Response Of Key Processes Of Carbon And Nitrogen Cycle To Nitrogen Addition In The Min River Estuarine Wetland

Coastal wetland, located in the interaction areas between land and sea, is a veryimportant ecosystem, which is sensitive to the global change and human activities.With the population growth and rapid development of industrial and agriculturalproduction in coastal zones, the estuarine wetlands often receive more and morenitrogen （N） nutrition originated from atmospheric deposition, river input, surface andunderground runoff and pollutant discharge, which may affect the carbon （C） and Nbiogeochemical process in the wetlands. In this paper, the native Cyperusmalaccensis and invaded Spartina alterniflora wetlands in the Min River estuarywere selected as study objects in order to understand the response of C and N cycle toexogenous N addition. The observations in situ, microcosm experiments andlaboratory control experiment were performed to study the effects of N addition ongreenhouse gas emission, transformation characteristics of C and N in wetland soils,the plant growth and C and N accumulation, the litter decomposition rules and C andN dynamics in decomposition process. The main results were drawn as follows:（1） The C. malaccensis and S. alternifora wetlands acted as sinks of CO₂,sources of CH₄and weak sources or sinks of N₂O in the growing season. Theinvasion of S. alternifora into the Min River estuary stimulated CO₂uptake andCH₄emission, while no significant effects were found on N₂O emission. Exogenous N（N1,21g N m-2a-1; N₂,42g N m-2a-1） promoted CO₂, CH₄and N₂O emissions bothin native and in invaded tidal marsh. Compared to N0treatment, the mean gas fluxesof N1and N₂treatments increased by25.80%and29.72%for CO₂,31.05%and123.50%for CH₄, and1604%and2706%for N₂O in the C. malaccensis marsh.However, in the S. alternifora marsh, the mean gas fluxes of N1and N₂treatmentsincreased by30.38%and28.35%for CO₂,63.88%and7.55%for CH₄, and1384%and1443%for N₂O.（2） Significant temporal variability of CO₂, CH₄and N₂O fuxes were observedafter the N was gradually added to the native and invaded marshes. Within3hours ofN addition, N₂O fuxes were signifcantly higher in plots receiving N additionsrelative to controls. After8days, when N concentration decreased, no significant differences were found between treatments. We conclude that the overall increase ofCO₂, CH₄and N₂O emissions affected by N addition in coastal regions may besignificantly underestimated due to the short-term temporal variations of gas fuxes. Inorder to better assess the global climatic role of salt marshes as affected by N addition,the short-term temporal variability of greenhouse gas emission should receive muchmore attention.（3） EC, pH and Eh in different soil depths were not significantly affected by Naddition during the entire experiment. There were some significant correlationbetween CO₂（CH₄） fluxes and temperature （Ec, pH and Eh）, while the correlationdecreased when N was added to the wetlands. The N₂O fluxes were not significantlycorrelated with temperature, Ec, pH and Eh.（4） Under60%WHC and submerged conditions, the added N of differentconcentrations and forms restrained the mineralization of soil organic carbon （SOC）in the C. malaccensis marsh and promoted the SOC mineralization in the S.alternifora marsh. Water condition was an important factor affecting the SOCmineralization in the Min River estuarine wetlands, the SOC mineralization ratesunder submerged condition were significantly lower than those under60%WHCcondition. There were significant correlations between the amounts of cumulative C,C0, k or C0k and the DOC contents or pH, suggesting that the changes of DOC and pHaffected by N input might be two of the important reasons that led to the differencesof SOC mineralization.（5） The CH₄production potential of different treatments in the S. alterniforamarsh were higher than those in the C. malaccensis marsh. N addition significantlyaffected the the CH₄production potential of the two marsh soils. The changes ofDOC and mineral N contents and EC in soils affected by N input might be theimportant reasons that led to the differences of CH₄production potential.（6） The CH₄oxidation potential were affected by the concentrations and forms ofadded N. There were interaction effects of N and salinity addition. Salinity additiondecreased the CH₄oxidation rate both in C. malaccensis and in S. alterniforawetland soils. The interaction of salinity and NH₄+-N addition inhibited the CH₄oxidation, while the interaction of salinity and NO3--N addition increased the CH₄oxidation.（7） The nitrifier denitrification is an important process of N₂O production inthe C. malaccensis and S. alternifora wetland soils, which can not be ignored when we study the N₂O production in wetland ecosystems. Nitrate N inputincreased the total N₂O production by promoting denitrification and other process.Low ammonium N addition increased the total N₂O production by promotingdenitrification, nitrifier denitrification and other process, however, the middle andhigh ammonium N addition decreased the total N₂O production by inhibitingnitrification, nitrifier denitrification and other proces in the C. malaccensiswetland soils. Three concentrations of ammonium N input increased the totalN₂O production by promoting nitrifier denitrification and other process in the S.alternifora wetland soils, but the increase were significantly lower than those ofnitrate N addition treatments.（8） Different forms of exogenous N input promoted the biomass of different organs,plant height and stem diameter of C. malaccensis and S. alternifora, and thepromotions of ammonium N were greater than those of nitrate N. There were nosignificant difference between treatments in TC contents of different organs of plants,while the N addition increased the N content in different organs. The C/N of differentorgans of C. malaccensis decreased as affected by the N addition, and the decreaseas effected by the ammonium N input were greater than those of nitrate N. The effectsof two forms of added N on C/N of S. alternifora were not consistent.（9） Nitrogen addition promoted the relative decomposition rate of C. malaccensislitters, leaf and leaf sheath litters of S. alternifora, while little effects were observedon the decomposition of stem litter of S. alternifora. In addition, relativedecomposition rate of litters were also affected by environment change, substratequality, temperature conditions, salinity and pH of soils. The C in litters in differentdecomposition sub-zones showed release at all times, the effects of N addition andchange of decompositon environment on C release in litters showed fluctuated change.The mass remaining rates significantly affected the C absolute contents. The N inlitters in different decomposition sub-zones showed release, the effects of N additionpromoted N release of C. malaccensis litter in the period of0⁴3d, while inhibited Nrelease in the periods of72³45d. However, N addition inhibited N release in S.alternifora litters.