Carbon isotopic composition of forest soil respiration in the decade following bark beetle and stem girdling disturbances in the Rocky Mountains

Abstract

Bark beetle outbreaks are widespread in western North American forests, reducing primary productivity and transpiration, leading to forest mortality across large areas and altering ecosystem carbon cycling. Here the carbon isotope composition (δ 13 C) of soil respiration (δ J ) was monitored in the decade after disturbance for forests affected naturally by mountain pine beetle infestation and artificially by stem girdling. The seasonal mean δ J changed along both chronosequences. We found (a) enrichment of δ J relative to controls (<1 ‰) in near-surface soils in the first 2 years after disturbance; (b) depletion (1‰ or no change) during years 3-7; and (c) a second period of enrichment (1-2‰) in years 8-10. Results were consistent with isotopic patterns associated with the gradual death and decomposition of rhizosphere organisms, fine roots, conifer needles and woody roots and debris over the course of a decade after mortality. Finally, δ J was progressively more 13 C-depleted deeper in the soil than near the surface, while the bulk soil followed the well-established pattern of 13 C-enrichment at depth. Overall, differences in δ J between mortality classes (<1‰) and soil depths (<3‰) were smaller than variability within a class or depth over a season (up to 6‰).

Disturbance Alters the Relative Importance of Topographic and Biogeochemical Controls on Microbial Activity in Temperate Montane Forests

“…Our study suggests that the environmental factors limiting microbial function shift depending on whether fire or pathogeninduced mortality is the disturbance agent. Our results are consistent with studies limited to single disturbances [31,32,40,50,92,94]. Soil biogeochemical processes can be represented through a variety of modeling approaches including a simple bulk substrate, moisture and Q10 response [105], a more complex treatment of soil substrate suitability [62,106], or even trait-based approaches [107,108].…”

“…Both disturbances contribute enough variation to the environmental drivers of microbial function to reduce the importance of topography and its correlates as determinants of the rates of CO 2 efflux and potential enzyme activities (Table 1). While beetle mortality alone may be complicated by the timing of when that mortality occurred [31,93,94], the environmental correlates of microbial function after fire alone (pH and DOC/TDN) are not the same as the correlates after compounded beetles and fire disturbance (DOC/TDN). In our previous work [32] and this current study, we have found limited effects of beetle mortality on soil pH itself, though we find some evidence that beetle kill apparently buffers soil pH (Figures 2e and A3).…”

Spatial and seasonal variabilities of the stable carbon isotope composition of soil CO2 concentration and flux in complex terrain

“…We did not collect location‐specific C a or δ a , and thus, we assumed that both values were homogenous across the study site. Previous studies have demonstrated that the variability of C a or δ a is negligible compared to the much larger variability of these quantities in the soil [ Riveros‐Iregui et al ., ; Bowling et al ., ; Mauer et al , ]. We then calculated the intercept of the regression ( δ R ) as shown in Figure .…”

Labile C dynamics reflect soil organic carbon sequestration capacity: Understory plants drive topsoil C process in subtropical forests

“…This is largely ascribed to the rapid accumulation of tree biomass in the plantations (Chen et al 2017). Soil LOC includes low-molecular-weight compounds from plant rhizodeposition, C leachates and decomposition products from plant litter, and microbial biomass (Jones et al 2004, Maurer et al 2016, and thus, the highly active C pool can be closely related to both soil heterotrophic respiration and soil C sequestration (K€ ogel-Knabner 2002). One of the key obstacles is that the huge background pool of soil organic carbon (SOC) is likely to impede the ready detection of SOC changes.…”

“…Here, we propose to explore SOC changes by monitoring the dynamics of soil labile organic carbon (LOC). Soil LOC includes low-molecular-weight compounds from plant rhizodeposition, C leachates and decomposition products from plant litter, and microbial biomass (Jones et al 2004, Maurer et al 2016, and thus, the highly active C pool can be closely related to both soil heterotrophic respiration and soil C sequestration (K€ ogel-Knabner 2002). As a result, changes in soil LOC concentration and/or turnover time may sensitively reflect changes in the C retention in soils (Cleveland et al 2007, Rousk et al 2016.…”