When considering the factors affecting the spatial and temporal variation of nitrogen and phosphorus in karst watersheds, the unique karst hydrogeology as an internal influencing factor cannot be ignored, as well as natural factors such as meteorological hydrology and external factors such as human activities. A watershed-scale field investigation was completed to statistically analyze spatial and temporal dynamics of nitrogen and phosphorus through the regular monitoring and collection of surface water and shallow groundwater in the agricultural-dominated Mudong River watershed in the Huixian Karst Wetland over one year (May 2020 to April 2021). Our research found that non-point source pollution of nitrogen (84.5% of 239 samples TN > 1.0 mg/L) was more serious than phosphorus (7.5% of 239 samples TP > 0.2 mg/L) in the study area, and shallow groundwater nitrogen pollution (98.3% of 118 samples TN > 1.0 mg/L) was more serious than surface water (68.6% of 121 samples TN > 1.0 mg/L). In the three regions with different hydrodynamic features, the TN concentration was higher and dominated by NO₃⁻-N in the river in the northern recharge area, while the concentrations of TN and TP were the highest in shallow groundwater wells in the central wetland core area and increased along the surface water flow direction in the western discharge area. This research will help improve the knowledge about the influence of karst hydrodynamic features on the spatial patterns of nitrogen and phosphorus in water, paying attention to the quality protection and security of water in karst areas with a fragile water ecological environment.

The Soil and Water Assessment Tool (SWAT) model is extensively used globally for hydrological and water quality assessments but encounters challenges in karst regions due to their complex surface and groundwater hydrological environments. This study aims to refine the delineation of hydrological response units within the SWAT model by combining geomorphological classification and to enhance the model with an epikarst zone hydrological process module, exploring the accuracy improvement of SWAT model simulations in karst regions of Southwest China. Compared with the simulation results of the original SWAT model, we simulated runoff and nutrient concentrations in the Mudong watershed from January 2017 to December 2021 using the improved SWAT model. The simulation results indicated that the modified SWAT model responded more rapidly to precipitation events, particularly in bare karst landform, aligning more closely with the actual hydrological processes in Southwest China’s karst regions. In terms of the predictive accuracy for monthly loads of total nitrogen (TN) and total phosphorus (TP), the coefficient of determination (R²) value of the modified model increased by 10.3% and 9.7%, respectively, and the Nash–Sutcliffe efficiency coefficient (NSE) increased by 11.3% and 9.9%, respectively. The modified SWAT model improves prediction accuracy in karst areas and holds significant practical value for guiding non-point source pollution control in agricultural watersheds.

Spatio-temporal dynamics of riverine nitrogen (N) and phosphorus (P) in karst regions are closely linked to hydrological conditions, human activities and karst features in upstream catchments. From October 2017 to September 2019, we undertook 22 sampling campaigns in 11 nested catchments ranging from 21.00 to 373.37 km² in Huixian karst wetland to quantify forms, concentrations, and fluxes of riverine total nitrogen (TN) and total phosphorus (TP), and to identify spatial and temporal variations of nutrients transfer from upstream to downstream, tributaries (Mudong River and Huixian River) to the main stem (Xiangsi River) in the dry and wet seasons. Considering the hydrological conditions, human activities and karst features within upstream catchments, the following three spatial and temporal variations of riverine nutrients were found over the monitoring period: (1) the dynamics of riverine nitrogen and phosphorus varied seasonally with hydrological conditions; (2) the spatial disparities of riverine nitrogen and phosphorus were induced by different human activities within catchment scales; (3) the dynamics of riverine nitrogen and phosphorus varied similarly at spatial scale restricted by karst features. The findings from this study may improve our understanding of the influence of hydrological conditions, human activities and karst features on nitrogen and phosphorus variations in river waters at different spatial and temporal scales in the Huixian karst wetland basin, and will help managers to protect and restore river water environments in karst basin from a catchment-scale perspective.

In this study, we investigated the influence of different simulations on the transport of shallow groundwater nitrogen in the Mudong River watershed of the Huixian Wetland, a karst wetland. Based on GMS (Groundwater Modeling System) software, the equivalent porous media model was used to simulate the transport of total nitrogen under different conditions in the study area. Two years of field monitoring data in the study area provided the input for the modeling. The SWAT (soil and water assessment tool) model was used to divide the study area into sub-basins. The initial concentration flux index W is first introduced in the equivalent porous medium model to calculate the initial concentration. The simulation results showed the difference between the simulated and monitored values of total nitrogen concentration was between 20% and 40% in 22.2% of the cases, and less than 20% in 66.7% of the cases, indicating that the solute transport model has good applicability in the Huixian Wetland. Parameter sensitivity analysis showed that fertilizer application was the main factor influencing total nitrogen. A 25% reduction in fertilizer application reduced total nitrogen emissions by 31.5% in sub-basin S3 and 22.5% in sub-basin S4. These reductions were greater than the abatement effect of changing land cover and managing river pollution. The pollution plume of total nitrogen was reduced by 38.5% in the southern part of sub-basin S3 (Mudong Lake) and by 40.2% in the western part of sub-basin S4 (Blacksmithing Village). The average concentration was reduced by 2.04 mg/L and 1.22 mg/L, respectively. This study shows that reasonable control of double-season rice nitrogen fertilizer application and appropriate land cover modification can help reduce total nitrogen emissions from wetlands in the Li River watershed and ensure the sustainable development of the local economy and groundwater.

To find out the nitrogen and phosphorus concentration and load emission regularity and its effect during the rainy season + irrigation period and dry season + non-irrigation period in the small watershed, five spatial scales (3.35 ∼27.25 km²) nested in the Jingui river watershed are selected and regularly monitored from October 2017 to September 2018. The results showed that the nitrogen and phosphorus concentrations in the rainy season + irrigation period and dry season + non-irrigation period increased with the increase of the scale. The total nitrogen and total phosphorus load per unit area in the rainy season + irrigation period decreased with the increase of the scale; the dry season + non-irrigation period increased slightly. The same nitrogen and phosphorus index concentration was significantly different at different spatial scales. The main reason for the scale effect was the distribution pattern of land-use. Affected by this, scale D, the critical scale of nitrogen and phosphorus load in Jingui river watershed, its average load of total nitrogen, ammonium nitrogen, total phosphorus and total dissolved phosphate reached the highest value of the watershed. The contribution rate of nitrogen and phosphorus load in rainy season + irrigation period and dry season + non-irrigation period of research areas with different area depends on precipitation and the distribution pattern of land-use. Therefore, the nitrogen and phosphorus emission control and pollutant reduction in the rice irrigation districts of Lijiang river basin should consider the spatial-temporal features and effects of nitrogen and phosphorus pollutant transport.

The accessibility of urban park green spaces holds significant importance for the daily lives of city dwellers and for evaluating urban liveability. This study integrates the Gaussian two-step floating catchment area method (G2SFCA) and network analysis to assess the accessibility distribution characteristics of park green spaces in Nanning City. The G2SFCA accounts for population density and park area to quantify per capita service capacity, while network analysis simulates real transport networks to evaluate distance- based and time-based accessibility. Results indicate significant disparities in accessibility across Nanning's districts: Qingxiu and Liangqing District exhibit the highest accessibility with average proportions of 65.84% and 45.54% respectively, while Xixiangtang and Xingning District record the lowest at 18.01% and 16.61%. Spatial Autocorrelation Analysis (Moran’s I = 0.928) further indicates pronounced clustering, with high–high clusters concentrated in the southeast and low–low clusters in the northern and western areas. The comparative analysis highlights the methodological complementarity between supply–demand based and distance–cost based evaluations, providing a more robust understanding of green space accessibility. Policy recommendations suggest prioritising the addition of community parks and green corridors in low-accessibility and high-density areas such as northern Xixiangtang and western Jiangnan, while consolidating the “large core–small node” layout along the Yongjiang River. This research offers scientific support for ecological spatial planning and contributes to advancing the development of a liveable and sustainable urban environment in Nanning.