N₂O Emission From Medicago Sativa Stands And Its Response To Nitrogen Application And Nitrification Inhibitor Use In Dryland Region

Global warming has accented the importance in mitigating greenhouse gas（GHG）emission.N₂O is considered to be one of potent GHG that contribute to the global warming largly.A large number of studies have shown that agricultural soil is an important source of N₂O emissions,as it accounts for 50%of global N₂O emissions.N₂O produced by soil can account for 70%to 90%of the total N₂O released from the biosphere to the atmosphere.At present,research on N₂O emission characteristics in China is mainly focused on paddy fields in the south and the main grian crop growing areas in the north.As the king of forage crop,alfalfa has good feeding value,and the functions of reducing nitrogen fertilizer input and improving soil quality.Therefore,it is widely planted in the Loess Plateau of East Gansu Province,which promotes local social,economic and ecological benefits.However,there are relatively few studies on the N₂O emission characteristics of alfalfa grasslands in the region,and the daily dynamics,seasonal characteristics,and influencing factors of N₂O emissions are not yet clear.The use of nitrification inhibitors in combination with nitrogenous fertilizers,by controlling the activity of nitrifying bacteria,enables the nitrogen source applied to the soil to exist in the form of ammonium nitrogen for longer periods for the crops to absorb and use,which not only increases the nitrogen fertilizer in the soil,but also reduces nitrogen leaching,and N₂O emissions from the soil.Therefore,the effects of nitrification inhibitors on N₂O emissions from alfalfa grassland need to be further explored.In view of this,based on the Qingyang loess plateau experimental station of Lanzhou University,we carried out the N₂O emission characteristics of dryland grassland in eastern Gansu and its response to nitrogen fertilizer application and nitrification inhibitor addition.A total of 5 treatments were set up in this study.Among them,nitrogen fertilizer application levels were set at 4 levels,namely N0（CK）,N50(50 kg N hm^-2),N100(100 kg N hm^-2),and N150(150 kg N hm^-2).In addition,a nitrification inhibitor,dicyandiamide（DCD）,N150+DCD,was added to the N150 treatment.Using a combination of LGR-N₂O/CO gas analyzer and traditional static chamber-gas chromatography to monitor the dynamics and characteristics of N₂O emissions from different treatments of alfalfa grassland.At the same time,dynamic monitoring of soil temperature,moisture,NO₃^-content and alfalfa production and other factors,in order to explore the influencing factors of N₂O emissions from alfalfa grassland,in order to obtain the characteristics of N₂O emissions and its application of nitrogen and nitrification inhibitor response mechanism.The results provided data support for accurate assessment of N₂O emission intensity and characteristics of dry-crop lands on the Loess Plateau,and can also quantify the effect of nitrification inhibitors on N₂O reduction of alfalfa grassland.The main results obtained from the study are as follows:1.In July and August of 2014,the daily dynamics and average N₂O emissions from the alfalfa stands under different nitrogen application rate(0 kg N hm^-22 and 150 kg N hm^-2)were monitored using the LGR-N₂O/CO gas analyzer.The results showed that N₂O fluxes had a distinct diurnal variation characteristic,showing a trend of decreasing first and then increasing.According to the daily emission characteristics,the N₂O fluxes at the daily or longer time scales may be underestimated when the N₂O flux values measured from 9:00 to 11:00.During the monitoring period,the amplitudes of N₂O emission fluxes of N0 and N150 were-0.0368-0.0665 mg m^-2 h^-1and-0.0093-0.0466 mg m^-2 h^-1.The average values were-0.0036.And 0.0118 mg m^-2 h^-1,N150treatment significantly increased N₂O emissions,compared with N0 treatment.The N₂O emissions under different nitrogen treatments increased with the increase of soil water content.2.From May to September of 2017,the N₂O emission characteristics of the alfalfa grassland at different nitrogen fertilizer gradients(0,50,100,and 150 kg N hm^-2)were measured using a conventional static chamber-gas chromatography method.The results showed that the amplitude of N₂O emission during the monitoring period ranged from-0.0021 to 0.0510 mg m^-2 h^-1.Among them,the N0,N50,N100 and N150 treatment N₂O average discharge capacity were 0.0035,0.0041,0.0050 and 0.0061 mg m^-2 h^-1,respectively.During the monitoring period,the cumulative N₂O emission from above treatments was 6.5,8.0,9.2,and 11.2 mg m^-2,respectively.The N₂O emission coefficients of N50,N100,and N150 treatments were 0.030,0.027,and 0.031,respectively.The N₂O emissions per unit of production of N0,N50,N100,and N150 were 6.5,7.8,11.3,and 12.5 mg kg^-1,respectivel.3.From May to September of 2017,the traditional static chamber method was used to determine the N₂O emission characteristics under nitrogen application at 0,150 kg N hm^-2 and the application of nitrification inhibitor DCD together with 150 kg N hm^-2.The results showed that the average N₂O emission fluxes under N0,N150,and N150+DCD during the growing season were 0.0035,0.0061,and 0.0026 mg m^-2 h^-1.The cumulative emissions were 6.5,11.2,and 4.3 mg m^-2,respectively.With the increase of nitrogen fertilizer gradient,N₂O emissions increased.Emission factors for N150 and N150+DCD treatments were 0.03 and-0.01 respectively.N150+DCD treatment had a significant emission reduction effect on N₂O emissions.Compared with N150 treatment,N150+DCD treatment reduced N₂O emissions by 62%,and inhibitors significantly reduced N₂O emissions.N₂O flux was significantly correlated with soil moisture（P<0.05）,and N₂O flux increased with increasing soil moisture.The N₂O emissions of N0,N150and N150+DCD treatment units were 6.5,12.5 and 6.0 mg kg^-1,respectively.In this study,we studied the N₂O emission characteristics of alfafa grassland and its response mechanism to nitrogen addition and application of nitrification inhibitor dicyandiamide.Changes in nitrate nitrogen content and aboveground plant yield revealed that N₂O emissions from alfalfa grassland increased with the increasing of nitrogen allicationa rate and soil moisture content.Nitrification inhibitor had a significant inhibitory effect on N₂O emission.