Effect Of Land-use Change On Greenhouse Gases Emissions In Freshwater Marshes In The Sanjiang Plain

The natural wetland is part of the eutrophic freshwater marsh, which covers more than 11,000km2 in the Sanjiang plain, making it the largest low-altitudinal wetland in China. With increasing the population, the demands of wetland for agricultural uses have been increasing. The conversion of wetland always starts with the construction of drainage ditches in order to reduce the excess water that is commonly associated with natural wetland. Both natural and converted marsh soil can potentially control the dynamics of carbon dioxide, methane and nitrous oxide. However, there is no quantitative data on the emission of these three gases from wetland and agricultural soil. The objective of this research was to quantify the effect of land-use change on greenhouse gases emission in marsh ecosystems in the Sanjiang Plain. The results showed that there were large seasonal and inter-annual variations of CH₄, N₂O fluxes and CO2 efflux rates from marshes. Emissions of CH₄ in 2003 and 2004 from Carex lasiocarpa marsh were 347.65 and 550.95 kg·ha^-1, 220.52 and 199.12 kg·ha^-1 from Deyeuxia angustifolia marsh respectively. Emissions of N₂O in 2003 and 2004 from Carex lasiocarpa marsh were 0.81 and 1.80 kg·ha^-1, 1.05 and 4.07 kg·ha^-1 from Deyeuxia angustifolia marsh respectively. And emissions of CO2 in 2003 and 2004 from Carex lasiocarpa marsh were 19.29 and 20.03 t·ha^-1, 27.62 and 38.01 t·ha^-1 from Deyeuxia angustifolia marsh respectively. Greenhouse gases emissions from wetlands were controlled by various physical, chemical and biological factors. Water depth, temperature, and plant are the main variables controlling gases emissions from wetlands. There existed significant special difference in CH₄, N₂O fluxes and CO2 efflux rates and hydrology is the main factor. CH₄ fluxes increase with water depth increasing, but N₂O fluxes and CO2 efflux rates decrease with water depth increasing. Changing marsh into cultivated lands (glebe and paddy field) decreased CH₄ emission. Ch4 emissions from paddy field and glebe were 94.82 and ^-1.37 kg·ha^-1·a^-1. Changing marsh into paddy field slightly reduced N₂O emission but into glebe markedly enhance N₂O emission. N₂O emission from paddy field and glebe were 2.09 and 4.90 kg·ha^-1·a^-1 respectively. The GWPs of the emission of CH₄ and N₂O under marshes and cultivated lands were assessed in an integrated manner. The integrated GWPs of the mean annual CH₄ and N₂O emission from paddy field and glebe was 0.27-0.32 and 0.05-0.24 of the marsh integrated GWPs respectively. From 1949 to 1999, large areas of marsh were reclaimed in the Sanjiang Plain. The reclamation resulted in the reduction of 1482.46 Gg·a^-1 of CH₄ emission to atmosphere, but increase of 1.71 Gg·a^-1 of N₂O emission to atmosphere. The degradation and reconstruction of wetland enhanced soil respiration rate. The reclamation of wetland could enhance or decrease soil respiration rate that depended on soil moisture and soil organic carbon content. The agricultural ecosystems showed carbon sink in plant growing period, but showed carbon source in ungrowing period. NEE (Net Ecosystem Exchange) were 0.80 and 1.12 t·ha^-1 in ungrowing periods from paddy field and glebe respectively. And NEE were –6.19 and –3.55 t·ha^-1 in growing period from paddy field and glebe respectively. Simultaneously applying eddy covariance technique and static chamber – gas chromatograph to measuring nighttime NEE of a paddy field. The results showed that there was significant difference between them. The seasonal nighttime measurement using by eddy covariance underestimated 28% than the measurement by static chamber. Soil C and N contents are very high in marsh. Soil C contents are 258.4 and 99.4 g·kg^-1 in Carex lasiocarpa marsh and Deyeuxia angustifolia marsh respectively. And soil N contents are 12.39 and 7.35 g·kg^-1 in these two marshes respectively. In marsh soil, C and N contents decrease rapidly from up to bottom which are affected by the distribution of plant roots in soil. The reclamation of marsh caused soil C and N contents greatly reduce, and C and N contents were 34.3 and 2.94 g·kg^-1 in agricultural soils. In glebe, soil total carbon, organic carbon, total nitrogen andorganic nitrogen presented logarithmic decrease with increasing of reclamation years, δ15N linear increase with increasing of reclamation years.