts of irrigation amount and water salinity level on soil water–salt dynamics, maize growth and photosynthetic parameters, grain yield, and irrigation water use efficiency; (2) to evaluate the response of soil CO₂ emissions and field carbon balance to varying irrigation amounts and water salinity levels; and (3) to identify an optimal saline irrigation water regime for the comprehensive assessment of maize productivity and field carbon balance.

2. Materials and Methods

2.1. Site Description

Two-year continuous field experiments were carried out at the Key Laboratory of Modern Water-saving Irrigation (85.59° E, 44.19° N, 412 m a.s.l.) in Xinjiang, northwest China. The area experiences a typical arid continental climate, characterized by limited precipitation and year-round dryness. Meteorological data for the experimental area during the maize growing seasons of 2023 and 2024 are presented in Figure 1. During these periods, the average air temperature was 23.56 °C with 54 mm of total precipitation in 2023, and 24.61 °C with 94.8 mm in 2024. The soil texture at the experimental site is classified as sandy loam according to the USDA soil taxonomy, with an average bulk density of 1.53 g cm⁻³ and field capacity of 0.27 cm³ cm⁻³ at a depth of 0–100 cm.

2.2. Experimental Design

A split-plot design arranged in a randomized complete block design was implemented, featuring two factors with three replications. Each factor was set at three levels, resulting in a total of nine treatments. The main plots corresponded to three irrigation amounts, while the sub-plots represented three salinity levels of irrigation water. To examine the effects of irrigation amount and water salinity on maize production and carbon emissions, we applied three irrigation amounts and three salinity levels, based on local conventional practices (5625 m³ hm⁻² for irrigation, W2; 0.85 g L⁻¹ for salinity, S1). Specifically, three irrigation amounts (W1: 4500 m³ hm⁻², W2: 5625 m³ hm⁻², and W3: 6750 m³ hm⁻²) and three salinity levels (S1: 0.85 g L⁻¹, S2: 3 g L⁻¹, and S3: 5 g L⁻¹) (

Table 1. Irrigation amount and salinity of irrigation water treatment design in this study.

Treatment	Irrigation Amount/m3 hm−2	Salinity of Irrigation Water/g L−1
W1S1	4500	0.85
W1S2	4500	3
W1S3	4500	5
W2S1	5625	0.85
W2S2	5625	3
W2S3	5625	5
W3S1	6750	0.85
W3S2	6750	3
W3S3	6750	5

) were considered to investigate main and interactive effects on soil water–salt distribution, maize growth and productivity, and carbon emission characteristics. The salinities of saline irrigation water (S2 and S3) were artificially adjusted by adding industrial-grade salt (NaCl ≥ 99.10%) to the local irrigation water sources. Irrigation amounts were distributed according to the specific growth stages of maize, with the following proportions: seedling stage (SS, one irrigation event) 10%, jointing stage (JS, three irrigation events) 30%, tasseling stage (TS, three irrigation events) 30%, grain-filling stage (GS, two irrigation events) 20%, and maturity stage (MS, one irrigation event) 10% (

Table 2. Observed maize growth stages and irrigation-fertilization regimes during the 2023 and 2024 growing seasons.

Year	Stages	Date	Duration /d	Irrigation and Fertilization Intervals/d	Frequency of Irrigation and Fertilization	Ratio of Irrigation and Fertilization/%
2023	SS	2 May–3 Jun.	32		1	10
	JS	4 Jun.–4 Jul.	30	10	3	30
	TS	5 Jul.–1 Aug.	27	10	3	30
	GS	2 Aug.–24 Aug.	22	10	2	20
	MS	25 Aug.–6 Sep.	12	12	1	10
	WS	2 May–6 Sep.	123		10	100
2024	SS	24 Apr.–26 May	30		1	10
	JS	27 May–27 Jun.	31	10	3	30
	TS	27 Jun.–25 Jul.	27	10	3	30
	GS	26 Jul.–16 Aug.	21	10	2	20
	MS	17 Aug.–30 Aug.	13	12	1	10
	WS	24 Apr.–30 Aug.	126		10	100

SS, seedling stage; JS, jointing stage; TS, tasseling stage; GS, grain-filling stage; MS, mature stage; WS, the whole growth stage.

). Fertilization followed local farming practices, applying 280 kg ha⁻¹ of urea (46% N), 100 kg ha⁻¹ of ammonium dihydrogen phosphate (61% P₂O₅), and 60 kg ha⁻¹ of potassium sulfate (52% K₂O). Fertilizers were mixed with water and applied through a pump aligned with the irrigation schedule, maintaining the consistent fertilization-to-irrigation ratio. The initial irrigation was conducted on the day of sowing, and subsequent irrigation and fertilization practices were systematically scheduled, commencing from the jointing stage (Table 2).

Following local planting schedules, the maize cultivar ‘Fengyu 33’ was sown on 29 April 2023 and 23 April 2024, with harvests occurring on 6 September, 2023 and 30 August 2024, respectively. The theoretical seeding density of the seeder was 82,500 plants/ha under the current planting pattern. Tillage involved mechanical deep plowing, followed by manual sowing at a depth of 3–5 cm. Maize was planted in an alternating wide-narrow row pattern, with wide rows spaced 60 cm apart and narrow rows 30 cm apart [33], and plant spacing set at 20 cm (Figure S1). Mulched drip irrigation was applied using a single-wing labyrinth irrigation system manufactured by Xinjiang Tianye Water-saving Company (Shihezi, China). Two drip tapes were placed at 90 cm intervals under the 1.45 m wide plastic mulch, with each tape irrigating two rows of maize.

2.3. Field Sampling and Laboratory Analysis

2.3.1. Soil Water Content (SWC) and Salt Content (SSC)

Soil samples were collected at depths of 0–100 cm within 48 h before and after each irrigation event, with sampling conducted at 10 cm intervals downwards. Soil water content (SWC, %) was determined using the oven-drying method at 105 °C for over 12 h. Soil salt content (SSC, g kg⁻¹) was calculated from soil electrical conductivity (EC₁:₅, dS m⁻¹), which was measured using a 1:5 soil-to-water extract and analyzed with an electrical conductivity meter (DDSJ-308A, Leici Instrument Factory, Shanghai Precision Scientific Instrument Co., Ltd., Shanghai, China). The extract was subsequently dried and weighed to obtain the SSC value. The EC₁:₅ values were converted to SSC using a calibration equation derived from experimental data (Figure S2). The calculation of SSC for this study is as follows:

$$SSC=1.0366{EC}_{1:5}$$

(1)

2.3.2. Soil CO₂ Emission and Carbon Sequestration

To measure soil carbon emissions in the maize field, a PVC chamber with a diameter of 19.5  cm and a height of 12  cm was inserted into the soil, leaving 6  cm of the chamber exposed above the surface to ensure airtightness. The vegetation and litter inside the PVC ring were carefully removed to minimize the potential impact from aboveground respiration and plant litter. After the field device stabilized, the Li-8100A automatic soil carbon flux analyzer (LI-COR Ltd., Lincoln, NE, USA) was used to detect soil carbon flux at each sampling point, with sampling intervals of approximately 10 days during the growing stages of maize. The CO₂ emission rate (R_s, mg m⁻² h⁻¹) was calculated using the following formula [23]:

$$R_s=0.001MA{R}_{si}$$

(2)

where R_s and R_si represent the CO₂ emission rate (mg m⁻² h⁻¹) and measured initial instrumental CO₂ emission flux (μmol m⁻² s⁻¹); M is the CO₂ molecule molar mass (g mol⁻¹); and A is the conversion coefficient for converting hours into seconds (3600). The cumulative soil CO₂ emissions (t hm⁻²) were calculated based on the CO₂ emission fluxes every two adjacent times. The formula for this is presented in Equation (3) [34]:

$$S_{CO2}={\sum }_i^n\left({\displaystyle \frac{R_{si}+R_{si+1}}{2}}\right)\times {\displaystyle \raisebox{1ex}{$\left(t_{i+1}-t_i\right)$}\!\left/ \!\raisebox{-1ex}{$1000000$}\right.}$$

(3)

where R_si and R_si+1 are the CO₂ emission fluxes at the sampling times i and i + 1, respectively, and (t_i+1 − t_i) is the hourly interval (h) between sampling times i and i + 1.

The net carbon sequestration (NCS) in maize fields during the growing season was calculated as [34]:

$$NCS=MIP-S_{CO2}$$

(4)

where MIP is the carbon initial productivity (kg C hm⁻²) of the maize field, accounting for approximately 45% of total maize biomass at harvest [35], and S_CO2 represents the total soil carbon emissions (kg C hm⁻²) during the maize growing season.

2.3.3. Plant Height and Leaf Area Index (LAI)

At the end of each growing stage, three representative maize plants that exhibited consistent development were randomly selected from each plot for investigation. Plant height and leaf area were determined using a steel ruler. Leaf length (L, measured from ligule to leaf tip) and width (W, measured at the widest portion of the leaf blade) were used to compute the single leaf area (LA) via the calculating formula LA = 0.75 × W × L [36]. LAI was then calculated as the ratio of total leaf area to the total land area.

2.3.4. Photosynthetic Parameters

Typical sunny days during the jointing stages (JS), tasseling stages (TS), and grain-filling stages (GS) were chosen to measure the leaf photosynthetic parameters, including net photosynthetic rate (Pₙ), transpiration rate (Tᵣ), stomatal conductance (gₛ), and intercellular CO₂ concentration (Cᵢ). These parameters were assessed via a portable photosynthetic measurement system (LI-6800, Inc., Lincoln, NE, USA) between 9:00 and 11:00 a.m.

2.3.5. Grain Yield, Total Biomass, and Irrigation Water Use Efficiency (I-WUE)

Upon maize maturity, three random sampling points were selected from each treatment. At each point, three maize plants were harvested and weighed to obtain the total biomass and grain yield [37]. Specifically, the entire maize plants were carefully uprooted, washed, and thoroughly dried to measure total biomass. For grain yield determination, maize ears from each treatment were naturally air-dried, threshed, and weighed. Irrigation water use efficiency (I-WUE) was calculated by dividing the grain yield by the total amount of irrigation water applied.

2.4. Statistical Analysis

To assess the main and interactive effects of irrigation amount and water salinity on maize growth parameters, water productivity, and carbon sequestration, a two-way analysis of variance (ANOVA) was performed using the “agricolae” package in R software (version 4.3.2). Differences between the treatments were evaluated using the least significant difference (LSD) test at a significance level of p < 0.05. Figures were generated using Origin 2025 (OriginLab, Northampton, MA, USA).

3. Results

3.1. Variations in SWC

The variations in SWC distribution under different irrigation amounts and salinity levels during maize growing seasons in 2023 and 2024 are illustrated in Figure 2. The differences in SWC among treatments became significant as the maize grew. Generally, the distribution of SWC was primarily influenced by both the irrigation amount and the salinity level of the irrigation water. The results indicated that, at the same salinity level of irrigation water, the maximum average SWC at a depth of 0–100 cm was observed in W3, followed by W2 and W1. Furthermore, the average SWC increased with the salinity level. Throughout the two-year observation, the W3S3 treatment recorded the highest overall SWC during the entire growth stage. At the mature stage in 2023 and 2024, the average SWC for W1 was 20.73% and 43.21% lower than that of W2, and 20.64% and 43.63% lower than that of W3, respectively. Additionally, the average SWC for S1 was 7.79% and 14.67% lower than that for S2, and 7.56% and 15.13% lower than that for S3, respectively.

3.2. Variations in SSC

Figure 3 illustrates the distribution of SSC under different irrigation treatments during the maize-growing seasons in 2023 and 2024. Similar distribution characteristics in SSC were generally observed from the seedling stage to the mature stage for most treatments in both seasons. Overall, the SSC was closely related to the salinity level of the irrigation water used. At the same irrigation amount, S3 exhibited the highest average SSC across the entire soil profile, followed by S2 and S1. At the seeding stage, the impact of salinity of irrigation water on SSC distribution were generally observed at the 0–40 cm depth. As the maize plants grew, this difference became evident throughout the entire 0–100 cm soil depth. At the mature stage in 2023 and 2024, the average SSC for S3 was 66.21% and 157.44% higher than that for S2, and 53.19% and 136.33% higher than that for S1, respectively. In contrast, the average SSC for W1 was 10.59% and 8.14% lower than that for W2, and 8.75% and 4.91% lower than that for W3, respectively.

The salt accumulation characteristics of different treatments were calculated based on the average SSC during the maturity and sowing periods. By comparing changes in SSC across different soil layers during the mature and seeding stages in 2023 and 2024 (

Table 3. The changes in average SSC (g kg⁻¹) in 0–40 cm and 40–100 cm soil depths at the mature stage and seedling stage under different treatments for 2023 and 2024.

Treatment	2023			2024
	0–40 cm	40–100 cm	0–100 cm	0–40 cm	40–100 cm	0–100 cm
W1S1	0.13c	0.07b	0.09c	0.12b	0.09b	0.10b
W1S2	0.21b	0.12b	0.16b	0.22b	0.11b	0.15b
W1S3	0.46a	0.34a	0.38a	0.47a	0.33a	0.39a
W2S1	0.16c	0.10c	0.13c	0.17b	0.12c	0.14c
W2S2	0.27b	0.18b	0.22b	0.22b	0.19b	0.20b
W2S3	0.53a	0.49a	0.50a	0.52a	0.51a	0.52a
W3S1	0.06c	0.05c	0.06c	0.06b	0.03c	0.04c
W3S2	0.22b	0.14b	0.17b	0.19b	0.15b	0.17b
W3S3	0.47a	0.56a	0.53a	0.46a	0.54a	0.51a
TWO-ANOVA
W	***	***	***	ns	***	***
S	***	***	***	***	***	***
W×S	ns	***	***	ns	***	***

Different lowercase letters indicate significant differences (p < 0.05) among different salinity of irrigation water under the irrigation amount. “***” indicate significant differences at the level of p < 0.001, ns indicates non-significance.

), we found that salt accumulated throughout the entire 0–100 cm soil profile for all treatments. Overall, the W3S3 treatment recorded the highest accumulation of salt, while the lowest was observed in the W3S1 treatment. In both years, salt accumulation in the 0–40 cm soil layer was mainly affected by the salinity of irrigation water, whereas salt accumulation in the deeper soil layer (40–100 cm) was affected by both the irrigation amount and salinity of irrigation water, as well as their interaction. At a constant irrigation amount, SSC increased with higher salinity levels of irrigation water. At relatively low irrigation amounts (W1 and W2), SSC tended to accumulate more in the 0–40 cm soil layer. However, as irrigation amount increased, SSC showed a tendency to accumulate in deeper soil layers.

3.3. Dynamic of Maize Growth

The response of maize growth indices to the salinity of irrigation water exhibited a similar pattern in both 2023 and 2024 (Figure 4). As the growth period progressed, plant height (Figure 4a,b) gradually increased, reaching its maximum at the mature stage. The leaf area index (LAI) under different treatments showed a rapid increase during the jointing and tasseling stages and peaked at the grain-filling stage before subsequently decreasing (Figure 4c,d). At the seedling stage, no significant differences were found in plant height and LAI under salinity gradients of irrigation water. However, as the growth period extended, both plant height and LAI declined with the increasing salinity of irrigation water. As illustrated in Figure S3, both irrigation amount and salinity of irrigation water, as well as their interaction, had a substantial effect on the maximum plant height and LAI during the growing seasons. Compared to S1, the maximum plant height and LAI for S2 and S3 were 11.98% and 17.07%, and 11.17% and 13.48% lower in 2023, respectively. In 2024, the reductions were 11.96% and 16.61% (plant height), and 11.77% and 14.12% (LAI), respectively.

3.4. Photosynthetic Characteristics

The photosynthetic characteristics, including net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (g_s), and intercellular CO₂ concentration (C_i), were measured at the jointing, tasseling, and grain-filling stages in 2023 and 2024 (Figure 5). Both irrigation amount and water salinity had significant impacts on these characteristics across both growing seasons. However, their interaction was inconsistent throughout these three stages over the two years. Generally, photosynthetic capacity increased with irrigation amount at the same salinity level, while it decreased with rising salinity of irrigation water at a constant irrigation amount. Consequently, the highest values for P_n, T_r, g_s, and C_i were observed in the W3S1 treatment, followed by the W3S2 treatment. As the growth period progressed, P_n, T_r, and g_s initially increased and then decreased, with their maximum values recorded at the tasseling stage. During this stage, the P_n at the S1 level was 7.92% and 23.05% higher than that at S2 and S3 in 2023, respectively, and 9.31% and 22.79% higher than S2 and S3 in 2024, respectively.

3.5. Soil CO₂ Emissions

Throughout the maize growth period, the soil CO₂ emission rate exhibited a trend of initially increasing and then decreasing in both 2023 and 2024 (Figure 6). With the extension of maize growth period, the maximum soil CO₂ emission rates were observed 80 days after planting (tasseling stage) (Figure 6a,b). Under different treatments, the maximum soil CO₂ emission rates ranged from 1218.10 mg m⁻² h⁻¹ to 2212.85 mg m⁻² h⁻¹ in 2023 and from 1293.65 mg m⁻² h⁻¹ to 2280.5 mg m⁻² h⁻¹ in 2024. Cumulative soil CO₂ emissions increased with maize growing days (Figure 6c, d), with the W1S3 treatment demonstrating the least total CO₂ emissions and the W3S1 treatment showing the most during the two-year investigation. As can be observed in Figure S4, total soil CO₂ emissions were found to be significantly influenced by irrigation amount and the salinity of irrigation water, and their interaction. Under the same irrigation amount, total CO₂ emissions declined with the increasing salinity of irrigation water. Compared to S1 of salinity, the total soil CO₂ emissions in S2 and S3 treatments were 10.08% and 27.53% lower compared with S1 in 2023, respectively; and 11.97% and 28.01% lower in 2024, respectively. Moreover, compared to W1, total soil CO₂ emissions in W2 and W3 treatments were 17.83% and 11.70% higher in 2023, respectively; and 13.73% and 7.70% higher in 2024, respectively.

3.6. Grain Yield, Irrigation Water Productivity, and Field Carbon Balance

Both irrigation amounts and the salinity of irrigation water significantly influenced grain yield and total biomass (p < 0.001). However, their interaction did not have a significant effect on grain yield. Throughout the two-year experiment, grain yield and total biomass exhibited an increasing trend with higher irrigation amounts, while they decreased with rising salinity of irrigation water (

Table 4. Grain yield (GY), total biomass (TB), harvest index (HI), irrigation water use efficiency (I-WUE), maize initial productivity (MIP), soil total CO₂ emission (S_CO2), and net carbon sequestration (NCS) under different treatments for 2023 and 2024.

Year	Treatment	GY (kg ha−1)	TB (kg ha−1)	HI	I-WUE (kg m−3)	MIP (kg C ha−1)	S-CO2 (kg C ha−1)	NCS (kg C ha−1)
2023	W1S1	14,252.76a	39,080.88a	0.36a	3.17a	17,586.39a	7773.20a	9813.19a
	W1S2	12,985.13ab	35,070.25b	0.37a	2.89ab	15,781.61b	7100.52b	8681.09c
	W1S3	11,294.95b	32,451.62c	0.35a	2.51b	14,603.23c	5281.67c	9321.56b
	W2S1	14,772.47a	42,405.96a	0.35a	2.63a	19,082.68a	8577.34a	10505.34a
	W2S2	13,966.78ab	39,575.67b	0.35a	2.48ab	17,809.05b	8037.2b	9771.85a
	W2S3	13,049.31b	38,323.33b	0.34a	2.32b	17,245.5b	6864.03c	10,381.47a
	W3S1	16,999.3a	45,340.05a	0.37c	2.52a	20,403.02a	8832.19a	11,570.84a
	W3S2	16,235.47a	41,177.92b	0.39b	2.41a	18,530.06b	7482.38b	11,047.68a
	W3S3	14,642.02b	34,640.83c	0.42a	2.17b	15,588.37c	6132.04c	9456.33b
	TWO-ANOVA
	W	***	***	***	***	***	***	***
	S	***	***	ns	***	***	***	**
	W×S	ns	***	*	ns	***	***	**
2024	W1S1	14,829.2a	40,639.01a	0.37a	3.3a	18,287.55a	7852.15a	10,435.4a
	W1S2	12,485.74b	33,774.33b	0.37a	2.77b	15,198.45b	7058.82b	8139.63c
	W1S3	11,485.93b	32,784.34b	0.35a	2.55b	14,752.95b	5189.90c	9563.06b
	W2S1	14,016.93a	40,348.39a	0.35a	2.49a	18,156.77a	8474.22a	9682.55a
	W2S2	13,654.88a	39,559.65a	0.35a	2.43a	17,801.84a	8026.83b	9775.01a
	W2S3	11,831.65b	34,956.48b	0.34a	2.1b	15,730.42b	6857.76c	8872.66a
	W3S1	17,428.38a	46,207.25a	0.38a	2.58a	20,793.26a	8616.53a	12,176.73a
	W3S2	16,542.77a	42,670.97b	0.39a	2.45a	19,201.94b	7366.48b	11,835.46a
	W3S3	14,937.05b	36,622.99c	0.41a	2.21b	16,480.35c	6129.81c	10,350.53b
	TWO-ANOVA
	W	***	***	***	***	***	***	***
	S	***	***	ns	***	***	***	***
	W×S	ns	***	ns	ns	***	***	***

Different lowercase letters indicate significant differences (p < 0.05) among different salinities of irrigation water under the irrigation amount. “*”, “**”, and “***” indicate significant differences at the level of p < 0.05, p < 0.01, and p < 0.001, respectively; ns indicates non-significance.

). Among the various irrigation amounts, regardless of salinity levels, W3 recorded the highest harvest index and net carbon sequestration, followed by W2 and W1. Irrigation water use efficiency (I-WUE) also significantly differed based on irrigation amount and salinity levels; however, their interaction did not show a significant effect. Notably, as both irrigation amount and water salinity increased, I-WUE exhibited a significant decline. Consequently, the maximum and minimum I-WUE were observed in the W1S2 and W3S3 treatments, respectively, in both growing seasons.

4. Discussion

4.1. Effects of Irrigation Amount and Water Salinity on Maize Productivity

Water is essential for plant growth, and appropriate irrigation can minimize water and nutrient loss, prevent soil salinity accumulation, and create optimal conditions for crop development. When formulating irrigation regimes for crops using saline water, it is crucial to consider the effects of irrigation water amount and salinity on soil environment and crop growth. In this study, varying degrees of salt accumulation were found under different treatments, each providing distinct water–salt environments to maize root. Salt accumulation in deeper soil layers (40–100 cm) was affected by both irrigation amount and water salinity, as well as their interaction (Table 3). On the one hand, the leaching effect of water on salt means that varying irrigation amounts can lead to salt accumulation at different layers [38]. On the other hand, differing salinities of irrigation water result in varying levels of salt content being introduced into the soil. For instance, when both irrigation amount and water salinity are high, the enhanced leaching effect can lead to increased salt carried by the water, maintaining high soil salinity level throughout the growth period. Traditionally, the application of saline irrigation water relies on providing excess water to leach salts from the root zone, thus minimizing yield loss [16]. However, increasing the use of saline water can also result in greater salt residues due to infiltration [39], reducing the benefits of saline irrigation water. In this study, the saline irrigation water used was made up with fresh water (0.85 g L⁻¹) and industrial-grade salt (NaCl ≥ 99.10%), with sodium accumulation potentially causing ion toxicity to maize plants. Therefore, when the irrigation amount is low but salinity is high, water stress and salt toxicity in the maize root zone can significantly inhibit the water absorption of crops, adversely affecting maize physiological growth and yield formation [13,40,41,42].

Previous studies have indicated that maize is a low salinity tolerance crop, with the optimal salinity threshold of irrigation water generally being below 3 g L⁻¹ [15,39]. Irrigation with saline water significantly increases salt accumulation in the soil, leading to many negative effects on maize growth. Our results showed that the morphological and productive parameters of maize under saline irrigation water treatments (S2 and S3) were remarkably lower compared to those under fresh irrigation water treatments (S1), particularly at low irrigation amounts (W1). In both years, significant interactions between irrigation amount and water salinity were observed on maximum LAI and plant height. This can be attributed to the persistent salinity stress throughout the growing period, which limited maize growth. However, we did not observe significant irrigation amount and water salinity interactive effects on leaf photosynthetic capacity across all three growth stages (Figure 5), which aligns with previous findings [43]. These discrepancies may arise from our treatments not reaching the threshold necessary for such interactions to occur. Prior research has shown that leaf photosynthesis is less likely to recover following combined water and salt stress [29,44], especially during the grain-filling stage. Water and salt stress significantly reduce the net photosynthetic rate in maize, primarily by decreasing stomatal conductance, which results from diminished leaf water potential [45]. Additionally, leaf area expansion slows under insufficient water and high salinity, leading to reduced solar radiation interception and a shorter duration of photosynthesis and grain filling [46,47]. Notably, reductions in these parameters may be a key reason for the observed decline in grain yield and total biomass under saline irrigation water. The accumulation of dry matter is predominantly influenced by leaf photosynthetic capacity and leaf area, both of which are highly sensitive to water and salt stress [48,49].

Irrigation water use efficiency (I-WUE) serves as a key metric for evaluating the applicability and productivity of saline irrigation water. Numerous studies have demonstrated that saline irrigation water (>3 g L⁻¹) can significantly reduce crop yield and water productivity [15,50]. Our study revealed that I-WUE decreased when grain yield approached its maximum, significantly affected by both irrigation amount and water salinity. When the irrigation amount is consistent, high soil salinity from saline irrigation water increases osmotic pressure in the root zone, impairing the efficient absorption and utilization of water and fertilizers, ultimately reducing crop yield and I-WUE [51,52]. Notably, I-WUE decreased with irrigation amount, regardless of salinity level, indicating that excess water was utilized for vegetative growth instead of reproductive growth. Additionally, water or salt stress disrupts the relationship between grain yield and water use, affecting how photosynthetic assimilation is allocated to reproductive organs [53,54]. Our results indicated that the net photosynthetic rate and stomatal conductance of leaves significantly declined under high salinity of irrigation water (5 g L⁻¹). Importantly, the reduction in grain yield under saline irrigation water observed was partially mitigated by higher irrigation amounts (Table 4), suggesting that increased irrigation could alleviate the negative impacts of salt stress on final yield. However, the long-term impact of salt accumulation from saline irrigation water on maize growth and yield cannot be overlooked.

4.2. Effect of Irrigation Amount and Water Salinity on CO₂ Emissions

Soil CO₂ emissions are primarily produced through soil respiration, which involves both autotrophic and heterotrophic activity [55]. Seasonal variations in soil CO₂ flux are closely linked to soil environmental conditions and rhizosphere respiration [33]. As maize grows and air temperature rises, soil transpiration and soil microbe activity increase, leading to higher CO₂ production [32]. Furthermore, we observed a significant interactive effect of irrigation amount and water salinity on total soil CO₂ emissions, which are expected to be closely related to the water–salt dynamics during the maize growth period. Soil water is a crucial factor influencing CO₂ emissions, as it alters soil aeration conditions and microbial activity [56,57]. In our study, total CO₂ emissions increased with higher irrigation amounts, indicating that soil water content across all irrigation levels remained within an optimal threshold. As soil water content rises, the contact between the soil soluble matrix and microorganisms also increases, enhancing soil enzyme and microbial activity, which in turn promotes soil CO₂ emissions [58]. Conversely, total soil CO₂ emissions significantly declined with increasing water salinity levels under the same irrigation amount, particularly in the low irrigation amount treatment (W1). This decline can be attributed to the persistent salinity stress during the entire maize growing period. Previous research has shown that salt accumulation slows the decomposition of crop residues and organic matter, leading to declined soil organic carbon levels as salinity increases, which subsequently diminishes the microbial reaction matrix [59,60]. Moreover, the ionic toxicity and osmotic potential imbalances caused by elevated sodium concentrations can inhibit the activity of soil heterotrophic microorganisms, thereby reducing soil CO₂ emissions [25,61,62].

4.3. Limitations

In this two-year field study, we analyzed the impacts of saline irrigation water on maize growth, photosynthesis parameters, water productivity, and carbon sequestration capacity to better evaluate the application prospects of saline irrigation water in maize production in arid regions. However, there are still some limitations to this work: (i) limited attention was given to the response of soil physiochemical properties under saline irrigation water; (ii) the potential benefits of growth stage-based deficit irrigation combined with saline water should be further explored, as this approach may stabilize yields and improve WUE; and (iii) long-term continuous investigations on saline irrigation water on soil environment and crop productivity are required in the future.

5. Conclusions

This two-year study revealed significant main and interactive effects of irrigation amount and water salinity on soil water–salt dynamics, maize growth and photosynthesis parameters, water productivity, and carbon sequestration capacity. Throughout the maize growing period, soil salt accumulation increased with higher salinity of irrigation water, and a significant interactive effect of water and salinity on salt accumulation in 40–100 cm layers was observed. Compared to fresh irrigation water, the physiological and morphological parameters of maize significantly decreased with increasing salinity when using saline irrigation water. Consequently, a reduction in grain yield under higher water salinity was expected. However, the yield reduction could be mitigated by increasing the irrigation amount, despite a decline in irrigation use efficiency. Although saline irrigation water significantly reduced the peak flux and total emissions of soil CO₂ compared to freshwater irrigation, it did not promote net carbon sequestration due to its negative impact on grain yield and the soil environment at relatively low irrigation levels. Therefore, in areas where both yield and carbon emission benefits need to be considered, sufficient water supply is still necessary when using saline irrigation water.