氮沉降对温带森林土壤甲烷氧化菌的影响

大量研究显示氮沉降影响森林甲烷吸收量,但其中的微生物驱动机制仍缺乏研究。基于长白山典型温带森林长期氮沉降模拟实验平台样地,采用定量PCR和克隆测序技术,研究了长期施加不同形态氮((NH_4)_2SO_4、NH_4Cl和KNO_3)处理下森林土壤甲烷氧化菌的数量和群落组成随季节变化的特征。结果表明,夏季,森林土壤甲烷氧化菌pmo A基因丰度在不同施氮处理之间无显著性差异(每克干土1.54×10~6-3.20×10~6拷贝数);秋季,pmo A基因丰度在施加NH_4Cl和(NH_4)_2SO_4处理小区(每克干土1.93×10~5-7.6×10~5拷贝数)与对照(每克干土(4.03×10~6±1.2×10~6)拷贝数)相比有所降低,尤其在(NH_4)_2SO_4处理小区(每克干土(4.61×10~5±2.61×10~5)拷贝数)显著降低;无论夏季还是秋季,施加不同形态氮处理土壤甲烷氧化菌均以Type I型为主(相对丰度在70.6%-85.4%之间),并以Methylobacter-group(Type I)为优势类群,占Type I型的55.1%-91.7%;Methylobacter-group(Type I)的相对丰度在夏季不同形态氮处理土壤样品中无显著差异,但秋季样品中在施加(NH_4)_2SO_4(52.7%±6.5%)和NH_4Cl(56.1%±8.9%)的处理显著低于对照土壤(77.0%±2.9%),Methylococcus-group(Type I)的相对丰度则在(NH_4)_2SO_4和NH_4Cl处理土壤呈增加的趋势。这些结果表明铵态氮肥添加对温带森林土壤甲烷氧化菌的生长具有抑制作用并导致其群落结构发生改变,受夏季温度和水分的影响,这种抑制作用在秋季表现更明显,而NO_3~--N添加对土壤甲烷氧化菌的群落组成和丰度无显著影响。这些结果解释了以往观测到的施铵态氮肥显著降低秋季温带林地土壤甲烷净吸收量,而在夏季无显著影响的观测结果,解释了长期氮沉降影响森林土壤甲烷吸收的微生物机制。

Effects of nitrogen addition on methanotrophs in temperate forest soil

Several studies have shown that increasing nitrogen deposition affects methane uptake in the forest ecosystems; however, the microbial mechanisms underlying this phenomenon remain unclear. In the present study, the seasonal variation in methanotroph abundance and community structure in a temperate forest receiving long-term differential nitrogenous compound addition treatments (45 kg N hm^-2 a^-1 using (NH₄)₂SO₄, NH₄Cl, and KNO₃) in Changbai mountains, were investigated by using real-time PCR, cloning, and sequencing. Methanotroph pmoA gene abundances were similar among all fertilized and control plots in summer (1.54×10⁶-3.20×10⁶ copies g^-1 dry soil); however, in fall, it was significantly lower in the (NH₄)₂SO₄ treatment (4.61×10⁵±2.61×10⁵ copies g^-1 dry soil) than that in the control plots (4.03×10⁶±1.2×10⁶ copies g^-1 dry soil). In both summer and fall, the methanotroph community was dominated by Type I methanotrophs, consisting of Methylobacter-group (Type I) and Methylococcus-group (Type I), with the relative abundance varying from 70.6%-85.4%. The dominant clade of Type I methanotroph, Methylobacter-group (Type I) showed no significant variance among all treatments in summer, but were present in significantly lower proportions in the (NH₄)₂SO₄ (52.7%±6.5%) and NH₄Cl (56.1%±8.9%) treatments than in the control in fall. In contrast, Methylococcus-group (Type I) showed an increasing trend in the (NH₄)₂SO₄ and NH₄Cl treatments compared to that in the control plots in fall. Overall, these results suggested that the addition of NH₄⁺-N fertilizers could inhibit the growth of methane oxidizing bacteria, and change their community composition. The inhibition effect became more obvious in fall than in summer, owing to the interactive effects of soil moisture and temperature in summer, whereas NO₃^--N addition showed no significant effect on the abundance and community composition of methanotrophs. These results were consistent with the previous observation that ammonium-based fertilizer addition reduced methane uptake from temperate forest soil in fall rather than in summer, and provided insights into the microbial mechanisms driving methane uptake in temperate forests under long-term nitrogen addition treatments.