Nitrogen Accumulation And Its Regulation In Soil Under Greenhouse Vegetable Cultivation

Substantial changes in land?use patterns have occurred in China since 1980s,mainly driven by economic development and rise in living standards of the people.Large areas of cereals have been converted to horticultural crops(eg,fruit trees and vegetables)due to their high economic value.Plastic greenhouse vegetable production,a unique vegetable system with high economic and ecological services,have successfully solved the problems of winter and spring vegetable supply in the North China and rain-proof cultivation in the South China.Plastic greenhouse production made a great contribution to ensure the annual supply of vegetables.Compared to the intensitive glass greenhouse,the plastic greenhouse is easily adopted,relatively inexpensive,and economically profitable for small-holder farmers in China.However,overuse and misuse of nitrogen(N)fertilization and improper irrigation in plastic greenhouse are common practices leading to the waste of N and water resources,and pose huge risks of soil degradation(eg.soil salinization and acidification)and N losses.However,there is little studies focused on:(1)the dynamics of N accumulation(especially nitrate accumulation),(2)the relationships between N inputs(or nitrogen surplus)and N accumulation,(3)the nitrate accumulation and controlling factors in regional scale,and(4)the effects of different measures such as reducing N and water inputs,and reducing N and water inputs combined with nitrification inhibitor(DMPP)or manure and straw on controlling nitrate accumulation.Therefore,field experiments and Meta-analysis were combined to study N accumulation and controlling factors under plastic greenhouse vegetable cultivation.The status of N inputs,surplus and accumulation were evaluated through a five-year crops cultivation cycle.The status of nitrate accumulation and leaching,and the effectiveness of strategies to control those were compared by Meta-analysis.At the same time,a two-year plastic greenhouse field experiment was conducted to evaluate strategies involving reduced N and water inputs,nitrification inhibitor(DMPP),manure and straw on vegetable yields,N uptake,N use efficiency,N accumulation and losses.The main conclusions are as follows:1.The results of the five-year experiment showed that the average annual N inputs were 1871 kg N ha^-1,with manure accounting for 61%of total N inputs.The annual N apparent balances were 1354 kg N ha^-1.Consequently,total N significantly increased in the0–100 cm soil profile.Nitrate accumulation in the 0–100 cm and 0–200 cm soil profiles were significantly increased,with annual average rates of 182 kg N ha^-1 and 225 kg N ha^-1,respectively.The pH in 0–20 cm soil was significantly reduced.However,the electrical conductivity(EC)in 0–100 cm soil significantly increased.In conclusion,excessive N surpluses resulted in rapid N accumulations in soil of plastic greenhouses,and attention should be paid to its negative effects on the environment.2.The average nitrate accumulation in 0-500 cm soil profile of plastic greenhouses was 2311-12157 kg N ha^-1,averaging 5860 kg N ha^-1 in Yangling,Shaanxi province.Based on a Meta-analysis of 773 observations from 117 published articles,the nitrate accumulation in 0-400 cm soil profile of plastic greenhouse was 950-1487 kg N ha^-1 in China,accounting for 13-17%of the accumulated N inputs.Of which,about 65%-70%storage was below the root zone(0–100 cm depth).The annual average growth rates of nitrate accumulation were 16-62 kg N ha^-1 yr^-1.N input rate and soil pH had positive effects and soil organic carbon,water input rates,and carbon-to-nitrogen ratio had negative effects on nitrate accumulation in 0–100 cm soil.N and water input rates had positive effects and soil organic carbon,soil clay content and carbon-to-nitrogen ratio had negative effects on nitrate accumulation in 100–400 cm soil.Thus,for a given vegetable field with relatively stable pH and soil clay content,reducing N and water inputs and increasing soil organic carbon,and thereby increasing carbon-to-nitrogen ratio,are effective measures to prevent nitrate accumulation.3.The results by a Meta-analysis showed that the main strategies to control nitrate leaching were:(i)improved N fertilizer management(INFM),(ii)reduced water management(RWM),(iii)comprehensive regulation of N fertilizer and water management(CFWM)and(iv)catch crops management(CCs),which decreased nitrate leaching by22%,24%,48%and 35%in plastic greenhouse production,respectively.The nitrate leaching scaled yields(defined as yields divided by the quantity of nitrate leaching)were significantly increased by 27%,31%,87%and 44%,respectively.However,RWM decreased vegetable yields.The efficacies of the strategies were dependent on soil properties.In conclusion,INFM and CFWM are suggested to decrease nitrate leaching from vegetable production.4.Flush of N₂O emissions was observed after chicken manure application and irrigation compared to a control during summer fallow season.The cumulative N₂O was1.13 kg N ha^-1,accounting for approximately 20%of the annual N₂O emissions from plastic greenhouse vegetable production.Similarly,the cumulative N₂O emissions from plastic greenhouse soil with water-soluble organic matter(WSOM)incorporation increased two to four times than those from unamended soil.The observed N₂O emissions from the plastic greenhouse soil are attributed to the WSOM and soil water content.The strong positive linear relationships between the CO₂ flux and N₂O emissions at 70%–90%water-filled pore space with WSOM addition of plastic greenhouse soil indicated that denitrification might be the dominant source of N₂O emissions.We concluded that N₂O emissions during summer fallow season should be included in the N₂O emission inventory of plastic greenhouse vegetable production.Moreover,measures for avoiding nitrate accumulation and improving water and manure management could be adopted to reduce the N₂O fluxes.5.The results of a two-year(three seasons)field experiment showed that compared with the farmers'conventional N and water inputs,reducing N and water inputs and reducing N and water inputs combined with nitrification inhibitor(DMPP),manure or straw significantly increased N use efficiency by 29%-88%,without compromising vegetable yields.Moreover,the above measures significantly reduced nitrate accumulation,NH₃ volatilization,N₂O emission and nitrate leaching by 25%-74%,1%-17%,50%-88%,respectively.Thus,there is a great potential for reducing N and water inputs in plastic greenhouse vegetable production.In addition,reducing N and water inputs combined with DMPP,manure or straw further reduced nitrate accumulation in soil profiles,but there is a risk of increasing the loss of NH₃ volatilization and N₂O emission.More attention should be paid on how to rationally combine different strategies(eg,DMPP,manure and straw incorporation).In conclusion,severe N inputs and surplus were common in plastic greenhouse production,leading to high nitrate accumulation in the soil profiles and leaching below the root zone.Therefore,it is urgent to optimize N and water application rate,and effective measures such as DMPP,manure and straw incorporation should be propagated for reducing nitrate accumulation and leaching.These strategies are of great significance to ensure sustainable and healthy development of the plastic greenhouse vegetable production.Although reducing N and water inputs combined with DMPP,manure or straw further reduce nitrate accumulation in soil profiles,there is a risk of increasing the loss of NH₃volatilization and N₂O emission.Therefore,more attention should be paid on how to rationally combine those effective measures.