The Controlling Factors And Mechanism Of Peatland Carbon And Nitrogen Accumulation In The Changbai Mountains Region

Boreal peatland are important ecosystems in modulating global biogeochemical cycles,and their C and N cycling were tightly linked,which are closely related to vegetation types and hydroclimate.During past decades,most studies concentrated on peatland C dynamic.However,few studies paid attention to peatland nitrogen cycle.Stable isotope technology has been used widely to explore peatland C and N dynamics in recent years.However,howδ¹³C andδ¹⁵N of bulk peat and plant residuals influence peatland C and N dynamics were still not reported.In this study,typical peatlands were selected in the Changbai Mountains Region influenced by East Asian Monsoon area and determined stable carbon and nitrogen isotopes of bulk peat and plant residues at different depths in the profiles to explore how environmental factors affect the changes of stable carbon and nitrogen isotopes of peat and plant residues in historical period.Combined with climate change and stable carbon and nitrogen isotope changes of plant residues,we explored the main controlling factors affecting peatland carbon and nitrogen accumulation.The main results and conclusions in this study are listed as follows:（1）δ¹³C of bulk peat was mainly affected by the preference of microbial decomposition to plant specific components,temperature and water table depth and different turnover rates of organic matter.Carex and mossδ¹³C values can indicate peatland hydroclimate,which was more related to peatland carbon cycling.Mossδ¹³C can be used to distinguish the balance of methanogenesis and methanotrophy.Mire plant types might have exerted a primary control on methanogenesis,methanotrophy and C dynamic accompanied by hydroclimate.（2）Atmospheric CO₂ concentration and DWT played important roles in affecting bulk peat and peat residualsδ¹⁵N variations.Our findings advance the working hypothesis that C and N exchanges between the symbiotic plants and microorganisms can help plants to alleviate their N limitation with rising atmospheric CO₂concentration.During the late Pleistocene and Holocene periods,moss symbiotic with cyanobacteria and Ericaceae symbiotic with mycorrhizal fungi increased nitrogen uptake with rising atmospheric CO₂concentration.The effects of rising atmospheric CO₂ were found to interact with DWT due to the effects of water on the diffusion of CO₂,which may in turn affect N₂ fixation.This longer-term feedback mechanism probably suggests that N constraints on terrestrial carbon storage with rising atmospheric CO₂ might be alleviated via the increase of symbiosis between plants and microorganisms.（3）Vegetation composition combined climate change had crucial importance in driving peatland C and N dynamics.We found CAR and NAR of bog phase were much faster than that of fen phase,which coincided with increased photosynthesis and nitrogen fixation of Sphagnum under elevated CO₂ concentration and higher DWT.Decomposition of different mire plants mainly controlled CAR and NAR of bog-fen pattern.Our results further highlight the relevance of peatlands as important long-term carbon sinks and reveals a high N sink capacity of bogs.（4）The long term carbon accumulation rate in our study was characterized as higher values in the early Holocene,decreased in the the mid-Holocene,and then increased to relatively high values over the last 2000 years in the late Holocene,which was coincided with other study sites of Northeast China.Enhanced methane oxidation characterized the late Holocene and the early Holocene,which played an important role in reducing carbon loss and higher CAR.More percent of Carex induced methanogenesis which emitted through aerenchymous and more decomposition under higher temperature and summer insolation,resulting much more C loss and lower CAR during the mid-Holocene.Nitrogen accumulation rate was significant positive with atmospheric CO₂ concentration during different periods of the Holocene.Ericaceae and moss symbiotic with ericoid mycorrhizal and cyanobacteria,respectively,which can help plants to alleviate their N limitation and accumulate more N of peatland.δ¹³C andδ¹⁵N of mire plants are now useful tools for investigating the relationships among peatland hydrology,vegetation,methanotrophy and nitrogen fixation over longer time scale,and of vital importance of understanding peatland C and N cycling.This study concluded the carbon and nitrogen cycle of typical peatlands in Changbai Mountains Region were mainly driven by vegetation succession and hydroclimatic change affected by climate change.For the ombrotrophic peatlands dominant by moss species,decomposition has a greater impact on the carbon cycle and accumulation than the production process.For minerotrophic peatland,decomposition and production simultaneously affect peatland carbon accumulation,which depends on the vegetation type.Ericaceae symbiosis with mycorrhizal fungi and moss symbiosis cyanobacteria bacteria respectively,which helps to alleviate the nitrogen restriction of plants and promote more nitrogen of the peatland.Drier environment and warming promote the decomposition of microorganisms and affect the nitrogen cycle in peatlands.