Response Of Nitrogen And Phosphorus Output Concentrations To Water Landscape Patterns In Agricultural Small Watersheds In Mountainous And Hilly Regions

There is a close interaction between landscape pattern and ecological processes.The nutrient load output concentration reflects directly the human activities and biogeochemical processes in the basin.In the context of non-point source pollution and increasing nutrient load output concentration,many researchers have attempted to reveal the relationship between landscape pattern dominated by land use and nitrogen and phosphorus output concentration in watershed.On the other hand,many studies have confirmed that all kinds of water bodies play an important role in the interception and purification of non-point source pollutants.However,existing studies pay more attention to the overall landscape pattern and nutrient element output of the basin,and there are few studies that separate water landscape pattern from the basin and analyze how water landscape affects nutrient element output of the basin.This study takes small agricultural watershed in mountainous and hilly areas as the research object,focuses on the water landscape within the basin,and studies the influence of water landscape pattern on nitrogen and phosphorus transport in the basin,which is of great significance for realizing the sustainable development of water ecology in small agricultural watershed in mountainous and hilly areas.In this study,107 small agricultural basins in Luzhou-Kaizhou,a hilly region in the upper reaches of the Yangtze River,were selected as research objects to explore the correlation between water landscape pattern and nitrogen and phosphorus pollutants and their influencing factors.Firstly,the water quality data of December 2021 dry season,April 2022 normal season and August 2022 wet season were collected in this study,the physical and chemical indexes of the drainage basin were determined,and the overall pollution and eutrophication levels of the drainage basin were evaluated.Secondly,the basin is divided into reservoir basin and river network basin,and the influence of landscape composition,landscape configuration and water structure difference on the water quality of the basin is discussed.Finally,based on the existing landscape pattern index and the constructed water landscape pattern index,the response rules of water landscape pattern to water quality index under three scales were explored.The results show:（1）The water quality of the study area was the worst in the normal period and the best in the wet period during the dry period in December and February 2021,the normal period in April 2022 and the wet period in August 2022.Through single factor evaluation,it was found that the total nitrogen of each basin in dry season,normal season and wet season in the study area exceeded the standard seriously,among which the dry season was classⅢ,normal season was class IV,and wet season was classⅢ.The comprehensive nutritional status index showed that the study area was a mild eutrophication level in the normal water period,and the nutritional status was general,showing a trend of eutrophication.During the wet season,the nutritive state of the whole study area was good,belonging to the medium trophic level.The results of single factor evaluation and comprehensive nutrient status index method show that the water quality of reservoir basin is better than that of river network basin.（2）In reservoirs and river networks,dry land,ponds,and settlements are the main"source"landscapes of nitrogen and phosphorus,while paddy fields and woodlands are the"sink"landscapes.During the normal flow season,there is a significant positive correlation between pond patch density and TN in a reservoir type watershed,and a significant positive correlation between the largest patch index in residential areas,the largest patch index in dry land,and TP;The shape index of wasteland in the river network type watershed during the normal water period was significantly positively correlated with NO₃^--N;The correlation between reservoir capacity and TP is strongest.During the flood season,there is a significant positive correlation between the patch density of residential areas and NO₃^--N in the reservoir basin,and a significant positive correlation between the largest patch index of residential areas and TN;There was a significant positive correlation between the largest patch index and NO₃^--N;The depth and hydraulic retention time of the reservoir are significantly negatively correlated with the NO₃^--N,while the capacity and hydraulic retention time of the reservoir are significantly negatively correlated with the TN.（3）Nitrogen and phosphorus pollutants in reservoir watersheds are jointly affected by river network structure,reservoir structure,aquatic plants,and microorganisms,enabling nitrogen and phosphorus substances entering the reservoir water body to be more fully digested and precipitated by microorganisms,aquatic animals,and plants,making the water quality in reservoir type watersheds better than in river network type watersheds.（4）At the 1-5 km2scale,water spatial distribution index（AD）and perimeter-area fractal dimension index（PAFRAC）are the key factors that affect the changes of water quality indicators;RDA sorting results display Number of patch（NP）,average storage capacity（A＿CAPACITY）and perimeter-area fractal dimension index（PAFRAC）affect5-15km2.Key indicators of watershed water quality change at scale;15～70km2scale water spatial distribution index（AD）,average water area（A＿WATER）and patch density（PD）are key indicators that affect water quality changes.（5）The water landscape has significant effect on the retention and purification of nitrogen and phosphorus pollutants in the basin.For the water landscape distributed at different locations in the basin,the effect on the retention of pollutants in the water body is different.In order to protect the water quality of the basin,it is necessary to build large and complex water landscape in the downstream of the basin,and focus on monitoring and management of the water landscape close to the drainage outlet.When developing and utilizing the land resources around the water landscape,it is necessary to pay special attention to protecting the surrounding ecological environment and reduce the development of high water consumption projects around the water landscape.To reduce the adverse effects of human activities on water quality.At the same time,it is necessary to strengthen the control of upstream pollutants,avoid building factories near lakes and reservoirs as far as possible,reduce the load of pollutants in the water landscape and curb its influence on the downstream.In this paper,the relationship between water landscape and water quality in the basin was studied,and it was proved that the reservoir basin had better interception and purification effect on nitrogen and phosphorus pollutants than the river network basin,and the key water landscape indexes affecting the water quality of different scale basins were revealed.It provides a scientific basis for controlling non-point source pollution in small agricultural watershed by managing and optimizing water landscape pattern.