Rapid Diagnosis For Nitrogen Nutrition Status Of Rice Leaves And Greenhouse Gas Emissions In Paddy Field

Nitrogen is one of the most important nutrient elements to rice cultivation.Appropriate rate of nitrogen fertilization contributes to rice yield,but excess rate application not only leads to lower efficiency of nitrogen usage,also aggravating environment pollution like,increment of greenhouse gas(GHG)emissions,eutrophication and so on.In this study,SPAD values of rice canopy leaf and its relationship between rice nitrogen nutrition diagnosis;variation of greenhouse gas(CO2,CH4,N2O)emissions and distribution of nitrogen fertilization in the plant soil and atmosphere systems were studied under six different nitrogen fertilization rates.Research was conducted to explore co-benefit of rice yield and reduction of GHG emissions by using rapid rice nitrogen nutrition diagnosis.The result provided scientific basis and practical guidance for field management and greenhouse gas emission reduction in paddy ecosystems,and it is meaningful for mitigating of global climate change and sustainable usage of paddy field.Six nitrogen fertilization rates(0,75,150,225,300,375 kg N/ha,2013-2014)and different fertilization program(2015)were set in experiment field to study the dynamic changes of SPAD value and its relationship between leaf nitrogen content,relevance of critical nitrogen concentration and nitrogen nutrition index(NNI),variation of GHG emissions and greenhouse gas index(GHGI)during the rice cultivation,distribution of nitrogen fertilization in the plant soil and atmosphere systems.The detailed results were as follows:(1)The SPAD values of 4 rice canopy leaves changed in an "M" shape during the growth period.This changing regular is decided by rice growth characteristics and won't change by fertilization rate.The SPAD values of low nitrogen treatment were lower than high nitrogen treatment.SPAD values in low nitrogen treatment varied severely in late growth period while trends of high nitrogen treatment changed more gently.The trend of correlation between SPAD values from L3(third top leaf),L4(fourth top leaf)and leaf nitrogen content presented decreasing in the first then increasing as in the tillering stage regression coefficients fitted the highest(0.80-0.80),followed by heading(0.68-0.69),lowest in booting stage(0.41-0.57).As L3 and L4 in response to a higher level of nitrogen nutrients,these two leaves are the best choice for plant nitrogen nutrition diagnosis.(2)Curve of rice critical nitrogen concentration can be over fitted by model Nc=aW-b.The fitting equation in 2013 is Nc=5.38W-0.49(R2=0.81)in 2013 and Nc=4.29W-0.55(R2=0.78)in 2015.The range of nitrogen application rate under rice critical nitrogen concentration is 225-300kg N/ha(NNI=1).In this case,the ideal SPAD value curve of L3 and L4 can be fitted by growth stage with a quadratic equation.The R2 in first stage(0-60 days)is 0.56-0.68,SPAD value variation ranged of 45-50;the fit coefficients were greater than 0.9 in the second stage(60-120 days),SPAD value variation ranged of 35-47.5.(3)The experiment of GHG emissions was conducted in 2013-2014 under different nitrogen application rates.And the results showed that nitrogen application rate promoted CO2 and N2O emissions,while showed no significant effect on CH4 emissions.CO2 emissions peaked in the middle of growth period and were significantly positively correlated with temperature.Emission rate ranged 26.85-2841.81mg/m2·h and emission flux ranged 1337.12-3263.51g/m2;CH4 emission peaked in the early rice growth and significantly associated with surface water,emission rate ranged 19.62-32.95mg/m2·h and emission flux ranged 145.38-404.63g CO2-eq/m2;N2O emission peaked in the late rice growth and significantly associated with nitrogen application rate.High emission rate appeared under high nitrogen rate treatment with 99.91?g/m2·h emission flux ranged 2.29-51.85g CO2-eq/m2.Total global warming potential(GWP)ranged 1514,31-3512.13g CO2-qe/m2,and the contribute ratio order was CO2>CH4>N2O.Field measured SPAD values combined with ideal curve can be used to predict appearance of N2O emission peak.(4)Greenhouse gas emissions index(GHGI)ranged 1.91-4.57kg CO2-qe/(kg rice)and the relationship between GHGI and nitrogen rate presented as an "U" shape.High GHGI appeared in low and high nitrogen rate treatment,while low GHGI appeared in appropriate nitrogen appropriate rate treatment Lowest GHGI appeared under the nitrogen rate with 225kg N/ha treatment for two years,and 3.69 kg CO2-qe/(kg rice)in 2013,2.23 kg CO2-qe/(kg rice)in 2014,with an average of 2.96 kg CO2-qe/(kg rice).Combining agricultural production and greenhouse gas emissions,225kg N/ha nitrogen rate is the best nitrogen rate in Taihu Lake,while ensuring rice yields and low emissions of greenhouse gases,achieving the co-benefit with agricultural efficiency and environmental benefits.(5)15N isotope was used to track the migration and distribution of nitrogen element in paddy field.The ratio of urea absorbed by rice and residue by soil decreased with nitrogen rate increasing.The ratio uptake by plants accounted for 24.02-44.23%,soil residual accounted for 6.88-21.82%,recovery ranged 30.90-66.05%;ammonia volatilization is the main way of nitrogen loss,ammonia volatilization loss of nitrogen accounts for the proportion with the increase of nitrogen rate increased,the loss ratio ranged 29.49-58.01%;N2O is another form of gaseous loss of nitrogen,N2O loss accounted for the proportion of nitrogen rises with increasing nitrogen rate,loss ratio ranged 0.14-0.94%.The main residual nitrogen conserved the soil in the layer of 0-20 cm,increasing with the amount of nitrogen rate.There was no significant difference between different nitrogen rate of nitrogen conservation in soil layers of 40-60cm.Excessive nitrogen input can't be effectively retained in the soil,but increased ammonia volatilization and N2O emissions.