Rapid Shifts in Soil Nutrients and Decomposition Enzyme Activity in Early Succession Following Forest Fire

Joseph Knelman,Steven Schmidt,Diana Nemergut,Scott Ferrenberg,Amanda Labrado,John Darcy,Emily Graham,Aurélien Lecoeuvre

doi:10.3390/f8090347

Abstract

While past research has studied forest succession on decadal timescales, ecosystem responses to rapid shifts in nutrient dynamics within the first months to years of succession after fire (e.g., carbon (C) burn-off, a pulse in inorganic nitrogen (N), accumulation of organic matter, etc.) have been less well documented. This work reveals how rapid shifts in nutrient availability associated with fire disturbance may drive changes in soil enzyme activity on short timescales in forest secondary succession. In this study, we evaluate soil chemistry and decomposition extracellular enzyme activity (EEA) across time to determine whether rapid shifts in nutrient availability (1–29 months after fire) might control microbial enzyme activity. We found that, with advancing succession, soil nutrients correlate with C-targeting β-1,4-glucosidase (BG) EEA four months after the fire, and with N-targeting β-1,4-N-acetylglucosaminidase (NAG) EEA at 29 months after the fire, indicating shifting nutrient limitation and decomposition dynamics. We also observed increases in BG:NAG ratios over 29 months in these recently burned soils, suggesting relative increases in microbial activity around C-cycling and C-acquisition. These successional dynamics were unique from seasonal changes we observed in unburned, forested reference soils. Our work demonstrates how EEA may shift even within the first months to years of ecosystem succession alongside common patterns of post-fire nutrient availability. Thus, this work emphasizes that nutrient dynamics in the earliest stages of forest secondary succession are important for understanding rates of C and N cycling and ecosystem development.

Highlights

Global change pressures have increased the prevalence of forest fires in western North America [1,2,3].a better understanding of the connection between resulting perturbations in environmentalForests 2017, 8, 347; doi:10.3390/f8090347 www.mdpi.com/journal/forestsForests 2017, 8, 347 factors and ecosystem processes, such as decomposition, will be vital to modeling ecosystem responses in the wake of such disturbance [4,5,6,7]
While past work has evaluated shifts in extracellular enzymes (EEA) [14] and environmental controls on enzyme potential across secondary succession in general [27], our study examines how relationships between edaphic properties and decomposition enzyme activity may change within the first years of succession following a severe forest fire
While BG showed significantly higher activity in 29-month soils than four-month soils, no differences in NAG activity or BG:NAG ratios over time were observed in reference soils (Table 1)

Summary

Introduction

Global change pressures have increased the prevalence of forest fires in western North America [1,2,3].a better understanding of the connection between resulting perturbations in environmentalForests 2017, 8, 347; doi:10.3390/f8090347 www.mdpi.com/journal/forestsForests 2017, 8, 347 factors and ecosystem processes, such as decomposition, will be vital to modeling ecosystem responses in the wake of such disturbance [4,5,6,7]. Fire disturbances dramatically alter soil pH, water holding capacity, and carbon (C) and nitrogen (N) pools [11,12], which may all continue to change through succession These factors may affect microbial investment in both C- and N-targeting decomposition enzymes across short time scales of ecosystem recovery (e.g., months). Work in similar forest ecosystems relating to mountain pine beetle kill has shown long-term effects of such disturbance, and immediate, short-timeframe impacts on microbial function, such as respiration [17] This body of work has demonstrated that even when succession is considered over just months to years after a disturbance, shifts in nutrient pools—such as in ammonium (NH4 + ) and C availability—can have strong effects on microbial function [16,17,18]. Even on a timescale of months to years after disturbance, these rapid and profound shifts in nutrient pools may influence microbial processes, such as the production of extracellular enzymes, which are central to nutrient cycling and ecosystem dynamics

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Conclusion