Estimation Of Rice-wheat Growth And Greenhouse Gas Emissions As Affected By Irrigation/sowing Date Under Nighttime Warming

Rice and wheat are two major food crops in China.Great attentions have been paid to the effect of climate warming on rice and wheat production and greenhouse gas emissions from farmland.Asymmetric diurnal warming is one of the main features of climate warming,with greater warming trend in the daily minimum temperature than in the daily maximum temperature.Water saving irrigation in rice and delay sowing in wheat are important field management measures to cope with climate change.However,the effect of water saving irrigation in rice/delay sowing in wheat on yield and the emissions of greenhouse gases from cultivated soils under nighttime warming remain unclear.Heading and flowering stages are critical not only to rice and wheat production but also to methane(CH₄)and nitrous oxide(N₂O)emissions.Therefore,real-time,accurate and rapid estimating rice heading,wheat flowering as well as CH₄and N₂O emissions from rice-wheat soil are important prerequisites for the production and the mitigation of greenhouse gases.Ground based remote sensing,with the advantages of high resolution,real-time,fast and non-destructive,has been widely used in monitoring growth status.However,few researchers have used ground based remote sensing to quantify the process of rice heading and wheat flowering stages,and greenhouse gas emissions under field conditions.Thus,field experiments were conducted to explore the combined effects of nighttime warming and water saving irrigation in rice season/delay sowing in wheat season on rice-wheat growth,CH₄and N₂O emissions from the field soil and canopy hyperspectral properties,and constructed models for estimating rice-wheat heading and flowering processes,and CH₄and N₂O emissions based on canopy hyperspectral data.Main results showed as follows:(1)Irrespective of irrigation or not,nighttime warming decreased rice tiller numbers,photosynthetic rate(Pn),and aboveground biomass and leaf area index(LAI)after tillering.The effect of nighttime warming on rice plant height varied with water management and phenophase.Under different sowing conditions,nighttime warming had different effects on wheat plant height,LAI,aboveground biomass and Pn.Compared with normal sowing,delay sowing significantly decreased pre-winter tiller numbers,plant height,aboveground biomass,LAI,and Pn at tillering-jointing stage.(2)The effect of nighttime warming on CH₄ and N₂O emissions in paddy field varied with irrigation conditions.Under flooding conditions,nighttime warming reduced CH₄flux and accumulative CH₄ emission,but increased N₂O flux and accumulative N₂O during draining-drying period.Under water saving condition,nighttime warming increased CH₄ flux and accumulative CH₄ emission,but reduced N₂O flux and accumulative N₂O in paddy soil.Furthermore,lower global warming potential(GWP)and GHG(greenhouse gases)emission intensity(GHGI)occurred in water saving irrigation.Nighttime warming increased CH₄ flux and accumulative CH₄emission in wheat soil.However,the effect of nighttime warming on N₂O flux varied with sowing date.Under normal sowing condition,nighttime warming significantly increased GWP and GHGI in wheat soil.Under delay sowing condition,nighttime warming had no significant effect on GWP and GHGI in wheat soil.(3)Nighttime warming increased near-infrared reflectance,red-edge amplitude and red-edge area on jointing stage,but decreased above parameters in subsequent sampling period in rice.Nighttime warming increased near-infrared reflectance,red-edge amplitude and red-edge area at jointing and booting stages,but decreased above parameters at heading and flowering stages in wheat.(4)Nighttime warming stimulated the heading process in rice.Water saving irrigation also promoted the heading process in rice.The newly developed spectral index(R₇₃₄-R₈₃₈)/(R₇₃₄+R₈₃₈-2R₈₁₂)was the optimal index for monitoring rice heading rate.Nighttime warming stimulated the flowering process in winter wheat.In contrast,delay sowing postponed the flowering process in wheat.The newly developed index(R₄₄₆-R₄₇₂)/(R₄₄₆+R₄₇₂)was the optimal index for monitoring winter wheat flowering rate.(5)Based on the model performances for estimating CH₄and N₂O emissions from the field soil with two-band spectral indexes(normalized difference spectral indexes,difference spectral indexes and ratio spectral indexes),three-band indexes(₄/)(₅)+₆))?(₄)-₅))_/(₄)+₆))?(₄)-₆))_/(₅)-₆))?(₄)-₅))_/(₄)+₅)-2₆)))and published indexes,the indexes with(R₁₁₃₉-R₉₆₀)/(R₁₁₃₉+R₉₆₀-2R₉₄₄)and(R₆₅₆-R₆₈₆)/(R₆₅₆+R₆₈₆)were optimal in monitoring CH₄flux;and the indexes with(R₉₅₆-R₄₅₆)/(R₉₅₅-R₄₅₆)and(R₈₄₉-R₈₅₀)/(R₈₄₉+R₁₃₀₀)were optimal in monitoring N₂O flux in rice field and wheat field,respectively.