Activated Carbon as a Restoration Tool: Potential for Control of Invasive Plants in Abandoned Agricultural Fields

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1 : 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 advantages by sequestering organic compounds. It is not known, however, if AC can effectively sequester organicsorreduceexoticplantgrowthunderfieldconditions. 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 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. 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 , U.S.A. 2 Address correspondence to A. Kulmatiski, andrew@biology.usu.edu Ó 2006 Society for Ecological Restoration International 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 JUNE 2006 Restoration Ecology Vol. 14, No. 2, pp

2 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 N, long W). 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 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-m 2 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 <0.8 mm; Los Angeles Chemical Company, Los Angeles, CA, U.S.A.) was raked into the top 10 cm of mineral soil in half of the plots. This created a surface soil with an AC concentration of approximately 1% by mass. Control plots were similarly raked, but no AC was added. One of four seed treatments, a seed-free control, an exotic seed mix, a native seed mix, and an additive exotic and native seed mix, was broadcast over each plot immediately following the application of AC. Each seed treatment was placed in 5 of 20 AC plots and 5 of 20 control plots in each field. The five replicate plots for each treatment were distributed randomly among the 40 plots. Seeds were planted at a rate suggested to saturate germination space (Sheley 1997). The exotic seed mix included 3,000 C. diffusa (6 g), 50 B. tectorum (0.2 g), 50 P. bulbosa (0.4 g), and 300 S. loeselii L. (0.1 g) seeds. The native seed mix included 2,600 P. spicata (6 g), 150 Lupinus spp. (2.9 g), 252 Restoration Ecology JUNE 2006

3 600 Stipa comata (Trin. and Rupr.) (needle-and-thread grass; 2.6 g), 160 Annual sunflower (Helianthus annuus (L.); 0.8 g), 45 Desert parsley (Lomatium dissectum (Nutt.); 0.8 g), and 30 B. sagittata (0.2 g) seeds. Native hand-collected seeds were mixed in a 1:1 ratio by mass with seeds harvested by Rainier Seed, Inc. (Davenport, WA, U.S.A.). Germination rates under laboratory conditions were greater than 90% for each species. In June 2003 and 2004, plant growth by species was determined using a point-intersect method with 81 points in each 1-m 2 plot, where the percent ground cover of a species was calculated as the proportion of the number of times a plant species intersected one of the 81 points in the grid. Soil Analyses To determine the impact of AC addition on soil organic and mineral concentrations, soil was analyzed for extractable organic C and N, extractable inorganic N (ammonium [NH 1 4 ]andnitrate[no 2 3 ]), and net N mineralization. At the beginning of the growing season in May 2003, 4-cmdiameter soil cores were collected to a depth of 15 cm from five seed-free plots of each treatment (AC and control) in each field. Samples were used to determine net N mineralization in a 1-month incubation, so half of the cores were placed in thin polyethylene bags and reburied (n ¼ 5per treatment 3 site combination). Net N mineralization was determined as the difference in total inorganic N extracted from preincubation and postincubation cores. All other soil concentrations were determined from preincubation cores. Core subsamples, 10-g dry-weight equivalent, were immediately extracted in 100 ml of 2.0 M KCl and 0.5 M K 2 SO 4 (Robertson et al. 1999). Extractable inorganic N was determined from colorimetric analysis of KCl extracts using a Lachat autoanalyzer (Lachat Chemicals, Mequon, WI, U.S.A.). Extractable organic N was determined by persulfate oxidation and colorimetric analysis of the K 2 SO 4 extracts (Robertson et al. 1999). Extractable organic C in K 2 SO 4 extracts was determined on a Dohrman TOC analyzer (Rosemount Analytical, Inc., Santa Clara, CA, U.S.A.). on nutrient concentrations in the soil was tested using single-factor ANOVAs. ANOVAs were conducted using SAS v.9 for Windows (SAS Institute, Cary, NC, U.S.A.). Significant differences were accepted at p < All values reported are means ± 1SE. We also conducted multivariate analyses of community composition using nonmetric multidimensional scaling (nmds) of species abundance data from each quadrat. nmds is particularly appropriate for data with high beta diversity (e.g., the communities observed in the three fields) but should be used only for descriptive or exploratory purposes (De ath 1999). A Bray Curtis dissimilarity matrix was used in a principle coordinates analysis (PCoA). The solution derived from the PCoA was used to provide a fixed starting point for a subsequent Kruskal nmds (De ath 1999). This approach guarantees that any local minimum in matrix stress derived from nmds will be at least as good as could be obtained using PCoA. This approach also provides a fixed starting point and therefore precludes the need for repeated iterations and subsequent searches for a global minimum in final stress values. All multivariate analyses were performed in the R programming language using isomds in the MASS library (R Research Core Team 2004). Results Native Plant Responses to AC After the first growing season, there was no difference in native plant cover between AC (9.4 ± 0.9%) and control (11.0 ± 1.7%; F [1,14] ¼ 4.30, p ¼ 0.057) plots, but after the second growing season, there was more native plant cover in AC (13.7 ± 1.7%) than in control (6.1 ± 1.0; F [1,14] ¼ 11.76, p ¼ ) plots. The growth of native grasses helped to explain this response. Native grasses grew better in AC plots than in control plots after the first (F [1,14] ¼ 14.81, p ¼ ) and second (Figs. 1 & 2; F [1,14] ¼ 4.31, p ¼ 0.057) growing seasons. Statistical Analyses The effects of treatment (AC or control) and seeding (none, exotic, native, or mixed) were tested on the percent cover of exotics, natives, annuals, perennials, native grasses, and the five most abundant exotic and native species using analysis of variance (ANOVA). We employed a two-factor (treatment and seeding), completely randomized, blocked (site) design model with subsamples (replicate plots within sites). Site, and site 3 treatment 3 seeding were treated as random effects. A Tukey adjustment was made to control for experiment-wise type I error within each ANOVA. Data from each year of the study were tested independently. Values of percent cover were arcsine square-root transformed to meet assumptions of normality and homogeneity of variance. The effect of AC Figure 1. Mean percent cover (±SE) of native grasses in all plots in the Methow Valley, Washington (n ¼ 60). An asterisk indicates a significant difference in treatment means at the 0.05 level. JUNE 2006 Restoration Ecology 253

4 Figure 2. Photograph of native bunchgrasses growing in a m plot treated with activated carbon and native plant seed; June 2005, Methow Valley, Washington, U.S.A. The vegetation surrounding the plot (yellow lines) is dominated by the exotics Poa bulbosa, Sisymbrium loeselli, and Centaurea diffusa. Two native grasses, Pseudoroegneria spicata and Stipa comata, comprised 96 and 92% of the native grasses observed after the first and second growing seasons, respectively. Pseudoroegneria spicata cover did not differ between treatments after the first growing season (F [1,14] ¼ 3.30, p ¼ 0.090), but it had greater cover in AC plots than in control plots after the second growing season (Fig. 3; F [1,14] ¼ 6.23, p ¼ 0.026). Stipa comata had greater cover in AC plots than in control plots after the first growing season (F [1,14] ¼ 6.26, p ¼ 0.025) but not after the second growing season (Fig. 3; F [1,14] ¼ 1.67, p ¼ 0.22). Many plots demonstrated no native grass growth, but 10 of the 60 AC plots demonstrated more than 10% native grass cover and one plot demonstrated 22% native grass cover. In contrast, only two control plots demonstrated 10% or greater grass cover (10 and 11%). Seeding improved the growth of native grasses during the first year (F [3,14] ¼ 5.25, p ¼ ) but not during the second year (F [3,14] ¼ 2.15, p ¼ 0.14). After the first year, in control plots, native grasses covered 0.5% of the ground in unseeded plots, 0.1% of the ground in exoticseeded plots, and 2.8% of the ground in native-seeded plots. Seeding treatments did not affect the growth of any other species or functional groups, nor did seeding interact with AC treatment effects for any species or functional groups. The other three most abundant natives in all experimental plots were annual forbs, Madia citriodora Greene, Epilobium minutum Lindl. ex Lehm., and Collinsia parviflora Lindl. Madia citriodora was less abundant in AC plots than in control plots in 2003 but did not differ in abundance between AC and control plots in 2004 (Fig. 3). Epilobium minutum was more abundant in AC (2.4 ± 1.0%) than in control (0.4 ± 0.3%) plots in 2004 (F [1,14] ¼ 8.72, p ¼ 0.011), and C. parviflora was less abundant in AC (0.1 ± 0.1%) than in control (1.5 ± 0.5%) plots in 2004 (F [1,14] ¼ 10.84, p ¼ ). Exotic Plant Responses to AC After the first growing season, there was less total exotic cover in AC plots than in control plots (F [1,14] ¼ 6.44, p ¼ 0.024; 25.5 ± 2.4% for AC plots and 33.6 ± 2.8% for Figure 3. Relationships between average percent cover (%) in control and in AC-treated plots at the end of (A) the first and (B) the second growing seasons for the 10 most abundant species in both years. Species plotted above the 1:1 line grew better in AC plots than in control plots. AC, activated carbon; BRTE, Bromus tectorum; CADR, Cardaria draba; CEDI, Centaurea diffusa; CHTE, Chorispora tenella; LASE, Lactuca serriola; MACI, Madia citriodora; POBU, Poa bulbosa; PSSP, Pseudoroegneria spicata; SISP, Sisymbrium spp., STCO, Stipa comata. An asterisk indicates that the abundance of the marked species was significantly different between control and AC plots (p < 0.05). 254 Restoration Ecology JUNE 2006

5 control plots), but after the second growing season, there was no difference in exotic cover between AC and control plots (F [1,14] ¼ 0.82, p ¼ 0.38; 36.4 ± 2.9% for AC plots and 32.0 ± 2.9% for control plots). The cover of individual exotic species differed between treatments after both growing seasons. Of the 10 most abundant species observed in all plots, 7 were exotic (Fig. 3). Two of these species demonstrated more cover on control plots than on AC plots in at least one of the 2 years (Fig. 3). Three of these species demonstrated more cover on AC plots in at least one of the 2 years (Fig. 3). The remaining two exotic species showed no significant response to AC treatments. The largest responses were a negative response to AC by Bromus tectorum in 2004 (Fig. 3; F [1,14] ¼ 2.33, p ¼ 0.14 and F [1,14] ¼ 4.56, p ¼ in 2003 and 2004, respectively), and a positive response to AC by Lactuca serriola L. (Fig. 3; F [1,14] ¼ 8.19, p ¼ and F [1,14] ¼ 16.61, p ¼ in 2003 and 2004, respectively). Annual and Perennial Responses to AC Total annual cover did not differ between AC and control plots after the first growing season (F [1,14] ¼ 3.72, p ¼ 0.074; 23.5 ± 2.7% for AC plots and 35.0 ± 3.0% for control plots) but showed a trend toward being greater on AC plots than on control plots after the second growing season (F [1,14] ¼ 4.44, p ¼ 0.053; 34.7 ± 3.3% for AC plots and 26.9 ± 2.9% for control plots). Total perennial cover did not differ between AC and control plots after the first (F [1,14] ¼ 0.19, p ¼ 0.67; 18.9 ± 1.7% for AC plots and 17.8 ± 1.8% for control plots) or the second (F [1,14] ¼ 2.37, p ¼ 0.15; 24.1 ± 1.9% for AC plots and 18.8 ± 1.7% for control plots) growing season. Community Responses to AC Total plant growth was lower in AC (31.9 ± 2.6%) than in control (41.1 ± 2.2%) plots after the first growing season (F [1,14] ¼ 5.01, p ¼ 0.042) but greater on AC (50.1 ± 2.8%) than on control (38.2 ± 2.8%) plots after the second growing season (F [1,14] ¼ 11.97, p ¼ ). These analyses are descriptive in nature and were not used to test for community-level differences among fields or between treatments. A plot of nmds values from axes 1 and 2 revealed that plant community composition varied by treatment and field (Fig. 4). Differences in community composition among fields and between AC and control plots were largely described by scores from nmds axis 1. The distribution of data along axis 1 suggested that community composition in Campbell and Haas fields responded similarly to AC addition and that the difference in community composition between AC and control plots was nearly as great as the difference in community composition among fields (e.g., the distance between AC and control points within a field was similar to the distance between control points among fields). Furthermore, although many AC plots demonstrated no native grass growth, 10 of the 60 Figure 4. Nonmetric multidimensional scaling (nmds) ordination graph of species composition in control and activated carbon plots in three fields in the Methow Valley, Washington, U.S.A., Each point represents the composition of vegetation in a single 1-m 2 plot. For clarity, only 12 randomly selected plots of the 20 plots in each site-treatment combination are shown. AC plots demonstrated native grass cover of at least 10%. This response is greater than that has been observed for any other management approaches tested at these sites (e.g., tillage, herbicide, fungicide, or shading). Soil Responses to AC Concentrations of extractable organic C and N were greatly reduced in plots treated with AC relative to control plots at the end of the first growing season (Table 1; F [1,24] ¼ 4.95, p ¼ 0.036; F [1,19] ¼ 5.17, p ¼ 0.035). Concentrations of extractable NO 2 3, however, were greater in AC plots than in control plots at the end of the first growing season (F [1,24] ¼ 5.60, p ¼ 0.033). No other differences in nutrient concentrations or cycling rates were detected between treatments. Discussion Plant Responses On soils that had been dominated by exotic species and resisted native plant growth for up to 47 years, AC additions decreased the cover of two dominant exotic Table 1. Soil chemical properties (±SE) in control and AC plots in three fields, Methow Valley, Washington in 2003 (n ¼ 15). Treatment Nutrient (Units) Control * AC Extractable organic 193 ± 21a 142 ± 12b C (mg/kg) Extractable organic 27.1 ± 6.7a 13.8 ± 1.8b N (mg/kg) NH 1 4 -N (mg/g) 0.40 ± 0.11a 0.33 ± 0.030a NO 2 3 -N (mg/g) 0.12 ± 0.037a 0.35 ± 0.097b Net N mineralization 37 ± 7.3a 25 ± 3.4a (mg m 22 day 21 ) Net nitrification (mg m 22 day 21 ) 34 ± 3.9a 54 ± 12a * Mean values followed by the same lowercase letters are not significantly different when comparing between treatments (p < 0.05). AC, activated carbon. JUNE 2006 Restoration Ecology 255

6 species, increased the cover of dominant native grass species, and induced consistent changes in plant community composition. More specifically, the addition of AC reduced cover of the exotic, Centaurea diffusa, from 6 to 0.1% in the first growing season and reduced cover of the exotic, Bromus tectorum, from 14 to 8% by the end of the second growing season. These results suggest that even under field conditions, the growth of C. diffusa can be reduced through the addition of AC (Callaway & Aschehoug 2000; Ridenour & Callaway 2001). The addition of AC also improved the growth of native species, in particular the growth of native grasses, Pseudoroegneria spicata and Stipa comata, especially when plots were seeded with natives. Although the response of these native grasses was relatively small, from 1 to 3% ground cover, we believed it to be important for two reasons. First, native bunchgrasses are long-lived perennials that can be expected to require more time to attain large ground cover than the time required by shorter-lived (exotic) species. Second, native bunchgrasses are rare in exotic-dominated fields, but where they are found, their growth is often vigorous, suggesting that, once established, they can be successful. For both these reasons, we expect that those grasses that have established in AC-treated plots will realize vigorous growth in the coming years. AC addition, however, did not reduce total exotic cover because some exotics (e.g., Cardaria draba and Lactuca serriola) showed positive responses to AC addition. These results underscore the complex nature of plant invasions. It is unlikely that any single management tool will be completely effective in restoring native plant communities because exotic and native species use many different mechanisms to succeed. However, a management tool that alters plant community composition can be important if it can be used to force successional change in exotic communities (D Antonio & Meyerson 2002). This is especially true where exotic communities are persistent (Stylinski & Allen 1999; Kulmatiski in press). For example, if native plants at this site are better competitors against L. serriola than B. tectorum or C. diffusa, then AC additions could be expected to continue to improve native plant growth over time. Thus, despite the persistence of some exotic species in AC-treated soils, this treatment holds potential as a restoration tool. Soil Responses Under laboratory conditions, AC has been found to have a weak affinity for inorganic nutrients in the form of electrolytes and a strong affinity for organically bound nutrients (Cheremisinoff & Ellerbusch 1978). As expected, under field conditions, extractable organic C and N concentrations were lower in AC plots relative to control plots, but extractable inorganic N concentrations were not lower in AC plots relative to control plots. Nitrate concentrations, however, were greater in AC plots relative to control plots. It appears likely that by sequestering organics, AC addition reduced heterotrophic microbial activity. Reduced heterotrophic microbial activity could explain decreases in nitrate immobilization and therefore increases in nitrate concentrations. Plant cover attheendofthefirstgrowingseasonwaslowerinacplots than in control plots, despite high nitrate concentrations. This suggests that effects of AC addition, other than increased nitrate availability, determined plant responses to AC. Though we did not test it directly, the reduction in C. diffusa cover in AC addition plots in 2003 provided circumstantial support for the hypothesis that allelopathy provides this species with a competitive advantage (Vivanco et al. 2004). Regardless of the exact mechanism, the decline in C. diffusa on AC plots is important because for decades, this species has been a dominant invasive in the study area. The dramatic decline in this species observed in both control and AC-treated plots in 2004 appeared to be the result of the introduction of the biocontrol agent Larinus minutus (Seastedt et al. 2003). Casual observation of experimental plots in the fall 2004 and spring 2005, however, suggests that L. minutus has disappeared from the study sites and that C. diffusa is regaining abundance in control plots but not in AC plots. Continued observations would be needed to determine the impact of both AC and the biocontrol agent on C. diffusa. Management Applications Further research on signaling between plants and soil microbes may reveal species-specific communication pathways that could be manipulated to enhance the growth of native plants. Even without species-specific approaches, it may be possible to improve native plant community restoration through more targeted uses of AC applications. For example, AC application may be more effective if positioned at greater soil depths, at greater concentrations near the surface, or at different times during the year. These application strategies should also be tested in combination with alternative restoration tools, such as N immobilization, tillage, burning, herbicide use, seeding, or grazing, to determine the most effective restoration strategy. We are currently working with local land managers and a commercial restoration company to expand testing of AC as a restoration tool. We suggest that AC additions increase total soil C concentrations by 20% (e.g., from 5 to 6 g C kg 21 of soil). If estimates are available for extractable carbon concentrations, we suggest AC additions of 5 g for every gram of extractable soil carbon in the top 10 cm of soil. In this study site, native grasses were most responsive to AC additions. We, therefore, suggest that AC be added in the form of an AC / native grass seed / water slurry early in the growing season. This slurry should be chained or disked into surface soils (10 15 cm deep) and possibly followed by a compaction treatment to maintain moisture, limit dust, and inhibit exotic seed establishment. This treatment would cost roughly $10,000 per hectare in food-grade AC. Bulk purchasing of lower quality AC, re-ac, or non-ac (charcoal) could be expected to lower this cost to between $2,000 and 5,000 per hectare. These costs are likely to be prohibitively 256 Restoration Ecology JUNE 2006

7 expensive for large-scale applications but should be reasonable for smaller areas of high concern especially where C. diffusa or B. tectorum are problem species. Acknowledgments This research was funded by USDA NRICGP Biology of Weedy and Invasive Plants (# ), the Utah State Agricultural Experimental Station, and the Switzer Foundation. We thank Jim Mountjoy, Kim Romain, and the Washington Department of Fish and Wildlife, for use of their land; Paul Grossl, for use of the TOC analyzer; and John Stark, for use of the Lachat Autoanalyzer. This research was supported by the Utah Agricultural Experiment Station, Utah State University, Logan, Utah. Approved as journal paper no LITERATURE CITED Bais, H. P., S. W. Park, T. L. Weir, R. M. Callaway, and J. M. Vivanco How plants communicate using the underground information superhighway. Trends in Plant Science 9: Bais, H. P., B. Prithiviraj, A. K. Jha, F. M. Ausubel, and J. M. 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