Harvesting Douglas-fir Stands Shifts Soil Microbial Activity and Biogeochemical Cycling

Self Cite

Core Ideas Nine forest stands in the Pacific Northwest were selected to detect at least a 5% change in mineral soil carbon. Change in mineral soil carbon content following harvest was negligible (+2% ns). There was an increase of 184% in forest floor carbon resulting from harvest residue inputs. Conventional methods of timber harvesting do not cause substantial short‐term loss in mineral soil carbon. Forest soil carbon represents a large, but slowly changing pool of carbon in forests. Understanding the response of soil carbon to forest management, including harvesting, is critical for determining overall stand and/or landscape carbon balance. This longitudinal study on nine randomly selected sites was designed to detect a 5% or greater change in mineral soil carbon storage following harvest. Soil samples were collected both pre‐harvest and 3 to 3.5 yr post‐harvest at 300 sample points on a fixed grid from each stand. A forest floor sample and mineral soil samples in depth increments of 0 to 15 and 15 to 30 cm were collected at every point. A soil sample at 30 to 100 cm (or 30–60 cm at two sites) was collected at 200 points. Each mineral soil depth fraction was separated into size fractions of <2 mm and 2 to 4.75 mm for analysis. At each site, these samples were spatially grouped into 25 composites by depth and size for the efficient analysis of carbon concentration. Soil mass and carbon concentrations were multiplied to calculate the carbon content in the soil per unit area. Across all sites combined, we found negligible change (+2% ns) in mineral soil carbon content following harvest and a 184% increase in forest floor carbon resulting from harvest residue inputs. This indicates that conventional timber harvesting methods in the Pacific Northwest do not cause substantial short‐term losses in mineral soil carbon, but the longer‐term trajectory of soil carbon storage following harvest remains to be investigated.

Section: Methodsmentioning

confidence: 99%

Soil Carbon Storage in Douglas‐Fir Forests of Western Oregon and Washington Before and After Modern Timber Harvesting Practices

Holub

Hatten

2019

Self Cite

Forest soil carbon storage in 10‐year‐old Douglas‐fir plantations of western Oregon and Washington remains similar to pre‐harvest

Holub,

Cattnach,

Littke

et al. 2024

Forests around the world, and in the case of this study, the coastal Pacific Northwest United States, store large amounts of carbon, both above ground in the trees and below ground in soils. Understanding the effects of forest disturbance, including timber harvesting, is important in order to evaluate the role that forestry plays in the global carbon cycle. Soil carbon can be difficult to assess with enough precision to detect the kinds of changes that are expected, yet a series of small changes over time in the same direction could have important cumulative effects. In this study, eight randomly selected Douglas‐fir forest stands in Oregon and Washington were sampled at 300 points each using a fixed‐depth sampling approach to attempt to detect a 5% or higher change in soil carbon storage to 1 m, longitudinally from pre‐harvest to 10 years post‐harvest. There was moderate variability in results over time at individual sites, with some sites decreasing slightly and others increasing slightly. Only two sites achieved lower than the 5% minimum detectible difference target. The remaining six sites were able to detect 5.7%–10.7% differences. In one case, an unexpectedly large increase in mineral soil carbon 10 years post‐harvest occurred without clear explanation. On average, forest floor carbon stores were 20% larger 10 years post‐harvest than pre‐harvest. Even with the large increases excluded, both the fixed‐depth approach and equivalent soil mass correction showed there was no significant change in mineral soil carbon stores to 1 m at 10 years post‐harvest in the region.

Postfire extracellular enzyme activity in a temperate montane forest

O'Kelley,

Evered,

Peter‐Contesse

et al. 2024

Wildfire is a disturbance expected to increase in frequency and severity, changes that may impact carbon (C) dynamics in the soil ecosystem. Fire changes the types of C sources available to soil microbes, increasing pyrogenic C and coarse downed wood, and if there is substantial tree mortality, decreasing C from root exudates and leaf litter. To investigate the impact of this shift in the composition of C resources on microbial processes driving C cycling, we examined microbial activity in soil sampled from an Oregon burn 1 year after fire from sites spanning a range in soil burn severity from unburned to highly burned. We found evidence that postfire rhizosphere priming loss may reduce soil C loss after fire. We measured the potential activity of C‐acquiring and nitrogen (N)‐acquiring extracellular enzymes and contextualized the microbial resource demand using measurements of mineralizable C and N. Subsurface mineralizable C and N were unaltered by fire and negatively correlated with hydrolytic extracellular enzyme activity (EEA) in unburned, but not burned sites. EEA was lower in burned sites by up to 46%, but only at depths below 5 cm, and with greater decreases in sites with high soil burn severity. These results are consistent with a subsurface mechanism driven by tree mortality. We infer that in sites with high tree mortality, subsurface EEAs decreased due to loss of rhizosphere priming and that inputs of dead roots contributed to mineralizable C stabilization.