No abstract

Abstract. Success of invasive annual grasses is often linked to increases in resources, and restoration ecologists have suggested that decreasing nitrogen (N) availability and restoring more conservative N cycles with lower N turnover should decrease the competitive advantage of these invaders and facilitate establishment of native perennials. We developed a multivariate conceptual model of the likely effects of environmental factors (precipitation and temperature) and potential management treatments (repeated burning, litter manipulation, and seeding an annual competitor) on soil N availability and success of cheatgrass, the most widespread invader in the western United States. We examined effects of the treatments in a five year field study in a cold desert shrubland. We used generalized linear mixed-effects models to examine differences among treatments over time, and structural equation modeling (SEM) to evaluate component hypotheses of the conceptual model. We hypothesized that burning would result in a pulse in soil available N due to heat-induced soil organic matter denaturation, but that subsequent fires would volatilize soil and biomass N resulting in progressive N deficiency. However, soil available N in burned plots was never lower than pre-burn levels, likely because burn temperatures were too cool to volatilize N from soils or aboveground biomass. Repeated burning decreased litter biomass, but there were generally no differences in soil available N between litter intact and litter removed plots. Litter removal had a negative effect on cheatgrass success likely due to effects on seed entrapment and plant establishment rather than N mineralization. Plots seeded with wheat generally had the lowest cheatgrass success, but available N was similar to other treatments, and decreased cheatgrass success was likely due to competition for other soil nutrients or water and reduced litter. In this semi-arid shrubland, long-term trends in available soil N and cheatgrass success were most strongly associated with precipitation and winter minimum temperatures. Treatments did not result in longer-term decreases in soil N, but indicated that integrated restoration approaches that reduce litter and seed banks and increase competitive interactions may be effective at reducing cheatgrass success and facilitating establishment of natives.

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and nonnative forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes.We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km 2 ) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grassdominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure-shrubs, grasses, and forbs-will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.