Studies On Nitrogen Transformation And Leaching Of Vegetable Soil In Wuhan Suburb, China

Nitrate leaching from ago-ecosystem is an important contributor of nitrate contamination of water resources,which has been extensively concerned by researchers of home and abroad for the recent decade.Based on the research of soil nitrogen transformation and nitrate leaching,we built large undisturbed soil columns and combined with the indoor cultivation and field trial to research the nitrogen and nitrate leaching of the Wuhan surbub vegetable soil.The test soils were two typical soil types (yellow-soil and fluvo-aquic soil) from Wuhan surbub,and vegetables of capsicum (Capsicum annuum L.),amaranth(Amaranthus mangostanus L.),radish(Raphanus sativus L.),spinach(Spinacia oleracea L.) and Chinese cabbage(Brassica Chinensis L.) were planted seasonly.The study reveals the nitrogen transformation and nitrate leaching law,sums up principles and measure of controlling nitrate groundwater pollution in Wuhan surbub,and provides a scientific base for the sustainable production of vegetables and control of nitrate groundwater pollution.The main results are as followed.1.The study of vegetable soil nitrate leaching in Wuhan surbub by large undisturbed soil core lysimeters(yellow-brown soil) revealed that much application of nitrogen significantly increased nitrate leaching risk and it was the main reason of nitrate groundwater pollution.The equation between the average nitrate concentration(Y mg N/L) and nitrogen application(x kg N/ha)(two years) were:Y=9E-06x²+0.0014x+3.51 (R²=0.99,P＜0.001).That is,when nitrogen application amount are 395.89 kg N/ha/yr and 646.78 kg N/ha/yr,the nitrate concentration reach to the drinking water control 10 mg N/L and groundwater control 20 mg N/L.Rainfall was the main reason for nitrate leaching,and nitrate leaching occurs mainly in spring and summer while the peak nitrate-N occurs mainly in autumn and winter.Nitrogen application increased the amount of calcium and magnesium leaching,and there is significant positive correlation between nitrate leaching and the leaching of calcium and magnesium2.The comparison of nitrate leaching law of yellow-brown soil and Fluvo-aquic soil by small undisturbed soil core lysimeters shows that the yellow-brown soil nitrate leaching loss was higher than fluvo-aquic under the condition of the same amount of nitrogen application while the nitrate concentration was higher than yellow-brown soil. The equation between the nitrate concentrations yellow-brown soil(Y₁) and Fluvo-aquic (Y₂) and nitrogen application(x) are:Y1=5E-06x²+0.0044x+3.55(R²=0.98 P＜0.0001)Y²=4E-06x²+0.0062x+4.41(R²=0.98 P＜0.0001)When the yellow-brown soil nitrogen application rate are 393.13 kg N/ha/yr and 722.22 kg N/ha/yr,and Fluvo-aquic soil 320.69 kg N/ha/yr and 680.18 kg N/ha/yr,the leachate nitrate concentration reach to the drinking water control 10 mg N/L and groundwater control 20 mg N/L.The small undisturbed soil core lysimeters simulates nitrate leaching better than the large one under the low nitrogen application.3.This paper studied the effect of nitrogen application on yield and nitrate content of vegetables by the combination of field and undisturbed soil trial.With the increase of the nitrogen application,the vegetable yield increased firstly,then decreased.They are significant positive correlation.The vegetable nitrate content increased with the nitrogen application and there is significant positive correlation between them.The vegetable nitrate content during the trail period is Chinese cabbage＞spinach and capsicum＞radish. The decrease of nitrogen application is the main method of reducing nitrate content of vegetables.Considering the yield and nitrate content of vegetables,we suggest that the optimum nitrogen application are 180 kg N/ha-270 kg N/ha,40 kg N/ha-80 kg N/ha,150 kg N/ha-300 and 120 kg N/ha-180 kg N/ha for autumn Chinese cabbage,winter spinach, spring and summer capsicum and autumn radish,respectively.The yield of vegetable in fluvo-aquic soil is higher than in yellow-brown soil on the same condition.4.The paper studied the effect of nitrogen application on vegetable soil nitrate leaching and vegetable nitrogen use efficiency in large undisturbed soil column using ¹⁵N and Br^- tracer.The result shows that nitrogen application was mainly used for the season vegetable,and was less absorbed by the post-seasons vegetable.The season vegetable average nitrogen use efficiency was 31.82%,and the post-seasons vegetable was only 4.72%.Besides,nitrogen application on the season vegetable contributes little to the nitrate leaching,which mainly occurs on the post-seasons vegetable.The use of Br^-showed that the Br^- leach completely when the leachate was 2.22 times than the field water capacity.5.The paper combined the small undisturbed soil column with field trials,the yellow-brown and fluvo-aquic soil mineralization law was studied.The soil net mineralization rates increase with the temperature.The fluvo-aquic soil mineralization rate was higher than yellow-brown soil.The mineralization potential of Yellow-brown soil and fluvo-aquic were 74.63 mg N/kg and 123.45 mg N/kg,respectively,and accounting for soil total nitrogen of 8.58%and 9.56%.Small undisturbed soil core lysimeters research showed that vegetable soil mineralization in spring-summer was higher than in autumn-winter.The yellow-brown soil mineralization rates were 0.13 mg N/kg/d and 0.02 mg N/kg/d in spring-summer and in autumn-winter seasons,respectively, and the fluvo-aquic were 0.18 mg N/kg/d and 0.02 mg N/kg/d.6.Research the atmospheric deposition of vegetable systems in Wuhan surbub.The results show that the average inorganic nitrogen input through atmospheric deposition was 29.58 kg N/ha/yr.Wet deposition was the main component of atmospheric deposition, and it occupies 81.05%of the total inorganic input.The atmospheric deposition in spring-summer was higher than in autumn-winter,and ammonium deposition was higher than nitrate.The average pH value was 4.47±0.59 during the trial period.7.In the paper,we proposed strategies to reduce nitrate-N leaching of vegetable soil in Wuhan suburb.(1) Reduce the nitrogen application,and the nitrogen applied no more than 722.22 kg N/ha/yr and 680.18 kg N/ha/yr in yellow-brown soil and fluvo-aquic soil, respectively;(2) Reduce the nitrogen applied amount in autumn-winter,and planted the vegetable or free filling crops before winter;(3) Rotate deep and shallow vegetable crops, choose long growing season vegetable crops and reduce farming;(4) Reduce the irrigation volume and frequency in the main rainfall seasons,and promote water-saving irrigation;(5) Promote vegetable fertilization soil testing technology to improve nitrogen use efficiency;(6) Applicate nitrification inhibitors.8.The use of PVC and Vaseline materials building the undisturbed soil core lysimeters fully meet the research need of the nitrogen transformation and leaching in agro-ecosystems.