RAPID: Do wildfire effects on roots and rhizosphere activity control both short- and long- term soil C stabilization?

This RAPID proposal is to examine soil carbon dynamics in forest stands affected by a large and catastrophic wildfire that burned three watersheds and surrounding forest in the H.J. Andrews Experimental Forest, OR. Old-growth forests in the USA hold significant amounts of carbon (C) in both tree biomass and in soils.

Because of this, disturbances such as forest harvest or fire that might release C into the air from vegetation or soil have the potential to impact the atmosphere. Both the number and the sizes of wildfires in the Pacific Northwest have been increasing in the last several decades, and increases in temperature are projected to lengthen fire seasons. Because so many Western U.S. ecosystems are expected to experience increasingly large and severe fires in the near future, understanding how wildfire impacts ecosystem C cycling is increasingly critical to future projections of global temperatures.

While fire effects on the C budgets of above ground vegetation are relatively easy to observe and measure, fire effects on soil organic matter C below the surface is less easy to measure or predict. Given that the soil C pool is over 3 times greater than the atmospheric C pool, it is critical to understand the mechanisms of soil response to extreme fire disturbance in order to have accurate models of global C concentrations under different scenarios of environmental change.

Results from this research will not only inform global earth system science models, but will also help inform ecosystem managers about potential benefits and costs of different fire management scenarios.

This research will test novel hypotheses about the influences of root/rhizosphere activity and above and belowground detrital inputs on soil C stabilization and destabilization after catastrophic disturbance. Hypotheses are based on a new model of soil C dynamics that starts with the argument that soil C in undisturbed ecosystems is a balance between rhizosphere-derived priming losses and root-derived soil C gains.

Priming of soil C occurs when inputs of labile C sources, usually from live root exudation, fuels increased respiration (= C loss) of stable soil organic matter (SOM) pools such as mineral-associated organic matter (MAOM); however, efficient microbial use of high-quality detrital inputs such as fine root litter are retained (= C gain) in soil as MAOM. These two competing processes keep MAOM levels both stable and below true mineral saturation.

However, when tree-mortality disturbance occurs, not only is there a significant influx of dead fine root material (aiding soil C stabilization through microbial use and microbial turnover), but rhizosphere priming ceases as root exudation of labile dissolved organic carbon ceases (removing soil C destabilization). This model predicts that change to soil C pools and C dynamics after significant disruption to detrital inputs is remarkably rapid due to immediate reduction in rhizosphere activity coupled with continued influx of detrital root materials.

This is contrary to current models of soil C after harvest or fire in which the most stable pools of soil C could increase until the forest regenerates. A sampling grid will be established based on tree mortality, burn severity, pre-fire aboveground biomass and LiDAR-derived geophysical variables throughout the region burned in the Holiday Farm fire at the Andrews Forest.

Soil cores will be taken along transects within identified sampling sites and density fractionated to separate out roots, particulate soil carbon, and MAOM. SOM in whole soils and soil physical fractions will be characterized using solid-state 13C nuclear magnetic resonance also allowing us to examine any movement of pyrogenic C into either POM or MAOM pools.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.