The Characteristics And Causes Of N₂O Flux Observed With Tall-tower Eddy Covariance Over A Typical Agricultural Landscape In The Yangtze River Delta

Agriculture is known to be the largest anthropogenic source of atmospheric N₂O.Derterming the N₂O emission and its influencing factors over a heterogenous agricultural landscape,including croplands,rivers,ditches,aquaculture ponds and other ecosystems,can reduce the uncertainty of regional N₂O budgets and provide important scientific basis for regional greenhouse gas emission reduction policy formulation.In this study,we reported the results of an experiment on the N₂O flux in a typical landscape(radius:15 km,area:707 km²)dominated by rice cultivation in the Yangtze River Delta,China.The observation was made with a closed-path eddy covariance(EC)system on a 70-m tall tower from October 2018 to December2021(39 months).The quality of flux data observed with the closed-path EC system on the tall tower was evaluated.The temporal variations of N₂O flux were clarified.Meanwhile,the N₂O emission and regional emission factor were calculated in the typical agricultural landscape.Furthermore,the influencing factors of N₂O flux were also analyzed at multiple time scales.The major results were as follows:(1)Quality assessment of the closed-path EC dataThe lag time that determined with the covariance maximization method for the closed-path system was 7.4±0.1 s.Based on the typical co-spectra,The N₂O co-spectrum was noisier than those of temperature and CO₂,and appeared to be lower than the modelled value for normalized frequency greater than 3.The frequency correction by the Eddy Pro software increased the N₂O flux by 9%on average.The observation data of the closed-path EC system agreed well with those of the parallel systems,indicating that the closed-path EC system achieved a high standard of performance.The flux footprint contribution from paddy fields,aquaculture ponds and other water bodies was 67.0%,5.4%and 8.9%,respectively,and those three accounted for 81%of the flux footprint contribution with 15-km radius of the EC tower.An obvious temporal variation trend was shown in the 80%contour of the footprint climatologies,with a strong diurnal and the seasonal variability,and the paddy fields accounted for the most important flux contribution on all time scales.(2)Temporal variations,total emission and regional emission factor of N₂O fluxBased on the 39-month N₂O flux measured with the tall-tower closed-path EC in this study site,we found an obvious temporal variation trend in the observed N₂O flux,with a strong diurnal,seasonal and interannual variability.The mean N₂O flux during the observational period was 0.91±0.76 nmol m^-2 s^-1,and the annual cumulative N₂O emission was 8.2 kg N₂O-N ha^-1y^-1,which was higher than those reported by other cropland studies.The annual mean emission factor was 4.2%±0.3%in the typical agricultural landscape,which was much higher than the IPCC default direct emission factors for rice paddies(0.3%)and dryland crops(1%),and also the indirect emission factor(0.75%).(3)Effects of environmental conditions on the N₂O fluxThe relationships of the N₂O flux with temperature(air temperature,water temperature and soil temperature)were exponential at the daily and monthly scales(p<0.01).The temperature sensitivity(Q₁₀)ranged from 1.4 to 1.6,indicating that the N₂O flux has weaker response to temperature at spatial scales consisting of multiple sources than at the plot scale consisting of a single soil source.Although precipitation and the N₂O flux in the typical agricultural landscape was insignificant on the daily and monthly scales(p>0.05),the daily mean N₂O flux fluctuated with the increasing precipitation of different intensities,and the case studies showed that the daily precipitation could stimulate the production and emission of N₂O.On the monthly and annual scales,the interpretation of the temperature and precipitation effects on the observed flux variability was up to 69%.