Effect Of Nitrogen Availability On Carbon And Nitrogen Dynamics In Wetland Ecosystems Of Northeast China

Nitrogen (N), as the richest element in atmosphere, is one of limiting elementsfor plant photosynthesis and primary productivity in terrestrial ecosystem, especiallyin high latitudes area in Northern Hemisphere. Global warming is an irrefragable factand north high latitudes are sensitive to this change. With global warming, the northpermafrost will thaw, active layer will deepen, which would change soil nitrogenavailability and carbon (C) storage. So in the present study, we selected typicalwetlands in different frozen areas in Northeast China, used the method of spatial scalereplace temporal scale to research soil nitrogen availability, litter decomposition andN₂O emission. This thesis would offer some basic information for us to predict soilnitrogen availability and the effects of nitrogen availability on carbon storage inwetland ecosystems in Northeast China under global warming conditions. The resultsare as follows:The total N contents were significantly different in various wetland types; soilmicrobial biomass carbon (MBC) contents had an obvious tendency of seasonalchange. The soil NH₄⁺-N, NO₃--N, dissolved organic nitrogen (DON) contents indepths were decreased as the depth increasing, but the decreased tendency wasvarious in different frozen areas. The average content of all N components fromhighest to lowest was continuous permafrost, island permafrost and seasonally frozenground. The net soil mineralization and nitrification in different wetland ecosystemhad evidently seasonal change, net mineralization rates were positive in the beginningof growing season, and negative in the end of growing season. In a word, Navailability was higher in continuous permafrost, but the N efficiency was low due tolow temperature. In the tendency of global warming, temperature increasing will hadmore intensive effect in continuous permafrost.The difference of total organic C was significant in three wetland ecosystems, continuous permafrost soil> island permafrost soil>seasonally frozen ground soil.MBC content had various temporal and spatial changes and had obvious correlationwith temperature and wetland types. The MBC content at depth was decreased. Thesoil dissolved organic carbon (DOC) content was decreased with decreased latitudes,continuous permafrost soil> island permafrost soil>seasonally frozen ground soil,and the difference was gradually reduced as soil depth increasing.The soil was collected in three frozen areas and investigated effects of exogenousnitrogen availability on carbon mineralization. The results showed that the cumulativeC mineralization of three types of soil under control treatment existed positivecorrelation with initial amount of soil organic C, total N and MBC in the end ofincubation, which indicated that C mineralization was effected by initial characteristicproperty and microbial communities. N input suppressed C mineralization and thesuppression increased as the amount of N increasing, but the suppression under Ninput was different in three soils. After incubation, cumulative mineralizationpositively related with MBN and negatively related with MBC/MBN, which indicatedthat N availability may affect the carbon mineralization by changing microbialstructure and composition.Through laboratory experiment to investigate the effects of N availability oncontinuous permafrost soil organic mineralization (SOC) in different depths, resultsindicated the SOC mineralization in boreal peatlands soil decreased with depth, whichmay be caused by soil initial characteristics in different soil layers. Water content, pHand total P content had evident correlation with SOC mineralization, while total SOCand total N had no or slight effect on SOC mineralization. Our results indicated that inboreal peatlands, SOC mineralization may not be limited by C or N energy, pavailability may be the mainly factor affecting SOC mineralization.A litter decomposition experiment was set in different wetland ecosystems. Theresults showed that the surface residues biomass from biggest to smallest wasseasonally frozen ground (SJ), island permafrost (YH) and continuous permafrost(TQ). The decomposition rates of SJM and SJX were evidently higher than YHM andTH, the difference between YHM and TH was not significant. Net C remaining of SJX residues was significant lower than other residues, net C remaining between SJMand TH was similar, but all lower than YHM residues. Net N remaining of all residueswas various in different experimental stages, but the difference among residues wassmall. In the end of experiment, the N remaining from highest to lowest was TH, SJM,SJX and YHM. The residues P remaining was evidently different after one yearexperiment, from highest to lowest was SJM, SJX, TH and YHM. These resultsindicated that inundated condition was favorable for carbon storage, but accelerated Prelease; increased temperature would improve C release.Eriophorum vaginatum litter decomposition was investigated in various wetlandecosystems. The results showed that decomposition rates were significant different indifferent wetland ecosystems (p<0.001), indicated environmental condition plays animportant role in litter decomposition. The tendency of C remaining was similar withlitter decomposition, in the end of one year decomposition, litter C remaining fromhighest to lowest was TQ, YH, SJ, however, the amount of N remaining was oppositewith litter decomposition, from highest to lowest was SJ, YH, TQ; P remaining in TQ,YH and SJ was similar, the amount of P remaining was89.8%,108.9%and124.2%,respectively.N addition suppressed residue decomposition, the inhibition enhanced as theamount of N addition increasing, but the response of different residue to four levels ofN addition was various. In the end of incubation, N addition increased residue Ncontent; a significant linear correlation existed between the amount of nitrogenaddition and N content of SJX, SJM and TH residue. P content of residues haddifferent effects on N addition, but high N addition decreased the P content of allresidues in different wetland ecosystems.Soil N₂O emission rates decreased at depths, and became a sink of N₂O in deepsoil. N addition evidently improved N₂O emission rates under different soil depth,especially in the initial stage of N addition. N addition had a priming effect on N₂Oemission, and the effect increased as N amount increasing. N addition had small effecton soil N₂O emission after20d incubation. In the end of incubation, cumulative N₂Oemission at depths was increased as N amount increasing, which of soil in deep depth was higher than in surface soil under high N treatment. These results indicated thatdeep soil was more sensitive to N addition than surface soil on N addition.Through laboratory experiment during134d, N addition enhanced N₂O emission,but the response of different residues to four levels of N addition was various. Theeffect of N addition on residues decomposition lasted longer period as the amount ofexogenous N increasing. The low N treatment impacted on N₂O emission only at first6-10days; the medium N treatment affected the N₂O emission at about the first month;while the high N treatment affected the N₂O emission in all incubated period. In theend of incubation, Low N treatment did not affect cumulated N₂O emissionsignificantly, while medium N and high N treatment evidently enhanced N₂Oemission (p<0.05), except the SJM residue under medium N treatment. CumulativeN₂O emission of TH residues was biggest under medium N addition treatment; thisresult indicated that the effect of N addition on TH residues was not linear.