Globally, soil salinization is intensifying, with alkalization coexisting. In particular, coastal ecosystems are more susceptible to salt problems due to their formation process and geographical locations. The nitrogen (N)-cycling processes of coastal ecosystems are bound to salt-alkali changes. Ammonia (NH 3) volatilization from agricultural ecosystems is one of the most important pathways of N loss and has also been considered the main contributor to air pollution in coastal ecosystems. As the most accessible land resource on earth, clarifying and quantifying the effect of saline-alkali on N content and on NH 3 volatilization in coastal ecosystems are pivotal to promote coastal agriculture productivity. The challenge in demonstrating the effect is how to identify the direct effects of salt-alkali and how these two factors indirectly impact NH 3 volatilization through interactions. By combining incubation experiments with the structural equation modeling method (SEM ‘element’ model), we revealed the net effects of salt-alkali on NH 3 volatilization and the roles of environmental factors in mutual interaction networks. Compared to the CK treatment, NH 3 volatilization increased by 9.31-34.98%, 3.07-26.92% and 2.99-43.61% with salt gradient increases from 1‰ to 15‰ at 0.05‰, 0.10‰ and 0.15‰ urea additions, respectively. With an increase in the alkalinity from 0.5‰ to 8‰, NH 3 volatilization significantly increased by 8.36-56.46%, 5.49-30.10% and 30.72-73.18%, respectively. According to the element model, salt and alkali both promoted NH 3 volatilization directly and had an indirect negative effect by altering the N contents and N transformations of microbes. The N contents in the incubation system showed a direct positive effect on NH 3 volatilization, with an obvious decrease under elevated salinity and alkalinity. Additionally, the gene abundance of N-transformed microbes strengthened NH 3 volatilization indirectly. The indirect prohibitory effect on NH 3 volatilization resulting from salt and alkali was compensated by the direct stimulating effects on the pH and NH 4 + contents, and the overall positive contribution of salt was less than that of alkali. Our results indicated that the potential of NH 3 emissions from coastal saline areas could be enhanced by concomitant soil alkalization.

Soil and water conservation measures, especially reforestation and check dam construction, have been progressively implemented on the coarse sandy hilly catchment region of the Yellow River basin for several decades, and climate conditions are also dynamic. Therefore, it is very urgent to understand how the precipitation variation, land use changes and check dams affect soil erosion and sediment yield in a large watershed. The sediment delivery distributed (SEDD) model was employed to quantitatively identify the impacts of the three factors on soil erosion and sediment yield in the Kuye River watershed. Significant land use changes, with the conversion of arable land and bare land to vegetation cover and construction land, occurred in the study watershed from 1987 to 2016. In addition, 306 key dams were built in the watershed, with a total storage capacity of 316.64 Mm3, according to the statistical data of 2011. Hot spot analysis showed that the high-risk regions for soil erosion and sediment yield were mainly concentrated in the middle reaches of the watershed. The simulation results showed that the check dams were the dominant factor, reducing total sediment load by 53.77% in 2006. However, from 1987 to 2016, the contribution of these three factors (precipitation variation, land use changes and check dams) to sediment reduction was 29.10%, 40.09% and 30.81%, respectively, which indicated that all of them had significant influence on sediment load. The results can serve as a reference for watershed management and policy implementation.

Reforestation and check dam construction have been progressively implemented on the Loess Plateau for several decades. However, it is still unclear how the two major sediment control strategies affect soil erosion and sediment yield in a large watershed. A combination of field investigation and model simulation was employed to quantitatively identify the impacts of the two measures on soil erosion and sediment yield in the Kuye River watershed. Significant land use changes, with the conversion of arable land and bare land to vegetation cover and construction land, occurred in the study watershed from 1987 to 2016. In addition, 306 key dams were built in the watershed, with a total storage capacity of 316.64 Mm3, according to the statistical data of 2011. Hot spot analysis showed that the high-risk regions for soil erosion and sediment yield were mainly concentrated on the periphery of Shenmu County and the outlet of the watershed. The simulation results showed that the land use changes from 1987 to 2016 remarkably reduced sediment yield by 51.14% without considering the action of check dams. In the 1987 scenario, the sediment yield was reduced by 50.44% when considering the action of check dams compared with the yield that was estimated without consideration of check dams. Under the combined effect of the two factors, the sediment yield decreased by 73.91% in 2016. More attention should be paid to check dams, and corresponding measures should be taken to protect them, especially in the flood period.