Department of Integrative Biology, University of Guelph, Science Complex, Guelph, ON, N1G 2W1, Canada.
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada.
Glob Chang Biol. 2017 Jun;23(6):2428-2440. doi: 10.1111/gcb.13612. Epub 2017 Jan 31.
Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (F ), ecosystem respiration (ER), net ecosystem exchange (NEE) and gross primary production (GPP) fluxes from a 9-years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing season F , GPP and NEE, thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing season GPP in subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence on ER, but dominant contribution to ER switched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter-annual lag effects on ER in this rich fen, as has been observed in several nutrient-poor peatlands. While ER was dependent on soil temperatures at 2 cm depth, F was linked to soil temperatures at 25 cm. Inter-annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higher F in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short-term fluctuations in wetness caused significant lag effects on F , but droughts caused no inter-annual lag effects on F . Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens.
富营养化湿地是常见的北方生态系统，具有独特的水文学、生物地球化学和生态学特征，影响着它们的碳（C）平衡。我们展示了来自阿拉斯加富营养化湿地 9 年地下水位控制实验的生长季土壤室甲烷排放（F）、生态系统呼吸（ER）、净生态系统交换（NEE）和总初级生产力（GPP）通量。该研究包括主要的洪水和干旱年份，其中湿润和干燥处理进一步改变了干旱的严重程度。结果支持了以前在泥炭地的研究结果，即干旱导致生长季 F、GPP 和 NEE 减少，从而减少或逆转其碳汇功能。实验加剧的干旱进一步通过导致植被变化，从而在随后的洪水年份将最大生长季 GPP 的幅度减少约 15%，相对于对照样本来减少湿地作为碳汇的能力，从而进一步减少了湿地作为碳汇的能力。相比之下，地下水位位置对 ER 的影响较弱，但 ER 的主要贡献者从湿润年份的自养呼吸转变为干旱年份的异养呼吸。与在几个养分贫乏的泥炭地观察到的情况相反，干旱在这个富营养化湿地中没有导致 ER 的年际滞后效应。虽然 ER 依赖于 2cm 深度的土壤温度，但 F 与 25cm 深度的土壤温度有关。深层土壤温度的年际变异性反过来又依赖于湿度而不是空气温度，因此，洪水年份 F 的增加同样是由于深部甲烷产量的增加和表面附近甲烷氧化的减少。湿度的短期波动对 F 产生了显著的滞后效应，但干旱对 F 没有年际滞后效应。我们的结果表明，干旱和洪水的频率和严重程度会对温室气体的交换产生特征性影响，并强调需要预测富营养化湿地的未来水文状况。
