Objective This study aims to investigate the specific mechanisms by which nitrogen application regulates rhizosphere bacterial denitrification pathways in salt-affected farmland， thereby promoting sunflower root growth and yield. Methods Field plot experiments with drip irrigation under plastic mulch were conducted in sunflower farmland from 2023 to 2024. Four nitrogen application levels were set： the local conventional nitrogen application rate as control （CK： 200 kg/hm²）， 25% nitrogen reduction （N1： 150 kg/hm²）， 50% nitrogen reduction （N2： 100 kg/hm²）， and 75% nitrogen reduction （N3： 50 kg/hm²）. Results The two-year experiments demonstrated that moderate nitrogen reduction improved the rhizosphere soil environment in salt-affected farmland and promoted sunflower root growth and yield formation. Compared with CK， both N1 and N2 treatments significantly increased Ca²⁺ （17.22%， 10.56%） and Mg²⁺ （34.75%， 25.40%） contents in rhizosphere soil at the budding stage， and reduced \mathrm{C}{\mathrm{l}}^{-} and \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} contents. Furthermore， N1 treatment significantly promoted oxalic acid and citric acid secretion by root at the budding stage （59.07% and 60.11%）， and N2 treatment significantly promoted formic acid secretion by root at both the budding stage and the maturity stage. Regarding functional genes， N1， N2， and N3 treatments reduced the relative abundance of denitrifying genes narG， nirK， and nirS， whereas N1 treatment increased the abundance of the nosZ gene and altered the composition of nosZ-type bacterial communities， including increasing the relative abundance of nosZ-type Acidobacteria and Proteobacteria. Regarding crop growth and yield formation， N1 and N2 treatments significantly increased root surface area density （RSAD） at the flowering stage and root length density （RLD） at the budding stage. Over two years， N1 treatment significantly increased sunflower 100-seed weight （9.07%）， seed weight per head （9.08%）， and yield （5.05%）. In contrast， N2 and N3 treatments significantly reduced RSAD values （23.66%， 29.46%）， 100-seed weight （21.67%， 29.51%）， and seed weight per head （21.66%， 29.49%） at the maturity stage. Conclusion A 25% nitrogen reduction treatment has a potential inhibitory effect on the initial stage of denitrification （\mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-}→NO） and a potential promoting effect on the complete denitrification process （N₂O→N₂） in rhizosphere soil. This not only suppresses nitrogen loss from salt-affected farmland at the source but also promotes N₂ reduction_. Meanwhile， it shows significant advantages in promoting plant root growth and yield formation， as well as improving the rhizosphere soil ionic environment. These findings provide a theoretical basis for scientific nitrogen application and nitrogen loss control in the Tumuchuan Plain irrigation district.

Objective This study aims to investigate the co-developmental relationship between the overlying soil and the underlying strongly weathered bedrock layer， thereby providing fundamental data for the quantitative prediction of subsurface rock-soil structures in karst areas. Methods In the Mulian small watershed of Huanjiang， Guangxi， intelligent photogrammetry technology was employed， using a 3 m high standard leveling staff as a reference， to investigate 68 rock-soil profiles， each 5 m in width， across four different geomorphic positions： upper slope， middle slope， lower slope， and depression （17 profiles per position）. Through image characterization in AutoCAD and vectorization processing using ArcGIS-Fragstats， structural indicators of the rock-soil profiles were obtained and compared， including equivalent thickness （T）， number of patches （NP）， shape index （SHAPE）， perimeter-to-thickness ratio （PTR）， fractal dimension （FRAC）， and coefficient of variation of thickness （CV）. Results 1） The equivalent thickness of the soil layer and the strongly weathered bedrock layer increased non-uniformly from the upper slope to the depression. The soil thickness on the upper slope was less than 25 cm， while the thickness of the strongly weathered bedrock layer in the depression ranged from 110 cm to 250 cm. 2） The number of patches （NP>4） and the shape index （SHAPE） on the middle slope were significantly higher than those at other slope positions， exhibiting fragmented and interlocked rock-soil mosaic characteristics. The number of trees and shrubs and the maximum root depth were significantly positively correlated with the thickness of the soil and the strongly weathered bedrock layer （r=0.86， r=0.65）， and increasing woody vegetation effectively reduced the spatial variability of soil thickness. 3） A linear model indicated that the thickness of the strongly weathered bedrock layer was approximately 69% of the soil layer thickness. The complex characteristics of the concealed subsurface structure could be effectively predicted through easily measurable surface indicators， such as soil thickness （T） and woody stem density （Sd）. Conclusion Quantitative research at the rock-soil profile scale reveals differences in structural characteristics among various geomorphic positions， providing a scientific basis for precise zonal management of geomorphic positions in karst ecological restoration.

Objective This study aims to clarify the spatiotemporal evolution characteristics of compound soil erosion in the source region of the Yellow River under the background of ecological restoration and reveal its response mechanisms to climate change. Methods Based on meteorological observations， remote sensing-derived vegetation index， and topographic data from 1980 to 2020， the RUSLE， RWEQ， and freeze-thaw erosion models were coupled to quantitatively assess the intensity of three types of soil erosion driving forces. Transition matrices were employed to analyze the spatiotemporal evolution pathways， while pixel-based partial correlation analysis was conducted to reveal the independent responses of compound erosion to annual precipitation， annual average temperature， and annual average wind speed. Results From 1980 to 2020， the climate in the source region of the Yellow River exhibited a warming and wetting trend， characterized by a significant increase in precipitation， a temperature rise of 1.5 ℃， and a wind speed reduction of 0.3-0.6 m/s， while vegetation coverage increased by more than 40.0%. The intensities of water， wind， and freeze-thaw erosion all decreased significantly. Partial correlation analysis between different climate variables and compound erosion driving forces revealed that， during the study period （1980-2020）， the positive driving effect of precipitation on erosion continuously strengthened （the proportion of positively correlated area increased from 27.4% to 56.0%）， the influence of temperature shifted from promoting to suppressing erosion （the proportion of positively correlated area decreased from 46.0% to 12.8%）， and the driving effect of wind speed markedly weakened （the proportion of positively correlated area decreased to 19.8%）. The alternating and synergistic effects of compound erosion driving forces in the source region of the Yellow River resulted in abnormal responses of climate variables and erosion intensity changes under specific spatiotemporal conditions. Conclusion The ecological restoration project effectively improves the compound soil erosion environmental characteristics in the source region of the Yellow River， thereby suppressing the compound soil erosion in this region. The alternating and synergistic effects of compound erosion driving forces lead to abnormal spatiotemporal responses of climate variables and erosion intensity changes. The research findings provide theoretical references for the intensity assessment of compound soil erosion and its response to environmental changes.

Objective This study systematically evaluates the estimation accuracy of a simple rainfall erosivity model across different regions and rainfall levels. Methods Based on observational data from 2 078 meteorological stations in humid， semi-humid， and semi-arid regions of China， rainfall erosivity of different magnitudes was calculated using the classic EI₃₀ model at hourly and daily rainfall scales as the baseline， and then compared with estimates from the daily rainfall erosivity model proposed. Results 1） Rainfall erosivity calculated by the two methods showed a spatial pattern of low values in the northwest and high values in the southeast， with significant positive correlations in humid， semi-humid， and semi-arid regions. 2） Without considering rainfall classification， estimation deviations of the daily rainfall erosivity model in humid， semi-humid， and semi-arid regions were 16.06%， 22.47% and 24.19%， respectively. However， under different rainfall levels， deviations of rainfall erosivity estimation in the above regions increased markedly， with systematic overestimation for moderate and heavy rain and systematic underestimation for rainstorms and extreme rainstorms. 3） Considering both data deviation and correlation coefficient， the daily rainfall erosivity model achieved the highest estimation accuracy in humid regions under rainstorm conditions， followed by semi-humid regions under rainstorm， and the lowest accuracy was found in humid regions under moderate rainfall conditions. Conclusion Accuracy differences and variation patterns of the daily rainfall erosivity model under different rainfall levels and in different dry and wet regions will provide an important basis for development of zoned and graded estimation models.

Objective To elucidate the pollution characteristics and environmental risks of polycyclic aromatic hydrocarbons （PAHs） in the Dawen River basin. Methods Surface water and sediment samples were collected from 24 sites in the main stream and major tributaries of the basin. The concentration levels， spatial distribution， source contributions， and ecological risks of 16 priority-controlled PAHs were analyzed. Results 1） The concentration of ΣPAHs in the water bodies of the Dawen River basin ranged from 59.05 ng/L to 105.27 ng/L， while that in sediments ranged from 414.90 ng/g to 2 931.17 ng/g. The spatial distribution trend of PAHs in surface water was： Yingwen River > Mouwen River > Daqing River and Dongping Lake > the main stream of Dawen River > Chaiwen River. The spatial distribution trend of PAHs in sediments was： Daqing River and Dongping Lake > Mouwen River > the main stream of Dawen River > Yingwen River > Chaiwen River. 2） Analysis using the Positive Matrix Factorization （PMF） model revealed that the main sources of PAHs in water were biomass combustion， industrial coal combustion， traffic emissions， and petroleum sources. The main sources of PAHs in sediments were petroleum sources， traffic emissions， coal combustion， biomass combustion， and coke sources. 3） Ecological risk assessment indicated that most PAHs in the basin were at a moderate risk level， while Nap and Ant in sediments were at a high-risk level. BaP and DahA were the primary contributors to the toxic risk of PAHs in the Dawen River basin. The results indicated that the Dawen River basin had been contaminated by polycyclic aromatic hydrocarbons （PAHs）， and its ecological risks could not be ignored. Based on the source apportionment results， it is recommended to prioritize the strengthening of emission controls on major contributing sources such as biomass burning， industrial coal combustion， and traffic emissions. Conclusion Future work should integrate the pollution characteristics of the basin， conduct long-term dynamic monitoring and source control strategy evaluation， and provide scientific support for regional water environment management and ecological security.

Objective To explore the spatiotemporal evolution patterns and main obstacle factors of cultivated land ecological security in the main grain-producing areas of the Yellow River basin， thereby providing a scientific reference for sustainable cultivated land use and improving ecological security. Methods Based on six-phase panel data from 60 prefecture-level cities in the Yellow River basin， the pressure-state-response （PSR） model was employed to construct an evaluation index system for cultivated land ecological security in the main grain-producing areas of the Yellow River basin. The entropy method and comprehensive index method were used to calculate the comprehensive index of cultivated land ecological security and classify its security level. The obstacle degree model was then used to identify the key obstacle factors affecting cultivated land ecological security. Results 1） The cultivated land ecological security index increased from 0.28 in 2000 to 0.43 in 2023， representing a relatively significant increase. The ecological security level shifted from "relatively unsafe" to "critically safe"， but there remained a considerable gap from the "relatively safe" level， indicating substantial room for future improvement. 2） The level of cultivated land ecological security exhibited significant regional differences. From 2000 to 2015， only three cities had cultivated land ecological security at the "critically safe" level. In 2023， 45 cities had cultivated land ecological security at the "critically safe" level. 3） From the perspective of criteria layers， the obstacle degree of response showed a positive declining trend in Henan， Shandong， Sichuan， and Inner Mongolia. However， the primary obstacle factor in Henan， Shandong， and Sichuan was the obstacle degree of pressure， whereas Inner Mongolia needs to focus on overcoming the obstacle degree of state. 4） Cultivated land area per capita， disposable income of farmers， share of primary industry in GDP， land reclamation rate， and forest coverage were the main obstacle factors affecting cultivated land ecological security. Conclusion Cultivated land ecological security in the main grain-producing areas of the Yellow River Basin shows an overall trend of continuous improvement， but the overall level remains low， and severe challenges persist. The research results can provide strong scientific support for promoting tailored governance and policy implementation in different regions.

Objective This study aims to reveal the differences in infiltration processes of soda saline-alkali soils with different degrees and their influencing factors， and to evaluate the applicability of different infiltration models in simulating the infiltration process of soda saline-alkali soils. Methods Using the one-dimensional vertical constant-head single-ring infiltration method， field in-situ infiltration experiments were conducted to study the characteristics of infiltration process parameters in soda saline-alkali soils with different degrees， analyze the effects of salinization-alkalization indicators on the infiltration process， and evaluate the applicability of different prediction models. Results 1） As pH increased from 9.31 to 10.35， exchangeable sodium percent （ESP） rose from 1.09% to 65.82%， and sodium adsorption ratio （SAR） increased from 1.24 to 115.71， the stable infiltration rates of the five tested soils decreased by 41.81% to 99.59% compared with the initial infiltration rates. Among them， the Suaeda salsa land with the highest degree of salinization and alkalization showed the largest decrease， with a stable infiltration rate of only 0.13 mm/h. 2） Correlation analysis indicated that the stable infiltration rate was significantly negatively correlated with ESP and pH （p<0.01）， and significantly negatively correlated with exchangeable Na⁺ （p<0.05）. The average infiltration rate was significantly negatively correlated with ESP， exchangeable Na⁺， SAR， pH， and salt content at different levels. 3） Based on the coefficient of determination （R²） of the models and the correlation with the saline-alkali indicators of soda saline-alkali soils with different degrees， the simulation performance of the Horton infiltration model was superior to that of the general empirical model when SAR<108.60， salt content （SC）<10 568.64 mg/kg， and electrical conductivity （EC）<1.70 mS/cm. When the degree of salinization and alkalization exceeded this range， the general empirical model performed better. Conclusion The degree of soda salinization and alkalization is a key factor constraining soil infiltration capacity. The applicability of infiltration models for soils with different degrees of salinization and alkalization can be optimized based on thresholds of typical saline-alkali indicators. These findings provide a theoretical basis and methodological support for simulating water movement and managing irrigation and drainage in soda saline-alkali soil regions.

Objective This study investigates the effects of drought stress on soil reinforcement performance of root systems in the water-level fluctuation zone of the Three Gorges Reservoir area， providing a theoretical basis for vegetation restoration， soil and water conservation， and ecological protection in this area. Methods Taking the root-soil composite of Cynodon dactylon as the research object， samples were collected from the field and subjected to adaptive maintenance. Drought conditions were simulated through controlled water supply in pot experiments. The experiment established treatments of different drought intensities （CK for control， moderate drought MD， and severe drought SD）， drought durations （0， 5， 10， 15， 30， and 60 days）， and root contents （0% for bare soil， 0.1%， 0.2%， and 0.3%）. Direct shear tests were conducted on the remolded samples to analyze the effects of drought intensity and drought duration on the shear resistance of the Cynodon dactylon root-soil composite. In addition， the microscopic mechanisms of the effects of drought stress on the soil reinforcement performance were revealed through microscopic tests of roots. Results 1） Samples under all conditions exhibited distinct strain-hardening failure characteristics， and drought stress and root incorporation had little effect on the variation patterns of stress-strain curves. 2） Short-term moderate drought improved the shear strength of the Cynodon dactylon root-soil composite， while long-term severe drought significantly reduced the shear strength （p<0.05）. Root incorporation significantly increased the soil shear strength， and the enhancement performance increased with the increase of root content. Compared with the bare soil treatment， the shear strength of the root-soil composite in CK， MD， and SD groups increased by 8.70%-51.52%， 4.35%-63.64%， and -13.04%-39.39%， respectively. 3） The response patterns of cohesion to drought stress and root incorporation were consistent with those of shear strength， while the internal friction angle remained relatively stable with slight fluctuations. Compared to bare soil treatments， the cohesion of the root-soil composite for CK， MD， and SD groups increased by 42.46%-63.99%， 15.08%-67.50%， and 15.08%-48.49%， respectively， while the internal friction angle increased by 1.55%-1.83%， -1.61%-1.68%， and -1.16%-1.43%， respectively. Conclusion Short-term moderate drought stress can improve the shear resistance of the Cynodon dactylon root-soil compositeto a certain extent， but excessive stress severely damages the root structure and reduces the shear strength of the root-soil composite.