The Impact Of Agroforestry Use And Underlying Surface Changes On CO₂ Fluxes Of The Reclamation Wetland At Dongtan Of Chongming Island

How does the CO₂ budget in coastal wetlands respond to dynamic and composite agroforestry reclamations is mostly unknown.For six consecutive years?from 2011 to2016?,CO₂ fluxes data were measured using the eddy covariance?EC?technique to determine the CO₂ budget changes and the key factors controlling the variations of net CO₂ ecosystem exchange?NEE?over a reclaimed coastal wetland in the Yangtze estuary,China.a homogeneity-of-slopes?HOS?model was utilized to partition sources of variation in CO₂ fluxes among meteorological effects,functional vegetation changes,and other effects.Meanwhile,the variability of different CO₂ fluxes is partitioned to functional vegetation changes,meteorological changes,and flux contribution area changes.The main conclusions of this study are as follows:1.The fluctuation law of CO₂ flux in coastal reclamation wetland is similar before and after agroforestry use.The diurnal dynamic characteristics show a typical u-shape distribution and the fluctuation is relatively stable.During the daytime?06:00-18:00?,it shows an obvious carbon sink?NEE<0?,at night?00:00-06:00,18:00-24:00?,it acts as a carbon source?NEE>0?,and the maximum NEE value in the agroforestry use process showed a decreasing trend.The average daily change of NEE in each year?2011?109-365 days only?-2016?presented a typical w-shaped curve.The effect of coastal reclaimed wetland began to fix CO₂ from the atmosphere in March.The maximum daily average NEE occurred on the 241st day(-14.14 g Cm^-2)of 2011,The240th day of 2012(-8.42 g Cm^-2),The 234th day of 2013(-4.86 g Cm^-2),The 188th day of 2014(-5.65 g Cm^-2),The 221th day of 2015(-4.90 g Cm^-2),and the 184th day of2016(-6.64 g Cm^-2).2.The average annual NEE value of reclaimed wetland decreased significantly before and after agroforestry use.The annual accumulation NEE decreased from-655.7?only 109-365 days?in 2011,558.4 g Cm^-2 yr^-1 in 2012,to-258.1 g Cm^-2 yr^-1?2013?,-198.1 g Cm^-2 yr^-1?2014?,-160.8 g Cm^-2 yr^-1?2015?and-141.2?2016?g Cm^-2 yr^-1.From2012 to 2016,the days that NEE accumulated to be negative occurred on day 85,day90,day125,day 152,and day 131,respectively.After agroforestry use?from 2013 to2016?,the days that NEE accumulated to be negative were delayed by 1-7 weeks compared with 2012.3.The decrease in NEE in 2013 was mainly due to the significant increase in ecosystem respiration?ER?.Since then,the continued decrease in NEE through 2016 is more due to the decrease in total primary production?GPP?.Compared with the continuous decline of NEE,the|ER/GPP|ratio rose from 0.40 in 2011 and 0.57 in 2012to 0.80 in 2013 and 0.83 in 2014,and continued to rise till 2015 and 2016,with the ratios of 0.85 and 0.89,respectively.4.The HOS model analysis results show that the inter-annual variations?IAV?of NEE are more attributed to the variation of vegetation function?19.0%?than the inter-annual meteorological variation?9.2%?in the agroforestry use of the reclaimed wetland.For seasonal variation and random error,the variability of NEE was explained by 27.2%and 44.7%,respectively5.The significant decrease of A_max and the small increase of?also verified the functional change of vegetation,which may be related to the change of dominant plant species and the decrease of the length of growing season.The dynamic changes of Q₁₀and R₁₀ responses to wetland drainage and agroforestry use reflect the seasonal changes of the underlying surface composition and respiration characteristics within the range of CO₂ flux footprint.6.The significant impact of the random error on NEE variability?44.7%?may be mainly due to the significant increase in the area within the contribution area of the young-age forest?YF?and artificial river?AP?after agroforestry use?2013-2016?.7.Based on Chamber method in 2017-2018,which conducted at the same place,we measured the NEE,Rh,and NEP in coastal reclaimed wetland.The results show that the VW with reed?p.australis?as the dominant species had higher NPP on an annual scale,compared with the VW which was dominated by imperata?I.cylindrica?.AP?artificial pond?and AR?artificial river?are both net CO₂ sources,while YF?young-age forest?and MF?mature forest?also showed high CO₂ emissions due to the low NPP and the relatively high R_h.The above research results show that agroforestry use on coastal reclamation wetland would lead to the continuous decline of carbon sink function by changing the structure and function of the ecosystem,including the decrease of vegetation productivity,the decrease of CO₂ flux response capacity of vegetation,the change of substrate respiration and the change of CO₂ flux response of vegetation to temperature.The structural change is mainly caused by the increase of other underlying surface type,which have a weak CO₂ absorption capacity and even act as the source of CO₂.Therefore,more sensible land use policies and management measures should be adopted in coastal reclamation areas to achieve regional low carbon emission reduction.The research results are helpful to understand the impact of agroforestry use on the carbon balance in coastal reclamation areas in the context of global climate change,and can provide theoretical basis and data support for government departments to take effective measures to manage different agroforestry use patterns,increase carbon absorption,and reduce carbon emissions.