Responses of CO₂ flux components of Alaskan Coastal Plain tundra to shifts in water table

Abstract

[1] The Arctic stores close to 14% of the global soil carbon, most of which is in a poorly decomposed state as a result of water-saturated soils and low temperatures. Climate change is expected to increase soil temperature, affecting soil moisture and the carbon storage and sink potential of many Arctic ecosystems. Additionally, increased temperatures can increase thermokarst erosion and flooding in some areas. Our goal was to determine the effects that water table shifts would have on the CO 2 sink potential of the Alaskan Coastal Plain tundra. To evaluate the effects of different water regimes, we used a large hydrological manipulation at Barrow, Alaska, where we maintained flooded, drained, and intermediate water levels in a naturally drained thaw lake basin over a period of three seasons: one pretreatment (2006) and two treatment (2007)(2008) seasons. To assess CO 2 flux components, we used 24 h chamber-based measurements done on a weekly basis. Increased water table strongly lowered ecosystem respiration (ER) by reducing soil oxygen availability. Flooding decreased gross primary productivity (GPP), most likely by submerging mosses and graminoid photosynthetic leaf area. A decrease in water table increased GPP and ER; however, the increase in root and microbial activity was greater than the increase in photosynthesis, negatively affecting net ecosystem exchange. In the short term, ER is the CO 2 flux component that responds most strongly to changes in water availability. Our results suggest that drying of the Alaskan Coastal Plain tundra in the short term could double ER rates, shifting the historic role of some Arctic ecosystems from a sink to a source of CO 2 .

Water-table height and microtopography control biogeochemical cycling in an Arctic coastal tundra ecosystem

“…As these DTLB's develop over the course of hundreds to several thousand years, ice wedge polygons create complex topography within these basins. One such DTLB near Barrow, Alaska was the location of a large scale water-table manipulation ("Biocomplexity") experiment (Zona et al, 2009;Olivas et al, 2010). Previous work at the Biocomplexity site showed that much of the basin is prone to anoxia, and that dissimilatory Fe(III) reduction is a particularly important anaerobic process that contributes significantly to the ecosystem C budget (Lipson et al, 2010).…”

“…), sedges (Carex aquatilis, Eriophorum spp. ), and grasses (Dupontia fisheri, Arctophila fulva), with mosses dominating topographically high areas such as polygon rims and Carex dominating low areas such as polygon centers (Zona et al, 2009;Olivas et al, 2010). The soils within the basin are classified as aquahaplels, orthels and histoturbels, with an organic horizon (∼12-15 cm thick) overlaying a silty mineral layer.…”

“…The active layer is typically about 30 cm deep (Shiklomanov et al, 2010). The water table manipulation experiment at this site has been described in detail previously (Zona et al, 2009;Lipson et al, 2010;Olivas et al, 2010). Briefly, the roughly elliptical basin (about 1.5 km long) was divided into three sections by plastic dikes buried in the permafrost.…”

Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure

“…Water saturation conditions affect heat flux rates into soils, and also alter thaw depth and the net ecosystematmosphere heat exchange (Jorgenson et al, 2010;Subin et al, 2013). Moreover, waterlogged conditions can increase solar absorption, contributing to increased thaw depths and potentially to a subsequent lowering of the water table with respect to the surface (Olivas et al, 2010). Conversely, lowering the water table can also preserve moisture in the ecosystem in the absence of plants with high leaf area index and deep roots, since higher albedo reduces net radiation, and poor thermal conductivity in dry soils keeps deeper layers colder .…”

“…Over the past two decades, several studies have documented the effect of an experimental manipulation of soil hydrologic conditions on biogeochemical cycles in Arctic ecosystems (e.g., Oechel et al, 1998;Olivas et al, 2010;Natali et al, 2015). Only a few of these studies examined longterm effects of manipulated water tables (e.g., Christiansen et al, 2012;Lupascu et al, 2014;Kittler et al, 2016), and none of them explicitly addressed the net impact of hydrologic disturbance on the energy budget; only shifts in thaw depth (Zona et al, 2011a;Kim, 2015) and indirect effects of heat fluxes on the carbon cycle (Turetsky et al, 2008) were reported.…”