Land Use Changes And Its Control Effect Of Nutrient Loss In Lanlingxi Watershed Of The Three Gorges Reservoir Area

The Three Gorges Reservoir area is the sensitive and fragile district of ecological environment in China.Since 2000,the implementation of a series of projects such as Returning Farmland to Forest and the Yangtze River Shelterbelt had made significant changes on land use in the small watershed of this area.Land use is closely related to the output of non-point source pollution,and land use changes have a marked impact on soil erosion and soil nutrient output.Therefore,it is urgently necessary to understand the change of land use after the Returning Farmland Project and its effects on soil erosion as well as nitrogen and phosphorus output in low mountainous upland region along Yangtze River in the Three Gorges Reservoir Area.In this study,Lanlingxi small watershed was taken as the research object,field runoff plots experiments,long term locating observation were used to discuss the difference of soil erosion,nitrogen and phosphorus nutrient output characteristics among various land use types,and GIS spatial analysis was used to clarify the land use structure change and its control effect of nutrient output.The causes of those differences were investigated,and the pathways of different forms of nutrients were also analyzed.Moreover,the main source of pollutants which were discharged into the river basin was identified,and the process of nutrient output was explored through typical rainfall.This study was conducted on plot scale,catchment scale and the whole watershed,aiming at providing the basis for the research of non-point source pollution control,water environment governance and other pertinent studies.The results obtained are listed as followings:On the plot scale,runoff and sediment output of typical reforestation patterns including garden plot?tea garden and citrus orchard?,forest land?Chinese Chestnut?and the original slope farmland were determined and then analyzed,as well as soil physical properties of these four patterns.Annual runoff loads of de-farming patterns?tea garden,Chinese Chestnut and citrus orchard?were greatly reduced compared with slope farmland,as well as runoff coefficients.The runoff volume was ranked in the order of Chinese chestnut(110.71m³·hm^-2)<citrus orchard(164.16 m³·hm^-2)<tea garden(171.74 m³·hm^-2)<slope farmland(181.38 m³·hm^-2).The runoff of the chestnut forest was significantly smaller than the other three models,and the average runoff volume of de-farming patterns decreased 17.92%.Annual sediment yield on different reforestation patterns presented the same rule,in the order of Chinese chestnut(141.92kg·hm^-2)<citrus orchard(343.78 kg·hm^-2)<tea garden(394.79 kg·hm^-2)<slope farmland(1210.77kg·hm^-2).De-farming patterns performed significantly better in holding soil and alleviating soil erosion than slope farmland,and the average sediment yield of de-farming patterns decreased 75.76%.After returning farmland to forest,soil water physical properties had been greatly improved.Compared with slope farmland,soil bulk density of de-farming patterns reduced,and soil capillary porosity,non-capillary porosity and total porosity increased accordingly,which therefore made the soil water storage capacity enhanced.After returning farmland to forest,soil physical properties had been some improved,and soil erosion had been effectively controlled.After the Returning Farmland to Forest Project the quantity of annual soil nutrient?nitrogen and phosphorus,the sum of them in sediment and runoff?loss decreased.The output of total nitrogen?TN?was in the order of slope farmland(5034.31g·hm^-2)>tea garden(3284.15g·hm^-2)>citrus orchard(2656.35g·hm^-2)>Chinese chestnut(1801.81g·hm^-2),and total phosphorus?TP?loss was slope farmland(1977.93g·hm^-2)>tea garden(629.72g·hm^-2)>citrus orchard(614.36g·hm^-2)>Chinese chestnut forest(205.13g·hm^-2).Compared with slope farmland,the load of TN and TP output of reforestation patterns decreased 48.74% and 75.58%,respectively.Compared with slope farmland,available nitrogen loss decreased in reforestation patterns.Total nitrate nitrogen?NO₃^--N?loss ranked in the order of slope farmland(616.04g·hm^-2)>tea garden(568.91g·hm^-2)>Chinese chestnut forest(302.75g·hm^-2)>citrus orchard(242.09g·hm^-2).The largest amount of ammonium nitrogen?NH₄⁺-N?was found in slope farmland(385.89g·hm^-2),then in tea garden(340.96g·hm^-2),the third in citrus orchard(311.39g·hm^-2),and the least in Chinese chestnut forest(205.26g·hm^-2).The main route of TN,NO₃^--N and NH₄⁺-N loss were all through runoff,the quantity of TN,NO₃^--N and NH₄⁺-N output in which accounted for 67.49%,98.56% and 97.91% of the total,respectively.The quantity of TP in sediment accounted for 90.60% of the total,TP loss showed extremely significant correlation with sendiment,which showed that sediment output was the main approach of TP loss.After the Returning Farmland to Forest Project the land use structure of Lanlingxi watershed changed dramatically.By using Arc GIS hydrological analysis module the watershed was divided into 9 catchments,and the land use type of this watershed was divided into 6 categories,including forest land,garden plot,cultivated land,residential land,water area and unused land.The study found that nitrogen output of each catchment was different from others.Spatial clustering analysis of nitrogen indexs showed that,9 catchments could be roughly divided into 3 categories,and there were significant differences among them: class I included S1 and S2,which was slightly effected by human activity and concentration of nitrogen nutrient output was low;class II was composed by S3,S4,S8 and S9,due to a certain level influence of agricultural activities and the lives of the residents,nitrogen concentration was higher than the former;class III was consisted by S5,S6 and S7,which was impacted greatly by agricultural activities and the lives.In addition,NO₃^--N concentration of class III excessed centralized drinking water sources of project standard limit,and TN concentration exceeded standard V for surface water.There were significant positive correlations between garden plot,residential land and N loss,and negative correlations between forest land,unutilized land and N export,which indicated that the formers were N sources and the latters were N sinks.Taking ecological benefits,economic benefits and the analyze of N source/sink into consideration,class II should be the pursuit of structural adjustment of this watershed,it was that forest land area should be increased and garden plot should be controlled,and the area ratio of residential land should be kept below 5%.Moreover,forest-tea and forest-fruits modes should be applied to change single land use type of garden plot in some catchments.The runoff and sediment output of Lanlingxi watershed in flood season were180246.73m³ and 199.10 t,respectively.The maximum monthly runoff and sediment output were both in May,the minimum of which were in August.The sediment output caused by heavy rainstorm event in May 11 th accounted for 44.91% of the total output,which revealed that frequency and intensity of rainstorm played a decisive role in sediment output.TN,TP,NO₃^--N and NH₄⁺-N output in flood season were 1879.76 kg,217.85 kg,37.71 kg and 1213.32 kg,respectively.The main route of TN loss was through runoff,TP mainly lost with sediment,and NO₃^--N loss made chief contribution to available nitrogen output.The erosion modulus of this watershed was 331.83 t·km^-2·a-1,and water erosion belonged to slight erosion.The average concentration of NH₄⁺-N,NO₃^--N,TN and TP was 0.21,7.62,10.75,0.10mg·L-1,respectively.NH₄⁺-N and TP concentration were under the II standard of the environmental quality standards for surface water,but TN concentration was over V standard.And NO₃^--N was below the centralized drinking water sources of project standard limit.In typical rainfall event,the flow rate was consistent with the change of rainfall,the peak value of which lagged behind in the rainfall,and the nutrient concentration curve had a strong response to the flow curve.According to the rainfall duration,the whole rainfall event was divided into three stages: the initial stage,the middle stage and the later stage.The result showed that: The discharge,NH₄⁺-N,NO₃^--N,TN and TP output load in middle stage accounted for the vast majority of the total,and the pecific proportion was 67.17%,75.99%,62.59%,63.41% and 75.95%,respectively.The relationship among three stages of indexes above was the middle >the later >the initial.In addition,the concentfration of NH₄⁺-N was the highest in the middle,then in the initial,and the lowest in the later.The order of NO₃^--N and TN concentration were both the later >the initial >the middle,and TP concentration gradually declined with the rainfall.The regression analysis showed that the output load of N and P can be simulated by polynomial,so the non-point source pollution load of this small watershed could be estimated by empirical equations.