| Unreasonable application of nitrogen fertilizer in the agricultural production process is one of the main causes of non-point source pollution.The problem of soil nitrogen(N)leaching caused by unreasonable water and fertilizer management practices is more seriously in vegetable planting areas.In order to study the characteristics of N leaching under different water and N management measures and the different control measures to reduce the N leaching in typical open-land vegetable fields in North China,a field experiment was established with the typical local cropping pattern of “spring-planted cucumber-later summer-planted eggplant” in this area.Different treatments were set up including different Nitrogen fertilizer level,controlled irrigation amount,combined application of organic fertilizer,and addition of N fertilizer synergists(ie.urease-inhibitors and nitrification inhibitors,biochar)and straw returning combined with the reduced N application rate by 20%,to investigate characteristic of nitrogen leaching,as well as phosthorous leaching,under different water and management practices.A kind of lysimeter was set at the 80 cm depth to monitor the nitrogen leaching,and the dynamic changes of nitrate nitrogen content in soil profiles at different depths were monitored.The major results are as follows:(1)The characteristic of N leaching for different N forms: During the growth period of cucumber-eggplant,the leached N was mainly nitrate-N,accounting for 76.92%-88.4% of the total leached N at the 80 cm soil depth;followed by organic-N accounting for 6.3-14.6% of the leached N.among the leached nitrogen,the ammonium-N in the leachate accounted for the least N forms with the leaching amount less than 1% of the total N leaching in the year-round.(2)The trait of total N leaching under different management practices: At the conventional fertilization level(890 kgN/ha/season),the total N leaching out of the 80 cm soil profile was 307.5 kgN/ha/a,accounting for 24.9% of the N application rate.There was a significant linear relationship between the N application rate and total N leaching.The leached N at cucumber season,and eggplant season accounted for 44.8% and 52.2% of yearly total leaching,respectively.Reducing the N application rate by 20% and 50% decreased the total N leaching by 15.8% and 37.1%,respectively.Amendment practices such as addition of the urease-inhibitor(ie.quinol)plus nitrification inhibitor(ie.DCD),(N2I),biochar(N2B),and straw incorporation(N2S)could decrease the total N leaching by 4.8% to 9.3%,respectively,compared with the treatment at the same N level(N2).Reducing the irrigation amount by 20%(N2LW)showed N leaching reduction by 30.55% at the 80 cm depth,compared with the treatment at the same N rate.Since the base fertilization was applied to the ridge line and covered with soil which avoiding the direct N leaching at the furrow,replacing 35% N with organic manure did not show significant effect on N leaching.(3)Nitrate eaching characteristics under different management practices: the concentration of nitrate-N in the leachate at 80 cm soil depth was between 8.0 mg/L and 54.4 mg/L during spring cucumber season and between 9.0 mg/L and 44.0mg/L during eggplant season,respectively.The concentration of nitrate-N increased significantly with the increase of fertilization amount: decrease of N rate by 20%(N2)and 50%(N3)showed reduction in nitrate-N concentration about 12.3% and 33.9%,respectively.Different control practices(N2I,N2 B,N2S,N2LW)also exhibited reduction in nitrate-N concentration at different degrees between 1.6% and 18.9%.(4)Dynamics of soil nitrate content and characteristics of acumulated mineral N in the soil profile: During the growth period of vegetable crops,the nitrate content in soil increased gradually with time and topdressing processes.The accumulated mineral N after cucumber harvest were about 30.1-449.0kgN/ha,24.4-544.2kgN/ha and 31.2-455.83kgN/ha at 0-20 cm,20-40 cm,and 40-60 cm layer,respectively.The nitrate content at soil profile decreased significantly with the increase of soil layer.Under conventional fertilization rate,the mean acumulated nitrate at the 0-20 cm layer and 20-40 cm layer were 1.6 times and 1.3 times of that at 40-60 cm layer,respectively during the cucumber growth period.After the vegetable crops were harvested,the acumulated mineral N(Nmin)at the 0-80 cm depth were 1257.6kgN/ha and 1552.6 kgN/ha at the harvest season of cucumbers and eggplants,respectively,for N3 treatment.Compared with N2 treatment,the Nmin increased at the harvest season of two vegetable crops for treatments of N1 and N2 at some degrees;and the addition of synergist(combines inhibitors,biochar),as well as straw incorporation also showed increase of Nmin in the soil profile compared than without amendments after vegetable crops were harvested.(5)Crop yield and N uptake: Compared with the conventional fertilization treatment(N3),reducing N application rate by 20% had no significant effect on vegetable yield and N uptake,except for reducing N by 50%.The apparent N utilization rate of applied N under different management practices ranged between 16.6% and 29.9% at the cucumber season,and between 1.6% and 18.9% at the eggplant season,respectively.(6)Apparent N balance: If the N leaching is considered as the only N loss path,,the N balance could be expressed using the equation of “N input-N output-N leaching=N balance”.expect non-fertilization treatment in cucumber season,applying nitrogen fertilizer treatments had different degrees of surplus;the eggplant treatment had different surpluses in each treatment,and the conventional fertilization treatment had the most nitrogen surplus,and the remaining control measures nitrogen were surplus or loss.Well,.biochar(N2B)is the closest to balance.Losses from other pathways(such as ammonia volatilization,nitrification and denitrification,etc.)also need to be addressed at the same time to more fully assess the fertilization strategy.In summary,nitrogen fertilizer synergist can significantly reduce nitrogen leaching level in vegetable field(eggplant)by 4.3%-9.8%.Nitrate accumulation in vegetable field soil layer decreases with depth increasing;Eggplant Yield decreases by 24.9% under 50% nitrogen reduction in the experiment,and other nitrogen reduction measures have no significant effect on yield.Considering the comprehensive economic and environmental benefits,it is recommended to apply combined inhibitors or straw surface application to control nitrogen leaching in vegetable fields at a fertilizer ratio of 20% organic fertilizer and 35% nitrogen reduction. |
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