| The water quality in the Three Gorges Reservoir was closely related to water safety and ecological economic development of reservoir area and the Yangtze River.The agricultural land use and fertilizer widespread application caused agricultural non-point source pollution was considered to be a important factor affecting water quality in the reservoir area.Understanding the occurrence strength and characteristics of agricultural non-point source pollution in the reservoir area could provide important scientific basis for prevention and control measures.A high frequency (daily) monitoring was carried out in a closed watershed in Fuling of the Three Gorges Reservoir for two years, and divided the river basin into two catchments A and B according to the distribution of the ditch. The aim to analyze nitrogen concentration and export fluxes in runoff. At the same time, using the mixed ion exchange resin adsorption method collected the soil nitrogen and phosphorus leaching exudates in different mulberry-crop systems on the slope farmland of the watershed for a year. The main research results were listed as follows:1. Dynamic characteristics of nitrogen and phosphorus in runoffThe land use in watershed was mainly corn/rice-mustard crop, in April and early May, corn and rice were respectively planted in rainfed cropland and rice paddy, and harvested in mid-August each year; in early October mustard were planted in all farmland, and harvested in at the end of February in the second year. Because after the fallow period was shorter quarterly harvest, regarded the end of March to mid-September as rice-corn season and the end of September to mid-March of second year as mustard season. In order to facilitate analysis and discussion, a corn/rice-mustard year rotation period was called a monitoring year.The TN concentration of catchment A ranged 0.92～32.18mg/L, mean concentration was 7.39 mg/L, the TN concentration of catchment B ranged 0.28～20.41mg/L, mean was 4.92 mg/L in the first monitoring year. By comparison, TN and NO3- -N concentrations of catchment A were 48%～53% and 39%～60% higher than catchment B, NH4+-N concentration was little difference. The TN concentration of catchment A ranged 0.39～28.58mg/L, mean concentration was 12.30 mg/L, the TN concentration of catchment B ranged0.86～28.90mg/L, mean was 6.78 mg/L in the second monitoring year. By comparison, TN and NO3- -N concentrations of catchment A were 58%-81% and 68%～109% higher than catchment B, NH4+-N concentration of catchment A was a time than catchment B.The TP concentration of catchment A ranged 0～2.19mg/L, mean concentration was 0.17 mg/L, the TN concentration of catchment B ranged 0-0.88mg/L, mean was 0.06 mg/L in the first monitoring year. The TP concentration of catchment A ranged 0～0.93mg/L, mean concentration was 0.13 mg/L, the TP concentration of catchment B ranged 0～1.02mg/L, mean was 0.06 mg/L in the second monitoring year. By comparison, the TP concentration of catchment A was higher about 2 to 3 times than catchment B; in different monitoring years, the difference of TP concentration between catchment A and B was not significant.In a two-year monitoring period, the TN and NO3- -N concentration in runoff of catchments A and B increased in April to May (early rice/corn season) and October to November (early mustard season) due to the impact of farmland fertilization activities. In early September after the rice/corn harvest, we also monitored a short peak on nitrogen concentration in runoff of in, which may be related to the summer heat soil mineralization. In addition, in February to March in the second year of planting, the highest peak on TN concentrations of two catchments appeared in, which may be decreased in rainfall comparing that of the same period in previous year, and the runoff also reduced, concentration and enrichment leaded to TN concentration was increased in runoff. The highest peak of TP concentration in runoff occurred in April to May (early rice/corn season), the rest of the time was not significant fluctuations, and PO43- -P concentration was monitored at a low level throughout the period. 2. Discharge load of nitrogen and phosphorus in runoffThe first monitoring year, the discharge load of TN、NO3--N and NH4+-N in catchment A was 16.10 kg/hm2、11.55 kg/hm2 and 0.007 kg/hm2, the discharge load of TP was 0.21kg/hm2; the discharge load of TN、NO3- -N and NH1+ -N in catchment B was 5.21 kg/hm2、3.60 kg/hm2 and 0.007 kg/hm2, the discharge load of TP was 0.08kg/hm2. The second monitoring year, the discharge load of TN、NO3- -N and NH4+-N in catchment A was 17.81 kg/hm2,13.37kg/hm2 and 0.11 kg/hm, the discharge load of TP was 0.24 kg/hm2; the discharge load of TN、NO3- -N and NH4+-N in catchment B was7.33kg/hm2、4.84kg/hm2 and 0.05kg/hm2, the discharge load of TP was 0.07 kg/hm2. As can be seen, in addition to NH4+ -N, the discharge load of TN, NO3- -N and TP in runoff of catchment A were significantly higher than those in catchment B. Comparison of different crop season, the discharge load of TN and NO3- -N in runoff of catchment A and B in rice/corn season was slightly higher than that in mustard season. Different crop growing seasons runoff of nitrogen output was NO3- -N, accounting for about 60%-81%.The volume of runoff in the first monitoring year was 1024 m3/hm2, discharge load of TN and TP were 8.60kg/hm2 and 0.12kg/hm2; The volume of runoff in the second monitoring year was 1221 m3/hm2, discharge load of TN and TP were 10.59 kg/hm2 and 0.12 kg/hm2. TN in the second monitoring year was more than the first monitoring year 23%, while the annual discharge load of TP was little difference, discharge load of TN was 72～88 times than TP, indicating Wangjiagou catchment nitrogen runoff loss is relatively serious, and we should focus on controlling water pollution caused by nitrogen loss.Due to the paddy wetland system to intercept runoff and runoff of nitrogen, phosphorus consumptive effects, two catchments have large differences in paddy spatial distribution pattern in this study, this factor may be the ultimate reasons that catchment B’s volume of runoff and the loss of nitrogen and phosphorus loads were significantly lower than the catchment A. 3. Different mulberry-crop systems nitrogen leaching characteristics in the slopingDifferent mulberry-crop planting systems soil TN leaching varied from 21.26-23.45 kg/hm2, the annual average was 22.47 kg/hm2. TN leaching in corn season was 10.66 kg/hm2, TN leaching in mustard season was 11.81 kg/hm2, was 10.8% higher than the former. Using differences analysis among the experimental fields of nitrogen loss was not significant. It can be inferred that different mulberry-crop planting systems effecting on nitrogen leaching of sloping land was not very significant.Different mulberry-crop planting systems soil NO3- -N leaching varied from 13.45-14.52 kg/hm2, the annual average was13.99 kg/hm2,,accounting for 60% to 64% of TN leaching; soil NH4+ -N leaching varied from 7.81-9.24 kg/hm2, the annual average was 8.51 kg/hm2, accounted for 36% to 40% of TN leaching. Comparison of different crop planting season, NO3- -N and NH4+-N leaching were 6.12 kg/hm2 and 4.56 kg/hm2 in corn season, and 7.87 kg/hm2 and 3.49 kg/hm2 in mustard season. NO3- -N leaching in mustard season was 28.6% higher than in corn season, and NH4+ -N leaching was contrast, the latter was 30.7% higher than the former. 4. Different mulberry-crop systems phosphorus leaching characteristics in the slopingDifferent mulberry-crop planting systems soil TP leaching varied from 0.189～0.209kg/hm2, the annual average was 0.197 kg/hm. Using differences analysis among the experimental fields of phosphorus loss was not significant. TP leaching in corn season was 0.096kg/hm2, TN leaching in mustard season was 0.100 kg/hm2.TN leaching was 110.4～124.0 times than that of TP, which indicating leaching leakage of nutrients was mainly nitrogen in test area. Therefore, a reasonable application of nitrogen fertilizer can improve nitrogen use efficiency, reduce nitrogen leaching, prevent and control the non-point source pollution in the catchment. |
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