Disturbance Alters the Relative Importance of Topographic and Biogeochemical Controls on Microbial Activity in Temperate Montane Forests

Rebecca Lybrand,Rachel Gallery,Nicole Trahan,David Moore

doi:10.3390/f9020097

Abstract

Fire and pathogen-induced tree mortality are the two dominant forms of disturbance in Western U.S. montane forests. We investigated the consequences of both disturbance types on the controls of microbial activity in soils from 56 plots across a topographic gradient one year after the 2012 High Park wildfire in Colorado. Topsoil biogeochemistry, soil CO2 efflux, potential exoenzyme activities, and microbial biomass were quantified in plots that experienced fire disturbance, beetle disturbance, or both fire and beetle disturbance, and in plots where there was no recent evidence of disturbance. Soil CO2 efflux, N-, and P-degrading exoenzyme activities in undisturbed plots were positively correlated with soil moisture, estimated from a topographic wetness index; coefficient of determinations ranged from 0.5 to 0.65. Conversely, the same estimates of microbial activities from fire-disturbed and beetle-disturbed soils showed little correspondence to topographically inferred wetness, but demonstrated mostly negative relationships with soil pH (fire only) and mostly positive relationships with DOC/TDN (dissolved organic carbon/total dissolved nitrogen) ratios for both disturbance types. The coefficient of determination for regressions of microbial activity with soil pH and DOC/TDN reached 0.8 and 0.63 in fire- and beetle-disturbed forests, respectively. Drivers of soil microbial activity change as a function of disturbance type, suggesting simple mathematical models are insufficient in capturing the impact of disturbance in forests.

Highlights

Forests account for half of Earth’s terrestrial organic carbon (C) stocks by storing carbon as plant biomass, forest litter, and soil organic matter (SOM) [1,2]
We show that the SAGA wetness index predicts exoenzyme activities in undisturbed landscapes, but that terrain-driven relationships with soil CO2 efflux and exoenzyme activities are altered by disturbance events that are predicted to intensify with climate change [5,8]
Soil moisture was not a strong predictor of exoenzyme activities after both disturbance types studied here; only potential leucine aminopeptidase (LAP) activity, involved in N-degradation, was significantly correlated with the SAGA wetness index following beetle infestation (Figure 3, Table 1)

Summary

Introduction

Forests account for half of Earth’s terrestrial organic carbon (C) stocks by storing carbon as plant biomass, forest litter, and soil organic matter (SOM) [1,2]. Forests 2018, 9, 97 water resources [9,10], nutrient cycling [11], and forest carbon storage [12]. These disturbance-caused changes require consideration when assessing global change risks to the stability of forest carbon [13,14]. Topography plays an integral role in the distribution of water, nutrients, and soil carbon in forested systems [16]; it is likely that hillslope-scale controls would influence soil biogeochemistry and microbial activity in disturbed systems

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Discussion

Conclusion