By fire or by hand, active carbon amendments inhibit flowering of an allelopathic invasive

Abstract Active carbon, whether by fire-deposition or manual remediation of the soil, may mitigate the negative effects of allelopathy by invasive plants on co-occurring native species. However, interactions between carbon and allelopathic exudates rarely consider how carbon interacts with other fire-induced changes to soil chemistry. Wildfire in Southern California’s fire-prone semi-arid habitats is reputed to reduce recruitment of the harmful, allelopathic invasive black mustard ( Brassica nigra ), but not until the second year of post-fire vegetative recovery. While previous research challenges the assumption that wildfire facilitates biological invasions, the multiple effects of wildfires on soil characteristics, such as soil pH, the mobilization of additional nutrients, and carbon deposition on B. nigra recruitment remains unknown. Here I examine the propositions that active carbon rather than changes to soil pH or nutrient availability inhibits reproduction of B. nigra , and that the effect of active carbon on heterospecifics is complicated by other fire-altered soil characteristics. In the field, carbon remediation of the soil inhibited flowering. In the greenhouse, carbon reduced the number of B. nigra inflorescences and suppressed the growth and survival of the native “fire-follower” deerweed ( Acmispon glaber ). However, additional nutrient availability and reduced soil pH appeared to mitigate carbon’s ability to neutralize allelopathy. Nevertheless, supplemental soil carbon, whether by fire or by manual remediation, may reduce the fitness, abundance, and therefore propagule pressure, as well as other negative effects, of B. nigra on native plant communities.