Carbon Deposition And Burial In Typical Coastal Wetlands Of The Northern Shandong Peninsula

Coastal wetlands are located at a critical interface between the terrestrial andmarine environments, which are generally known to be biogeochemical reactors formaterial sinks, sources, and transformations in landscapes. Coastal wetland plays animportant role in the global carbon budget. The research of soil carbon deposition andburial is of great significance for clearly understanding global carbon cycle. Thisstudy investigated the vertical variations of carbon pool and their influencing factorsin different coastal wetland through the comparison of the biogeochemicalcharacteristics of carbon in sediments from two typical coastal wetlands of Shandongpeninsula, and characterized the labile carbon pool and recalcitrant carbon pool in thecoastal wetlands，then estimated the storage of stabile carbon pool and discussedstability of the sediment organic carbon pool. Our study would be helpful for futureresearch on the carbon biogeochemical cycle and accurate assessment of carbon burialability for the coastal wetlands. Major results and conclusions are as the following:1. Biogeochemical characteristics of carbon in sediments from different coastalwetlands were quite different, while vegetation and tidal creek had greatinfluence on sediments organic carbon and inorganic carbon retention. Generally,the inorganic carbon contents in Guangrao wetland were homogeneous and quitehigh, while the variation of organic carbon was obvious. It could be explained bythe point that the capacity of wetland soils for organic matters retention andplant litters input amounts were very distinct within this wetland, and themineralization and decomposition of organic matter were quite slow under thelong-term anaerobic situation. Inorganic carbon contents in Changyi wetlandsediments were lower, and the profile distributions of TOC might be dominantlycontrolled by the aboveground and belowground biomass of local vegetation. Soilorganic carbon mineralization rates were higher in Changyi wetland, and thecarbon burial capacity still needed to be improved.In Guangrao wetland, the TOC contents in GRB1slightly increased in the profileuntil a depth of30-32cm firstly, and then it showed a decreased trend with depth with the lowest value of1.139mg g-1at72-74cm. TOC contents in GRC2profile werehigher than that in GRB1. An irregular increase of TOC contents was observed alongGRC2profile. In the upper10cm, SOC contents gradually decreased with depth, thenit showed a gradually increase trend except for two obvious accumulation peaks at the12-14cm and38-40cm depth. Firstly, the GRC2area had a tidal creek nearby, whichcan bring much allochthonous nutrients deposited. Secondly, GRC2surface wascovered with P.australis, which had much higher biomass and sufficiently strongerroots than GRA2with S.salsa. Generally, the inorganic carbon contents in Guangraowetland were homogeneous, the average of the inorganic carbon contents in GRB1and GRC2were11.25g/kg and10.93g/kg, respectively. In Changyi wetland, TOCcontents at most study sites that located at supralittoral zone showed a regularlychange pattern that generally decreased with depth along about an upper10cm depthprofile and then fluctuated within a limited range. There was only occasionally floodfrequency when spring tide or storm surge came. Therefore, the profile distributionsof TOC might be dominantly controlled by the aboveground and belowgroundbiomass of local vegetation. Compared to Guangrao wetland, inorganic carboncontents in Changyi wetland sediments were lower.2. Dissolved organic carbon (DOC), hot water extractable carbon (HWC) andacid hydrolyzable carbon distribution characteristics in different coastalwetlands were obtained, and the HWC content was quite higher than DOCcontent. While the HWC content in Changyi wetland were lower than that inGuangrao wetland, its proportion in total organic carbon was higher. Tidal,vegetation and landscape were of significance for the distribution of HWC.Hydrochloric acid extraction of organic carbon content varied differently, and itsenvironmental significance was not clear. However, different concentrationsulfuric acid could divided the labile organic matter into two groups ofhydrolysate extraction that had great potential value for indicating the organiccarbon degradation.Dissolved organic carbon content in different coastal wetlands varied greatly, with higher contents in the sediments of Guangrao wetland. The average contents ofDOC in GRB1and GRC2were637.37mg/kg and712.705mg/kg, respectively, andthe variation of DOC content in GRC2was more significantly than GRB1. In ChangyiWetland, the maximum value of74.68mg/kg appeared in the surface0-2cm, andDOC in the shallow depth decreased. Water content affected the allocation of DOC inthe TOC, and the proportion of DOC in TOC in different wetlands was of greatdifference. The proportion of DOC in the GRB1and GRC2profiles were34.86%and24.08%, while the CYA3and CYA9profiles were only2.42%and1.54%. While theHWC content in Changyi wetland sediment were lower than that in Guangrao wetland,its proportion in total organic carbon was higher. The mean content of HWC in GRB1was0.632g/kg, and it was higher in GRC2with an average value of0.751g/kg. InChangyi wetland, the average HWC content in CYA9was higher than that of CYA3.The vertical variation of HWC content in CYA3which was not influenced by tide isthe smallest, while the variation coefficients in the other three stations were quitesimilar. In addition, only in CYA3the content of HWC was significantly related to thetotal organic carbon. These results implied that tidal fluctuations could bringconsiderable impact on the HWC in the sediments of the coastal wetlands, and thecomprehensive effects of different vegetation and landform conditions caused thestrong spatial heterogeneity of HWC distribution. Hydrochloric acid extraction oforganic carbon content in GRC2was significantly higher than that of GRB1. The acidhydrolyzable carbon in GRB1was within the range of0.527g/kg-1.280g/kg, whilethe GRC2was in the range of1.087-2.309g/kg. In Changyi wetland, the verticalvariation of acid hydrolyzable carbon in CYA3showed obvious downward trend withthe average of0.92g/kg. In CYA9, the variation of acid hydrolyzable carbon was verysmall, and it only had a certain degree of volatility within the0-12cm shallow layer.Different concentration sulfuric acid could divide the labile organic matter into twogroups. In different wetland, labile pool II were significantly lower than labile pool I,and changes of pool II were more stable with depth than pool I. In Guangrao wetland,the mean content of GRB1pool I and pool II were0.583g/kg and0.290g/kg, whilethat of the GRC2were0.860g/kg and0.409g/kg, respectively. In Changyi wetland, pool I sharply decreased within the first10cm along CYA3profile. Pool II increasedwith the depth within the first16cm, after which the fluctuations became smaller.3. The recalcitrancy indices of sediment organic carbon were analyzed, andvertical distribution characteristics of recalcitrant carbon pool in differentcoastal wetlands were illuminated. Generally, stability of organic carbon pool inGuangrao wetland sediments was higher than that of Changyi wetland. InGuangrao wetland, the stability of organic carbon of two sediment profiles werevery close to each other, and the tidal played very important role in maintainingthe stability of sediments organic carbon pool. However, due to environmentalconstraints, the recalcitrancy indices were quite low, and the organic matter inChangyi wetland were easily decomposed. We estimated the stabile carbonstorage capacity of Guangrao and Changyi wetland within1m depth were21.9kg/m2and7.5kg/m2, respectively. The results indicated that the carbonsequestration ability of Changyi wetland had higher potential for improvement.In Guangrao wetland, recalcitrant carbon contents in GRB1and GRC2were0.919g/kg and1.671g/kg, respectively, and this may due to the significantly differentquality of organic matter input in profiles from different vegetation. The recalcitrantcarbon increased with depth firstly and then decreased in GRB1, while it decreasedfirstly and then increased in GRC2. Recalcitrant carbon content was lower in Changyiwetland sediments. The distribution pattern of recalcitrant carbon contents in CYA3showed a dramatic reduction with depth firstly and a slight fluctuation then. Theaverage content of recalcitrant carbon in CYA9was0.384g/kg, with smaller change.In the shallow layer, the stability of organic carbon in CYA3was decreased withdepth, this probably due to more vegetation litter accumulated in the surface layer,besides, the leaching process of the active organic matter in the shallow layer causingthe increasing accumulation. We estimated the stabile carbon storage capacity ofGRB1, GRC2and CYA3, CYA9within1m depth were15.074kg/m2,28.748kg/m2,9.371kg/m2and5.652kg/m2. In tidal wetland, the sediment organic carbon storagestability was relatively high, while the Changyi wetland, due to environmental constraints, the stable organic carbon storage was very low. CYA9was located at thebare tidal flat, however, its stable organic carbon distribution was more uniform, andit can reach more than half of CYA3stable carbon reserves within1m depth.