Effects Of Hydrological Changes On Soil Carbon Sequestration Of Marsh In The Sanjiang Plain

Wetland is important carbon pool in terrestrial ecosystem and changes of hydrologicalconditions would influence the carbon sequestration function of it. Shortly decrease ofwater table would lead to loss of carbon sink, while the long term effect of the watercondition changes on carbon cycling depend on the wetland type, climate and the extentof water table fluctuations. Hydrological conditions affect the soil physical characters andvegetations cover, and decide the ecosystem productivity. Wetland exploitation and waterdeprivation decreased the area and ecology function of wetland. The paper focused on thefresh water marshes in Sanjiang Plain, and synthesized the methods of filed investigation,field control and simulation indoor experiment, to study the effect of water conditionchanges on soil carbon content, plant community succession and biomass accumulation,and also to estimate the response mechanism of carbon cycling between soil, vegetationsand atmosphere to water table fluctuations.Vegetation structure and plant biomass were affected by the flooding conditions.Biomass and absolute growth rate of Calamagrostis angustifolia community were lowerin seasonal flooded marsh than in wet meadow. While biomass of the Carex lasiocarpacommunity increased by47.41%under17–30cm flooding water table than10–20cm. Todifferent plant communities, increase in water table improve the biomass carbonaccumulation but inhibited the growth potential of them, then shorten the growth period.Species richness was negatively related to flooding water table, which suggested incertain content less specie number in a plant community would increase the palnt carbonaccumulation in wetland.Decompositions of litters of C. angustifolia and C. lasiocarpa responded differently towater table changes. During the growing season the flooding condition accelerated theboth litters' decompositions, while during the non-growing season the frozen water bodyinhibited the process. Specie richness was influenced by water condition, and it alsoaltered mixed litter decomposition dynamic in the mix stand. Non-additive effect wasfounded in the decomposition of mixed litters, and litter identity and species amount takeimportant part in the decompositions. It could be concluded that mixing of two speciesbelong to different family would improve the decomposition, while those belong to thesame family do the opposite. We also found that when litters of C. angustifolia and Carexmeyeriana were mixed, decomposition rate didn't change much no matter the relativeproportion is. Mixed litters of3plant species decreased decomposition rate compared tothe expected value, indicating that higher species richness of plant community would bebenefit the soil carbon sequestration function in wetland ecosystem. Hydrological circle had apparently driving effect on SOC and active carbon fractionsin marsh soil during the growing season, and the effect varied with the marsh type. Lightfraction organic carbon content (LFOC) in top soil in marshes increased with water tableelevated. During growing season the organic carbon contents of both density fractions inC. lasiocapa marsh increased continuously, but LFOC was slower, indicating that soilcharacters in C. lasiocapa marsh are beneficial to the maintenance of heavy fraction (HF).Meanwhile, there was a significant correlation between the contents of LFOC and SOC(r=0.816) in the C. angustifolia wet meadow, suggesting that higher LFOC content wasfavorable to the soil carbon accumulation. Besides, the lower MBC and microbialquotient in C. lasiocapa marsh with17–30cm standing water suggested that higher watertable elevate the stability of soil carbon pool.Soil respiration in the C. angustifolia marsh was improved by seasonal floodingenvironment, and was negatively correlated to flooding water table, showing thatinundated conditions inhibited soil organic matter mineralization process. But when thewater table was lower than-5cm, CO₂emission rate didn't increased, which means thatlow soil water content would also restrain soil respiration. On the other hand, CO₂emission rates increased with water table rises when the soil was inundated, indicatingthat in flooded condition mineralization of organic matter would be affected byenvironmental factors except soil air permeability. Incubation experiment showed thatinundated condition accelerated CH₄emission and DOC release. The latter would be thereason for higher mineralization rate in deepest stagnant water. Meanwhile, higherflooding water table decreased soil MBC and MBN, while didn't affect microbial C/N,suggested that microflora in marsh soil could adapt their activity to the of changes waterenvironment.Flooding condition significantly increased soil CH₄emission, total amount of CH₄emitted under15cm flooding water table was2and30times of those under0cm and-10cm. Water table was positively correlated to CH₄emission amount (p<0.01). But whenthe water table was beneath the soil surface, increase of water table (from-10cm to0cm)didn't change the emission rate much (p>0.05), while CH₄emission rates differed fromeach other notably under5cm and15cm water table (p<0.05), Inundated environmentlead to lower total carbon emission during the growing season was, but it also result inhigh greenhouse potential. The lowest greenhouse potential appeared in the conditionwith0cm water table, which had low carbon release just a little higher than5cmcondition. The result would be guide the technique of wetland restoration andconservation that the saturation condition is beneficial for carbon conservation in regionalenvironment.Root activity altered soil MBC, MBN and DOC contents, and improved microbial C/Nin deep soil layer significantly (p<0.05), illustrating that root activity would influence microbial structure in soil and particularly in deep layers. Contribution of root activity tosoil respiration was affected by environmental conditions. Soil respiration in2011werehigher than that in2010(p<0.01), while the contributions of root to them were47.71%(2010) and40.14%(2011), which didn't differ much with each other (p>0.05). Rootrespiration had no remarkable relations to water tables, indicating low moisturesensitivity. Root could improve soil CH₄emission and most notable in August. CH₄derived from root activity (1.51g·m^-2) contributed31.84%to the total emission (3.15g·m^-2) during growing season in2010. But in2011root had no distinct effect on methaneemission, and the soil of C. angustifolia marsh behaved as CH₄sink (-0.08g·m-2). Theseresult suggested that promotion of root activity on CH₄emission was more significant inwet condition.Flooding water table affected root contributions to soil carbon emissions in the C.angustifolia marsh. Steady water table decreased root respiration, and the total CO₂emission amount displayed as-5cm> CK>10cm>0cm>5cm, while the contribution ofroot was highest under10cm steady water table. Water table also limited the effect of rooton CH₄emissions. Under the conditions of-5cm and0cm flooding water table, growth ofC. angustifolia root inhibited soil CH₄emission, especially under saturated condition(p<0.01). Root behaved as promotion factor under the condition of10cm stagnant water,indicating that strength of root effect on methane production or oxidation vary withchanges of water conditions.