Postfire extracellular enzyme activity in a temperate montane forest

Authors: O'KELLEY, REGINA - Oregon State University; EVERED, ABIGAIL - Oregon State University; PETER-CONTESSE, HAYLEY - Oregon State University; Moore, Jennifer; LAJTHA, KATE - Oregon State University

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2024
Publication Date: 11/7/2024
Citation: O'Kelley, R., Evered, A., Peter-Contesse, H., Moore, J.M., Lajtha, K. 2024. Postfire extracellular enzyme activity in a temperate montane forest. Soil Science Society of America Journal. 88(6):2277-2294. https://doi.org/10.1002/saj2.20745.
DOI: https://doi.org/10.1002/saj2.20745

Interpretive Summary: Soil microbes are important decomposers that drive nutrient cycles in forests and other terrestrial ecosystems. Organic matter (leaves, wood, etc.) is a food source for microbes from which they obtain carbon (C), nitrogen (N) and other nutrients. Like flora and fauna, the soil microbiome can be profoundly impacted by disturbances. Fire changes the type and amount of C sources available to soil microbes, increasing charred and woody material, and decreasing C from root secretions and leaves, which are also richer in N. We studied the impact of this shift in the composition of resources on microbial processes that are involved in decomposition. We compared microbial activities in a burn complex in Blue River, Oregon, late in the first year of recovery after fire. We sampled soils from burned and unburned sites and measured potential activity of enzymes that soil microbes release to decompose organic matter, these are related to the microbial demand for C and N. We also measured the total amount of C and N available for microbial consumption to understand how the supply of C and N relates to the microbial demand for them. Measurements of these activities uncovered differing responses to fire across burn severity and enzymes in the 0 – 5 cm, 5 – 10 cm and 10 – 30 cm depths. Some microbial activities were decreased by fire, but only in the 5 – 10 cm and 10 – 30 cm depths, and mainly in sites with high soil burn severity. Decreased microbial activity is often associated with a decrease in available C and N. However, the C and N available to microbes were broadly unaltered by fire, suggesting that another process prevented changes to soil organic matter in the subsurface. Rhizosphere priming, a phenomenon where root secretions trigger microbes to decompose organic matter faster, is a potential explanation for this result. We suggest that microbial activities decreased below 5 cm due to a loss of rhizosphere priming, and that inputs of dead roots in sites with high tree mortality contributed to stabilizing C and N. Measuring microbial activities and C and N in the soil throughout vegetation recovery will help to clarify the role of disturbances in modulating organic matter decomposition.

Technical Abstract: The soil contains vast biodiversity and functionality in terrestrial ecosystems. Like flora and fauna, the soil microbiome can be profoundly impacted by disturbances. Fire changes the types of carbon (C) sources available to soil microbes, increasing pyrogenic C and coarse woody debris, and if there is substantial tree mortality, decreasing C from root exudates and leaf litter. We aimed to investigate the impact of this shift in the quantity and quality of C resources on microbial processes driving nutrient transformations and C stabilization and destabilization. We evaluated microbial activity in a burn complex in Blue River, Oregon, late in the first year of recovery after fire. We sampled soils from sites spanning three levels of burn severity alongside unburned sites, measured the potential activity of a suite of extracellular enzymes associated with resource acquisition of C and nitrogen (N) rich substrates, and contextualized the microbial resource demand using measurements of mineralizable C and mineralizable N. Measurements of potential extracellular enzyme activity (EEA) uncovered differential responses to fire across burn severity and enzymes in the 0 – 5 cm, 5 – 10 cm and 10 – 30 cm depths. Hydrolytic EEA was affected by fire, but only in the 5 – 10 cm and 10 – 30 cm depths, and mainly in sites with high soil burn severity. Meanwhile, mineralizable C and N were broadly unaltered by fire, suggesting that a compensatory mechanism stabilized soil organic matter in the subsurface. We suggest that EEAs were lower below 5 cm due to a loss of rhizosphere priming, and that inputs of dead roots in sites with high tree mortality contributed to stabilizing potentially mineralizable C. Measuring EEA and mineralizable resources throughout vegetation recovery will help to clarify the role of different organic matter fractions in modulating organic matter decomposition.