| Rice-wheat rotation is the general plant pattern in the Taihu region, where the high input-high output mode is one of the main characteristics of agricultural production. To assessquantitatively the impact of fertilization changes on agricultural nitrogen balance and its loss in the Taihu region, field experiment was conducted in a rice-wheat rotation system in this region. The concentration of different forms of nitrogen, both in the surface runoff and subsurface flow, were measured comparatively. Moreover, the impacts of fertilization changes on soil nitrogen balance and its loss were analyzed by collecting monitoring data of leaching, rainfall intensity and concentration of nitrogen in soil, crops and rain. The main results are as follows:(1) In a complete cycle of rice-wheat rotation system, the soil nitrogen surplus intensity of the 2000s is 134.0 kg/ha,32.8% higher than that (100.9 kg/ha) of the1980s. Nitrogen surplus mainly occurred in the rice season, while in the wheat season the nitrogen surplus intensity was quite low. It is 17.9 kg/ha of 2000s, and even negative of 1980s with a value of -6.4 kg/ha; the nitrogen surplus intensity of 1980s and 2000s are 107.3 kg/ha and 116.1 kg/ha respectively. The conclusion we can reached is that the farmland soil nitrogen surplus intensity rose by 32.8% since 1980s with the background of fertilization changes. The nitrogen surplus occurred mainly in the rice season.(2) Nitrate nitrogen is the mainform of nitrogen loss through surface runoff at two sample plots. In the wheat season, the concentration of total nitrogen runoff loss in 2000s was 32.11 mg/L, while nitrate nitrogen concentration was 19.64 mg/L, increased by 47.9% and 40.6% than that of 1980s respectively. In the rice season, the concentration of total/nitrate/ammonium nitrogen of surface runoff was 7.85 mg/L,3.44 mg/L and 2.03 mg/L separately, increased by 160.7%,157.9% and 193.5% than that of 1980s. The proportion of nitrate nitrogen in total nitrogen in 2000s was evenly higher than that in 1980s. It shows that the intensity of nitrogen loss through surface runoff increased due to fertilization changes since 1980s. The main form is nitrate nitrogen. The ratio of nitrate nitrogen and total nitrogen concentration in furface runoff increased due to fertilizer type changes. The risk of nitrogen loss through surface runoff in the wheat season is higher than that in the wheat season.The surface runoff loss intensity of total nitrogen and nitrate nitrogen at 2000s sample plot was higher than that at 1980s sample plot in both wheat and rice seasons. In the wheat season, the loss intensity of total and nitrate nitrogen in surface runoff at 2000s sample plot were 8.7 kg/ha and 5.3 kg/ha, respectively, increased by 53.3% and 48.2% than that at 1980s sample plot. In the rice season, On paddy fields in 2000s, the loss intensity of total/nitrate/ammonium nitrogen in surface runoff were 3.3 kg/ha,1.4 kg/ha and 0.8 kg/ha, respectively, increased by 194.6%,187.8% and 212.0% than that in 1980s sample plot separately. It shows that fertilization changes leads to the increase of surface runoff loss of nitrogen, in which the nitrate nitrogen is the main form. The proportion of nitrate nitrogen in total nitrogen tends to increase simultaneously.(3) The concentrations of total/nitrate/ammonium nitrogen of subsurface flow at 2000s sample plot increased evidently due to fertilization changes. It is about 1.8～2.2times,1.9～ 2.4 times, and 2.6～5.1 times in the wheat season of 1980s, while 1.9～3.8 times,1.1～8.6 times and 1.7～4.5 times in the rice season of 1980s.In the wheat season, the main form of nitrogen loss through subsurface flow is nitrate nitrogen at both plots. The proportion of nitrate nitrogen in total nitrogen was 72.6%～84.8% for 1980s and 83.8%～91.0% for 2000s respectively. Soil nitrate is easier to loss through subsurface flow in the wheat season due to fertilization changes since 1980s. Long-term accumulation of soil nitrate leaching will have a negative impact on the groundwater environment unavoidably.In the rice season, the concentrations of ammonium and nitrate nitrogen in subsurface flow at 2000s sample plot are higher than those at 1980s plot. The proportion of ammonium in total nitrogen was 21.1%～23.8% for 1980s and 9.6%～43.2% for 2000s respectively. Soil ammonium is easier to loss through subsurface flow in the rice season due to fertilization changes since 1980s. The loss of soil ammonium nitrogen leaching will lead to reduced soil fertility in the long term.The concentration of nitrogen in subsurface flow in the wheat season is higher than that in the rice season for both plots. For the 1980s plot, the concentration of total/nitrate/ammonium nitrogen in the wheat season is 14.4～25.6 times,63.2～120.0 times and 1.4～2.6 times that in the rice season.The numbers are 13.2～26.3 times,17.5～ 146.0 times and 0.1～1.2 times for the 2000s plot. It shows that the risks of soil nitrogen loss in the wheat season are higher than that in the rice season. The main form of nitrogen loss in the wheat season is nitrate nitrogen, while it was mainly ammonium nitrogen loss in the rice season. |
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