Effect Of Aerated Irrigation On CO₂, N₂O And CH₄ Emissions From Soils Of Greenhouse Tomato Fields

Aerated irrigation,seeping the water-gas mixture into the crop root zone soil through the venturi equipment,is a new high-efficiency and water-saving irrigation technology that can improve the growing environment of the root zone.Aerated irrigation has been proved to have a positive effect on crop growth,fruit quality and water productivity.However,the ecological effects of aerated irrigation have not been paid close attention to.The increase of oxygen content in soil pores caused by aerated irrigation is bound to affect the soil microorganism activity,decomposition of soil organic matter,nitrification and denitrification processes,and eventually lead to the changes of soil CO₂,N₂O and CH₄ emissions.At present,studies on the effect of aerated irrigation on CO₂,N₂O and CH₄ emissions of farmland soils have not been reported,which needs to be carried out.Therefore,studying the influence and regulation mechanism of aerated irrigation on CO₂,N₂O and CH₄ emissions of farmland soil is of great significance to correctly evaluate the farmland ecological effects of aerated irrigation technology,and to realize farmland greenhouse gas emission reduction from the perspective of irrigation regulation.In this paper,three irrigation levels(taking crop-pan coefficient(k_cp)as 0.6,0.8 and 1.0,respectively)and two irrigation modes（aerated irrigation and subsurface drip irrigation）were established.Hence,six treatments were contained.The 3-year field experiment was carried out to study the seasonal changes of soil CO₂,N₂O and CH₄ emissions from greenhouse tomato fields,tomato growth,nutrient accumulation,yield,irrigation water use efficiency（IWUE）,net global warming potential（NGWP）and environmental factors influencing greenhouse gas emissions（water-filled pore space（WFPS）,soil temperature,NO₃^-content,enzyme and microbial activities,and tomato dry weight）,to discuss the applicability of aerated irrigation in greenhouse tomato fields in northwest semi-humid arid area,and to put forward the suitable irrigation mode for greenhouse tomato planting when synthetically considering high yield,water saving and emission reduction.The main conclusions are as follows:（1）Soil CO₂ fluxes from greenhouse tomato fields fluctuated seasonally.Compared to the control,aerated irrigation increased soil CO₂ emissions by 0.7%～29.4%.Soil CO₂emissions with k_cp of 1.0 were increased by 0.7%～13.4%and 1.2%～16.7%compared to the treatment with k_cp of 0.8 and 0.6,respectively.（2）Higher N₂O fluxes were observed during the early growth period,followed by relatively low and stable N₂O flux levels.Top dressing caused peaks of fluxes.Aerated irrigation decreased soil N₂O emissions during the spring-summer season in 2015,with a decreasing rate of 6.7%and 2.5%for irrigation level of k_cp=0.6 and 1.0,respectively.However,aerated irrigation increased soil N₂O emissions by 1.5%～51.2%for the other 5growing seasons.In addition,irrigation had a positive effect on soil N₂O emissions.Soil N₂O emissions with k_cp of 1.0 were increased by 0.9%～33.2%and 2.6%～79.1%compared to the treatment with k_cp of 0.8 and 0.6,respectively.（3）Soil CH₄ fluxes from greenhouse tomato fields fluctuated seasonally and were a sink of CH₄ in most time.Aeration had a negative impact while irrigation had a positive effect on soil CH₄ emissions.During the spring-summer season in 2015,soil CH₄ emissions under the control were 3.00 and 1.67 times higher than aerated irrigation for irrigation level of k_cp=0.6 and 1.0,respectively.Meanwhile,soil CH₄ emissions under aerated irrigation were decreased by 3.5%～25.7%with respect to the control during the other 5 growing seasons.Soil CH₄ emissions with k_cp of 1.0 were increased by 5.3%～25.4%and 9.7%～122.2%compared to the treatment with k_cp of 0.8 and 0.6,respectively.（4）Compared to the control,aerated irrigation decreased WFPS by 0.6%～5.2%during the spring-summer season,while it increased WFPS by 0.9%～5.6%during the autumn-winter season in the 3 years.Relative to the control,aerated irrigation decreased soil NO₃^-content(k_cp=0.6:4.9%;k_cp=1.0:0.4%)and urease activity(k_cp=0.6:1.4%;k_cp=1.0:0.8%)during the spring-summer season in 2015,while it increased soil NO₃^-content by0.3%～4.4%and urease activity by 0.8%～3.6%for the other growing seasons.Aerated irrigation generally increased soil catalase activity by 0.1%～2.0%,cellulase activity by0.1%～1.9%,nitrifying bacteria by 1.1%～6.5%,soil microbial biomass C（MBC）by0.3%～1.7%and soil microbial biomass N（MBN）by 2.0%～4.1%,but it decreased soil denitrifying bacteria by 4.2%～25.9%.With the increase of irrigation amount,WFPS,soil urease activity,catalase activity,cellulase activity,soil nitrifying bacteria,soil denitrifying bacteria,MBC and MBN presented an overall increasing trend.（5）WFPS,soil temperature and catalase activity were major factors influencing soil CO₂ emissions.WFPS,soil temperature,NO₃^-content,nitrifying bacteria and denitrifying bacteria were main factors affecting soil N₂O emissions.However,the main controlling factors affecting soil CH₄ emissions in different growing seasons were slightly different,which needed to be further studied.（6）Compared to the control,aerated irrigation increased plant height,stem diameter,dry weight and nutrient（N and K）absorption,which increased tomato yield and IWUE by14.7%～47.6%.Tomato yield was increased while IWUE was decreased with irrigation amount increasing.Tomato yield with k_cp of 1.0 were increased by 5.2%～16.6%and19.8%～35.5%compared to the treatment with k_cp of 0.8 and 0.6,respectively.IWUE with k_cp of 1.0 were decreased by 6.7%～15.8%and 18.7%～28.1%compared to the treatment with k_cp of 0.8 and 0.6,respectively.（7）Compared to the control,aerated irrigation increased NGWP by 11.8%,17.4%and15.8%for irrigation level of k_cp=0.6,0.8 and 1.0,respectively.NGWP with k_cp of 1.0 were increased by 20.1%and 34.7%on average compared to the treatment with k_cp of 0.8 and 0.6,respectively.（8）As a new water-saving irrigation technology,the increase of tomato yield was much larger than the increase of greenhouse effect caused by aerated irrigation.Therefore,it is reasonable that aerated irrigation technology is widely used in field crops.The application of aeration with k_cp of 0.8 was the preferable irrigation mode when synthetically considering tomato yield,water saving（IWUE）and greenhouse gas reduction（NGWP）.The results can be used as a reference for providing certain theoretical foundation and scientific basis of water-saving and emissions-reducing.