Drainage enhances modern soil carbon contribution but reduces old soil carbon contribution to ecosystem respiration in tundra ecosystems

Abstract

Warming temperatures are likely to accelerate permafrost thaw in the Arctic, potentially leading to the release of old carbon previously stored in deep frozen soil layers. Deeper thaw depths in combination with geomorphological changes due to the loss of ice structures in permafrost, may modify soil water distribution, creating wetter or drier soil conditions. Previous studies revealed higher ecosystem respiration rates under drier conditions, and this study investigated the cause of the increased ecosystem respiration rates using radiocarbon signatures of respired CO2 from two drying manipulation experiments: one in moist and the other in wet tundra. We demonstrate that higher contributions of CO2 from shallow soil layers (0–15 cm; modern soil carbon) drive the increased ecosystem respiration rates, while contributions from deeper soil (below 15 cm from surface and down to the permafrost table; old soil carbon) decreased. These changes can be attributed to more aerobic conditions in shallow soil layers, but also the soil temperature increases in shallow layers but decreases in deep layers, due to the altered thermal properties of organic soils. Decreased abundance of aerenchymatous plant species following drainage in wet tundra reduced old carbon release but increased aboveground plant biomass elevated contributions of autotrophic respiration to ecosystem respiration. The results of this study suggest that drier soils following drainage may accelerate decomposition of modern soil carbon in shallow layers but slow down decomposition of old soil carbon in deep layers, which may offset some of the old soil carbon loss from thawing permafrost.

Alaskan carbon-climate feedbacks will be weaker than inferred from short-term experiments

“…Ecosys has been applied to model the ecological controls on ecosystem processes and the whole ecosystem response to changing climate across different ecosystems, including within high-latitude soils; Arctic tundra 11,12 , and fens 13,14 , boreal forests 8,[15][16][17][18] . We note that the Ecosys model performs extremely well in replicating contemporary flux tower estimates of carbon exchange in Alaskan ecosystems 19,20 , and is therefore an appropriate model for testing how Arctic ecosystems response to changing climate. The parameters and algorithms governing the modeling of the tundra ecosystem have been described in detail previously 21 .…”

“…Respiration rates can also increase due to a modeled decline in soil moisture under short-term warming. A small decline in soil moisture can increase oxygen diffusion into soils which can further stimulate heterotrophic and autotrophic respiration 19 . The rapidity with which belowground activity increases tends to exceed vegetation responses and leads to a net decline in SOC.…”

“…However, the microbial community response to warming is complex and depends on concurrent changes in soil hydrology. For example, while drying or anoxic microsite formation under saturating conditions can reduce microbial activity 6,9 , a slight decrease in soil moisture has also been shown to increase microbial activity 19 .…”

Negative feedback processes following drainage slow down permafrost degradation

“…We observed an overall increase in ER by a factor of 1.35, implying a flux shift of −79.7 ± 71.6 g CO 2 ‐C/year (Figure a), where negative values indicate that more carbon was released from the ecosystem to the atmosphere (Kittler, Heimann, et al, ). In this context, based on ER partitioning methods using natural abundance of radiocarbon, Kwon et al () determined that a strong increase in respiration within shallow soil layers (drainage: 55% of total ER; control: 31%) dominated over a decrease in colder, deeper layers (drainage: 11% of total ER; control: 43%; see also left panel of Figure ). At the same time, higher photosynthetic uptake led to a moderate increase in GPP by a factor of 1.11 (29.5 ± 74.2 g CO 2 ‐C/year), thus counterbalancing part of the respiration losses to the atmosphere.…”

Soil carbon release responses to long-term versus short-term climatic warming in an arid ecosystem

“…Soil respiration, according to previous studies, is expected to increase with soil water content (SWC) (e.g., Chen et al, 2008;Song et al, 2015;Wan et al, 2007;Yan et al, 2013). However, when the SWC exceeds the optimal point to reach saturated levels, Rs decreases (Huxman et al, 2004;Kwon et al, 2019;Moyano et al, 2012;Moyano et al, 2013;Wang et al, 2014). In a study conducted in a tall grass prairie, water addition dramatically increased soil CO2 efflux (Liu et al, 2002).…”