Observation,Simulation And Potential Risks Of O₃ And CO₂ Fluxes In Farmland Ecosystems

In order to tackle China's severe air pollution issue,the government has released the"Air Pollution Prevention and Control Action Plan"(known simply as the"Action Plan")since 2013.A recent study reported a decreased trend in PM_2.5 concentrations after the implementation of the"Action Plan",but surface O₃ pollution has become more serious.For terrestrial ecosystems,high surface O₃ can inhibit the growth of plants,resulting in reduced yield.In addition,the absorption of atmospheric carbon dioxide(CO₂)by vegetation has been weakened due to the stress effect of O₃,thereby changing the balance of greenhouse gases and exacerbating global warming.To quantify the potential risks of surface O₃ pollution to plants,meteorological factors,crop leaf stomatal conductance,and O₃ and CO₂ fluxes were observed continuously in a major winter wheat and rice planting area of China.Here,we evaluated the applicability of stomatal conductance models for winter wheat and rice,and distinguished the distribution of O₃ dry deposition fluxes between stomatal and non-stomatal sinks.At the same time,we analyzed the dynamic changes of CO₂ flux and carbon budget during the two crops growing seasons,and established a CO₂ flux model driven by various environmental factors.Based on surface ozone observation data covering 362 cities across China,we also evaluated the effects of surface O₃ on the yield and economic losses of winter wheat and rice in the main growing regions of China in recent years.The main results of these studies were as follows.(1)The diurnal variations of stomatal conductance for winter wheat and rice both increased first and then decreased,and reached their maximum values around noon.The growth stage changes of stomatal conductance for the two crops showed a unimodal pattern,and the peaks appeared at the flowering and booting stages,respectively.The revised Jarvis stomatal conductance model had a better simulation effect and was applicable to the calculation stomatal O₃ flux for the two crops in this region.In the future climate change scenario,the values of stomatal O₃ absorption fluxes for the two crops will decrease if the variation of ozone concentration is not considered.If we consider the future changes in ozone concentration,the values will increase.(2)During the observation period of the two crops,the rate and flux of O₃ dry deposition displayed a relatively gentle change during the nighttime and a more intense change during the daytime.The peaks of the rate and flux for O₃ dry deposition appeared at 08:30?09:00am and 12:30?14:00pm,respectively.The average values of O₃ deposition rate in the winter wheat and rice growing seasons were 0.31cm·s^-1and 0.34 cm·s^-1,respectively,and the average values of O₃ deposition flux were-0.0044?mol·m^-2·s^-1 and-0.0049?mol·m^-2·s^-1.The accumulated O₃ flux,stomatal flux and non-stomatal flux during the observation period of winter wheat were 26.1 mmol·m^-2,7.7 mmol·m^-2,and 18.4 mmol·m^-2,respectively.The accumulated O₃ flux,stomatal flux and non-stomatal flux during the observation period of rice were 27.8 mmol·m^-2,9.8 mmol·m^-2,and 18.0 mmol·m^-2,respectively.(3)During the growing season of winter wheat,the ratios of average stomatal O₃flux and non-stomatal O₃ flux to total O₃ flux were 29.5%and 70.5%,respectively,and the ratios during the daytime were 39.7%and 60.3%.During the growing season of rice,the ratios of average stomatal O₃flux and non-stomatal O₃ flux to total O₃ flux were 34.0%and 66.0%,respectively,and the ratios during the daytime were 49.0%and 51.0%.(4)During the whole observation period,the diurnal variation curves of CO₂concentration for the two crops showed a typical one-peak-one valley pattern,which was low during the day and high during the night.The diurnal variation curves of CO₂concentration for winter wheat was relatively flat,while that of rice was more severe.The mean values of CO₂ fluxes during the daytime for winter wheat and rice were-13.4?mol·(m²·s)^-1 and-12.9?mol·(m²·s)^-1,respectively.The highest fluxes occurred at the booting stage of winter wheat and at the flowering stage of rice.In addition,the CO₂ fluxes of the two crops during the growing season showed a diurnal variation characteristic of a U-shaped curve.Two farmland ecosystems absorbed CO₂ mainly during the daytime,and the absorption peaks of winter wheat and rice appeared at12:00 and 11:30,respectively.The CO₂ flux changes during the night were relatively stable,which manifested as respiratory CO₂ emission.(5)The CO₂ fluxes and leaf stomatal conductance of the two crops driven by environmental factors such as photosynthetically active radiation,air temperature,vapor pressure deficit,phonological period,time,and CO₂ concentration showed similar limiting mechanisms.The modified Jarvis multiplication model can be applied to simulate the CO₂ fluxes for winter wheat and rice.Two farmland ecosystems were carbon sinks.For winter wheat,the net ecosystem carbon exchange was 188.2 g C·m^-2,the total primary productivity was 459.1 g C·m^-2,and the carbon emission 270.9 g C·m^-2.For rice,the net ecosystem carbon exchange was 233.8 g C·m^-2,the total primary productivity was 689.3 g C·m^-2,and the carbon emission 455.5 g C·m^-2.(6)O₃-induced yield reductions were estimated to range from 20.1 to 33.3%for winter wheat,7.3-8.8%for single rice,3.9-6.8%for double-early rice and 5.9-7.1%for double-late rice.O₃-induced production losses for winter wheat,single rice,double-early rice,and double-late rice totaled 39.5-99.7 million metric tons,9.5-11.9million metric tons,1.2-1.8 million metric tons,and 2.2-2.7 million metric tons,respectively,and the corresponding economic losses totaled 14.3-36.2 billion US$,3.9-4.9 billion US$,0.5-0.7 billion US$,and 0.9-1.1 billion US$,respectively.