| It is reported that there are50%nitrogen and60%phosphorus runoff from cropland into rivers and lakes resulted in eutrophication, and most of them are from paddy fields. In Tai Lake region, the paddy fields which accounts a large proportion of total cropland has become an important source of agricultural non-point pollution because of the large amount of rainfall in rice growth season, resulting in increased nitrogen and phosphorus runoff loss. As consequence, recognizing the characteristic and risk of nutrient runoff loss from the paddy fields in Tai Lake region, and find out good technologies and measures to control or reduce the nutrient runoff loss from paddy fields is an urgent problem to solve at present.In order to clarify the characteristic and risk of nitrogen and phosphorus runoff loss from the paddy fields in Tai Lake region, and find proper agricultural management practices, the fertilization and nutrient balance in Tai Lake region were analyzed, and the characteristic of the dynamic changes of nitrogen and phosphorus in the surface water and runoff loss under different farming practices with straw return in paddy fields were studied through questionnaire survey, data collection> simulation experiment, field control experiment and field monitoring experiment. Taking Liyang city as an example, we assessed the risk of nitrogen and phosphorus runoff loss from paddy under different farming practices with straw return in the region. In addition, in order to control and reduce the pollution runoff from paddy fields to external water, the purification effect of water hyacinth on removal ability of nitrogen and phosphorus from differently concentration paddy discharge water were comparatively studied. Based on the paddy-pond system with water cycling constructed, we studied the effects of the system on controlling the nitrogen and phosphorus runoff loss, so as to provide the technical supports relieve the eutrophication of Tai Lake for. The main conclusions are as follows:1. The annual average application of nitrogen in two cropping patterns in Taihu Lake region including Liyang, Yixing and Suzhou Wuzhong are556.2and488.0493.3kg/hm2respectively, and the surplus rates are100.2%and62.5%. In addition, the average application of nitrogen of rice in two cropping patterns is298.0kg/hm2, which is higher than wheat at229.1kg/hm2and rape at218. kg/hm2.Furthermore, the deficit rate of phosphorus in wheat-rice cropping pattern is9.0%, and it is-7.4%in rape-rice cropping pattern, with annual application being117.6and116.7kg/hm2, respectively. Meanwhile, there is more than50.0%deficit of potassium in both two cropping patterns, and the least annual application of potassium is107.3and105.3kg/hm2. In the two rice cropping patterns, the application of nitrogen had largest surplus resulting in eutrophication, however, the least application of potassium contributed to critical nutrient deficit, while the phosphorus was reasonable for nutrient balance.2. The dynamics of TN and TP concentrations of surface water in paddy field was monitored after fertilizing on the experiment plots in Liyang and Yixing in Taihu Region. Concerning on the TN concentrations in surface water, they both reached its highest level at the first day after fertilizer application. Then they descended after a few days, and then maintained at a constant level. It took7-8days to be constant after base fertilizer application, and took about5days after tilling fertilizer application. Either after base or tilling fertilizer application, it could be observed that Y (TN concentration of surface water of paddy field) and t (time after fertilizing) could be fitting by the linear equation (y=CO×e-kt) in both two experiment plots, and the fitting were highly significant. Concerning on the TP concentrations in surface water, they both raised rapidly after fertilizer applied in the two experiment plots. They reached the peak on the first day and then descended gradually to a constant level within8-9days. While there were a few tiny drifts in the following days until the paddy got out of drowning.3. Whether straw return or not, the runoff volume from different tillage mothods were sequenced as plough treatment<rotary tillage treatment<non-tillage treatment. However the nutrition concentrations of TN and TP in runoff water showed the reverse tendency. The runoff volume in non-straw treatments was higher, whereas the nutrition concentration was lower than that in straw treatments. In the non-straw treatments, the total amount of runoff nitrogen from paddy field during rice growth period were6.78,8.05and11.09kg/hm2respectively for plough, rotary tillage and non-tillage. And in the straw treatments, they were4.82,6.44and8.87kg/hm2respectively. Meanwhile the total amount of runoff phosphate were0.05,0.63and0.78kg/hm2respectively for plough, rotary tillage and non-tillage in the non-straw treatments. And they were0.39,0.51and0.70kg/hm2respectively in the straw treatments. In all treatments, the runoff volume of plough with straw was the lowest. For purpose of controlling the agricultural Nitrogen and phosphorus widespread pollution, the plough treatment combined with straw return was better to reduce the concentrations and the amount of nitrogen and phosphate in runoff, and contributed to conserve fertility and moist in paddy field.4. The nitrogen and phosphorus runoff loss in rice season were evaluated under different tillage methods and straw return, based on the precipitation data(1991-2010) in Liyang in Tai lake region through model. The results show that during last two decades, the average rate of nitrogen runoff loss under different treatments are from1.66%to2.15%, while the largest rate are from2.87%to3.76%, and the least rate are from0.75%to1.06%. The average rate of phosphorus runoff loss which is lower than nitrogen are from0.59%to0.82%, and the largest rate are from1.45%to1.90%, while the least rate are from0.10%to0.14%. Whether straw returns or not, the nitrogen and phosphorus loss rates are higher under no-tillage than those under rotary tillage and rotary are higher than plowing. The nitrogen and phosphorus runoff loss are significantly lower with straw return compared to those without it under the same tillage method. The nitrogen and phosphorus runoffloss are high or low under one treatment in certain year is similar to those under other treatments obviously. In addition, higher precipitation results in higher nutrient loss, however, mistakes could exist to sometimes.5. The removal rates of TN by water hyacinth from different concentration paddy discharge water were62.04%,76.75%and88.46%, respectively, while the removal rates of TP were93.52%,88.46%and78.57%. There was a significant correlation between the concentration of nitrogen and phosphorus in the paddy water and purification by water hyacinth. The average removal rate of TN by water hyacinth dropped with the increase of initial concentration of TN, while the average removal rate of TP increased with the increase of initial concentration of TP. In the paddy-pond circulation system, there was2592.42m3/hm2paddy runoff during the rice growth period, however, outward runoff emissions was0because of storage by pond; the irrigation in the system was6750.55m3/hm2, the added irrigation water from outside was3594.32m3/hm2, and reduced the water from outside was3156.23m3/hm2; Rainwater impounded by pond was5228.44m3/hm2during the whole growth season, which contained TN and TP of1.49and0.03kg/hm2, respectively. |
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