The Effects of Temperature, Flooding, and Goose Feces Addition on Greenhouse Gas Emissions and Ammonification in Four High Latitude Soils. Yukon Delta National Wildlife Refuge, Alaska, 2022.

The large carbon (C) stock of wetlands is vulnerable to climate change, especially in high latitudes that are warming at a disproportional rate. Likewise, low-lying Arctic areas will experience increased coastal flooding under climate change and sea-level rise, which may alter goose herbivory and fecal deposition patterns if geese are pushed inland. While temperature, flooding, and feces impact soil C emissions, their interactive effects have been rarely studied. Here, we explore the impact of these interactions on carbon dioxide (CO2) and methane (CH4) emissions and nitrogen (N) mineralization (ammonification) in soils collected from four plant communities in the Yukon-Kuskokwim (Y-K) Delta, a high latitude coastal wetland in western Alaska. Communities included a Grazing Lawn, which is intensely grazed and susceptible to flooding, a Lowland Wetland and an Upland Wetland that experience moderate grazing and frequent (Lowland) and less frequent (Upland) flooding, and a rarely grazed and flooded Tundra community, located at the highest elevation. Soils were incubated for 16 weeks at 8 degrees Celsius (°C) or 18°C in microcosms and subjected to flooding and feces addition treatments with no-flood and no-feces controls. We quantified C emissions weekly and ammonification over the course of the experiment. While warming increased ammonification and C demand in the Lowland Wetland and always increased CO2 and CH4 emissions, interactions with flooding complicated warming impacts on C emissions in the Grazing Lawn and Tundra. In the Grazing Lawn, flooding increased CH4 emissions at 8°C and 18 °C, but in the Tundra, flooding suppressed CH4 emissions at 18°C. Flooding alone reduced CO2 emissions in the Upland Wetland. Feces addition increased CO2 emissions in all communities, but feces impacts on CH4 emissions and ammonification were minimal. When feces and flooding occurred together in the Lowland Wetland, CH4 emissions decreased compared to when feces was added without concomitant flood. Feces decreased the immobilization of ammonium and N demand in the Tundra only. Our results suggest that flooding could partially offset C loss from warming in less frequently flooded, higher elevation communities, but this offset could be negligible if flooding and warming drastically increase C loss in more flooded lowland areas.

湿地（wetlands）拥有庞大的碳库（carbon stock），其稳定性极易受气候变化影响，在升温速率不成比例的高纬度（high latitudes）地区尤为突出。同样，在气候变化与海平面上升的背景下，地势低洼的北极（Arctic）区域将面临愈发频繁的海岸洪水（coastal flooding）威胁；若雁类因环境变化向内陆迁徙，其植食作用（goose herbivory）与粪便沉积（fecal deposition）模式或将发生改变。尽管温度、洪水与粪便均可对土壤碳排放（soil C emissions）产生影响，但三者的交互效应（interactive effects）却鲜有研究。本研究以位于阿拉斯加西部的高纬度滨海湿地——育空-库斯科奎姆三角洲（Yukon-Kuskokwim (Y-K) Delta）的4种植物群落土壤为实验材料，探究上述三者的交互作用对土壤二氧化碳（carbon dioxide, CO₂）、甲烷（methane, CH₄）排放以及氮（nitrogen, N）矿化作用（mineralization，即氨化作用ammonification）的影响。本次研究涵盖4类植物群落：其一为放牧草坪（Grazing Lawn），该群落受高强度放牧且易受洪水侵袭；其二为低地湿地（Lowland Wetland）与高地湿地（Upland Wetland），二者均受中度放牧，其中低地湿地洪水频发，高地湿地洪水发生频率较低；其三为苔原（Tundra）群落，该群落受放牧与洪水干扰均极少，且处于最高海拔区域。将土壤置于微宇宙培养装置（microcosms）中，分别在8摄氏度（°C）与18°C条件下培养16周，设置淹水、粪便添加两种处理，以及无淹水、无粪便添加的对照组。研究人员每周定量测定碳排放量，并在整个实验周期内监测氨化作用进程。研究结果显示，增温可提升低地湿地的氨化作用强度与碳需求，且始终显著提高CO₂与CH₄排放；但洪水与增温的交互作用，使得放牧草坪与苔原群落的碳排放对增温的响应更为复杂。在放牧草坪群落中，淹水在8°C与18°C条件下均提升了CH₄排放；而在苔原群落中，淹水则在18°C条件下抑制了CH₄排放。仅在高地湿地群落中，单独施加淹水处理便降低了CO₂排放。粪便添加处理可提升所有群落的CO₂排放，但对CH₄排放与氨化作用的影响均较为微弱。在低地湿地群落中，当粪便添加与淹水处理同时施加时，CH₄排放量相较于仅添加粪便时有所下降。仅在苔原群落中，粪便添加降低了铵态氮的固持作用与氮需求。本研究结果表明，在洪水发生频率较低、海拔较高的群落中，淹水或可部分抵消增温引发的碳损失；但在洪水频发的低地区域，若淹水与增温共同大幅加剧碳损失，则该抵消效应或可忽略不计。