Study On The Environmental Risk Of Nitrogen And Phosphorus And Its Control Measures In The Concentrated Planting Region Of Pomelo

Guanxi honey pomelo in Pinghe County,Fujian Province,has a planting area of 4.33×10⁴ hm² and a yield of 120×10⁴ t.The planting area,yield and export volume rank the first in the country.After more than 30years of large-scale development,the problems of soil acidification,soil nutrient accumulation and nutrient imbalance caused by long-term excessive fertilization in Pinghe county have attracted wide attention and related research.However,there is still lack systematic research on the environmental risks of non-point source pollution caused by pomelo concentrated cultivation.In this study,fertilization investigation of 291pomelo orchards,nitrogen?N?and phosphorus?P?determination of soil and fruit samples from 313 pomelo orchards,determination the runoff of mountainous pomelo orchards,the groundwater of cultivated pomelo orchards and water of Huashan river were carried out to evaluate N and P loss risk of pomelo orchards.Through field runoff plot experiment and interior simulated soil column leaching experiment,the control measures of N and P loss were studied.The aim of study was to provide the basis of pomelo rational fertilization,the prevention and control measures of non-point source pollution in large-scale citrus planting areas in China.The main conclusions were as follows:1.N and P input characteristics and environmental risk assessment in pomelo orchard?1?The average N application rate of 291 pemolo orchards in Pinghe county was 1042.3 kg·hm^-2.The N through chemical fertilizer application in orchards was much higher than that from organic fertilizer,the percentage of chemical fertilizer N accounting for 93.6%of total N input amount.N surplus of more than 500 kg·hm^-2 was used as an evaluation index of N environmental pollution risk,the results showed that the average amount of N surplus was 800.9 kg·hm^-2,81.4%of the pomelo orchards had an N surplus of more than 500 kg·hm^-2,22.3%had a surplus of more than 1000 kg·hm^-2.The results of soil alkali-hydrolyzed nitrogen content analysis in 313 pomelo orchards showed that the ratios of low(<100 mg kg^-1),adequate(100-200 mg kg^-1)and high(>200 mg kg^-1)were48.6%,49.8%and 1.6%,respectively.The results indicated that the content of soil alkali-hydrolyzed N did not increase with the increase of N input amount,and there was a large loss of N fertilizer in pomelo orchard.?2?The average application rate of P in 291 honey pemolo orchards in Pinghe county was 837.2 kg·hm^-2,the percentage of chemical fertilizer P accounting for 96.3%of total P input amount.The average amount of P surplus was 671.2 kg·hm^-2,87.3%of orchards had a P surplus of more than 300 kg·hm^-2.The content of soil available P increased with the increase of P input amount.The proportion of available P low(<20mg·kg^-1),adequate(<20-40 mg·kg^-1),high(>40 mg·kg^-1)in 313 pomelo orchards were 9.5%,9.7%and 80.8%,respectively.The soil available P content of 62.3 mg·kg^-1 and 96.3 mg·kg^-1 were used as thresholds for assessing the environmental risk of P in cultivated and mountainous pomelo orchards,respectively,the results showed that 73.8%of cultivated and 62.6%of mountainous pomelo orchards were at high risk of phosphorus leaching.2?Investigation on N and P loss and water quality of pomelo orchard?1?The total N content and ammonia N content in the runoff of three mountainous pomelo orchards were higher than the limits of category V(2mg·L^-1)and category II(less than 0.5 mg·L^-1)of surface water environmental quality,respectively,the total P content were higher than that of category V water standard(0.4 mg·L^-1).?2?The groundwater nitrate content of three cultivated pomelo orchards were higher than the groundwater quality limit category III(20mg·L^-1),and the total P content of six cultivated pomelo orchards were higher than Canada drinking water limit(0.01 mg·L^-1).?3?The total N content of three water samples from Huashan River Basin were higher than the limit of surface water quality category V(2.0mg·L^-1).The total P content accorded with category II(<0.1 mg·L^-1),category III(<0.2 mg·L^-1)and category V(<0.4 mg·L^-1)of surface water standard,respectively.3.Control measures of N and P loss in pomelo orchards3.1 N and P runoff lossField runoff plot experiments were conducted to study the effects of dolomite(150 kg·666.6m^-2),zeolite(600 kg·666.6m^-2)and ground cloth mulching on N and P runoff loss in pomelo orchards.?1?The application of dolomite and zeolite could reduce the loss of NO₃^--N,NH₄⁺-N,dissolved organic nitrogen?DON?,total nitrogen?TN?,soluble phosphorus?DRP?,granular phosphorus?PP?and total phosphorus?TP?.?2?Ground cloth mulching significantly reduced the rate of runoff and the loss of NO₃^--N,NH₄⁺-N,DON,PN and DRP,PP and TP.?3?The effect of different measures on total N and total P losses showed that ground cloth mulching was better than dolomite and zeolite application.3.2 N and P leachingBased on the same N and P level,indoor simulated soil column leaching experiment was carried out to study the effects of dolomite,zeolite,combined organic manure with chemical fertilizer,combined organic manure and chemical fertilizer with zeolite treatments on soil N and P leaching.?1?Compared with chemical fertilizer treatment,combined chemical fertilizer with dolomite,combined chemical fertilizer with zeolite treatments could significantly reduce NH₄⁺-N and DRP,DOP,DTP leaching,but significantly increased NO₃^--N leaching.?2?Compared with chemical fertilizer treatment,combined organic manure with chemical fertilizer treatment could reduce NO₃^--N,NH₄⁺-N and DTN leaching,whereas significantly increased DRP,DOP and DTP leaching,indicating that combined organic manure with chemical fertilizer treatment could reduce N leaching,but significantly increase P leaching.?3?Compared with chemical fertilizer treatment and combined organic manure with chemical fertilizer treatment,combined organic manure and chemical fertilizer with zeolite treatment could significantly reduce DRP,DOP and DTP leaching.In summary,the application rate of N and P is large in Pinghe County,and the environmental risk of N and P loss is high.The application of dolomite,zeolite and mulching cloth is an effective measure to control the runoff of N and P in mountainous pomelo orchards.For cultivated pomelo orchards,combined organic manure with chemical fertilizer is an effective measure to control N leaching,dolomite and zeolite are effective measures to prevent P leaching,but dolomite and zeolite can increase nitrate nitrogen leaching,and combined organic manure with chemical fertilizer can increase P leaching.