The experiment was conducted at the Irrigation Experimental Station (37°23′N, 104°08′E) of the Jingtaichuan Electric Power Irrigation Water Resource Utilization Center in Gansu Province from July to September 2022. This region has a temperate continental arid climate characterized by intense sunshine, infrequent rainfall, and a dry climate. The annual average sunshine duration, frost-free period, radiation amount, temperature, precipitation, and evaporation are 2652 hours, 191 days, 6.18 ×10⁵ J·cm⁻², 8.6°C, 201.6 mm, and 3028 mm, respectively. The soil texture at the experimental site was loam, and the dry bulk density of the soil was 1.63 g·cm⁻³. The field water capacity was 24.1% (mass water content), and the pH was 8.11. Groundwater depth was > 40 m. The initial soil properties of the study site were as follows: total nitrogen 1.62 g·kg⁻¹, total phosphorus 1.32 g·kg⁻¹, total potassium 34.03 g·kg⁻¹, available nitrogen 74.51 mg·kg⁻¹, available phosphorus 26.31 mg·kg⁻¹, available potassium 173 mg·kg⁻¹, and alkali-hydrolyzed nitrogen 55.2 mg·kg⁻¹ in the 0–60 cm soil layer. Meteorological data were collected by a small intelligent agrometeorological station installed at the experimental station. The total amount of precipitation, daily maximum temperature and daily minimum temperature during the experiment were 77.01 mm, 35.07°C and 8.71°C, respectively ( Figure 1 ).

The selected wolfberry (Ningqi No.5) was a two-year-old seedling transplanted on 12 April 2021, with a plant spacing of 1.5 m and row spacing of 3.0 m. Based on local production practices and previous studies (Tian et al., 2023), the experiment utilized a completely randomized block design, with irrigation and nitrogen application levels as two factors. Among them, the irrigation levels [the upper and lower limits of irrigation were set to control the percentage of soil volumetric moisture content to field water capacity (θ_f), and the planned depth of humid layer was 60 cm] included 75%–85% θ_f (W0, full irrigation), 65%–75% θ_f (W1, slight water deficit), 55%–65% θ_f (W2, moderate water deficit) and 45%–55% θ_f (W3, severe water deficit). The nitrogen application (pure nitrogen) levels included 0 kg·ha⁻¹ (N0, no nitrogen application), 150 kg·ha⁻¹ (N1, low nitrogen application), 300 kg·ha⁻¹ (N2, medium nitrogen application), and 450 kg·ha⁻¹ (N3, high nitrogen application) ( Table 1 ). Thus, there were 16 treatments in total, with each treatment repeated three times. The residential area measures 76.5 m² (10.2 m × 7.5 m). Drip irrigation was then applied. Valves and water meters (with an accuracy of 0.0001 m³) were independently installed in the water-delivery pipes of each district to regulate the amount of irrigation effectively. The spacing of the drip irrigation belt layout was 0.3 m, the designed flow rate of the drip head was 2.0 L·h⁻¹, and the spacing of the drip head was 0.3 m. The irrigation process during the wolfberry growth is illustrated in Figure 2 . The main growth period of wolfberry in 2022 was divided into four stages: the vegetative growth period (26 April to 28 May), the full flowering period (29 May to 30 June), the full fruit period (1 July to 14 August) and the autumn fruit period (15 August to 10 September). In growing season, nitrogen fertilizer (urea and nitrogen content 46%) according to 6:2:2 was applied during the vegetative growth period (21 May), the full flowering period (7 June), and the full fruit period (4 July). Phosphate (superphosphate, with a phosphorus content of 12%) and potassium (potassium chloride, with a potassium content of 60%) at a rate of 130 kg·ha⁻¹ were applied as the base fertilizer in a single application during the vegetative growth period on 21 May. Field management includes pest control and other measures consistent with those of local growers.

Microsoft Excel 2010 was used for data organization, Equations (1–20) were used to calculate the relevant indicators, and the CRITIC-entropy weights-TOPSIS model was used for comprehensive evaluation. IBM SPSS Statistics software (version 25.0) was used for statistical analysis. One-way ANOVA and the Duncan method were used for variance analysis and multiple comparison of indicators in different treatments (P < 0.05). Two-way ANOVA was performed to examine the effects of water and nitrogen, as well as their interactions, on soil NO₃ ⁻–N accumulation, nitrogen uptake by wolfberry plants, and soil N₂O emission characteristics (P < 0.05). The drawing was created using the Origin 2021 software.

The total nitrogen content and total nitrogen uptake of wolfberry plants, soil NO₃ ⁻–N accumulation, soil temperature, water-filled pore water content of the soil, N₂O emission flux, and other indicators were consistently analyzed ( Table 3 ). The weights of the wolfberry indices based on the CRITIC-entropy weight method were as follows: F > N_ua > NR_a > N_ia > f > GHGI > EF ( Table 4 ). According to the comprehensive evaluation results of the CRITIC-entropy weights-TOPSIS model ( Figure 9 ), the W1N2 treatment ranking was the best, followed by the W3N2 treatment. These results indicate that when a mild water deficit (W1, 65%–75% θ_f) was coupled with medium nitrogen application (N2, 300 kg·ha⁻¹), the nitrogen uptake of wolfberry plants was higher, and the accumulation of NO₃ ⁻–N and N₂O emission in the soil was lower.

Soil NO₃ ⁻–N is an important component of soil soluble nitrogen and serves as the primary mineral nitrogen source directly absorbed by crops. It is commonly used to assess the nitrogen supply capacity of the soil (Cui et al., 2013). This study revealed that the soil NO₃ ⁻–N content in the 0–100 cm soil layer of each treatment decreased during the harvest period initially decreased, then increased, and finally decreased with increasing soil depth. Additionally, it decreased with increasing horizontal distance from the wolfberry plants. The soil NO₃ ⁻–N content at 30 cm and 60 cm from the wolfberry followed the order of W0 > W2 > W1 and W3. This can be attributed to two factors: Firstly, the negative charge of the NO₃ ⁻ ions causes repulsion with the negatively charged soil colloidal particles, leading to the downward movement of NO₃ ⁻–N through leaching and infiltration, thereby affecting the vertical redistribution of NO₃ ⁻–N. Secondly, water migration and root absorption affect the vertical and horizontal redistribution process of NO₃ ⁻–N, resulting in intensified NO₃ ⁻–N leaching near the wet body in the drip irrigation zone and the phenomenon of “enrichment” near the roots (Vijayalakshmi et al., 2013). Furthermore, this study revealed a positive correlation between soil NO₃ ⁻–N content and the amount of nitrogen applied. The NO₃ ⁻–N content increased with the amount of irrigation and nitrogen application, and the peak value of NO₃ ⁻–N increased and gradually moved downward. This finding was similar to that reported by Gu et al. (2018) for winter oilseed rape in the Guanzhong area of Shaanxi Province. This result indicated that urea applied to the soil through irrigation rapidly hydrolyzes into NH₄ ⁺–N and is oxidized to NO₃ ⁻–N under the action of ammonia oxidizing microorganisms. However, NO₃ ⁻–N is highly soluble in water and tends to accumulate at a specific depth within soil pores (Wu et al., 2022b).

Crops absorb a portion of NO₃ ⁻–N during their growth, whereas unabsorbed NO₃ ⁻–N accumulates in the soil, thereby increasing the potential risk of groundwater pollution. In this study, the accumulation of soil NO₃ ⁻–N in the 0–100 cm soil layer decreased as the horizontal distance increased under different water and nitrogen treatments (except for the N0 nitrogen application level). The average soil NO₃ ⁻–N accumulation in the 0–100 cm soil layer at each horizontal distance followed the order of W0 > W1 > W2 > W3 (except for the N2 nitrogen application level), and N3 > N2 > N1 > N0. Both water and nitrogen affected the accumulation of NO₃ ⁻–N in the soil, with nitrogen application having a greater influence on soil NO₃ ⁻–N accumulation than the irrigation amount ( Table 3 ). This contradicts the findings of Wang et al. (2008) in Shandong Province, which indicated that both irrigation and nitrogen application significantly affected NO₃ ⁻–N accumulation in the 0–100 cm soil layer of wheat, and the contribution of irrigation to NO₃ ⁻–N accumulation was greater than that of nitrogen application. This may be attributed to the more developed root system of wolfberry than that of wheat. Additionally, the soil resource endowment characteristics in the Yellow River irrigation region of Gansu Province may not fully support the growth and development of wolfberry, leading to the increased reliance on exogenous nutrients to maintain normal physiological growth activities of the plants (Dou et al., 2021). At the same time, this study concluded that the single factor of water and nitrogen significantly affected the accumulation of soil NO₃ ⁻–N. The interaction effect of the two factors only had a significant impact on NO₃ ⁻–N accumulation in the soil 30 cm away from the horizontal distance of the wolfberry ( Table 2 ).This may be related to the root distribution of wolfberry plants, and multiple irrigations may have caused downward leaching of nitrogen accumulation. Nitrogen accumulation tends to decrease away from the drip irrigation belt, making wolfberry roots closer to the belt more sensitive to water and nitrogen responses (Iranzi et al., 2023).

A reasonable water and nitrogen supply pattern is beneficial for increasing nitrogen uptake by plants (Dai and Zhang, 2020). The results of this study revealed that the total nitrogen content of each organ of the wolfberry at the time of harvest followed the order leaves > roots and stems > fruits. The highest values were observed under the W1N2 treatment, with the nitrogen content reaching 3.25% in the roots, 3.30% in the stems, 3.91% in the leaves, and 2.42% in the fruits. This suggests that the appropriate water-nitrogen combination has a synergistic effect on nitrogen allocation and uptake in crops (Qin et al., 2021). In addition, this study found that the total nitrogen content of wolfberry leaves and fruits initially increased and then decreased with increasing irrigation and nitrogen application. This contradicts the findings of Zhen and Zhen (2006) and Qin et al. (2017) who concluded that the nitrogen content in wheat leaves and fruits is regulated by exogenous nitrogen and increases with increasing nitrogen application. The difference in absorption capacity between wolfberry and wheat could be attributed to the more developed roots and stronger absorption capacity of wolfberry. Additionally, nitrogen application in this experiment reached the absorption threshold of wolfberry (medium nitrogen application of 300 kg·ha⁻¹). With an increasing in nitrogen application (high nitrogen application of 450 kg·ha⁻¹), the antagonism between the absorbed elements and ions of wolfberry plants was enhanced. This leads to extravagant nitrogen absorption, by the plants, ultimately resulting in a decrease in the nitrogen content of crops (Filipović et al., 2016).

Irrigation and nitrogen application can affect the accumulation of plant nutrients by improving soil water and fertilizer conditions (Liu et al., 2007). In this study, it was concluded that the nitrogen uptake of wolfberry showed the following trend: N2 > N3 > N1 > N0. Specifically, N2 showed significant increases of 85.32%, 26.80%, and 15.39% compared with N0, N1, and N3, respectively. These finding are consistent with those of a study on cotton in the Xinjiang region conducted by Zhang et al. (2021). This may be due to the excessive application of nitrogen, which can lead to an imbalance in plant nutrient uptake, reduce plant nitrogen uptake and accumulation, and cause loss of nitrogen resources. However, appropriate nitrogen application can increase the levels of inorganic nitrogen, such as NO₃ ⁻–N and NH₄ ⁺–N in the soil of the root zone, and facilitate nitrogen uptake and accumulation in plants (Gu et al., 2018). The study concluded that the nitrogen uptake of wolfberry initially increased and then decreased with increasing irrigation amount. The total nitrogen uptake of W0 was significantly lower than that of W1 by 11.31%–12.50%. However, the accumulation of NO₃ ⁻–N showed a fluctuating trend and W0 significantly increased by 7.54%–46.61% compared with W3. This suggests that a reasonable increase in irrigation can enhance crop nitrogen uptake, whereas excessive irrigation can lead to the “dilution effect” of crop nitrogen and exacerbate soil nitrogen loss, including the leaching of NO₃ ⁻–N and NH₄ ⁺–N, N₂O emission, and NH₄ volatilization (Ma et al., 2023). In addition, this study also revealed that the nitrogen uptake of wolfberry was the highest in the fruits, followed by the stems, roots, and leaves (except for the W3 treatment). Among these, the nitrogen absorption rate of wolfberry fruit (63.56 kg·ha⁻¹) was the highest, and the total nitrogen absorption rate of wolfberry plants (160.45 kg·ha⁻¹) was the highest under W1N2 treatment. It can be observed that using the appropriate water and nitrogen management strategy can produce a synergistic effect on water and nitrogen, enhancing the activity of nitrogen metabolism enzymes and the uptake of nitrogen by plants (Sun et al., 2009).

The application of irrigation and nitrogen not only affects the distribution and accumulation of NO₃ ⁻–N in the soil and distribution and the absorption of nitrogen by plants, but also affects soil N₂O emissions. The fluxes of N₂O emission and total emissions during the harvest period of wolfberry were 28.68–177.91 ug·m⁻²·h⁻¹ and 0.40–1.67 kg·ha⁻¹, respectively. Moreover, three peaks of N₂O emission flux (177.91 ug·m⁻²·h⁻¹, 158.55 ug·m⁻²·h⁻¹, and 150.93 ug·m⁻²·h⁻¹) occurred after irrigation. This suggests that irrigation can increase soil microbial abundance and soil enzyme activity, leading to an increased mineralization rate of soil organic matter, higher soil nitrogen content, and consequently, greater release of N₂O (Li et al., 2020). In line with Du et al. (2018), this study also found that the N₂O emission flux from fully irrigated fields increased by an average of 7.97%–53.69% compared with deficit irrigation. This may be because higher soil moisture reduces soil porosity and soil O₂ diffusion capacity, enhances soil denitrification, and promotes N₂O emissions. The addition of exogenous nitrogen increases the substrate concentration for soil nitrification and denitrification, leading to enhanced N₂O emissions (Stehfest and Bouwman, 2019). The results of this study revealed a significant positive correlation between total N₂O emissions and nitrogen application ( Figure 7A ). Additionally, the total N₂O emissions at 300 kg·ha⁻¹ were 13.03% higher than those at 150 kg·ha⁻¹. Similar results were also found by Zheng et al. (2021) in their study on wheat in Northwest China. This study also concluded that the interaction between water and nitrogen had a significant impact on the N₂O emission intensity. The N₂O emission intensity of the W1N2 treatment was significantly reduced by 31.8% and 11.76% compared with the W0N3 and W2N1 treatments ( Figure 7B ). These results indicate that the intensity of N₂O emissions can be effectively reduced by appropriately reducing the amount of irrigation water and nitrogen application (Zhang et al., 2015). Previous studies found that the N₂O emission coefficients of vegetable plots under different water and nitrogen supplies ranged from 1.09%–1.63% under different water and nitrogen conditions. Additionally, the N₂O emission coefficients of orchards initially decreased and then increased with increasing nitrogen application (Cao et al., 2006; Wang et al., 2019). In this study, the N₂O emission coefficient (0.17%–0.39%) for all treatments was generally low and decreased with increasing nitrogen application. This reason may be because the irrigation frequent during the growth period of wolfberry lower than that in vegetable fields, and frequent irrigation can lead to frequent alternations of dry and wet soil, promoting N₂O emissions.

N₂O emission fluxes are influenced by various factors, such as WFPS, soil temperature, and soil NO₃ ⁻–N content. It has been found that soil wet-dry cycles promote the emission of N₂O by stimulating nitrification and denitrification (Peyron et al., 2016). The emission flux of N₂O is exponentially correlated with WFPS (Du et al., 2018). WFPS is maintained at 45%–75% when the N₂O emission rate is at its maximum, during which soil nitrification-denitrification produces the same proportion of N₂O (Shelton et al., 2000). This study demonstrated that soil WFPS in the 0–15 cm soil layer ranged from 37.53%–52.90% under varying water and nitrogen regulations. Additionally, a significant positive linear correlation was observed between N₂O emission flux and WFPS ( Figure 8A ). This indicates that when WFPS was low, N₂O emissions were mainly from the nitrification reaction, and WFPS gradually increased with increasing irrigation levels. When the WFPS exceeded a certain threshold, the denitrification rate gradually accelerated and contributed to N₂O emissions along with the nitrification reaction. Soil temperature is an important factor that influences plant root respiration and soil microbial activity. Chen et al. (2018) found an exponential positive correlation between N₂O emission flux and soil temperature in their study of greenhouse tomato in northwest China. This finding contradicted the conclusion of the present study, which found a linear and negative correlation between N₂O emission flux and soil temperature (R ^{2 =} 0.15ns). The variation in soil temperature (23.1°C–26.6°C) during the wolfberry harvest period suggested a decrease in soil nitrogen mineralization and soil microorganism respiration. Simultaneously, the soil NO₃ ⁻–N content can not only promote the denitrification rate but also inhibit the reduction of N₂O to N₂. In this study, it was found that soil NO₃ ⁻–N increased with increasing nitrogen application ( Figure 3 ; Table 2 ). Additionally, N₂O emission flux showed a significant and linear positive correlation with soil NO₃ ⁻–N content (R ² = 0.64**). This result further demonstrated that when the nitrogen application rate exceeds the nitrogen requirement of the plant, excess nitrogen remains in the soil and is eventually lost in the form of N₂O. Therefore, water and nitrogen inputs should be properly controlled to minimize N₂O emissions during agricultural production. In addition to WFPS, soil temperature, and soil NO₃ ⁻–N content, the intermediate products of nitrification-denitrification, N₂, and NH₄, are also important factors that affect soil N₂O emissions. Subsequent monitoring of N₂ and NH₄ should be conducted to further investigate the emission characteristics of soil N₂O under different water nitrogen regulations.