Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline

Abstract

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space-for-time’ approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg) is significantly lower at shrub (2.98 ± 0.48 kg m−2) and forest (2.04 ± 0.25 kg m−2) plots than at heath plots (7.03 ± 0.79 kg m−2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.

Simulating ectomycorrhiza in boreal forests: implementing ectomycorrhizal fungi model MYCOFON in CoupModel (v5)

“…7). This corresponds well with findings from field measurements and recent modeling studies that ECM are able to degrade complex N polymers in humus layers, thus enhancing soil N transformation under low-N conditions (Hartley et al, 2012;Moore et al, 2015;Lindahl and Tunlid, 2015;Parker et al, 2015;Baskaran et al, 2016). The modeled higher soil respiration further explains the minor losses of soil C and N at the southern sites and also a higher mineral N pool and thus higher N leaching in the explicit approach (Figs.…”

Removal of grazers alters the response of tundra soil carbon to warming and enhanced nitrogen availability

“…, Parker et al. , ). In our study, deciduous shrub abundance under light grazing was 79% higher in warmed than control plots, while organic soil carbon stocks were 33% lower, which is comparable to the observed natural differences in soil organic carbon under varying deciduous shrub cover (Parker et al.…”

“…, Parker et al. ). This warming and fertilization‐induced shift in shrub abundance, and thereby in the shrub‐mediated carbon sequestration processes, is unlikely to take place in high herbivore densities.…”

Does Caragana korshinskii plantation increase soil carbon continuously in a water‐limited landscape on the Loess Plateau, China?

“…Afforestation in semiarid areas has been proposed as an effective way for improving soil carbon accumulation and restoration of degraded ecosystems (Lu et al, ; Shi et al, ). Nevertheless, field studies have shown diverse response of carbon stock to afforestation, observing increase (Garcia‐Franco et al, ), decrease (Berthrong, Jobbágy, & Jackson, ; Parker, Subke, & Wookey, ), and no change (Yang, Luo, et al, ). Our results indicated that afforestation of C. korshinskii only increased soil carbon stock in short term (10 years) compared with an abandoned field, but the carbon sink diminished during the late two decades (Figure a), which supported our primary hypothesis.…”