Activated Carbon as a Restoration Tool: Potential for Control of ...

Activated Carbon as a Restoration Tool: Potential for Control of Invasive Plants in Abandoned Agricultural Fields Andrew Kulmatiski1,2 and Karen H. Beard1 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 advantages by sequestering organic compounds. It is not known, however, if AC can effectively sequester organics 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, Bromus tectorum and Centaurea diffusa, decreased on AC

Introduction Despite a large number of biological, chemical (herbicide), and cultural control methods, exotic plant species continue to expand their ranges (Sheley & Petroff 1999). Furthermore, the unintended consequences of some control 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 management 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.

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 [email protected]

Ó 2006 Society for Ecological Restoration International

JUNE 2006

Restoration Ecology Vol. 14, No. 2, pp. 251–257

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 control 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 negatively 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 exotic plant control tool, especially for B. tectorum and C. diffusa. Key words: allelopathy, Bromus tectorum, Centaurea diffusa, exotic grass, invasive species, native grass, nutrient availability, shrub-steppe restoration.

AC is a nontoxic, highly adsorptive compound that could reduce exotic plant growth through several mechanisms. 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 exudates 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 phytotoxic 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 concentrations 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 indicating that exotic plants benefit from positive plant–soil feedbacks, whereas native or rare plants are susceptible to

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weakly positive or negative feedbacks (Brussaard et al. 2001; Klironomos 2002; van der Stoel et al. 2002; Reynolds et al. 2003; Callaway et al. 2004a, 2004b; Kulmatiski et al. 2004). If the addition of AC reduces microbe populations involved in these feedbacks, then AC would reduce the growth of plants that rely on strong positive plant–soil feedbacks (e.g., exotics). Furthermore, AC additions would also be expected to increase the growth of plants that are susceptible to negative plant–soil feedbacks (e.g., natives). Alternatively, it is possible that AC may sequester those root exudates that inhibit the growth of pathogenic fungi or bacteria (Bais et al. 2005). It is not known whether pathogen defense via root exudates would benefit natives or exotics. Finally, by sequestering organic nitrogen (N) or phosphorus (P), AC may decrease N and P mineralization rates and therefore reduce nutrient availability to plants. It has been suggested that decreased nutrient availability could remove the competitive advantage of exotic plant species that rely on fast growth rates (Zink & Allen 1998; Davis et al. 2000; Lake & Leishman 2004). Therefore, a reduction in nutrient availability may benefit native plant species. However, experiments that reduce N availability directly or that attempt to immobilize soil nutrients by adding reduced forms of carbon (e.g., sawdust) have not consistently reduced exotic plant growth (Morghan & Seastedt 1999; Blumenthal et al. 2003; Lowe et al. 2003; Corbin & D’Antonio 2004). The addition of AC provides a novel approach to reduce N and P availability because unlike C additions, which immobilize soil nutrients by increasing the biomass of C-limited microbes, AC additions sequester organics directly and can therefore be expected to reduce both microbial biomass and N and P availability. Thus, AC addition may reduce the competitive advantage of some exotic species and, therefore, assist in native plant restoration. Our goals were to test this prediction in a site that has been invaded by Diffuse knapweed (Centaurea diffusa Lam.), a species that has been shown to exude the phytotoxin 8-hydroxyquinoline (Vivanco et al. 2004). It was our goal to determine whether the addition of AC to soils dominated by exotic species results in (1) decreased abundance of exotic species; (2) increased abundance of native species; and (3) decreased concentrations of extractable soil organic and inorganic N compounds.

Methods Site Description

Research was conducted in the northern shrub-steppe ecotype of the Methow Valley, Washington (lat 48°379N, long 107°109W). Soils at the study sites are of the Newbon soil series (coarse-loamy, mixed mesic Typic Haploxeroll). Precipitation is seasonal, with 250 of 360 mm of annual precipitation falling from October through March (NOAA 2004). The growing season begins with snowmelt in mid- to late April and continues through June, with

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some deep-rooted plants growing until snowfall in mid- to late November. Experimental Design

Three abandoned agricultural fields were used to determine if AC could reduce the amount of exotic cover in these fields. The three fields were located between 680 and 880 m above sea level, on north- and south-facing slopes, and separated by 5–15 km. Prior to the experiment, the fields had been abandoned from agricultural use and dominated by exotic plants for 4 (Half), 22 (Campbell), or 47 years (Haas). Sites with variable abandonment age and aspect were chosen to allow the greatest inference to the landscape. In 2002, the dominant exotic species in these fields was Centaurea diffusa. In 2003 and 2004, the dominant exotic species was Cheatgrass (Bromus tectorum L.). Other dominant exotics in these fields included Bulbous bluegrass (Poa bulbosa L.), Smooth brome (Bromus inermis Leyss.), White-top (Cardaria draba (L.) Desv.), Sisymbrium loeselii L., and Tumble mustard (Sisymbrium altissimum L.). Between 2 and 50 m from each of the three fields was an adjacent field that had never been used for agriculture. The adjacent fields were dominated by native species, including Bluebunch wheatgrass (Pseudoroegneria spicata Pursh.), Arrowleaf balsamroot (Balsamorhiza sagittata Pursh.), Lupinus spp. (Lupinus arbustus Dougl. ex Lindl., Lupinus latifolius Lindl. ex J.G. Agardh, Lupinus sericeus Pursh; lupine), Big sagebrush (Artemisia tridentata Nutt.), and Bitterbrush (Purshia tridentata Pursh.). These fields served as potential seed sources for native species. Ten 1-m2 plots were placed at 1-m intervals in four randomly located transects in each of the three abandoned agricultural fields (n ¼ 40 plots/field). Transects within a field were parallel and separated by 5–50 m. In the spring 2002, each plot was treated with 30 mL glyphosate herbicide (Roundup; Monsanto, St. Louis, MO, U.S.A.) at a rate of 0.5 kg/ha active ingredient. Standing dead vegetation in the plots was clipped by hand and removed, leaving bare soil. In September 2002, 1 kg of AC (particle size of