Wildfire-Produced Charcoal Directly Influences Nitrogen Cycling in Ponderosa Pine Forests

Fire is the primary form of disturbance in temperate and boreal forest ecosystems. However, our knowledge of the biochemical mechanisms by which fire stimulates forest N cycling is incomplete. Charcoal is a major byproduct of forest fires and is ubiquitous in soils of most forest ecosystems, yet the biological function of charcoal in soils of forest ecosystems has been greatly overlooked. We conducted a suite of laboratory experiments on soils from ponderosa pine (Pinus ponderosa Laws) forests to determine the influence of charcoal on soil N dynamics and in particular, nitrification. The addition of NH4 to forest soils had absolutely no effect on nitrification demonstrating that this process is not substrate limited. The amendment of these soils with NH4 and field collected charcoal (1% w/w) significantly increased the nitrification potential, net nitrification, gross nitrification, and decreased the solution concentrations of plant secondary compounds (phenolics). Charcoal had no effect on nitrification in soils (from a grassland site) that had naturally high rates of nitrifier activity. The increase in gross nitrification in forest soils and lack of effect on grassland soils suggests that charcoal may alleviate factors that otherwise inhibit the activity of the nitrifying microbial community in forest soils. These results reveal the biological importance of charcoal and advance our mechanistic understanding of how fire drives nutrient cycling in temperate and boreal ecosystems. CHARCOAL is one of the most significant byproducts of forest fire and has the potential to influence physical, chemical, and biotic processes in soil (Tyron, 1948; Ishii and Kadoya, 1994; Zackrisson et al., 1996; Pietikäinen et al., 2000; Glaser et al., 2001); however, there is virtually no understanding of the effect of natural fire deposited charcoal on forest productivity or soil biotic processes. Recent studies allude to the potential importance of charcoal in soil ecosystems by demonstrating that soils amended with charcoal and manure centuries ago sustain some of the highest biodiversity and productivity of any soils within the Amazon basin (Glaser et al., 2001, 2002; Mann, 2002). The mechanism by which charcoal enhances this long-term soil productivity is not clear, but may be linked to the base saturation status (Bélanger et al., 2004), exchange potential (Glaser et al., 2002), or surface adsorption potential of organic compounds (Zackrisson et al., 1996). The lack of information regarding charcoal in soils is a noted oversight given the sheer volume of charcoal that accumulates in soils (Schmidt et al., 1999; Skjemstad et al., 2002) and the potential biological importance of charcoal (Zackrisson et al., 1996). Charcoal is generated by the partial combustion of organic materials and is created during most natural fire events. The polycyclic, aromatic structure of charcoal or ‘‘black carbon (C)’’ makes it chemically and biologically stable allowing it to persist in the environment for centuries. Both activated C (industry produced charcoal) and charcoal C effectively function as adsorptive surfaces for organic compounds. This functional aspect of charcoal has resulted in its use in a myriad of applications from water purification (Ivancev-Tumbas et al., 1998) to odor reduction. The majority of ecological literature available regarding charcoal has focused on its use as a paleoecological tool (Clark, 1990) or as a form of recalcitrant C that is sequestered in marine sediments (Massiello and Druffel, 1998; Dickens et al., 2004) and soil organic matter (Schmidt et al., 1999). There are only a limited number of studies that address the biological or biochemical importance of charcoal in ecosystem function. Charcoal may enhance seedling germination (Keeley et al., 1985; Keeley and Pizzorno, 1986), increase N uptake by certain plant species (Wardle et al., 1998), increase nutrient availability (Tyron 1948; Glaser et al., 2001), and enhance humus formation (Glaser et al., 2001). Charcoal or activated C has the capacity to deactivate phytotoxic compounds via adsorption to the char surface (Callaway and Aschehoug, 2000; Nilsson et al., 2000; Yang and Sheng, 2003). Although charcoal additions to soil do not appear to influence total microbial biomass or activity (Tyron, 1948; Wardle et al., 1998; Steiner et al., 2004), there is some indication that charcoal may alter the presence or activity of specific soil microorganisms (Wardle et al., 1998; Pietikäinen et al., 2000). We currently have only very limited understanding of the influence of charcoal on soil processes and specifically those involving N transformations. Natural forest ecosystems are often considered to be N limited (Vitousek and Howarth, 1991) and respond rapidly to the addition of inorganic N (e.g., Tamm, 1991; Mandzak and Moore, 1994). Although little NO3 accumulates in natural forest ecosystems via the autotrophic process in which NH4 is converted to NO3, it is thought to be formed and rapidly immobilized by microorganisms (Stark and Hart, 1997), taken up by plants, or lost to leaching and denitrification (Neary et al., 1999). Fire results in an increase in net mineralization and nitrification in boreal and temperate forest ecosystems (Tamm, 1991; Stark and Hart, 1997; Kaye and Hart, 1998; Neary et al., 1999; DeLuca and Zouhar, 2000; Choromanska and DeLuca, 2001; DeLuca et al., 2002). Surprisingly, those factors that control nitrification and the activity of nitrifying organisms in forest ecosystems have remained T.H. DeLuca, M.D. MacKenzie, and M.J. Gundale, Dep. of Ecosystem and Conservation Sciences, The Univ. of Montana, Missoula, MT 59812; W.E. Holben, Division of Biological Sciences, The Univ. of Montana, Missoula, MT 59812. Received 29 Mar. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:448–453 (2006). Forest, Range & Wildland Soils doi:10.2136/sssaj2005.0096 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 448 Published online February 2, 2006