Forest soil C and N responses to salvage logging and belowground C inputs in bark beetle infested stands
Date
2020
Authors
Avera, Bethany N., author
Cotrufo, M. Francesca, advisor
Rhoades, Charles, committee member
Rocca, Monique, committee member
van Diepen, Linda, committee member
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Abstract
Managing forest ecosystems in this era of global change requires a fundamental understanding of forest soil properties and processes. Forest disturbance events are projected to increase in severity and frequency, requiring a better understanding of how post-disturbance management will impact ecological processes such as soil nutrient dynamics and stocks of soil carbon (C). The research in this dissertation focused on areas of widespread mortality in lodgepole pine (Pinus contorta var. latifolia) in northern Colorado due to the most recent outbreak of the endemic mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins). The goal of this research was to examine soil nitrogen (N) stocks, plant N uptake, and changes in forest soil C stocks in soil organic matter (SOM) due to tree mortality and subsequent salvage logging and from different belowground C inputs. To achieve this aim, I compared the three most prevalent management options: 1) uncut beetle-infested lodgepole pine stands and clear-cut salvage logged areas with either 2) post-harvest residue retention or 3) post-harvest residue removal. To determine the impacts of MPB-infestation and salvage logging on ecosystem N stocks and plant N uptake, I implemented an experimental field study by adding 15N-labeled ammonium sulfate to research plots centered over lodgepole pine seedlings. Measuring N stocks and 15N recovery in soil and vegetation pools over two growing seasons highlighted the coupled nature of forest C and N cycling between plant and soil forest ecosystem compartments. The majority of the 15N label was recovered in the soil and was not impacted by the management treatments. In contrast, the N uptake by lodgepole pine seedlings was driven primarily by the amount of C fixation and the patterns of C fixation, in turn, related to other environmental factors modulated by the management treatment, such as available light. An observational field study sought to quantify changes in forest soil C stocks in the bulk soil and SOM fractions and detect any changes in C chemistry as a result of management that may impact C persistence. In the dry, high elevation forests studied, soil C increased with salvage logging likely due to mixing of surface residues and O horizon C into the mineral soil during logging. The distribution of C stocks among the mineral soil fractions and the chemistry of those fractions indicated that root C accumulation in the particulate organic matter (POM, >53 μm) is an important mechanism of soil C accumulation in these forest soils. A mechanistic laboratory incubation evaluated the efficiency of mineral-associated organic matter (MAOM, <53 μm) formation from root and hyphal necromass inputs with different C chemistries. This study showed that rye root necromass with more labile and less structural C than pine roots, was processed most in the 38-day incubation and contributed much more efficiently to the formation of MAOM than did the pine roots. Despite less processing, the arbuscular and ectomycorrhizal fungal necromass both contributed as efficiently as rye roots to MAOM formation. These results indicate that both C chemistry and C/N ratio exert controls on residue processing and MAOM formation. Together, this dissertation work showed that salvage logging stimulated the growth of lodgepole pine seedlings, resulting in increased storage of both C and N in the plant biomass above- and belowground. As this pine root biomass turns over, the root necromass will contribute C to the POM fraction, the largest pool of soil C in this system. The net increase of forest soil C with salvage logging found in this study is notable as it suggests that the MPB-infested lodgepole pine forests of Colorado can be salvage logged with a low risk of significant soil C loss. Additionally, the highest recovery of the N label was in the soil, thus the high soil N recovery with higher soil C supports SOM is a sink of N reducing N losses. Finally, pine seedling colonization by ectomycorrhizal fungi may further aid with nutrient retention and the efficient formation of MAOM during regeneration.
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Subject
nitrogen
soil carbon
woody residue
salvage logging
belowground inputs
soil organic matter