Variations Of Nitrogen And Phosphorus In The Subsurface Flow Of Flue-cured Tobacco Slope Land In The Fuxian Lake Watershed

Fuxian Lake is the second deep lake in China, as well the largest storage capacity lake in Yunnan Province. But in recent years, the water quality has declined. The major sources of pollution are agricultural areas, sewage runoff and other non-point source pollution. As the major tourism resources and freshwater sources in Yunnan Province, it is essential to reinforce the protection and management for Fuxian Lake.According to the Chinese water resource bulletin of 2007, the overall water quality of Fuxian Lake has dropped from original Grade I to Grade II which belongs to the typical case of poor nutrition type lakes. Pollution of Fuxian Lake mainly comes from non-point source pollution, which takes about 90% of the total load and most of it comes from surface and subsurface runoff of farmland. In the area of plentiful rainfall, soil has high degree of saturation and obvious interflow. The probability that contamination of non-point source transfers through interflow is high, mechanism and effect between interflow and transmission of contamination must be paid more attention.This paper selected the typical river basin of Fuxian Lake in the southwest of our country--Jianshan River as experimental area. We chose the typical land use type tobacco slope farmland as research object. Under natural rainfall condition, we monitored the concentration vertical distribution feature and the temporal dynamic changes of N and P in the interflow of different slope position( 0² m ) in slope land soil throughout the growing season of crops in original position .We also monitored the effect to the concentration of N and P under different fertilization level. The purpose of monitoring results is to control the loss of agricultural nitrogen and phosphorus and prevent eutrophication of water. Meanwhile, it will offer scientific basis on the efficient use of fertilizer, agricultural sustainable development. The results showed that:(1) In routine apply fertilizer level, partial correlation among TN and TP in interflow, rainfall and cumulative rainfall was negative correlation in rainy season. Partial correlation between TN and TP in interflow and rainfall was negative correlation in the dry season, while it was positive correlation between TN and TP in interflow and cumulative rainfall. Average concentration of TP was approximate. Under different fertilization level, concentration of TP was approximate that had no significant difference. It was positive correlation between the concentration of TN and fertilization.With the increase of fertilization, partial correlation between concentration of TP and TN and rainfall was still negative correlation in rainy season, but partial correlation between concentration of TP and TN and cumulative rainfall was positive correlation. Therefore, we inferred that there was a fertilizer critical value between concentration of TP and TN and fertilization. When it was lower than the critical, partial correlation between concentration of N, P and cumulative rainfall was negative correlation with a certain rainfall; while it was contrary when it was higher than the critical value.( 2 ) Dynamic changes on concentration of P and N in interflow①Concentration of TN in interflow was basically identical with time in different slope position. The trend of concentration of N in interflow was first increased and later decreased with the tobacco growth process. Concentration of TP in interflow decreased fluctuated in a certain range with time in different slope position.About half a month after tillage, concentration of TP and TN at 200cm on lower slope was 6.657 mg·L^-1 and 0.030 mg·L^-1 that was 2.4 and 1.5 times than with no tillage which was beyond the nitrogen and phosphorus concentrations of the required standard. So we should avoid the rainy season during tillage as much as possible to reduce the degradation of soil fertility on slope.②Concentration of TN on different slope position from surface of soil layer to deep layer changed largely at firt and got to uniform level later. The changing range was 2.791¹4.656 mg·L^-1 at the depth of 2 m and the average was 6.371 mg·L^-1 which was largely beyond the requirements of the standard nitrogen concentration in water. It was another important way that soil nutrients went into the adjacent water through soil runoff other than surface runoff.③Concentration of TP on different slope position from surface of soil layer to deep layer decreased wavelike. Coefficient of variation got lower. With the increasing of depth in the soil, concentration of TP got stable which showed that phosphorus transported in the vertical direction. The changing range of concentration of TP at 200 cm was 0.016⁰.030 mg·L^-1 and the average was 0.024 mg·L^-1 which was little higher than phosphorus concentration of international standards. Otherwise, as phosphorus could be long term accumulated in the soil, it may have potential impact to the environment of groundwater and surface water which will increase the risk of eutrophication in Fuxian Lake.④Between 0 and 50 cm, slope position was the most influential factor to the concentration of P and N and the low slope was obvious than the middle slope which showed that nutrient distribution on slope had obvious spatial heterogeneity. At the time of fertilization, we could determine the optimal fertilization based on different slope positions.( 3 ) The effect to concentration of N and P in interflow under different fertilizer levels①Under three different fertilizer treatments, concentration of NO3+-N in soil at T3 and T1, T2 had significant difference at 77 cm. Concentration of N in soil showed an increasing trend at 77cm with the increasing of fertilizer treatments, while it had small difference at 200 cm with different fertilizer treatments. Concentration of N in interflow at 77 cm was higher than that at 200 cm. To the leaching loss of nitrogen on tobacco slope field, nitrate nitrogen (NO3+-N) was the main part.②Content of TP under different phosphorus fertilizer treatments was not positive correlation with the content of fertilizer and had no significant differences between treatments. Concentration of P in interflow at 77 cm was higher than it at 200 cm. The average changing range of concentration was 0.02 mg·L^-10.03 mg·L^-1 and 0.01 mg·L^-10.02 mg·L^-1.( 4 ) At the beginning of fertilization, fertilization had great influence on concentration which showed that leakage loss of N, P was relatively large at the beginning. Concentration of nutrient tended to steady state and the effect of fertilizer was smaller and smaller with the time prolonged.