Activated Carbon as a Restoration Tool: Potential for Control of Invasive Plants in Abandoned Agricultural Fields Andrew Kulmatiski 1,2 and Karen H. Beard 1 Abstract Exotic plants have been found to use allelochemicals, positive plant–soil feedbacks, and high concentrations of soil nutrients to exercise a competitive advantage over native plants. Under laboratory conditions, activated carbon (AC) has shown the potential to reduce these ad- vantages by sequestering organic compounds. It is not known, however, if AC can effectively sequester organ- ics or reduce exotic plant growth under field conditions. On soils dominated by exotic plants, we found that AC additions (1% AC by mass in the top 10 cm of soil) reduced concentrations of extractable organic C and N and induced consistent changes in plant community composition. The cover of two dominant exotics, Bro- mus tectorum and Centaurea diffusa, decreased on AC plots compared to that on control plots (14–8% and 4– 0.1%, respectively), and the cover of native perennial grasses increased on AC plots compared to that on con- trol plots (1.4–3% cover). Despite promising responses to AC by these species, some exotic species responded positively to AC and some native species responded neg- atively to AC. Consequently, AC addition did not result in native plant communities similar to uninvaded sites, but AC did demonstrate potential as a soil-based ex- otic plant control tool, especially for B. tectorum and C. diffusa. Key words: allelopathy, Bromus tectorum, Centaurea dif- fusa, exotic grass, invasive species, native grass, nutrient availability, shrub-steppe restoration. Introduction Despite a large number of biological, chemical (herbi- cide), and cultural control methods, exotic plant species continue to expand their ranges (Sheley & Petroff 1999). Furthermore, the unintended consequences of some con- trol methods may be more costly than inaction (Pearson & Callaway 2003; Thelen et al. 2005). Thus, there is a need to discover control methods that are effective in reducing growth of invaders and improving the growth of natives, with few adverse nontarget effects. Recent research on novel weapons (Callaway & Aschehoug 2000; Bais et al. 2003; Vivanco et al. 2004), positive plant–soil feedbacks (Klironomos 2002), and competitive interactions (Davis et al. 2000; Booth et al. 2003) has highlighted the potential role of plant–soil interactions in the invasion process; yet, relatively few control methods take advantage of these relationships by manipulating the soil environment. There remains, therefore, a large potential for soil-based man- agement in the restoration of native plants to invaded communities. The addition of activated carbon (AC) to soils provides one example of a soil manipulation that may reduce exotic growth. AC is a nontoxic, highly adsorptive compound that could reduce exotic plant growth through several mecha- nisms. First, AC adsorbs phytotoxic root exudates (Inderjit & Callaway 2003). Although AC indiscriminately binds organics, there is a reason to expect that this would benefit native species more than exotics. Common native species are likely to have evolved resistance to root exu- dates of plants from the same region but are likely to be naive to root exudates of plants from other parts of the world (Bais et al. 2003; Vivanco et al. 2004). Furthermore, it is unlikely that the removal of phytotoxic root exudates released by natives would improve exotic growth because exotic species that are susceptible to allelopathy by natives are unlikely to be successful invaders. Under greenhouse conditions, AC has successfully removed phy- totoxic root exudates (Mahall & Callaway 1992; Callaway & Aschehoug 2000) and, consequently, the competitive advantage of exotics species (Callaway & Aschehoug 2000; Ridenour & Callaway 2001). However, under field conditions, it is not known if AC can adsorb sufficient quantities of phytotoxins to prevent allelopathy or even if allelopathy is important. AC also may reduce microbial activity by reducing con- centrations of organic molecules that are either used as substrate by microbes or used as signals to encourage their growth (Bever 2003; Bais et al. 2004; Duffy et al. 2004; Gage 2004). There is a growing number of studies indicat- ing that exotic plants benefit from positive plant–soil feed- backs, whereas native or rare plants are susceptible to 1 Department of Forest, Range, and Wildlife Sciences and the Ecology Center, Utah State University, Logan, UT 84322-5230, U.S.A. 2 Address correspondence to A. Kulmatiski, email andrew@biology.usu.edu Ó 2006 Society for Ecological Restoration International JUNE 2006 Restoration Ecology Vol. 14, No. 2, pp. 251–257 251