Multiscale responses and recovery of soils to wildfire in a sagebrush steppe ecosystem

Abstract

Ecological theory predicts a pulse disturbance results in loss of soil organic carbon and short-term respiration losses that exceed recovery of productivity in many ecosystems. However, fundamental uncertainties remain in our understanding of ecosystem recovery where spatiotemporal variation in structure and function are not adequately represented in conceptual models. Here we show that wildfire in sagebrush shrublands results in multiscale responses that vary with ecosystem properties, landscape position, and their interactions. Consistent with ecological theory, soil pH increased and soil organic carbon (SOC) decreased following fire. In contrast, SOC responses were slope aspect and shrub-microsite dependent, with a larger proportional decrease under previous shrubs on north-facing aspects compared to south-facing ones. In addition, respiratory losses from burned aspects were not significantly different than losses from unburned aspects. We also documented the novel formation of soil inorganic carbon (SIC) with wildfire that differed significantly with aspect and microsite scale. Whereas pH and SIC recovered within 37 months post-fire, SOC stocks remained reduced, especially on north-facing aspects. Spatially, SIC formation was paired with reduced respiration losses, presumably lower partial pressure of carbon dioxide (pCO₂), and increased calcium availability, consistent with geochemical models of carbonate formation. Our findings highlight the formation of SIC after fire as a novel short-term sink of carbon in non-forested shrubland ecosystems. Resiliency in sagebrush shrublands may be more complex and integrated across ecosystem to landscape scales than predicted based on current theory.

Figure 1

Predicted plant and soil carbon pools and processes following disturbance. Immediately after disturbance, loss of soil organic carbon and plant material are predicted (above panel). Prior to disturbance, net primary productivity (NPP) is in balance with hetetrophic respiration (R_H) such that NEP equals zero. A pulse in respiration (R_H) (bottom panel) is predicted after disturbance such that NPP < R_H and NEP is negative (source of carbon to the environment) and eventually the ecosystem comes back into quasi-steady state (modified from Gorham et al. ).

Figure 2

Aspect and microsite properties contrast in study catchments at the Reynolds Creek Experimental Watershed-Critical Zone Observatory (RCEW-CZO) in Southwestern Idaho, USA. (a) North- and south-facing aspects within Mack’s Creek (burned) and Babbington Creek (unburned) in the RCEW-CZO. (b) Image highlights burned biomass and remanent plant-interplant spaces after the Soda fire in August 2015.

Figure 3

Soil properties exhibit scale-dependent responses and recoveries after fire. Comparison of (a) soil pH, (b) soil organic carbon (SOC), (c) fraction of pyrogenic carbon (PyC), and (d) soil inorganic carbon (SIC) on burned (red and pink) and control (dark and light grey) north- (squares) and south-facing aspects (triangles) 2–37 months after fire.

Figure 4

Soil pH and carbonate isotopes values provide possible mechanisms for SIC formation. (a) A threshold associated with soil inorganic carbon (SIC) formation was observed as pH increases following fire, burn soils (red), control (grey), (b) Plot of δ¹³C- and δ¹⁸O-carbonate values depicting depleted fire carbonate isotope ratios relative to observed soil carbonates at depth in the RCEW-CZO, and (c) the correlation of δ¹³C-carbonate values with δ¹³C-SOC values.

Figure 5

Respiration on burned aspects was reduced relative to controls in both lab and field measurements. (a) Soil carbon (C) mineralization rates with time (months) since fire on burned (red and pink) and control (dark and light gray) for north- (squares) and south-facing aspects (triangles) 2–37 months after fire, (b) total C mineralization that occurred during this period, (c) respiratory fluxes (R_H) with reduced and even negative fluxes immediately after fire in September (~ 30 days post-fire) on the burned north-facing aspect (red line), (d) total losses from each site showing variable losses from different burned and control aspects, and (e) average total fluxes from burned and control aspects were not significantly different owing to difference in aspect.

Figure 6

Cross-scale responses to fire were observed. Observed main burn response compared to predicted response based on disturbance theory and Fellows et al. in the case of C mineralization. We found ecosystem and landscape scale dependent responses and interactions across different scales. Recovery after 3-year post-fire (yes or no) is reported showing that recovery varies with soil property and process.