The effects of nitrogen addition on the growth of two exotic and two native forest understory plants

Abstract

Many studies have linked the spread of exotic, invasive species to high nitrogen supply, but most of this work does not distinguish between various inorganic forms and different concentrations of available nitrogen. Previous research has suggested that exotic, invasive species common in eastern deciduous forests may preferentially utilize nitrate in contrast to native species that preferentially make use of ammonium. We tested this hypothesis by comparing the growth response of two common forest invaders, Berberis thunbergii and Microstegium vimineum, and two co-occurring native species (Vaccinium pallidum and Hamamelis virginiana) to different forms of nitrogen under varying concentrations in a greenhouse experiment. Two forms of nitrogen (nitrate or ammonium) were added at different concentrations (22, 106, and 212 mg N l -1 ) to all species. Growth response variables included survival, stem length, stem diameter, above and belowground biomass, and estimated seed production (Microstegium only). Unexpectedly the exotic species did not respond preferentially to nitrate addition. Microstegium responded most strongly to both nitrogen forms depending on the response variable. Berberis and Hamamelis surprisingly reacted similarly to nitrogen additions. As expected, Vaccinium fared poorly under most treatment conditions, but did show some growth in NH 4 ? treatments. Our findings suggest species response to nitrogen addition is complex, and that exotic species do not all respond similarly to nitrogen inputs. In this study, the response of exotic and native species to available nutrients does not provide a general mechanism of invasion success.

Nitrogen addition increases intraspecific competition in the invasive wetland plant Alternanthera philoxeroides, but not in its native congener Alternanthera sessilis

“…Nitrogen addition significantly increased the growth of the two Alternanthera species, while intraspecific competition markedly decreased it (Table ; Figs ), agreeing with previous findings on other plant species (Xia & Wan ; Ross et al . ; Gough et al . ; Ladwig et al .…”

Amplicon pyrosequencing reveals the soil microbial diversity associated with invasive Japanese barberry (Berberis thunbergii DC.)

“…Nitrogen addition (NO 3 − and NH 4 + ) to the substrate of container grown Japanese barberry seedlings collected from invasive stands in New Jersey had no significant effect on aboveground or belowground plant biomass (Ross et al . ), which may result if the plant's primary N source is derived from a symbiotic N fixer. N 2 fixation is probably occurring in the rhizosphere of Japanese barberry, whether by symbiotic bacteria or free‐living diazotrophs, although the quantity of microbial‐derived N used by the plant remains unknown.…”

Increased nitrogen cycling facilitates native forest regeneration: Potential for restoring extinct ecological processes?

“…Our finding contrasts with the many studies documenting facilitative effects of elevated N availability on plant invasions (Huenneke et al 1990, Ostertag and Verville 2002, Brooks 2003, Leishman and Thomson 2005, Tyler et al 2007, including from avianderived feces (Mulder et al 2009). An explanation for this difference is that on early successional soils, particularly those with high pH where nitrification readily occurs, NO 3 À is often the dominant form of available N (Vitousek et al 1982), and so only plants that are adapted to these sites, such as the native species in our study, may benefit from increased NO 3 À availability (e.g., Kronzucker et al 1997, Ross et al 2011. Native species may therefore persist following the stimulation of germination because they can uptake available forms of N, while other species, such as nonnatives, may rapidly decline, as we observed across plots by 2009.…”