CH₄ And N₂O Emission In Response To Nighttime Warming And Its Hyperspectral Estimation In Rice-wheat Field

Global warming is an important environmental problem we are facing in recent years,which is related to the massive emissions of greenhouse gases due to human activities.The global climate warming,mainly characterized by nighttime temperature increasing greater than daytime.The nighttime warming not only affects crop growth and production,but also greenhouse gas emissions in soils,with likely exacerbating greenhouse effects.Rice and winter wheat are two main food crops,rice-winter wheat rotation is the main planting pattern in the region of Yangtze River Delta,and the rotated paddy soils(rice-wheat soil)are important CH4 and N2O emission sources.It is an effective measure to monitor and estimate the emissions of CH4 and N2O in rice-wheat rotated soils with remote sensing,and to increase the capacity of rice and wheat productions to climate change.A field experiment with rice and wheat was conducted to investigate the effects of passive nighttime warming on CH4 and N2O emissions,and canopy hyperspectral characteristics in rice-wheat rotation system,at the Station of Agricultural Meteorology,Nanjing University of Information Science and Technology,Nanjing,China.CH4and N2O emission fluxes were estimated with hyperspectral data.This study provides a theoretical basis for and demonstrates the feasibility of non-destructively monitoring of CH4and N2O emissions in the rotated paddy field.The experiment was designed with two warming levels,i.e.,nighttime warming(NW)and control(CK).NW was created by covering the rice canopy with an aluminum foil reflective film at night(19:00-6:00).The main findings are listed as follows:(1)CH4 and N2O emission fluxes in response to temperature,precipitation(irrigation)and fertilization in rice-wheat rotated soilIn rice field,CH4 and N2O emission fluxes were significantly correlated to the duration of continuous flooding(drainage)(P<0.05).During the wheat growth period,the correlation between CH4flux and temperature was extremely significant(P<0.01),while the emission peak of N2O was related to fertilization in wheat growth period.(2)CH4 and N2Oemissions in rice-wheat rotated soilNighttime warming did not change the seasonal variation trend of CH4 and N2O emissions in rice-wheat rotation system.Nighttime warming significantly increased CH4emission flux at rice jointing stage,but had different influences on CH4source and sink at different growth stages in wheat cultivated soil.Nighttime warming decreased N2O emission in rice and wheat at different growth stages.Therefore,sustained-flux global warming/cooling potentials(SGWP/SGCP)were used to assess the combined warming effects of nighttime warming on CH4 and N2O emissions fin the rotated field.Nighttime warming increased the total contribution of CH4 and N2O emissions,CH4emission in rice paddy is the dominant contributor.Combined with the yields of rice and wheat,the increasing trend of the total emission intensity of CH4 and N2O in the rotated field was significant.(3)Canopy hyperspectral of rice and wheatIn rice field,nighttime warming increased the reflectance of the near-infrared spectrum on the canopy at the tillering and jointing stages,but decreased at the grain filling and maturity stages;in wheat field,nighttime warming increased the values from the heading stage to maturity stage.Nighttime warming significantly reduced the ’Area of Red Edge’ and‘Red edge amplitude’ of rice canopy at rice jointing stage;significantly increased the values on wheat canopy from heading to maturity stage.At rice tillering stage,nighttime warming made the ’red edge position’ a significant‘red shift’;At wheat ear-ripening stage,the‘red edge position’ continuously showed‘blue shift’,while the control showed ’red shift’ and then‘blue shift’.(4)Estimation of CH4 and N2O fluxes based on the canopy hyperspectral data of rice and wheatThe positive correlations were observed in the relationships of CH4emission with spectral reflectance and the first derivative and the characteristic value of the spectrum.After comparing correlation coefficients(R2)of the fitting models and prediction models,the quadratic polynomial model by ’Area of Blue Edge’(SDb)was found to be the best model(fitting model R2=0.70,prediction model R2=0.72)for estimating CH4 emission.The spectral parameters of rice canopy were closely related with N2O emissions at grain filling stage and mature stage,being up to 0.83(P<0.01);Models were constructed based on the‘continuous drainage or irrigation days(ConD)’and spectral data,of which the linear regression model based on‘ratio of the green peak(Rg)and the red vale reflectance(Rr)’(Rg/Rr)and ’ConD’ demonstrated the best simulation accuracy and tested accuracy(fitting model R2=0.57,prediction model R2=0.60).In wheat field,the emission fluxes of N2O was obviously related with canopy spectra at filling stage.