| 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 cabbage”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（i.e.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 phosphorus leaching,under different water and management practices.A kind of lysimeter was set at the 80cm depth to monitor the nitrogen leaching,and the ceramic suction cups combined with tensiometers were used to measure the concentration of leached N at different depths.In addition,the possible leaching thresholds of phosphorus from three typical vegetable fields from Henan,Hebei,and Beijing were tested to check the risk of phosphorous leaching in these areas.The major results are as follows:（1）The characteristic of N leaching for different N forms:During the growth period of cucumber-cabbage,the leached N was mainly nitrate-N,accounting for 86.9-94.1%of the total leached N at the80cm 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（780 kgN/ha/season）,the total N leaching out of the 80 cm soil profile was 585.3kgN/ha/a,accounting for 37.5%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,fallow period,and cabbage season accounted for 27.8%,20.0%,and 52.2%of yearly total leaching,respectively.Reducing the N application rate by 20%and 50%decreased the total N leaching by 18.3%and 43.0%,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 23.5%,15.3%,and 7.1%,respectively,compared with the treatment at the same N level（N2）.Reducing the irrigation amount by 20%（N2LW）showed N leaching reduction by 38.4%at the 80cm 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 leaching characteristics under different management practices:the concentration of nitrate-N in the leachate at 80cm soil depth was between 4.8 mg/L and88.6 mg/L during spring cucumber season and between 14.0 mg/L and 114.8mg/L during cabbage 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 20.3%and 46.7%,respectively.Different control practices（N2I,N2B,N2S,N2LW）also exhibited reduction in nitrate-N concentration at different degrees between 1.6%and18.9%.（4）Characteristics of nitrate-N concentration in the leachate at different depths:The concentration of nitrate-N in the soil leachate at 20cm,50cm,and 80cm depth was 28.1-257.8mg/L,16.1-73.8mg/L,and 9.4-55.0mg/L,respectively during the cucumber season.The concentrations of nitrate-N in the leachate were in the order of N3>N2>N2LW.The average concentration of nitrate-N in the leaching solution at 50 cm and 20 cm for N3 treatment were 1.5 times and 4.8 times higher than at the80cm depth,respectively.The concentration of nitrate-N in the soil leachate decreased with the increase of depth.The concentration of nitrate-N at the depth of 20cm was much higher than that in the deep layer,especially at the last two top dressing stages.The nitrate-N concentration in the soil leachate at the 50cm,80cm and 100cm depths during the cabbage season was about 10.0 241.7mg/L,47.2-167.5mg/L,and10.8-138.0mg/L,respectively.The concentrations of nitrate-N showed liner decrease with the increase of soil depth,especially after the base fertilizer and the two topdressing stage,the nitrate concentration of the soil solution decreased linearly with the increase of depth;the nitrate concentration of leachate at50 cm and 20 cm depth were 2.2 times and 1.7 times of the that at 80 cm depth,respectively,for the N3 treatment.（5）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 3.6-191.6kgN/ha,6.3-145.2 kgN/ha and 2.7-114.7 kgN/ha at 0-20cm,20-40cm,and 40-60cm 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 0-20cm layer and 20-40cm layer were1.6 times and 1.3 times of that at 40-60cm 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 327.6kgN/ha and 354.9 kgN/ha at the harvest season of cucumbers and cabbages,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.（6）Phosphorus leaching in the leachate:the concentration of total phosphorus（TP）in the soil leachate at 80 cm depth was lower.In the whole vegetable growth period,the TP concentrations in the leachate were lower than 1 mg/L,and the average leaching amount of the typical treatments were 1.28 kg P2O5/ha at the vegetable growth season,accounting for 0.16%of phosphorus application.Amendment with biochar and straw incorporation could decrease the TP leaching at certain degree.For the three typical cinnamon soils with different soil texture,the Olsen-P leaching threshold for the soil sampled at Luoyang Henan,Baoding Hebei,and Changping Beijing were 114.9 mg/kg,105.3 mg/kg,and;117.3mg/kg,respectively,perhaps associated with the specific soil properties including the concentration of calcium,aluminum,and iron etc.（7）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 12.3%and 18.1%at the cucumber season,and between 10.7%and 22.9%at the cabbage season,respectively.Reducing the N application rate by 20%did not have obvious impacts on P and K uptake,furthermore amendment with combined inhibitors and biochar could improve the uptake of P and K.（8）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 cabbage 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.Nitrogen surplus on the 20%reduction of nitrogen 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. |
PDF Full Text Request