Response Of Nutrient Distribution Characteristics To Land Use Change In Manas River Basin

Land use and cover change（LUCC）reflects the relationship between the Earth’s environmental system and human social production system,and causes various natural phenomena and ecological process changing,including soil nutrient content,biogeochemical cycles and biodiversity distribution,etc.In recent years,the impact of land use change caused by human activities on surface nutrient loading has attracted widespread attention,especially for small watersheds,unreasonable land use caused soil erosion intensified,affected the natural distribution of nutrients,destroyed the cycle of global biogenic elements（C,N,P,S）and caused eutrophication of the water.Therefore,the land use change,surface nutrient loading and the the concentration of nitrogen and phosphorus in lakes reveals the relationship between land use change and soil,hydrology and other ecological environment,and it was of great significance to optimize on land structure and improve the social and natural ecological environment.Taking Manas River Basin as the research object,analyzed the land use change from1990 to 2015 with GIS software.Analyzed the content of nutrients and element stoichiometric characteristics under different land use,and compared the distribution differences.Finally,by estimating the contribution rates of different land-use types to total nitrogen（TN）and total phosphorus（TP）input waters,the effects of land-use type changes on the nutrient loss and pollution on the water environment were evaluated.The main results as follows:（1）Land use have changed significantly in the Manas River Basin.From 1990 to 2015,the Manas River basin was dominated by unutilized land,grassland and cultivated land,and the area of cultivated land and construction increased continuously by 3,558 km² and 306 km²respectively.The grassland and unused area decreased by 2,530 km² and 1317km².The change rate of single land use dynamics shows that the rate of increase of cultivated land from2010 to 2015 has dropped to 1.85%,the rate of increase in construction land has reached a maximum of 5.35%,and the rate of reduction of grassland has continuously increased,with a minimum of-2.03%.The unused land changed positively from 2010 to 2015,and the dynamic degree of single land use was 0.17%.The area increased;from the shift direction,the cultivated land mainly came from grassland and unused land,and the construction land mainly came from cultivated land,the transfer of arable land to construction land increased from 2010 to 2015.The area of unused land increased from 2010 to 2015,mainly transfer from cultivated land and grassland,and forest area changed little.The dam breakage of the middle reaches of the early 20th century led to water increased,transfer to unutilized land.The Lorentz curve of land use in 1990 and 2015 shows that the spatial distribution of land use types was relatively large,and the Gini coefficient was greater than 0.4.With the change of time,the distribution gap of cultivated land and water declined and grassland increased.From the perspective of land use comprehensiveness index,the land use efficiency of the Manas River Basin was low,but as time changes,unutilized land was continuously developed,and the comprehensive utilization of land was improved.（2）There are differences in nutrient distribution and elemental stoichiometric characteristics under different land use patterns.In the middle reaches of the Manas River,the pH values under the different land use patterns were generally higher,and the alkalinity was stronger;The contents of clay and fine silt in the Mushroom Lake Reservoir,Daquangou Reservoir,and cultivated land were higher,49%and 54%and 60%respectively,content of sand in wetlands,wasteland and sand dunes was relatively higher,accounting for 44%,32%and 91%respectively.The contents of total organic carbon（TOC）,total nitrogen（TN）and total phosphorus（TP）were significantly affected by land use patterns.The content of TOC and TN was higher in the sediments and farmland of Mushroom Lake Reservoir and Daquangou Reservoir,and the trend of the two changes with depth was similar;The content of TP was high in cultivated land and Mushroom Lake Reservoir.Due to its special sedimentary characteristics,it varies irregularly with depth.The carbon/nitrogen（C/N）,carbon/phosphorus（C/P）,and nitrogen/phosphorus（N/P）ratios vary with land-use change,in general,the effectiveness of N and P in the study area was high.Land use patterns and grain size characteristics affect the distribution of nutrients to a certain extent.There were also significant correlations among various nutrient salt indicators,reflecting the balance and coupling relationship between nutrients.（3）Land use change has an impact on the distribution of nutrients and the quality of water environment.Through the output coefficient model,the contribution rates of land use patterns to the TN and TP of the water body within the 10km buffer zone of Mushroom Reservoir and Daquangou Reservoir were estimated.Among them,the contribution rate of cultivated land was the highest by 85%and 70%,followed by construction land,being 5%and 15%,while the contribution rate of grassland relative to its area was relatively low,below5%,had a certain interception and absorption effect on N and P,indicating that land use changes will affect TN and TP in the environment.The natural distribution.The contents of TN and TP in both reservoirs were higher than the agricultural irrigation water standards(Type II water,TN<0.5 mg·L^-1,TP<0.1 mg·L^-1),and the eutrophication of Mushroom Lake Reservoir was higher than that of the Daquangou reservoir;the TN and TP contents in the sediment were contaminated,but they were not toxic to benthic organisms.In short,land-use changes will lead to the loss of surface sources of nutrients,resulting in enrichment in lake sediments,affecting the cyclical laws of C,N,P in land and water,destroying soil and aquatic ecosystems,and putting pressure on the environment.