Study On Phosphorus Balance And Its Loss Following The Water Migration In Rice-wheat Rotation System

The Taihu Lake area is dominanted by fertile farmlands with rice-wheat rotation.It is a well-known high-yield agricultural production area in China,which also consumed the highest amount of chemical fertilizers.Long-term application of large-scale of chemical fertilizers has indeed brought great achievements to agricultural modernization,but excessive fertilization also brings potential environmental risks.Since the limited ability of crops to absorb nutrients in soil,the most of the phosphorus input from fertilizers accumulates and enriches in the soil.After Pi enters the water systems through rainfall runoff or leaching,it will promote the growth of algae and threaten rivers and lakes and causes eutrophication.Therefore,the problem of agricultural non-point source pollution in the Taihu Lake Basin has attracted more and more attention.This research selected the Yixing area of the Taihu Lake Basin to conduct field experiments to study the migration of phosphorus in the rice-wheat rotation system.Through the effect of fertilization and the loss of phosphorus in farmland,the threshold of phosphate fertilizer application in rice-wheat rotation system is studied,which is used to guide agricultural fertilization,achieve the effect of weight loss and efficiency,and reduce environmental pollution caused by excessive fertilization.The field experiment was conducted from the 2018 wheat season to the 2020 rice season,with 4 seasons in total.The experiment set up two sub-experiments to explore the phosphate fertilizer migration,which are the experiment of different phosphate fertilizer application rates and the optimization experiment of phosphorus reduction combined with organic fertilizer.Experiments with different amounts of phosphate fertilizer:6 gradient treatments were set for both rice and wheat crops.The treatments were:（1）P0,no phosphate fertilizer;（2）P25,application of 25 kg P₂O₅ hm^-2 chemical phosphate fertilizer;（3）P50,application of 50 kg P₂O₅ hm^-2chemical phosphate fertilizer（4）P75,apply 75 kg P₂O₅ hm^-2 chemical phosphate fertilizer;（5）P100,apply 100 kg P₂O₅ hm^-2 chemical phosphate fertilizer;（6）P150,apply 150 kg P₂O₅ hm^-2 chemical phosphate fertilizer.Optimization experiment of phosphorus reduction combined with organic fertilizer:5treatments were set up for both rice and wheat seasons,and the treatments were:（1）CK,no fertilizer;（2）NK,no phosphate fertilizer;（3）FFP,farmer’s conventional fertilization(the local farmer’s customary fertilization rate was 90 kg P₂O₅ hm^-2);（4）FFP-20%P,chemical fertilizer with 20%phosphorus reduction;（5）FFPM-36%P,chemical fertilizer with 20%phosphorus reduction,using organic fertilizer to replace 20%chemical phosphate fertilizer.Research indicates:1.The application of phosphate fertilizer can significantly increase the yield of rice and wheat,and promote the growth and development of rice and wheat.When the phosphorus application reaches 75 kg P₂O₅ hm^-2,the annual yield of rice and wheat is the best.If the threshold is exceeded,the annual yield increase effect will be significantly weakened.On the basis of conventional phosphorus application,reducing the application of20%phosphorus fertilizer or reducing phosphorus combined with organic fertilizer can significantly improve the agronomic efficiency of phosphorus fertilizer and phosphorus fertilizer use efficiency of rice and wheat,and promote the absorption and utilization of phosphorus by rice and wheat.2.The content of available phosphorus in the soil during the harvest period of rice and wheat increased with the increase of the phosphorus application rate.After the phosphorus application rate reached 75 kg P₂O₅ hm^-2,the growth rate decreased.Reducing phosphorus by 20%or replacing some chemical phosphate fertilizers with organic fertilizers after phosphorus reduction can significantly increase the soil available phosphorus content.The annual phosphorus surplus of the soil under different phosphorus levels increased with the increase of phosphorus application,and changed from a deficient state to a surplus state.Replacing some chemical phosphate fertilizers with organic fertilizers after reducing phosphorus by 20%can effectively reduce the annual soil surplus of phosphorus.Therefore,it is recommended to apply 75 kg P₂O₅ hm^-2 of phosphate fertilizer.3.（1）The concentration of TP and DP in the field surface water reached the highest peak on the first day after basal fertilizer was applied.The concentration of TP and DP in the surface water in the first 9 days decreased rapidly,and then gradually stabilized.Therefore,about one week after the application of phosphate fertilizer is the key period to control the runoff loss of phosphorus in the field water,and the field water is mainly lost in the form of dissolved phosphorus.（2）The amount of phosphate fertilizer applied is positively correlated with the loss of TP runoff and the loss of TP leakage.Within the range of 0～75 kg P₂O₅ hm^-2 of phosphorus application,the cumulative TP loss is under reasonable level.If the threshold is exceeded,the TP runoff and leaching loss will increase significantly.Reducing the application of 20%phosphate fertilizer can effectively reduce the runoff loss and leaching loss of phosphorus from paddy fields.Substituting organic fertilizer for some chemical phosphate fertilizers has reduced the leaching of phosphorus from paddy fields,but it may increase the risk of runoff loss.Taken together,this study get the conclusion of the phosphorus migration law in the rice-wheat rotation system in the Taihu Lake basin in a quantitative manner,and found that the threshold of phosphorus fertilizer application in Yixing area is 75 kg P₂O₅ hm^-2.If the threshold is exceeded,phosphorus will be lost more and more.It is recommended to reduce phosphate fertilizer by 20%on the basis of conventional fertilization for farmers(FFP=90kg P₂O₅ hm^-2),and at the same time,organic fertilizer can be used to replace 20%rest chemical phosphate fertilizer,which is more conducive to ensure rice and wheat yield and relative high phosphorus use effiency to reduce phosphorus loss.