| Mangrove ecosystems are among the most productive natural ecosystems in the world, and global mangrove wetland is one of major contributors to earth’s blue carbon sink. Systematic study of mangrove carbon cycle will improve our understanding of the regulatory mechanisms of mangrove carbon sink, and prediction of their responses to global change. However, there are limited studies on mangrove carbon sequestration and sink strength, especially for subtropical mangrove ecosystems. In this study, eddy covariance technique was used to quantify mangrove carbon sequestration and their determining factors in Yunxiao, Zhangzhou and Gaoqiao, Zhanjiang, China; then traditional survey, field control experiment and stable isotope technique were applied jointly to investigate litter decomposition and their consumption processes. The main findings and conclusions from this study were summarzied as follows:(1) The total biomass was82.17-118.74Ton ha-1for the mangrove forests in Yunxiao (August2008) and85.04-161.08Ton ha-1for the mangrove forests in Gaoqiao (October2009), respectively, with branch and taproot having the highest proportions. Below ground biomass were mainly distributed in a depth of0-30cm, and their below to above-ground ratios ranged from0.35-1.47. Mean annual litter production was843.20-1011.51and964.98g m"2for the Yunxiao and Gaoqiao mangrove forests from2009to2012, with the leaf litter having the highest proportion. Litter production showed obvious seasonal and inter-annual variation, attributed to the changes in temperature, rainfall and wind conditions.(2) During the last four years (2009-2012), the net ecosystem CO2exchange (NEE) was687and721g C m-2yr-1for Yunxiao and Gaoqiao mangrove forests, respectively. The mean annual gross ecosystem productivity (GEP) was1871and1763g C m-2yr-1, respectively for Yunxiao and Gaoqiao mangrove forests during the same period, with corresponding mean annual total ecosystem respiration (RE) of1287and1096g C m-2yr-1and RE/GEP of0.69and0.63, respectively. Solar irradiance, cloud, vapor pressure deficit (VPD) and temperature co-regulated daytime NEE. Both Yunxiao and Gaoqiao mangrove ecosystems reached their maximum NEE under moderate cloudiness (clearness index of0.3-0.6and0.4-0.6). Net primary productivity (NPP) estimated by traditional survey (litter fall method) was much higher than that from eddy flux measurements, likely due to the underestimation of litter proportion in NPP by the traditional survey method, which underscored the importance of research on litter fall for understanding mangrove carbon cycle processes.(3) Frequent typhoons with strong wind and intensive rainfall caused defoliation and local cooling effect during typhoon season. Results from our synthesis of23typhoons during2009-2011demonstrated that wind speed, duration and distance of typhoon from mangrove ecosystem were the most important factors controlling the mangrove ecosystem carbon and water fluxes. In2010, the maximum NEE was slightly decreased following typhoon. Except for category13typhoon Megi (October2010), the apparent quantum yield was significantly greater than that before typhoon. Daytime RE also exhibited significant decrease after typhoon landfalls, particularly following typhoon Megi. Daily total NEE values were decreased by26%and57%following category12typhoon Conson-Chanthu (July2010) and category10typhoon Namtheum-Fanapi (September2010), respectively, while they were significantly increased by140%following typhoon Megi. These changes were quickly reversed within a few weeks right after the typhoon events indicating that mangrove ecosystems have strong resilience to the frequent typhoon disturbances.(4) The leaf litter of Kandelia obvata decomposed faster than that of Spartina alterniflora at the mangrove decomposition site, with a decay constant and half life time of0.021,0.012d-1and34,59d for K. obvata and S. alterniflora, respectively. However, there was no significant difference between species at the salt marsh decomposition site. The leaf litter of S. alterniflora leached faster than that of K. obvata with a leaching rate of0.14,0.09g d-1, respectively. The dry mass loss rates were similar for the leaf litter of both plant species and both decomposition site, degrading rapidly in the first28days, then following by a slower, steady decrease for the remaining period of the experiment. Total nitrogen content of leaf litter decreased during the decomposition, and the C/N ratio was negatively correlated with nitrogen content for the decomposed leaf litter. The δ13C values of decomposing leaf litter decreased during decomposition, about1%o for K. obvata and1.5%o for S. alterniflora).(5) Polychaetes depended on mangrove leaf litter, microphytobenthos and particular organic matter for their foods at the mangrove site, but mainly used S. alterniflora leaf litter at the salt marsh site. The preliminary results from a field control experiment showed that the δ13C values of polychaetes were affected significantly by added leaf litter during decomposition. After adding exotic S. alterniflora leaf litter, polychaetes at the mangrove site had larger enrichment in813C values than that at the salt marsh site. Mangrove polychaetes assimilated more carbon from exotic S. alterniflora leaf litter than from native mangrove leaf litter. However, the significant carbon consumption only occurred within the half-life time (<2months) of exotic S. alterniflora leaf litter indicating significant impact of Spartina invasion on the food sources of local polychaetes. Decomposition rate and nutrient content might contribute to the selective utilization of litter from exotic S. alterniflora and native mangroves by the polychaetes in the mangrove wetlands. |
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