Nitrogen And Phosphorus Discharge Load And Balance In A Small Typical Agricultural Watershed In Three-Gorges Reservoir Area

As the most distinguished hydraulic engineering construction, Three Gorges reservoir attracts great attention all over the world. Since June 2003, after begining of water reserving, many have reported that eutrophication frequently occurred in certain areas of water body, and even more serious situations found in partial tributary steams, which resulted in threatening the water envrionemtnal safety and sustainable development of society in areas of down streams. With the development of financial investment and investigation including controls and treatments for eutrophication, the effect of eutrophication on water quality in Three Gorges reservoir will be much more significant. Thus for deeply understanding the characteristics and intensity of eutrophication occurring in reservoir areas, a typical watershed called Wang Jiagou, having the main crops planted in the areas are rice,corn and tuber mustard. which is a relatively topographic confined in Fu Lin sect of reservoir areas, was selected as the investigation field in this study. Based on above mentions, the surfacewater changes and farming activities were investigated consecutively in one year period. The detailed targets in this study were:1) understanding the change rule of nitrogen (N) and phosphorus (P) concentrations in various types of surfacewater; 2) clearly investigating the discharge dynamic and load intensity of N and P in runoff in this research field; 3) comparing and analyzing effect of N and P budget of this farmland on N and P in non-point losses, in this watershed. The main research results were listed as follows:Ⅰ. Surfacewater QualityDuring the investigating period, total 293 samples were collected from various types of water including ditch runoff, pool and well water in the watershed, and analyzed, which showed over 74.7% of samples exceeded total N standard of V type water, and 62.1% of all were below total P standard ofⅢtype water, respectively. The results showed that, in this studied areas, N pollution (especially total N) was serious in surfacewater, as compared to P pollution.Ⅱ. N and P concentration Comparsion and analysis Three types of surfacewater including ditch runoff, pool and well water were all investigated in our setting study period, which showed the total N (TN) concentration ranges were 0.2-26.4,0.5～16.6 and 3.3～34.7mgN/l, respectively, and the average TN concentrations were 5.9,4.6 and 11.9mgN/l respectively. In certain one sampling time, average TN concentrations of well sufacewater were higher than from ditch and pool. As the important part of nitrogen element in water, averagely NO3--N accounted for TN concentrations of ditch, pool and well surfacewater were 59.5%,55.5% and 86.6% respectively; as compared to NH4--N, which only accounted for TN concentrations of three types of surfacewater were 3.5%,9.9%and 1.1% respectively. In addition, the dissolved organic and particle nitrogen (PON) contribution rate for TN were in the ranges of 11.6-35%. Thus, in above three types of surface water, dissolve inorganic nitrogen (DIN) (especially NO3--N) was the mainly fraction of TN, however, the PON existence still can not be negligible.For P investigation, the ranges of ditch, pool and well surfacewater in the watershed, were 0.01～3.82, 0.01～3.31 and 0.01～1.93mgP/l, and average concentrations were 0.28,0.32 and 0.27mgP/l respectively. As contrast to TN, which mainly were composed of DIN, particle P (PP) mainly accounted for TP, following dissolved P (DP). Additionally, the PP accounted for TP ranged from 59.3% to 65.4% in three types of water, however the DP only accounted less than 41% of TP concentrations.Ⅲ. N and P discharge loadOne discharge was setted for estimation of N and P losses and yearly load in the watershed. The total N and P losses load yearly were approximately 3985kgN and 112kgP respectively, and average N and P losses per area unit of farmland were 61kgN/(hm2.a) and 1.7 kgP/(hm2.a) respectively, which N losses in runoff was 36 times higher than P losses. According to crop growth seasons for calculation, in spring and summer period of the seedtime and early period of the growth of rice and corn,which contributed 80.5% of the whole yrea N losses in runoff and 75.9%of the whole year P losses in runoff, respectiely.In autumn and winter of the seedtime and early period of the growth of tuber mustard,which contributed 18.9% of the whole yrea N losses in sunoff and 23.2%of the whole year P losses in sunoff.Thus, for decreasing the agricultural non-point pollution risk, key control treatment is to decrease the soil N and P (expecially N) losses from runoff in spring and summer time, which time is for rice and corn plantation and growth. Meanwhile, reasonable and rational distribution of land usages is also the one of most important ways to effectively control N and P losses derived from agricultural non-point sources. In addition, this study suggested that the N and P losses load in sub-catchment of surfacewater in lower stream parts of watershed, which were distributed more rice fields, was lower than from sub-catchment,which mainly accounted for drylands. This may be explained by that rice fields played a role of barrier and interception for collect nutrient elements in runoff from middle or high land as a pool. IV. Relationship between N and P budget and losses in runoffIn this investigated field, the net incomes of N and P (caculated) were 819 and 115 kg/hm2.a, and net outlets were 507 and 33kg/hm2.a, and the net balance were 312 and 82kg/hm2.a respectviely. The application of fertilizers in farmlands is the mainly inlet source, which accounted for 72.3% and 82.9% of N and P incomes. Secondly, straw returing for fertilizer contributed only 14.7% and 9.2% of total N and P. The soil N and P discharge mainly was uptake of vegetation such as crops, which accounted for 42.1% and 94.8% of N and P discharges. In addition,12.0% and 5.2% of total N and P discharges were due to runoffs in watershed, and 10.3% and 1.8% of N and P fertilizers application amount also lose by runoffs. For consideration of three typical farmland usage types in this studied field, effect of tuber mustard plantation was greatest significant, which accounted for 12.0% and 80.8% of N and P net incomes, following corn plantation, which accounted for 27.9% and 19.2%, and the effect of rice field was lower significant. Thus, applying rational technology and measures such as mixtures application of various fertilizers, decresing soil N and P net incomes in dry farming system (e.g. tuber mustard and corn plantation), would be helpful for decreasing the effect of agricultural activities on surrounding water environment.