Terrestrial ecosystems may act as a source or a sink for the atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), depending on land use and management. This paper reviews the literature on carbon, CH4, and N2O fluxes from terrestrial ecosystems in China, and analyzes its national greenhouse gas budget. Carbon storage in biomass and soils in Chinese terrestrial ecosystems decreased in the past 300 years, due to deforestation and expansion of cultivated land, and reached a minimum in the late 1970s. Since then, carbon storage has increased at an estimated rate of 0.19 to 0.26 Pg C yr-1, mainly owing to reforestation and afforestation. CH4 emission from natural wetlands decreased from 6.65 Tg CH4 yr-1 in 1990 to 5.71 Tg CH4 yr-1 in 2000 owing to the decrease in wetland area. CH4 emission from flooded rice fields was 7.41 Tg CH4 yr-1. At the same time, aerobic soils took up atmospheric CH4 at a rate of 2.56 Tg CH4 yr-1. Nitrous oxide emission from forestlands, grasslands, and farmlands was positively correlated with precipitation at a national scale, and the emission rate was positively correlated with the CH4 uptake rate of forestlands and grasslands (P<0.01). Natural N2O sources were estimated to be 419 Gg N yr-1 and anthropogenic sources (from farmlands) to be 292 to 476.3 Gg N yr-1, with a mean of 372.6 Gg N yr-1. The integrated budget of greenhouse gasses indicates that Chinese terrestrial ecosystems act as a small net sink for global warming potential (GWP), ranging from 0.04 to 0.32 Pg CO2-eq yr-1, in a striking contrast to terrestrial ecosystems globally, which are a source of 2.75 to 6.78 Pg CO2-eq yr-1. The ratios of anthropogenic to natural sources of CH4 and N2O are much larger in Chinese terrestrial ecosystems than they are in global averages, reflecting greater human disturbance of terrestrial ecosystems in China.
采用淹水厌氧密闭培养-乙炔抑制法测定了长白山4种森林土壤的反硝化势以及气态产物N2O、NO和N2的产生量。供试4种森林土壤均呈酸性,pH(H2O)4.5～5.3,有机碳含量24.6～83.0 g kg-1。结果表明,长白山4种典型森林土壤反硝化势及反硝化产物中氮氧化物的比例差异很大。土壤反硝化势强弱顺序依次为:阔叶红松>白桦>红松云杉冷杉>兴安落叶松,与土壤有机碳和全氮含量呈显著的正相关关系(p<0.01),与土壤pH的相关性不显著。比较加乙炔和不加乙炔处理发现,在培养过程中,N2O始终是反硝化的主要产物,占反硝化产物的比例变化于50%～85%之间,不随培养时间而发生显著的变化,与土壤pH呈弱负相关关系(p=0.22)。反硝化产物中,NO仅占0.2%～2.4%,随培养时间也未发生有规律的变化,与土壤有机碳含量呈显著的对数负相关关系(p<0.05)。上述结果表明,长白山森林土壤反硝化过程并不能有效地将活性氮转化为惰性氮,反硝化作用的生态环境意义需要重新评估。
