Differential Response of Greenhouse Gas Evasion to Storms in Forested and Wetland Streams

Abstract:

Greenhouse gas evasion from inland waters is a globally significant yet highly uncertain flux, especially in regard to effects of wetlands and hydrologic variability. We sampled five first-order and two second-order streams with variable wetland influence during storm events for dissolved CO2, CH4, and N2O. We also calculated gas evasion rates. In first-order streams, pCO(2) and pN(2)O were significantly higher in the stream with the most wetland influence (mean1std: 3,9651,504 and 1.180.37atm, respectively) than the forested stream (2,927439 and 0.470.08atm, respectively). In second-order streams, pCO(2), pCH(4), and pN(2)O were higher in the 14% wetland stream (3,274 +/- 825, 501 +/- 207, and 1.37 +/- 0.43atm, respectively) than in the 2% wetland stream (1,858 +/- 423, 137 +/- 53, and 0.37 +/- 0.08atm, respectively). In first-order streams, pCO(2) in streams with wetland influence increased during rain events, while pCO(2) in streams with little to no wetland influence decreased or remained constant. Generally, pCH(4) and pN(2)O followed the same trend, except in one stream with intermediate wetland influence. Gas transfer velocity increased in all streams during storm events. However, the forested streams had higher gas transfer velocities than the wetland streams due to steeper topography. CO2, CH4, and N2O evasion peaked in one of the intermediate wetland streams at high flow (maximum: 66gC.m(-2).day(-1), 177mgC.m(-2).day(-1), and 9.7mgN.m(-2).day(-1), respectively). These findings suggest that gases are shunted downstream in flatter, wetland streams, while gases are evaded closer to their source in steeper, forested streams. Plain Language Summary Greenhouse gas evasion from inland waters is an important component of the global carbon cycle. However, this flux has only just been included in the most recent IPCC carbon cycle (AR5). Headwater streams are areas where gases, such as carbon dioxide, methane, and nitrous oxide, tend to collect and are released to the atmosphere. Gas dynamics in headwater streams must be understood to improve global evasion estimates. We sampled streams covering a gradient of wetland to forest land covers during rainstorms to examine the effects on gas evasion. We found that gas concentrations generally increased during storms in wetland streams, while concentrations tended to decrease or remain constant in forested streams. However, we found that gases were more easily evaded from forested streams than from wetland streams. Forested streams tend to be found in steeper areas and have more turbulent flows than wetland streams, which promote evasion. Conversely, gas inputs to wetland streams are carried downstream and released farther from the source. The significance of this work is that greenhouse gas evasion from headwater streams is a spatially and temporally variable flux from the landscape. Inclusion of this variability in global models may support higher global evasion estimates.