温带针阔混交林土壤碳氮气体通量的主控因子与耦合关系

中高纬度森林地区由于气候条件变化剧烈,土壤温室气体排放量的估算存在很大的不确定性,并且不同碳氮气体通量的主控因子与耦合关系尚不明确。以长白山温带针阔混交林为研究对象,采用静态箱-气相色谱法连续4a(2005—2009年)测定土壤二氧化碳(CO2)、甲烷(CH4)和氧化亚氮(N2O)净交换通量以及温度、水分等相关环境因子。研究结果表明:温带针阔混交林土壤整体上表现为CO2和N2O的排放源和CH4的吸收汇。土壤CH4、CO2和N2O通量的年均值分别为-1.3 kg CH4hm-2a-1、15102.2 kg CO2hm-2a-1和6.13 kg N2O hm-2a-1。土壤CO2通量呈现明显的季节性规律,主要受土壤温度的影响,水分次之;土壤CH4通量的季节变化不明显,与土壤水分显著正相关;土壤N2O通量季节变化与土壤CO2通量相似,与土壤水分、温度显著正相关。土壤CO2通量和CH4通量不存在任何类型的耦合关系,与N2O通量也不存在耦合关系;土壤CH4和N2O通量之间表现为消长型耦合关系。这项研究显示温带针阔混交林土壤碳氮气体通量主要受环境因子驱动,不同气体通量产生与消耗之间存在复杂的耦合关系,下一步研究需要深入探讨环境变化对其耦合关系的影响以及内在的生物驱动机制。

The controlling factors and coupling of soil CO2, CH4 and N2O fluxes in a temperate needle-broadleaved mixed forest

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are three major greenhouse gases, accounting for 80% of global warming. Forest ecosystems comprise the largest carbon stocks in terrestrial ecosystems, and function as important sinks and sources of atmospheric CO₂, CH₄ and N₂O. Complicated interactions occur during the generation and absorption of soil CO₂, CH₄ and N₂O, including synergies, tradeoffs, and randomness. High-latitude forests are experiencing the effects of significant global change (e.g., warming, changed precipitation, and increased nitrogen deposition), leading to great uncertainty in estimates of soil greenhouse gas fluxes. Furthermore, the factors controlling the coupling of soil CO₂, CH₄ and N₂O fluxes remain unclear. This study was conducted in the temperate needle-broadleaved mixed forest of Changbai Mountain, Northeast China. The net exchange fluxes of soil CO₂, CH₄ and N₂O, as well as soil temperature and soil moisture, were measured over four years (2005-2009) using static chamber and gas chromatograph techniques. The results showed that temperate needle-broadleaved mixed forest soils behaved as a source of atmospheric CO₂ and N₂O but a sink of atmospheric CH₄ over the course of the study. The average soil CH₄, CO₂ and N₂O fluxes were estimated at -1.30 kg CH₄ hm^-2 a^-1, 15102.2 kg CO₂ hm^-2 a^-1, and 6.13 kg N₂O hm^-2 a^-1, respectively. In addition, soil CO₂ flux exhibited significant seasonality, and was mainly affected by soil temperature, followed by soil moisture. Seasonal variation in soil CH₄ flux was less significant than that of soil CO₂ and N₂O fluxes;moreover, it was positively correlated with soil moisture. When soil temperatures were within a threshold range, soil moisture determined CH₄ production and oxidation in soil profiles by regulating CH₄ and O₂ diffusion as well as methanotrophic community activity. Similar to soil CO₂ flux, soil N₂O flux was significantly correlated with soil moisture and soil temperature. Furthermore, there were no significant relationships between soil CO₂ flux and soil CH₄ flux, or between soil CO₂ flux and soil N₂O flux, which exhibited a random relationship. However, a significant negative relationship between soil CH₄ uptake and N₂O emission was found, indicating a tradeoff between them. The random relationship between soil CO₂ and CH₄ fluxes was attributed to their different pathways and substrate utilization. The trade-off relationship between soil CH₄ and N₂O fluxes was related to moisture control and competition for mono-oxygenase (MMO) between ammonia-oxidizers and methanotrophic communities. The weak synergy between soil CO₂ and N₂O fluxes reflected that they were driven by same environmental factors, such as soil temperature and moisture, and that no microbial mechanisms drove their production or consumption. These results suggest that soil carbon and nitrogen gas fluxes are mainly driven by environmental factors and substrate availability, and their complicated couplings are related to the activity and functional composition of microbial communities. It is necessary to further explore the effects of environmental change on the coupling of soil CO₂, CH₄ and N₂O fluxes as well as the microbial mechanisms underlying these using molecular biology and metagenomic analyses.