Contributions Of Nitrification And Denitrification To N₂O Production As Affected By C And O₂ Availability In Greenhouse Vegetable Soils

Agricultural soil is a major source of N₂0 and it is generally accepted that both nitrification and denitrification are the most important processes for N₂0 production.Based on the flux measurements for decades in situ,N₂O emissions have been attributed to N₂0 emission peaks resulted from natural and anthropogenic perturbations,more important in the greenhouse vegetable cropping system,where there are more frequent irrigations and fertilizations.Therefore,further exploration of the processes and the facrors related for N₂0 production during N₂O flushing periods are of great significance for N₂O mitigation and reduction of gaseous N losses from the soil.In the present studies,soil samples were collected from two on-set experiments in a typical concentrated greenhouse vegetable production region,Shouguang,Shandong Province,in the North China plain for laboratory incubations.Soil samples from the surface layer were incubated in a robotized incubation system together with parallel static incubations for monitoring the kinetics of gaseous products and the changes in mineral N simultaneously.The treatments included manure amendment,oxygen concentrations,and various soil moisture contents.Moreover,the effect of pH and initial C/NO₃^-on N₂0 production and product ratio in response to denitrification and the relative contribution of abiotic process on N₂0 production when NO₂--N was applied as substrates were also investigated.The main results and conclusions are as follows:?1?N₂O and N₂ production and product ratio,N₂O/?N₂O+N₂?,were increased by manure addition and increase in soil moisture content.Denitrification was the predominant process for N₂O production under the high WFPS,and the occurrence of denitrification was triggered by adding manure.Dinitrogen is an important component of the loss of nitrogen gas in the greenhouse vegetable soil.The N₂ production accounted for the proportion of the sum of manure and inorganic nitrogen as high as 30.7%at 85%WFPS after manure application.Accumulation of nitrite was significant increased by application of manure and watering,and accumulation of nitrite was significantly related with gas production and N₂0 production ratio.?2?The loss of gaseous nitrogen gas from vials with initially low oxygen to microanaerobic stage was significantly lower than that in the microanaerobic and anaerobic stage in the greenhouse vegetable soils.As a whole,the N₂O,NO and N₂ production and production rate in the 1%oxygen concentration were significantly higher than that in the 3%oxygen concentration.The maize straw addition accelerated oxygen consumption in soils,and promoted the transition from initial low oxygen to microanaerobic and strictly anaerobic stages,and enhanced the CO₂,N₂0,and N₂ emissions and the potential of denitrification.The loss of nitrogen was higher in greenhouse vegetable soil in compare with farmland soil,and the N gas loss of farmland soil is mainly restricted by soil nitrate content.?3?In the high-input greenhouse vegetable cropping system,the NO₂-was significantly positively correlated with N₂0 production rate and explained about 76.3%and 49.8%of the N₂0 variability under aerobic and anaerobic conditions,respectively.There was a strong linkage between nitrite and N₂O emission in the greenhouse vegetable soil.The contribution of aerobic nitrification process for N₂O and NO emissions was very low,and the range of N₂0 emission factor was from 0.06%to 1.61%under different levels of N application.N₂O and NO production were significantly increased in anaerobic condition,and 26.5-60.4 mg N kg-1 nitrate was reduced to N₂0 during incubated 120 h.Adding straw could decrease the N₂O emission factor,but it could significantly increase the potential of denitrification and CO₂ production rate.?4?Increase in soil pH significantly reduced N₂0 and NO production in the vegetable soils,and with increase in initial labile C,production of N₂0 was reduced in neutral and alkaline soil,but kept unchanged in acidic soil.A significant interaction on N₂O production was observed between soil pH and initial labile C/NO₃^-ratio?P<0.001?.Addition of sodium glutamate could promote the reduction of N₂0 to N₂ in denitrification process at different soil pH.The N₂0 product ratio was significantly higher in acidic soil than that in alkaline soil under different initial labile C/NO₃^-?P<0.01?,and a significant linear negative correlation relationship was observed between soil pH and IndexN₂O?P<0.01?.Both soil pH and carbon availability were crucial factors for estimation of N losses of denitrification by multiplying N₂0 flux and N₂0 product ratio.?5?The abiotic processes have certain contribute on N₂O production by nitrite as the substrate in the greenhouse vegetable soil.The ranges of N₂0 production was 0.14-9.44 nmol N g-1 in sterilized soil under low pH?pH=6.59?after short-time aerobic incubation and abiotic processes accounted for 34.5-63.7%of total N₂O production.However,abiotic processes accounted for only 0-10.7%of total N₂0 production in the high pH?pH=8.03?soil.Under anaerobic condition,abiotic processes contributing to N₂O production is very low,only accounted for 0-0.24%of total N₂O production,and meanwhile,an inverse correlation was found between N₂ production and NO₂-application rate in non-sterilized soil during the short-time incubation.?6?Compared with different soil layers,the N₂O production mainly occurs in the surface soil under aerobic condition.Higher soil water content could increase N₂0 production in surface soils,but there was no difference in the bottom soils?73-78 cm?for N₂O production.Under anaerobic condition,the largest denitrification potential was observed in surface soil,and vegetable soil also had the capability of denitrification with soil depth above 20 cm.The denitrification product ratio decreased after glucose addition in 0-20 cm soil layer and the loss of denitrification in the bottom soils also increased.Meanwhile,there was a lag-effect in the process of denitrification with the increase of soil layer depth.