Wildfire severity: Environmental effects revealed by soil magnetic properties

Abstract

Strong wildfires pose significant damage to all soil qualities and lead to land degradation. The complex nature and properties of fire‐derived materials require multidisciplinary efforts for their reliable characterization. The main objective of our study was to evaluate the suitability of magnetic properties of fire‐affected soils as proxy parameters for wildfire severity and to relate magnetic signature of burnt soils to carbon and nitrogen contents as influenced by wildfires. We present mineral magnetic investigation of 22 sites with wildfire‐affected soils and 17 nonburnt soils from nearby locations. We employed measurements of magnetic susceptibility and anhysteretic remanence in combination with scanning and transmission electron microscopy observations on magnetic particles from burnt soils and ashes. Bulk soil and vegetation ash analyses of total carbon and nitrogen, organic carbon, and elemental content were carried out as well. We show that pyrogenic magnetic enhancement is restricted to the uppermost 0‐ to 2‐cm soil depth and can be used as a proxy for wildfire severity. Strong wildfires lead to the production of nanometer‐sized superparamagnetic magnetite and/or maghemite particles and smaller amount of single‐domain fraction. These strongly magnetic minerals have typical characteristics of high‐temperature combustion products with spherical shape and diameters between 0.1 and 2 μm. Fire‐affected soils show relative enrichment with phosphorous, manganese, and heavy metals (Cu, Pb, Zn, Ni, Co, and As) calculated with respect to soils from nonburnt nearby localities. Our results demonstrate the potential of environmental magnetic methods as an additional tool for assessment of wildfire severity and the content of main soil nutrients.

Magnetic Properties of Plant Ashes and Their Influence on Magnetic Signatures of Fire in Soils

“…It follows that other mass-specific magnetic properties, such as susceptibility, that appear to increase in soils following wildfire events may also dominantly reflect a reduction in organic matter mass instead of pyrogenic mineral formation or transformation. Magnetic enhancement in fire-affected soils may therefore be a reasonable proxy for organic matter loss due to burning, which supports the suggestion of Jordanova et al (2019a) that fire intensity is directly correlated with the resulting degree of magnetic enhancement. Our results further indicate that elevated mass-specific magnetic parameters following high-intensity burning, such as forest fires, primarily occur because mass loss associated with organic matter combustion serves to increase the concentrations of inorganic magnetic particles incorporated into plant material and secondarily occur due to thermal transformation of weakly magnetic inorganic phases to strongly magnetic phases (see Section 4.3 below).…”

“…If present on plant surfaces, these phases will easily alter on heating and can transform rapidly to magnetite and maghemite, especially in the reducing environment created by combusting organic matter (Till et al, 2017;Till and Nowaczyk, 2018). For weakly magnetic ferric phases such as goethite and hematite in soils affected by low-intensity fires, the modest rise in temperature, which is restricted to the uppermost few cm, is likely insufficient to produce thermal alteration of these minerals (Roman et al, 2013;Jordanova et al, 2019a). However, high-intensity fires can heat soil down to depths of at least 10 cm to temperatures high enough to trigger reductive alteration of goethite and hematite to magnetite or maghemite Clement et al (2011);Nørnberg et al (2009); Ketterings et al (2000).…”

“…Wildfires and anthropogenic burning affect many aspects of soils, including their magnetic properties. Wildfires have long been suggested as a possible source of topsoil magnetic enhancement (e.g., Le Borgne, 1960;Kletetschka and Banerjee, 1995) and many examples of increased magnetic susceptibility following fires have been observed (Clement et al, 2011;Jordanova et al, 2019a). Because magnetic enhancement in soils relative to unweathered parent material often correlates closely with various environmental factors (Maher et al, 2003), understanding the impact of fires on soil magnetism is important for accurate interpretation of magnetic paleoenvironmental proxies.…”

Identification and Classification of Archeological Materials From Bronze Age Gold Mining Site Ada Tepe (Bulgaria) Using Rock Magnetism

“…Grain‐size sensitive magnetic parameters (Figure 7) show that samples from this cluster contain moderate amount of SP fraction ( χ fd % ∼ 8%; Figure 7a), as well as remanence‐carrying SD fraction (Figures 7c and 7d). Except as a common product of pedogenic development, strongly magnetic fraction consisting of SD and SP magnetite‐like grains is also characteristic of soils/sediments which experienced burning (Jordanova et al., 2019; Longworth et al., 1979; Vendelboe et al., 2005). On the other hand, natural soils usually show much lower natural pedogenic magnetic enhancement (Jordanova, 2016).…”

“…This observation, along with the highest range of the IRM 2T /χ ratio (Figure 7d) shows that the stable remanence-carrying fraction dominates in these materials, while the superparamagnetic component is less significant. Such situation is often observed in fired soils, where stable SD fraction increases as a relative contribution in the bulk magnetic mineralogy (Jordanova et al, 2019). Therefore, we attribute cluster #1 to fired soils (archeological structures, burned soils, etc.…”