Nitrous oxide (N₂O), a long-lived greenhouse gas, is primarily produced in agricultural soils through biological nitrification and denitrification processes. However, the effects of soil salinity on nitrogen transformation processes remain insufficiently understood, hindering the development of effective nitrogen management in salt-affected farmlands. In this study, laboratory incubation experiments combined with a ¹⁵N stable isotope tracing technique were conducted to quantify the effects of salt stress on nitrification and denitrification rates and their contributions to N₂O emissions. The results showed that during the first week of incubation, slight and moderate salinity (NaCl contents of 0.04% and 0.10%; EC_1:5≤1.10 dS/m) enhanced both nitrification and denitrification rates, whereas strong salinity (0.20% NaCl; EC_1:5≥1.42 dS/m) inhibited these processes. In the first week of incubation, nitrification and denitrification contributed approximately 65%-70% and 30%-35% to the total N₂O emissions under 60% water-filled pore space, respectively. These results indicate that nitrification represents the predominant source of N₂O production in saline soils during the first week following fertigation. The findings suggest that nitrogen management practices for inhibiting the nitrification process (e.g., the addition of nitrification inhibitors in companion with fertigation) may mitigate N₂O emissions in saline fields. This provides a scientific basis for optimizing nitrogen management in saline soils and reducing greenhouse gas emissions.

Inappropriate irrigation and fertilization have led to high carbon (C) emissions and degradation of ecosystem services in the saline-alkali sunflower farmland in the arid upper Yellow River basin of Northwest China. This study aims to determine the rational irrigation and fertilization strategy for the sustainable production of saline-alkali sunflower farmland in this area. A field experiment was carried out to investigate the effects of organic fertilizer application on the net ecosystem carbon budget (NECB) and ecosystem service values of saline-alkali farmland under drip irrigation in 2021 and 2022. Two factors were considered, including the lower limit of irrigation and the fertilization mode. Among them, the lower limit of irrigation (drip irrigation) was divided into two soil matric potential thresholds (SMPT) (W1: −20 kPa SMPT and W2: −30 kPa SMPT). Three fertilization modes were selected (CK: Chemical fertilizer, LBF: Lignite C-based organic fertilizer 4.5 t/hm², SMF: Sheep manure compost 5 t/hm²) under each SMPT using a completely random block design. In addition, the border irrigation combined with chemical fertilizer was set as the control treatment (MCK), corresponding to the local farmer practices. A comparison was performed on the NECB of different ecosystems and the components, as well as three ecosystem service values (supply function value of agricultural products, accumulated functional value of organic matter, and gas regulation value) among different treatments. Results indicated that the LBF and SMF treatments effectively promoted crop growth and biomass accumulation, compared with the CK. There was also an increase in the soil moisture content and soil organic matter (SOM) content under the drip irrigation. Among all treatments, better performance was achieved in drip irrigation with −20 kPa SMPT combined with 4.5 t/hm² lignite C-based organic fertilizer application treatment (W1LBF). The highest aboveground biomass and yield also increased by 32.30% and 44.13%, respectively, compared with the MCK. These soil and crop indicators depended mainly on the NECB. The net primary productivity (NPP) and grain C output were the main components of the carbon sink and carbon source in the farmland system, respectively. NPP, SOM content, grain C output, and soil C emission in the drip irrigation treatment were higher than those in the surface under the same fertilization, where the values of these indicators increased with the increase of SMPT. Moreover, the organic fertilizer application (LBF and SMF) also significantly improved NPP, SOM content, and grain C output under the same irrigation, but reduced the soil C emissions, compared with the chemical fertilizer application (CK). The W1LBF treatment obtained the highest NPP and lower soil C emissions, thus achieving the highest NECB. Compared with the W1CK and MCK treatments, the W1LBF treatment increased NECB by 26.24%-159.98% and 151.74%, respectively. In addition, the changes in NPP, SOM, and soil carbon emissions also significantly affected the ecosystem service value. W1LBF treatment increased the supply function value of agricultural products, SOM accumulation, and gas regulation by 8004.20, 923.90, and 2094.70 Chinese Yuan/hm², respectively, compared with the MCK. In conclusion, the drip irrigation with −20 kPa soil matric potential thresholds combined with 4.5 t/hm² lignite C-based organic fertilizer application can effectively increase the C sink and service values from the saline-alkali sunflower farmland ecosystem in the arid upper Yellow River basin of Northwest China. The research can provide the scientific basis for the saline-alkali farmland C sequestration and ecosystem service value, while greenhouse gas emission reduction in the arid and semi-arid areas.

Soil salinization is one of the major limiting factors for the sustainable development of agriculture in the arid northwest China. Among them, flue gas desulfurization (FGD) gypsum from the coal-fired power plant can be widely used to improve the saline-sodic soil. At the same time, the lignite bioorganic fertilizer (LBF) has also been expected as an effective amendment for the better saline-sodic soil in recent years. However, little is known about the combined application of LBF and FGD gypsum on the physical and chemical properties of saline-sodic soil and sunflower growth. In this study, a two-year field experiment was conducted in 2021-2022, in order to investigate the effect of combining both LBF and FGD gypsum on the saline-sodic soil quality, crop yield, water use efficiency (WUE), and partial-factor productivity of N (PFPn) in the sunflower farmland at the Hetao Experimental Station of China Agricultural University, located in the Bayannur City, Inner Mongolia Autonomous Region, China. Soil samples of 0-40 cm soil layer were collected at the sunflower maturity stage. The sunflower yield, WUE, and PFPn were measured at the same time. Six treatments were considered, i.e., a chemical fertilizer treatment (CK), and 2 t/hm² LBF combined with 0 (LBF), 5 (LBF+F5), 10 (LBF+F10), 15 (LBF+F15), and 20 (LBF+F20) t/hm² FGD gypsum application treatments. The results showed that the LBF treatment decreased the average content of Na⁺ by 23.33% in the 0-40 cm soil layer in 2022, compared with the CK treatment, but there was no effect on the other water-soluble ions. The LBF+F20 treatment increased the average contents of Ca²⁺, SO₄²⁻ and Mg²⁺ by 450.71%, 123.41%, and 391.90%, respectively, compared with the CK, whereas, there was the a decrease in the average contents of CO₃²⁻+HCO₃⁻ by 48.29% in 0-40 soil layer. The LBF combined with the FGD gypsum treatments significantly decreased the average soil pH by 0.29-0.88 in the 0-20 cm soil layer. The soil organic matter content in the 0-20 cm soil layer was higher than that in the 20-40 cm soil layer. The LBF treatment significantly increased the soil organic matter content by 39.89%-131.65% in the 0-20 soil layer in 2021-2022. In addition, the LBF combined with FGD gypsum treatments decreased the soil ESP and soil bulk density by 18.34%-43.42%, and 5.91%-11.06% in the 0-40 cm soil layer, respectively, compared with the CK. The LBF+F10, LBF+F15, and LBF+F20 treatments increased the soil aggregate mean weight diameter by 102.00%, 127.78%, and 91.95%, respectively, but there was no effect on the soil percentage aggregate destruction. Although the application of FGD gypsum decreased the soil pH and ESP, however, the excessive application of FGD gypsum would increase the soil salt content, which may inhibit the sunflower growth and yield improvement. The LBF+F10 treatment increased the sunflower yield, WUE, and PFPn by 44.45%, 44.62%, and 80.46%, respectively, compared with the CK. In general, the combination of 2 t/hm² LBF and 10 t/hm²FGD gypsum shared the greatest effect on the physical and chemical properties of saline-sodic soil, indicating the largest sunflower yield, WUE, and PFPn. The finding can provide the scientific implication for the saline-sodic soil remediation and crop productivity improvement of salinized farmland in HID.