The Effects Of Temperature And Water Table On The Carbon Processes In Coastal Reclaimed Wetland

Natural coastal wetlands have high vegetation productivity, low soil organic matter decomposition rate and fast carbon sequestration rate, therefore, they are always considered as the outstanding carbon sink (at least at regional scale). The carbon sink function of coastal wetlands is affected by many environmental factors. Temperature and hydrology are the most important environmental factors controlling the carbon process of coastal wetlands. In recent decades, large quantities of natural coastal wetlands were reclaimed by human-constructed dikes, so as to meet the land resources demand derived from the economy development and population growth in the coastal zone. A distinctive disturbed coastal wetland——coastal reclaimed wetland had high possibility to appear in the reclaimed area where agricultural managements were not yet implemented. Compared with natural coastal wetlands, reclamations stopped tidal influences on the coastal reclaimed wetland, cut off the exchange of water, heat and materials between these areas and the sea, as well as changed the hydrothermal conditions and nutrient status. The coastal reclaimed wetland had a lower water table, altered soil salinity, and weaker soil anaerobic environment. All the environmental alterations of coastal wetlands (especially alterations of hydrological factors) caused by reclamations can greatly affect the soil carbon emission and plant community’s carbon fixation of the wetlands. In the context of climate warming, the anthropogenic disturbances to natural coastal wetlands caused by reclamations may potentially influence the responses of the soil carbon emission and plant community’s carbon fixation to air warming. Besides, the controlling mechanisms of temperature, water content and other important environmental factors on carbon flux may differ between natural coastal wetlands and reclaimed coastal wetlands.In order to provide scientific basis for sustainable management and utilization of carbon sink function of coastal reclaimed wetland in the context of climate change, taking a coastal reclaimed wetland at Dongtan of Chongming Island in the Yangtze Estuary as the object, the effects of air warming on the soil carbon emission and plant community’s carbon fixation in the coastal reclaimed wetland were studied by applying open-top chambers (OTC) to simulate air warming conditions. The effects of water table on the soil carbon emission and plant soil respiration in the coastal reclaimed wetland were studied by the comparisons among three different groundwater table gradients. The effects of temperature, water content as well as some other environmental factors on the CO2flux in the coastal reclaimed wetland were studied using in situ carbon flux monitoring based on eddy covariance technique. In terrestrial ecosystem, the soil carbon emission can be reflected by the soil respiration, soil respiration represents the vast majority of soil carbon flux of CO2to the atmosphere. The plant community’s carbon fixation ability is determined by its productivity, and the responses of plant community’s productivity are mediated by the dominant species to a large extent. Carbon flux generally refers to CO2flux between ecosystem and the atmosphere.This dissertation focused on the following questions:1) the effects of air warming on the soil respiration in the coastal reclaimed wetland;2) the effects of air warming on photosynthesis, morphology and growth of two dominant grass species which belong to different functional type in the coastal reclaimed wetland;3) the effects of water table on the soil respiration in the coastal reclaimed wetland;4) the effects of water table on photosynthesis, morphology and growth of two dominant grass species which have different water adaptability in the coastal reclaimed wetland;5) the effects of temperature, water content as well as other important environmental factors on CO2flux between the coastal reclaimed wetland and the atmosphere(net ecosystem productivity).The main results of this dissertation were as follows:1) During a two-year study period, The OTCs significantly increased the mean air temperature by1.53±0.17℃. The air warming resulted in no significant stimulation of the mean soil respiration averaged across the entire study period, warming had no significant effect on soil respiration in the growing season, but it markedly reduced soil respiration by16%in the non-growing season. Air warming had no significant effect on either the mean soil temperature or volumetric moisture at0-5cm depth, but it increased the mean soil porewater salinity by119%averaged across the entire study period. Air warming had no significant effect on total organic carbon, total nitrogen or the molar C/molar N ratio of the soil in the uppermost10cm layer during the two years of soil respiration measurement. The warming treatment also had no significant effect on aboveground biomass or fine root (<2mm) density during the second year of soil respiration measurement. Soil temperature at0-5cm depth was positively correlated with soil respiration in both the control and elevated temperature plots. No significant correlation between soil volumetric moisture and soil respiration was observed in either the control or elevated temperature plots. Soil porewater salinity was positively correlated with soil respiration in the control plots, but such a positive correlation was not found in the elevated temperature plots. No change of Q10value of soil respiration was observed. Soil porewater salinity may be the key factor controlling the effects of air warming on soil respiration in the coastal reclaimed wetland.2) In the peak growing season, air warming significantly decreased the leaf photosynthetic capacity of Phragmites australis (Cav.) Trin. ex Steud (a C3grass), but it did not markedly alter the leaf photosynthetic capacity of Imperata cylindrica (Linn.) Beauv.(a C4grass). Averaged across the entire growing season, at the shoot level, air warming generally reduced the morphological and growth traits of aboveground part of P. australis, but it did not markedly affect those traits of I. cylindrica. At the population level, air warming significantly increased shoot density, leaf area index and aboveground biomass of P. australis, but it markedly decreased those traits of I. cylindrica. In the early growing season, air warming had no significant effect on the rhizome biomass of P. australis at0-20cm depth, but it markedly reduced that of I. cylindrica at the same depth. During the entire growing season, air warming had no significant effect on inorganic nitrogen content in the uppermost10cm soil layer, suggesting that air warming had no effect on the available nutrients content at the top soil layer.In the environment with elevated air temperature and warming-increased soil salinity, P. australis may enhance aboveground biomass by increasing shoot density and specific leaf area despite of their decreased leaf photosynthetic capacity and shoot-level growth, while the clonal propagation of I. cylindrica is likely to be suppressed by the increased soil salinity and the enhanced inter-specific competition from P. australis. 3) During a one-year study period, the mean soil respiration under the three water table gradients decreased in the sequence of MW(medium water table)> LW(low water table)> HW (high water table). The mean soil respiration in HWT was lower than those in LW and MW by18.3%and27.5%. Soil temperature at0-5cm depth was the key microclimate factor driving soil respiration across the three gradients, which could explain more than70%of the seasonal variations of soil respiration in the coastal reclaimed wetland by fitting an exponential model. There was no obvious difference in Q10value of soil respiration among the three gradients. It was likely that the lowest soil respiration rate in HWT was due to its lowest soil temperature and highest soil volumetric water content. Meanwhile, the higher soil respiration rate in MW than LW might be caused by the lower soil porewater salinity and bulk density, in combination with the higher aboveground biomass and live fine root density.4) In the peak growing season, leaf photosynthetic capacity of the hygrophyte P. australis in HWT was significantly lower than those in LW and MW, but no difference was observed in leaf photosynthetic capacity of the mesophyte I. cylindrica among the three water table gradients. Averaged across the entire growing season, at the shoot level, the morphological and growth traits of P. australis got the optimum in MWT, but most of the morphological and growth traits of I. cylindrica were not markedly different among the three gradients. At the population level, the shoot density, leaf area index and total aboveground biomass were highest in HWT for P. australis, but all of the three traits were highest in LW for I. cylindrica. In the early growing season, no difference in the rhizomes biomass of P. australis at0-20cm depth was observed among the three water table gradients; the rhizomes biomass of I. cylindrica at0-20cm depth in HWT was significantly lower than those in LW and MWT. During the entire growing season, inorganic nitrogen content in the uppermost10cm soil layer in HWT was lower than those in LW and MW, suggesting that to raise water table may have significantly reduce the available nutrients content at the top soil layer. The variations of performance of P. australis among the three gradients were probably attributed to the differences in the soil environmental factors as well as the competition intensity from I. cylindrica. 5) CO2fluxes between the coastal reclaimed wetland and the atmosphere at Dongtan of Chongming Island behaved strong daily and seasonal variations. At the daily scale, photosynthetically active radiation and air temperature determined the fluctuation patterns and amplitudes of daytime CO2fluxes and nighttime CO2fluxes, respectively. At the seasonal scale, photosynthetically active radiation and air temperature were both the main environmental factors driving the seasonal variations of daily CO2fluxes, leaf area index and live aboveground biomass were other two biotic factors that potentially affecting the seasonal variations of CO2fluxes. In the coastal reclaimed wetland, the seasonal increase of temperature can significantly increase ecosystem CO2flux. The seasonal increase of soil moisture cannot show significant restriction or stimulation on ecosystem CO2flux, however, it may affect the responses of wetland vegetation photosynthetic capacity to photosynthetically active radiation variation, and ecosystem respiration to temperature variation. At the whole year scale, the coastal reclaimed wetland at Dongtan of Chongming Island behaved as a CO2sink. The annual net ecosystem productivity was558.4gC·m-2·yr-1, with gross ecosystem productivity and ecosystem respiration to be1297.9and739.4gC·m-2·yr-1, respectively. The annual net CO2fixation of the coastal reclaimed wetland at Dongtan of Chongming Island was generally higher than those studies conducted in inland freshwater wetlands located at middle and high latitudes and estuarine/coastal wetlands located at higher latitudes than this study, but lower than those conducted in natural salt marshes at Dongtan of Chongming Island in the same region and tropical mangrove swamps. The annual net CO2fixation of estuarine/coastal wetland showed an increase tendency with the decline of latitude where these wetlands locate. Although the reclamations would reduce its ability of net CO2fixation, the coastal reclaimed wetland at Dongtan of Chongming Island in the Yangtze Estuarine was still an important CO2sink.Based on the above results, this dissertation can further give out several important conclusions for sustainable management and utilization of carbon sink function of coastal reclaimed wetland in the context of climate change.1) In coastal reclaimed wetland, under a warming climate, the enhanced evapotranspiration derived from higher soil water content and the warming-enhanced leaf area index will dissipate and obstruct most of the energy that used for soil warming. Therefore, a moderate air warming may not lead to soil temperature elevation, and thus it may not significantly stimulate soil respiration rate.2) In reclaimed coastal wetland, because the ground water is high in salinity, air warming will accumulate salts in the root zone by enhancing evapotranspiration, and the selective absorption of water in root zone by plant roots will increase soil salinity. The warming-enhanced soil salinity may not only affect the response of soil respiration to air warming, but also increase the dominance of the native dominant marsh plant (P. australis in this study) by restricting the growth and propagation of the dominant non-marsh plant (I. cylindrica in this study) in the plant community, and thus affecting the vegetation productivity.3) In coastal reclaimed wetland, to raise water table will decrease soil respiration rate by lowering soil temperature and increasing soil water content, and thus mitigating the loss rate of soil carbon reserve in coastal wetland caused by the reclamations. In addition, to raise water table will increase the dominance of the native dominant hygrophyte (P. australis in this study) by restricting the growth and propagation of the dominant mesophyte (I. cylindrica in this study) in the plant community, and thus increasing the vegetation productivity. Therefore, to appropriately manipulate water table will have good chance to increase the carbon sink function of coastal reclaimed wetland.4) In coastal reclaimed wetland, too high soil water content may indirectly restrict ecosystem CO2fixation ability by reducing the response of wetland vegetation photosynthesis to photosynthetically active radiation; too high soil water content may not significantly alter temperature sensitivity of ecosystem respiration, but may to some extent decrease the basal ecosystem respiration rate. Therefore, in order to increase net ecosystem productivity of coastal reclaimed wetland, the managers should take measures to manipulate its soil water content. On the one hand, the potential measures should decrease ecosystem respiration; one the other hand, the potential measures should maintain the reasonable response of wetland vegetation photosynthesis to photosynthetically active radiation.