Although the Chinese government has conducted much work in recent years to reduce land-based pollutant discharge, eutrophication continues to occur frequently in many rivers, estuaries, and coastal waters. This may indicate that sediment is a major source rather than a sink for nitrogen (N) and phosphorus (P). To clarify the endogenous mechanisms of eutrophication in coastal river basins, the eutrophication status, physicochemical properties, and bacterial parameters of overlying waters and sediments in the catchment (CA), estuarine (EA), and offshore (OA) areas in the Duliujian River Basin of Bohai Bay were investigated. The results showed that the eutrophication index (EI) of CA, EA, and OA were 62.71, 57.86, and 36.51, respectively. The EI was more sensitive to increases in P (slope = 3.887) than to increases in N (slope = 0.734) of the overlying water, indicating that P is the main factor driving eutrophication in the coastal river basin. However, a nonlinear relationship was found between P in sediments and overlying waters, suggesting that bacterial mediation may occur during P release. As speculated in this study, P in the overlying water increased more quickly than N with increasing bacterial diversity and metabolic abundance, indicating that the allometric release of N and P mediated by bacteria increases the risk of eutrophication. Redundancy analysis showed that organic matter and total N in sediment have positive effects on bacterial communities, which explains 21.8% and 23.7% variation in bacterial diversity, and explains 31.3% and 7.1% variation in bacterial metabolism. This also suggests that the accumulation of N in the sediment promotes the release of P and further aggravates the eutrophication of water. Therefore, simultaneous control of N and P is necessary to control water eutrophication in coastal river basins.

In arid and semiarid regions with water shortage, forestry development is limited by water availability. Understanding how tree sap flow responds to water stress and microclimatic variables is essential for the management of trees and the understanding of the eco-physiological properties of trees in arid areas. In the city of Tianjin in northern China, we measured the sap flow of Fraxinus pennsylvanica, a widely distributed urban greening tree species in semiarid regions of China. We measured the sap flow in four F. pennsylvanica trees over 6 months (April–September 2021), using a thermal diffusion probe method, and simultaneously monitored microclimatic variables and soil moisture. Results indicated that high nighttime sap flow velocity might be produced under the water stress condition. In addition, the nighttime sap flow velocity under the water stress condition was more susceptible to the combined effects of meteorological factors at night. The daytime sap flow velocity exerted a highly significant positive effect on the nighttime sap flow velocity during the whole research period, and the model fit was higher in the early growing season than that in the late growing season (early growing season: R² = 0.51, P < 0.01; late growing season: R² = 0.36, P < 0.01). Vapor pressure deficit had a positive effect on daytime sap flow. However, net vapor pressure deficit restrained daytime sap flow velocity when the intercorrelation between the microclimatic variables was removed. Our study highlights that drought areas perhaps have higher nighttime sap flow and that more emphasis should be placed on nighttime sap flow and the response of nighttime sap flow to microclimatic variables. In addition, the influence of other microclimatic variables on vapor pressure deficit needs to be considered when analyzing the relationship between daytime sap flow and vapor pressure deficit. An increase in net VPD can suppress the daytime sap flow.

Plant water use can have a profound impact on the regional water cycle and water balance. A great deal of research has been conducted in this area in recent years. However, plant nighttime sap flow and non-growing season water use have rarely been addressed. These two components should not be neglected in accurately predicting the water use of urban landscape trees and large-scale plantation forests. In this study, the thermal diffusion probe (TDP) method was used to observe the water use of Fraxinus pennsylvanica Marshall, a common tree species in northern China. Continuous observations of sap flow were made from November 2020 to September 2021, while meteorological conditions in the region were recorded. We analyzed the sap flow changes in different months and their responses to environmental factors at the daily scale. The results showed a clear circadian rhythm phenomenon of sap flow during the growing season, with strong correlations between nighttime sap flow and daytime sap flow, as well as environmental factors. Transpiration and refilling stem water storage were also observed at night. In the non-growing season, the average whole day sap flow rate is less than 0.5 cm/h. The difference in average sap flow rate between daytime and nighttime is less than 0.3 cm/h. At the daily scale, temperature (Ta), relative humidity (RH), and vapor pressure deficit (VPD) were the main influences on nighttime sap flow. Solar radiation had a significant effect on the overall water use strategy of the trees.

Coastal riparian zones are important transitional areas for carbon cycling between terrestrial and aquatic ecosystems. However, the associations among litter-derived dissolved organic matter (DOM), soil DOM composition, and soil organic carbon (SOC) fractions across habitats remain insufficiently understood. This study aimed to clarify SOC fraction distribution in woody and herbaceous habitats and evaluate its associations with litter-derived DOM, soil DOM composition, and soil environmental factors in different seasons from a coastal riparian zone. In this study, woody habitats had higher SOC content in March, whereas herbaceous habitats showed greater SOC content in November. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) were generally higher in different habitats. MAOC accounted for a large proportion of the measured SOC-related pools and showed a consistent positive association with SOC content. Soil DOM consisted of protein-like and humic-like components, with herbaceous habitats showing a stronger protein-like component and woody habitats showing a stronger humic-like component in March. Woody litter showed greater DOC and DON release potential than herbaceous litter leachates, while litter-derived humic-like DOM was closely associated with a soil humic-like component. The labile SOC pool was mainly associated with vegetation type and electrical conductivity, whereas MAOC was positively associated with soil moisture content and total phosphorus. Overall, the distribution of SOC fractions in coastal riparian habitats is shaped by habitat-specific environmental factors, including vegetative carbon inputs, litter leaching and soil physicochemical properties. This finding is critical for developing targeted management strategies to facilitate SOC accumulation in coastal zones.

Soil degradation in the agro-pastoral ecotone of North China severely constrains the productivity of vital forage crops like oat (Avena sativa L.). While biochar and humic acid are known soil amendments, their combined synergistic effects and optimal application ratios remain poorly understood. This study aimed to investigate the integrated effects of biochar and humic acid on soil properties and oat productivity to identify an optimal co-application strategy for this fragile ecosystem. A three-year field experiment (2023–2025) was conducted using a randomized complete block design. Treatments included a control (CK), sole biochar (B1H0), sole humic acid (B0H1), and three combined biochar-humic acid ratios (7:3, 5:5, and 3:7 by mass; B7H3, B5H5, B3H7). We measured key soil physicochemical and biological properties, as well as oat growth, forage yield, and quality parameters. Partial least squares structural equation modeling (PLS-SEM) was employed to elucidate the causal pathways linking amendments, soil quality, and crop performance. The combined application of biochar and humic acid demonstrated significant synergistic effects, outperforming sole applications in improving soil health and oat productivity. The B5H5 (1:1 ratio) treatment was the most effective, significantly reducing soil bulk density by up to 22.4% and increasing the mean weight diameter of water-stable aggregates by 40.7% compared to the CK. This treatment also led to the greatest increases in soil organic matter (68.9%), microbial biomass carbon (142.2%), and the activities of sucrase (139.8%) and urease. Consequently, the B5H5 treatment enhanced oat forage yield by 56.1% and improved its nutritional quality by reducing acid detergent fiber and neutral detergent fiber content by 22.9% and 12.7%, respectively. The PLS-SEM analysis confirmed that the positive effects of the amendments on crop performance were primarily mediated through the enhancement of soil quality (path coefficient β = 0.75, p < 0.001). The integrated application of biochar and humic acid, particularly at a 1:1 mass ratio, is a highly effective strategy for restoring degraded soils in the agro-pastoral ecotone. These findings provide a strong scientific basis for promoting this co-amendment approach as a sustainable solution to enhance agricultural resilience and productivity in fragile environments.