Prescribed Fire Alters Dissolved Organic Matter and Disinfection By-Product Precursors in Forested Watersheds - Part I. A Controlled Laboratory Study

Abstract

Detritus material in forested watersheds is the major terrestrial source of dissolved organic matter (DOM) and disinfection by-product (DBP) precursors in source waters. Forest fire reduces the thickness of detritus layer and changes foliar litters into pyrogenic organic matter (PyOM) on the forest floor, resulting in different quantity and quality of DOM exported from forested watersheds. Many studies have examined DBP precursors exported from forested watersheds; however, DOM leaching from PyOM, or dissolved black carbon (BC), could have different reactivity in DBP formation compared to the DOM leaching from unburned detritus layer. Using controlled laboratory burning in this study, characteristics of foliar litters before and after burn were compared. Quality and quantity of water extractable organic matter (WEOM) from raw and burned foliar litters commonly found in the southeastern United States, including baldcypress ( Taxodium distichum ), boxelder ( Acer negundo ), longleaf pine ( Pinus palustris ), pop ash ( Fraxinus caroliniana ), sweetgum ( Liquidambar styraciflua ), and water tupelo ( Nyssa aquatica ) was compared, and evaluated for their disinfection by-product yield and specific DBP formation using a uniform formation condition test. Laboratory analysis using pyrolysis gas chromatography mass spectrometry showed that foliar litters after burn contain significantly less lignins, nitrogenous compounds, and polysaccharides because of loss of organic matter. However, both the formula numbers and relative abundance of PAHs increased, which can be a potential health concern. Water extractable organic carbon (WEOC) and total nitrogen (WETN) decreased after burn and in particular raw material had less humic acid and fulvic acid-like compounds. PyOM aromaticity may be either enhanced or lowered by prescribed fire and highly depending upon the plant species. The specific formation of chloroform (a prevalent DBP species) decreased by 12%-60% for all plant species after burn. However, dissolved BC was more reactive in the formation of nitrogenous disinfection by-products (N-DBPs), and the specific haloacetonitriles formation increased after burn for all species.