Biochar is a carbon-rich product formed through the pyrolysis of organic matter. A meta-analysis of 371 independent studies from 114 published manuscripts determined that the addition of biochar to soils resulted in increased crop yields, soil microbial biomass, rhizobia nodulation, soil phosphorus (P), soil potassium (K), and total nitrogen (N) and carbon (C) (Biederman and Harpole, 2013). Similarly, a meta-analysis conducted by Jeffery et al. (2011) reported that biochar additions to the soil resulted in an average increase of 10% in crop productivity. Biochar has been proposed as a soil amendment to sequester carbon and improve soil properties and crop yields. However, biochar may increase weed growth in addition to crop productivity. Although weeds are known to reduce crop yields, there is little published research on the effect of biochar on weeds (Major et al., 2005; Quilliam et al., 2012; Smith and Cox, 2014; Soni et al., 2014). Smith and Cox (2014) showed a neutral effect of biochar on parasitic yellow rattle (Rhinanthus minor) abundance. Biochar did not increase the biomass of palmer amaranth (Amaranthus palmeri), sicklepod (Senna obtusifolia), and southern crabgrass (Digitaria ciliaris), although it did reduce germination in palmer amaranth (Soni et al., 2014). Quilliam et al. (2012) reported no effect of biochar on weeds 3 years after incorporation at 25 or 50 t biochar/ha in a sandy clay loam soil. However, weed emergence was reduced when biochar was reapplied. The authors were unable to explain the reduction in weed emergence but suggested that increased soil microbial activity might play a role (Quilliam et al., 2012). Biochar applied alone did not increase weed cover but biochar plus an inorganic fertilizer increased weed cover more than the inorganic fertilizer alone in low fertility, highly acidic soils of the central Brazilian Amazon (Major et al., 2005).
Biederman and Harpole (2013) determined that annual plants (no response was detected for perennial plants) responded to biochar with increased belowground growth but they did not detect an effect of biochar on biomass partitioning. They therefore suggested that, in annuals, biochar increases both above- and belowground growth. However, this conclusion was based on a small number of studies (n = 10) (Biederman and Harpole, 2013), and the effect of biochar on RSA, which is the spatial arrangement or topology of plant roots, has been examined in only a few studies (Abiven et al., 2015; Olmo et al., 2016; Prendergast-Miller et al., 2011). Khan and Shea (2012) concluded that biochar can increase turf root growth in the Jandakot sands of West Australia. We are unaware of any published studies in which the effect of biochar on weed RSA was examined.
The objective of this study was to examine the effects of two types of biochar on root growth and RSA of large crabgrass (D. sanguinalis L. Scop.) using a rhizobox mesocosm. Large crabgrass is a common and problematic annual weed in turfgrass and agricultural settings (Mitich, 1988; Turner et al., 2012). Large crabgrass has fibrous roots, an often prostrate growth habit, and can produce adventitious roots at stem nodes and form thick mats (Mitich, 1988). Crabgrass appears to have relatively high P and K requirements, and reduction in soil P and K availability can significantly reduce large crabgrass growth (Hoveland et al., 1976). We hypothesized that large crabgrass roots would proliferate in soil amended with biochar, particularly in soil amended with the high-nutrient biochar.
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