Study On The Ecological Rice Farming Systems With Low Inputs Of Chemical Fertilizers And Pesticides Around Dianshan Lake In The Suburbs Of Shanghai, Southeast China

Located at the western suburbs of Shanghai,Dianshan Lake is the only freshwater lake in Shanghai,and the source of Huangpu River,which is an important drinking water source of Shanghai.In order to protect the water environment of the lake,any industries and livestock breeding around Dianshan Lake were forbidden since the early history.However,the lake is still under a eutrophic condition,whose water quality is classified as Type IV^V,or even worse than Type V in some parts according to China National Water Quality Standards.There are six towns around Dianshan Lake,and rice cultivation is the main farming method in the areas due to low-lying landform.High inputs of chemical fertilizers and pesticides have posed great threat to the water body of Dianshan Lake.It is significant importance to develop ecological rice farming with low inputs of chemical fertilizers and pesticides around the lake to reduce the agricultural diffuse pollution and protect the water body in Shanghai.Taking Jinze Town,one of the six towns around Dianshan Lake as an example,a questionnaire to 21 farms in the town was conducted on the inputs of the conventional rice farming for consecutive five years.Field experiments were conducted to study the effects of different fertilizers on rice growth and disease-resistant capability,and to probe the occurrence regularity of rice diseases and pests.Further experiments were carried out to study the mechanisms of the rice-frog and rice-duck integrated systems,and to evaluate the effects of promotion of the ecological rice farming with low inputs of chemical fertilizers and pesticides around the lake on reducing agricultural diffuse pollution and protecting the lake water.The main results are as follows:?1?According to the questionnaire to farmers for consecutive five years,the amounts of N,P₂O₅ and K₂ O applied in the conventional rice farming around Dianshan lake were 336.6 kg ha^-1,76.9 kg ha^-1 and 46.7 kg ha^-1 on annual average,respectively,with the ratios of N: P₂O₅: K₂ O being 100: 23: 14,respectively,suggesting an excessive input of N and low inputs of P₂O₅ and K₂ O.The input of N was much higher than the recommended value,210 kg ha^-1,in the study areas.The amount of pesticides consumed in the conventional rice farming around Dianshan Lake was 27.80 kg ha^-1 on annual average,which was significantly higher than the national average of cultivated land,13.81 kg ha^-1.The amountsof pesticides,fungicides and herbicides consumed were 15.45 kg ha^-1,7.5 kg ha^-1 and 4.85 kg ha^-1 on annual average,respectively,with the ratio of them being 320: 155: 10⁰.The rate of pesticides was much higher than the national and world averages.?2?The results of field experiments indicated that the application of chemical fertilizers promoted rice growth and tillering,and increased the contents of chlorophyll,soluble sugar and proteins in rice plants more effectively.However,the incidence of rice pests and diseases treated with CF was significantly higher than in the other fertilizer treatments and CK?p<0.05?.In particular,the density of planthoppers on the plants in CF treatment in 2014,145.0 per rice hill on average,was 1.3,1.6 and 3.1 times higher than in CMH,CML and CK treatments,respectively,resulting in a decrease of grain yield.In contrast,the application of organic fertilizer,increased soil microbial biomass,raised soil enzyme activity,improve the capability of rice disease resistance,and finally increased grain yield partially due to its slow and sustained release of nutrients.Therefore,lower or no inputs of chemical fertilizers were beneficial to reduce the incidence of rice pests and diseases.?3?The rice-frog field experiments indicated that introducing frogs in the fields not only promoted rice growth and tillering,but also reduced the incidence of rice pests and diseases effectively,especially for rice leaf roller?Cnaphalocrocismedinalis?and stem borer?Chiosuppressalis?.Moreover,the activities of frogs in the paddy fields improved the ventilation of the soil-rice system,depressed pathogenic bacteria,and reduced the incidence of rice sheath blight significantly.Immigrant frogs in the fields also increased soil microbial biomass,and improved the activities of soil enzymes.As a result,thehe grain yield with the frog treatment in 2013 and 2014 were 1.41 and 1.43 times that with the control.This suggests that introducing frogs in the paddy fields not only reduced the incidence of rice pests and diseases,but also was beneficial to improve the ecosystem of farmland,thus reducing the application of chemical fertilizers and pesticides and increasing grain yield significantly.?4?The rice-duck field experiments indicated that raising ducks in the fields could not only promote rice growth,but also control the occurrence of rice diseases and pests effectively.In particular,the populations of rice planthoppers on the plants with duck treatment decreased by 57.40% and 60.01% in 2014 and 2015,respectively.Moreover,the duck activities in the fields could also restrict weeds from growing.The number of weeds in the plots with the duck treatment decreased by 92.19% and 89.36% in 2014 and 2015,respectively.The duck activities could also promote organic matter decomposition and nutrient mineralization,improve soil enzyme activity,increase the quantities of soil nematodes,and enrich soil biodiversity.The population of soil nematodes with the duck treatment,10.0 g-1 on average,was significantly higher than those with the non-duck control,5.6 g-1 on average,in the late growing stage?p<0.01?.Raising ducks in paddy field not only increased rice grain yield by 52.94%⁸9.68%,but also produced purely natural duck products,which seems more profitable and should be encouraged for promotion in southern China.?5?There are 5975 ha paddy fields in the six towns around Dianshan Lake in western suburb of Shanghai.The amounts of N and P₂O₅ consumed by the conventional rice farming around Dianshan Lake were 2.01×10⁶ kg a^-1 and 4.59×10⁵ kg a^-1 on annual average,respectively;these leached into the environment were 0.99 × 10⁵ kg a^-1 and 0.23 × 10⁵ kg a^-1 on annual average,respectively;these flowing into the lake were 0.99 ×10⁴ kg a^-1 and 0.23 ×10⁴ kg a^-1 on annual average,respectively.The agricultural diffuse pollution caused by excessive applications of chemical fertilizers,pesticides and herbicides under the conventional rice farming system have posed serious threat to the lake.The amounts of N and P₂O₅ flowing into Dianshan Lake can be reduced by 75.76% and 95.24%,respectively,if implementing the rice-duck integrated farming around the lake,which is of significant importance for protecting the drinking water source in Shanghai.Moreover,the net income from the rice-duck farming was 3.03⁴.56 times that of the conventional rice farming,which seems more profitable.