Microbial and nitrogen pool response to fuel treatments in Pinyon-Juniper woodlands of the southwestern USA

• Mastication of dead trees did not significantly increase surface organic horizons.
• Mastication residues increased O-horizon carbon:nitrogen ratio.
• Mastication residues increased the available inorganic N in the surface mineral soil.
• Fuel treatments altered microbial communities compared to control for 3 years.
• Drought and insect mortality greatly influenced treatment outcomes.

Wildfire mitigation in Pinyon-Juniper woodlands in the Colorado Plateau region is a management priority. Two wildfire mitigation treatments, mastication and thin-pile-burn, are often chosen based on costs and availability of equipment, yet there are ecological concerns with either treatment. Ecological outcomes from additions of low quality organic residues following mastication can potentially alter microbially mediated soil nitrogen availability, therefore we wished to evaluate the effects of the additions of low quality woody residues to the soil surface compared to thin-pile-burn treatments at three Ips confusus affected woodland sites in the Colorado Plateau region. Thin-pile-burn treatments consisted of thinning between 40% and 60% of the canopy cover, while mastication treatments entailed shredding dead trees and 50% of shrub canopy while retaining live pinyon trees and designated snags. Both fuels treatments achieved wildfire mitigation goals, yet twice as much basal area and three times as much volume were removed by thinning relative to mastication. Surface additions of woody residues favored bacteria, while fungal populations that typically colonize and decompose woody organic residues did not increase in the upper mineral horizon. Neither fuel treatment affected mineral soils at the plot level, but the carbon to nitrogen (C:N) ratio of the organic horizon was significantly greater with mastication compared to the control. Nitrogen availability was reduced by almost half during the non-growing season, yet fuel treatments did not significantly influence nitrogen availability compared to control. Fuel treatment as performed in our study following the large mortality event demonstrated that microbial populations and their effects on nitrogen availability are resistant to disturbances produced by wildfire mitigations treatments.