PARENT SESSION
Contributed Oral Session 57: Polar and Alpine Ecology: Carbon and Nutrient Dynamics
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Plant and microbial seasonal uptake of inorganic nitrogen in a degraded arctic salt marsh.

Buckeridge, Kate^*,1, Jefferies, Robert², ¹ Queen's University, Kingston, Ontario, Canada² University of Toronto, Toronto, Ontario, Canada

ABSTRACT- Nitrogen (N) is the limiting nutrient for plant growth in most terrestrial ecosystems. If competition for inorganic N occurs, microbes are expected to have a short-term advantage. Although plants may acquire sufficient N over the growing season, it is unclear if this is a result of short-term competition with microbes for organic and inorganic N, or the result of physical persistence at a time when microbial demand for N may be limited. Short-term allocation of inorganic N to plants has implications for successful plant reestablishment following disturbance. In an Arctic coastal salt marsh at La Pérouse Bay, Manitoba, the effects of heavy grazing by migratory geese on N competition were investigated. Grubbing of vegetation by geese in the tidal and supratidal marshes has exposed sediments. Degraded areas represent an alternate stable state, where high soil salinities, low redox potentials and soil compaction prevent reestablishment of vegetation. In this study, plots with intact vegetation were contrasted with areas in the two marshes devoid of vegetation. ¹⁵NH₄⁺ was injected into soil cores to measure N allocation between soil, plants and microbial biomass in spring, summer and winter. In vegetated marshes, microbes were 1.5 (intertidal) to 3 (supratidal) times as successful as plants at ¹⁵N uptake (g ¹⁵N cm^-3 d^-1). Plants in the intertidal marsh with a lower microbial soil biomass were more successful than plants in the supratidal marsh at ¹⁵NH₄⁺ acquisition. Microbial ¹⁵N uptake and gross ¹⁵N immobilization were lower in degraded soils devoid of plants, and thus net mineralization and ¹⁵N in the soil solution were higher. The proportion of injected ¹⁵N taken up by microbes was broadly consistent across the entire season in each marsh. Although plants may be competing for N in vegetated sites, the lack of plants in the degraded sites did not result in increased uptake of N by microbes. In these degraded soils microbial growth may be carbon-limited, or suppressed because of high soil salinity and low redox potentials. Hostile soil characteristics in these degraded soils appear to limit microbial metabolism and prevent plant reestablishment.

Key words: nitrogen cycling, coastal salt marsh, goose herbivory, plant-microbial competition