Fire severity effects on forest soil C and N in northern Minnesota, USA

Background/Question/Methods

Fire is a key natural disturbance in the Laurentian mixed forests of the Great Lakes region, USA. Recent patterns of prolonged regional drought suggest that the ecological effects of wildfire may become increasingly severe due to increased forest susceptibility to fire.  Although soil organic matter content and characteristics are important controls on post-fire forest soil nutrient availability and forest recovery, our current understanding of how contrasting levels of fire severity influence soil organic matter is limited. The 2011 Pagami Creek wildfire in northern Minnesota was a historical fire event and resulted in a range of fire severity levels determined via remote sensing and field measurements. To evaluate the effects of fire severity on forest soil organic matter, we quantified post-fire (October-November 2011) total soil carbon (C) and nitrogen (N) using elemental analysis, and black (pyrogenic) C using nuclear magnetic resonance (NMR) spectroscopy in areas classified as low to high fire severity.

Results/Conclusions

Relative to unburned reference soils, soil total C content in the organic soil layer decreased in high-severity areas, whereas no significant effect occurred in low- or moderate-severity areas. There were no effects of fire or fire severity level on soil total C content in the 0-10 cm or 10-20 cm mineral soil depths, or on soil total N content in organic or mineral soil.  Fire decreased soil C:N ratio in organic and mineral soil layers. The magnitude of fire effect on soil C:N ratio increased with fire severity in the organic soil layer, and decreased with depth. Results on pyrogenic C content and relationships with soil total C and N contents, and C:N ratio will be presented.  Our results indicate that fire effects on soil C content and C:N ratio are limited primarily to the organic soil layer, and that effects on mineral soil are minimal. These results represent short-term fire effects, and provide a baseline for evaluating longer-term effects due to altered organic matter composition. Understanding the environmental effects of forest fire as a function of fire severity is critical for developing appropriate forest management policies and practices that minimizing detrimental effects and promote long-term resilience of fire-prone forests.