寒温带针叶林土壤CH4吸收对模拟大气氮沉降增加的初期响应

N沉降作为驱动因子会改变森林土壤－大气界面CH4净交换通量和方向。然而,对引起北方森林土壤CH4吸收发生转变的大气N沉降临界负荷及其响应机制知之甚少。为此,本研究以我国大兴安岭北方寒温带针叶林土壤作为研究对象,参照大兴安岭站实际大气N沉降通量,构建了低剂量、多形态和高频率的大气N沉降模拟增加控制实验,研究了2010年6-10月生长季土壤CH4吸收通量及其驱动因子对增N的初期响应。研究表明：整个生长季,大兴安岭寒温带针叶林土壤作为大气CH4净汇,CH4平均吸收通量为51.5?4.70 ugm-2h-1,主要受0-10cm土壤水分驱动。短期内,0-10cm矿质土壤NH4 -N含量对增N响应敏感；0-10cm矿质土壤NO3--N含量则受NO3--N输入影响较为明显。相反,0-10cm矿质土壤pH对增N的响应不敏感。总体上,低剂量的N输入对大兴安岭寒温带针叶林生长季土壤－大气界面CH4净交换通量影响不显著,而不排除NO3--N 输入尤其是低N处理情形所呈现出促进土壤CH4氧化的趋势。大兴安岭寒温带针叶林土壤CH4吸收对增N的响应敏感程度可能和土壤CH4活性氧化区域,土壤NH4 -N、NO3--N含量空间分布格局和相对比例有关。未来长期低水平的大气N沉降是否会改变大兴安岭北方森林土壤氧化大气CH4趋势,有待研究。

Early responses of soil CH4 uptake to increased atmospheric nitrogen deposition in a cold-temperate coniferous forest

N deposition as a driving factor will alter the magnitudes and directions of CH4 uptake in forest soils. However, the critical load of atmospheric N deposition which causes shift from stimulation on CH4 uptake in forest soils, especially for N-limited boreal forest soils, to inhibition and mechanisms responsible for these trends are poorly understood. In this study, we conducted a low-level simulated N deposition experiment in a boreal cold-temperate coniferous forest soil. Net exchange of CH4 was measured on a ten-day basis using closed clambers on field plots fertilized repeatedly with different forms of N during the 2010 growing season. Except for soil moisture and temperature, environmental variables including soil inorganic N and surface soil pH were also monitored. Our results showed that throughout the growing season, the average rate of CH4 uptake in the cold-temperate coniferous forest soil was 51.5?4.70 ugm-2h-1, which was mainly control by surface soil moisture. In the short term, soil NH4 -N content in the 0-10cm mineral layer increased greatly with increasing N input and soil NO3--N in the 0-10cm mineral layer was significantly influenced by NO3--N input while surface soil pH showed insensitivity to N input. Overall, our lower doses of N input had no significant effects on the net flux of CH4 uptake from the cold-temperate coniferous forest soil in the growing season. However, NO3--N input showed a trend of stimulation on CH4 oxidation in the N-poor soil. The sensitivity of the cold-temperate coniferous forest soil CH4 uptake in response to N input may be closely associated with the active CH4 oxidizing zone, the vertical distribution and relative proportions of soil NH4 -N and NO3--N. Whether long-term chronic atmospheric N deposition will alter the trends in CH4 oxidation in the boreal cold-temperate coniferous forest soil remains uncertain, and needs to be investigated.