Invasive perennial forb effects on gross soil nitrogen cycling and nitrous oxide fluxes depend on phenology

Abstract

Invasive plants can increase soil nitrogen (N) pools and accelerate soil N cycling rates, but their effect on gross N cycling and nitrous oxide (N2O) emissions has rarely been studied. We hypothesized that perennial pepperweed (Lepidium latifolium) invasion would increase rates of N cycling and gaseous N loss, thereby depleting ecosystem N and causing a negative feedback on invasion. We measured a suite of gross N cycling rates and net N2O fluxes in invaded and uninvaded areas of an annual grassland in the Sacramento‐San Joaquin River Delta region of northern California. During the growing season, pepperweed‐invaded soils had lower microbial biomass N, gross N mineralization, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification‐derived net N2O fluxes (P < 0.02 for all). During pepperweed dormancy, gross N mineralization, DNRA, and denitrification‐derived net N2O fluxes were stimulated in pepperweed‐invaded plots, presumably by N‐rich litter inputs and decreased competition between microbes and plants for N (P < 0.04 for all). Soil organic carbon and total N concentrations, which reflect pepperweed effects integrated over longer time scales, were lower in pepperweed‐invaded soils (P < 0.001 and P = 0.04, respectively). Overall, pepperweed invasion had a net negative effect on ecosystem N status, depleting soil total N to potentially cause a negative feedback to invasion in the long term.

Soil microbial communities are not altered by garlic mustard in recently invaded central Illinois forests

“…Because invasive plants are often less susceptible to native pathogens than are native plants (Agrawal et al 2005;Engelkes et al 2008;Klironomos 2002;Mitchell and Power 2003), this "enemy release" can provide another relative fitness benefit to invasive plants over natives (Kardol et al 2007;Keane and Crawley 2002). Invasive plant effects on soil microbial communities and their functions can vary with invasion age (Lankau 2011), plant phenology (Portier et al 2019), and soil properties (Li et al 2006). Therefore, it is important to investigate these plant-microbe interactions in different environmental contexts to avoid biases in the documented patterns (Hulme et al 2013).…”

“…Garlic mustard studies often focus on plant-microbe interactions during the active life stage of garlic mustard earlier in the growing season (e.g., Anthony et al 2017;Burke et al 2019). However, invasive plant effects on soil microbial communities (Arthur et al 2012) and microbial processes (Portier et al 2019) can vary with plant phenology. As garlic mustard exhibits an early phenology relative to native competitors (Engelhardt and Anderson 2011), its effects on soil microbial communities may depend on its own life stages or broader temporal changes associated with surrounding plants and ecosystem conditions (Bennett et al 2013;Saikkonen 2007).…”

“…However, garlic mustard’s influence on microbial communities changes with invasion age because garlic mustard populations produce less allelochemicals over time (Lankau et al 2009), thereby weakening its influence on soil microbes (Lankau et al 2010). Furthermore, invasive plant effects on soil microbial communities (Arthur et al 2012) and microbial processes (Portier et al 2019) can also vary with plant phenology. Much of our understanding of garlic mustard impact of microbial communities is derived from studies on persistent invasions with high plant densities, mostly located in the north-eastern United States (Stinson et al 2006, Wolfe et al 2008, Anthony et al 2017, Anthony et al 2019, Burke et al 2019a) and based on observations during garlic mustard’s active life period.…”

“…Invasive plant effects on soil microbial communities and their functions can vary with invasion age (Lankau 2011), plant phenology (Portier et al 2019), and local soil properties (Li et al 2006). After initial invasion, garlic mustard population densities increase rapidly then become relatively stable (Anderson et al 1996, Nuzzo 1999).…”

“…Many studies on microbial aspects of garlic mustard invasion also focus on early growing season, active life cycle plants, as these are most likely to have the greatest allelochemical production (Smith and Reynolds 2015). However, post-senescent or inactive invasive plants may still have significant impacts on microbial communities and processes (Arthur et al 2012, Portier et al 2019). Garlic mustard exhibits an early phenology relative to native competitors (Engelhardt and Anderson 2011), and so its effects on soil microbial communities may depend on its life stages or broader temporal changes associated with surrounding plants and ecosystem conditions (Saikkonen 2007, Bennett et al 2013).…”