Thermal alteration of water extractable organic matter in climosequence soils from the Sierra Nevada, California

Abstract

In the next decades, the influence of wildfires in controlling the cycling and composition of soil organic matter (SOM) globally and in the western U.S. is expected to grow. While the impact of fires on bulk SOM has been extensively studied, the extent at which heating of soil affects the soluble component of SOM remains unclear. Here we investigated the thermal transformations of water‐extractable organic matter (WEOM) by examining the changes in the distribution of carbon (C) functional groups in WEOM from soils heated at low and intermediate temperatures. WEOM (<0.7 µm particle size) was extracted from topsoils (0–5 cm depth) of five soil series formed from a nonglaciated granitic bedrock and sampled along a Sierra Nevada climosequence. Soils were heated in a muffle furnace at 150°C, 250°C, and 350°C for 1 h. The extracted solution was analyzed for WEOM aromaticity, mean molecular weight, organic C (OC) concentration, and major structural components by employing optical spectrophotometry and liquid‐state 1H‐NMR spectroscopy. At 150°C and 250°C, OC concentrations increased relative to the thermally unaltered samples, with losses of oxygenated functional C groups and enrichment of aliphatic C structures observed at 250°C. Conversely, OC concentration and mean molecular weight decreased as heating increased from 250°C to 350°C, whereas WEOC became more enriched in aromatic C structures. Our results suggest that low and intermediate fire intensities significantly alter the nature of dissolved organic matter exported from soils to rivers in the Sierra Nevada and beyond.

Fire severity, time since fire, and site-level characteristics influence streamwater chemistry at baseflow conditions in catchments of the Sierra Nevada, California, USA

“…Spectra were processed using MNova (Mestrelab Research, Santiago de Compostela, Spain). For each 1 H-NMR spectrum, we integrated the total signal within each of the five regions (Hertkorn et al 2013, Santos et al 2016, as shown in Fig. 2: aliphatic ( δ H~0.0 to 1.9 ppm), functionalized aliphatic ( δ H~1.9 to 3.1 ppm), oxygenated ( δ H~3.1 to 4.3 ppm), unsaturated ( δ H~5.3 to 7.0 ppm), and aromatic functional groups ( δ H~7.0 to 10 ppm).…”

“…Fires can alter the cycling, concentration, and chemical composition of OM in soil (Santín et al 2015, Araya et al 2016, Santos et al 2016, Araya et al 2017 and aquatic ecosystems (Jaffé et al 2013, Masiello and. The thermal degradation of soil organic C and biomass produces a heterogeneous mixture of organic residues that are collectively referred to as pyrogenic (fire-derived) C (PyC).…”

“…We hypothesized that nutrient, cation, and anion concentrations would be positively correlated with high-severity fire (>1 yr following wildfire), whereas DOC concentration would be negatively correlated with high-severity fires. We also expected to see an increase in aromaticity and a decrease in molecular weight (MW) of DOM corresponding with high-severity fire, as an increase in fire severity would lead to higher proportions of PyC (Knicker 2007) and low molecular weight DOM (Norwood et al 2013, Santos et al 2016 transported from soils to streams. Additionally, we hypothesized that streamwater solute concentrations would decrease with an increase in time since fire due to ecosystem recovery; whereas aromaticity would increase with time since fire as soil PyC would become progressively more oxidized and hydrophilic, therefore highly mobile, over time (Knicker 2007, Abiven et al 2011, Näthe et al 2017.…”

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

“…The OM that is left behind may be more complex biomolecules, such as lignin that are more difficult to breakdown, or possibly more susceptible to leaching. Santos et al (2016) previously showed that thermal alterations of topsoil can include leaching of dissolved PyC from soil, in particular after low and medium severity fire temperature regimes. It is possible though, in particular in soils with high clay content, the leaching process can be relatively slow.…”

Thermal alteration of soil organic matter properties: a systematic study to infer response of Sierra Nevada climosequence soils to forest fires

“…The degree of alteration caused by fires depends on the fire intensity and duration, which in turn depend on factors such as the amount and type of fuels, properties of aboveground biomass, air temperature and humidity, wind, topography, and soil properties such as moisture content, texture, and SOM content (DeBano et al, 1998). The first-order effects of fire on soil are caused by the input of heat, causing extreme soil temperatures in topsoil (Badía and Martí, 2003b;Neary et al, 1999), which results in loss and transformation of SOM, changes in soil hydrophobicity, changes in soil aggregation, loss of soil mass, and addition of charred material and other combustion products (Albalasmeh et al, 2013;Araya et al, 2016;Mataix-Solera et al, 2011;Rein et al, 2008;Santos et al, 2016).…”