Katherine J. Stewart

Nitrogen Inputs by Associative Cyanobacteria across a Low Arctic Tundra Landscape

Abstract Available soil N is a key factor limiting plant productivity in most low arctic terrestrial ecosystems. Atmospheric N2-fixation by cyanobacteria is often the primary source of newly fixed N in these nutrient-poor environments. We examined temporal and spatial variation in N2-fixation by the principal cyanobacterial associations (biological soil crusts, Sphagnum spp. associations, and Stereocaulon paschale) in a wide range of ecosystems within a Canadian low arctic tundra landscape, and estimated N input via N2-fixation over the growing season using a microclimatically driven model. Moisture and temperature were the main environmental factors influencing N2-fixation. In general, N2-fixation rates were largest at the height of the growing season, although each N2-fixing association had distinct seasonal patterns due to ecosystem differences in microclimatic conditions. Ecosystem types differed strongly in N2-fixation rates with the highest N input (10.89 kg ha−1 yr−1) occurring in low-lying Wet Sedge Meadow and the lowest N input (0.73 kg ha−1 yr−1) in Xerophytic Herb Tundra on upper esker slopes. Total growing season (3 June–13 September) N2-fixation input from measured components across a carefully mapped landscape study area (26.7 km2) was estimated at 0.68 kg ha−1 yr−1, which is approximately twice the estimated average N input via wet deposition. Although biological N2-fixation input rates were small compared to internal soil N cycling rates, our data suggest that cyanobacterial associations may play an important role in determining patterns of plant productivity across low arctic tundra landscapes.

Bryophyte-cyanobacterial associations as a key factor in N2-fixation across the Canadian Arctic

Nitrogen inputs via biological N2-fixation are important in arctic environments where N often limits plant productivity. An understanding of the direct and indirect theoretical causal relationships between key intercorrelated variables that drive the process of N2-fixation is essential to understanding N input. An exploratory multi-group Structural Equation Modeling (SEM) approach was used to examine the direct and indirect effects of soil moisture, plant community functional composition, and bryophyte and lichen abundance on rates of nitrogen fixation at a low arctic ecosystem, two high arctic oases and a high arctic polar desert in the Canadian Arctic. Increasing soil moisture was strongly associated with an increasing presence of bryophytes and increasing bryophyte abundance was a major factor determining higher N2-fixation rates at all sites. Shrubs had a negative effect on bryophyte abundance at all sites with the exception of the polar desert site at Alexandra Fjord highland. The importance of competition from vascular plants appears to be greater in more productive sites and may increase at lower latitudes. Moisture availability may have an indirect effect on ecosystem development by affecting N input into the system with bryophyte-cyanobacterial associations playing an important intermediary role in the process.

Spatially explicit structural equation modeling

Structural equation modeling (SEM) is a powerful statistical approach for the testing of networks of direct and indirect theoretical causal relationships in complex data sets with intercorrelated dependent and independent variables. SEM is commonly applied in ecology, but the spatial information commonly found in ecological data remains difficult to model in a SEM framework. Here we propose a simple method for spatially explicit SEM (SE-SEM) based on the analysis of variance/covariance matrices calculated across a range of lag distances. This method provides readily interpretable plots of the change in path coefficients across scale and can be implemented using any standard SEM software package. We demonstrate the application of this method using three studies examining the relationships between environmental factors, plant community structure, nitrogen fixation, and plant competition. By design, these data sets had a spatial component, but were previously analyzed using standard SEM models. Using these data sets, we demonstrate the application of SE-SEM to regularly spaced, irregularly spaced, and ad hoc spatial sampling designs and discuss the increased inferential capability of this approach compared with standard SEM. We provide an R package, sesem, to easily implement spatial structural equation modeling.

Physical, chemical and microbial soil properties of frost boils at Browning Peninsula, Antarctica

In a pilot study, we examined whether frost boils grossly mask the effect of site by comparing physical, chemical and microbial soil properties between the middle and edge of frost boils on remote and understudied Browning Peninsula, Antarctica. In addition, we determined the degree to which soil microbial community structure can be attributed to frost boil physical and chemical soil properties. Principle components (PCs) of physical and chemical soil properties and of microbial soil properties were compared between the middle and edge of frost boils and between frost boil replicates. Despite higher soil moisture, % C, % S and % N, pH, total DNA, 16S copy number, total phospholipid fatty acids, bacteria, fungi, proteobacteria, anaerobes, gram-positive bacteria and eukaryotes at frost boil edges, PCs were not significantly different between edge and middle positions. Physical and chemical soil properties between frost boil replicates differed significantly, but not microbial communities. In non-metric multidimensional scaling (NMS) ordinations, soil microbial community structure was affected by differences in frost boil replicates and some frost boil replicates were well separated. Therefore, site factors may play a critical role in determining the soil characteristics of frost boils. It remains to be seen whether stronger gradients in physical, chemical and microbial soil properties will develop across frost boils with continued warming of Antarctic soils. Frost boils may have subtle effects and require consideration in long-term sampling design for understanding the effects of climate change.

Herbicide Toxicity Testing with Non-Target Boreal Plants: The Sensitivity of Achillea millefolium L. and Chamerion angustifolium L. to Triclopyr and Imazapyr

Terrestrial plant toxicity tests were conducted to determine the sensitivity of two boreal plants, yarrow (Achillea millefolium L.) and fireweed (Chamerion angustifolium L.), to the herbicides imazapyr and triclopyr. Both plants are common non-target species on northern powerline rights-of-way where the impacts of proposed herbicide applications are of concern. In the vegetative vigour test, triclopyr foliar spray caused extensive damage to A. millefolium at <50% of the maximum field application rate (inhibition concentration (IC)50 = 1443.8 g a.i. ha−1) and was lethal to C. angustifolium at the lowest dose tested (1210.9 g a.i. ha−1). Both species demonstrated extremely high sensitivity to imazapyr foliar spray: IC50s = 8.29 g a.i. ha−1 and 4.82 g a.i. ha−1 (<1.5% of the maximum field rate). The seedling emergence and seedling growth tests were conducted in the organic horizon of five boreal soils. Few differences in herbicide bioavailability between soils were detected. Triclopyr limited growth of A. millefolium, C. angustifolium and standard test species Calamagrostis canadensis at low levels (most IC50 estimates between 2–20 µg g−1). For imazapyr, IC50 estimates could not be calculated as there was >75% inhibition of endpoints at the lowest doses of ~2 µg g−1. A foliar application of triclopyr or imazapyr for woody species control would likely cause significant damage to boreal non-target plants. The high sensitivity of both species to herbicide residues in soil indicates long term impacts are dependent on herbicide degradation rates in northern conditions. A. millefolium performed well and is recommended for use in toxicity testing relevant to boreal regions.

Linking Herbicide Dissipation to Soil Ecological Risk along Transmission Rights-of-Way in the Yukon Territory, Canada

In the Yukon Territory, transmission rights-of-way (ROWs) are managed using brushing and mowing techniques alone. When cut, target species such as Michx. and spp. grow rapidly shortening maintenance cycles. Long-term vegetation control may be improved by integrating herbicide application. However, prior to implementation, the dissipation and toxicity of herbicides in northern latitudes needed to be assessed. The dissipation of Garlon XRT (triclopyr) and Arsenal Powerline (imazapyr) in soils was assessed at five ROW locations representative of the main ecoregion types where ROWs occur within the Yukon Territory. Soils from four sites were collected at 1, 30, and 365 d after treatment to determine persistence of herbicides for each of three application methods (backpack spraying, cut stump, and point injection). Increased sampling intervals were added to better determine the dissipation rate of each herbicide in Yukon Territory soils. Soil dissipation data were linked to a series of standardized toxicity tests, including three soil invertebrates (, , and ). Additionally, the dissipation of both herbicides from the target species L. was assessed at one site. Herbicide residues persisted in soils for longer than 365 d after treatment and longer than 30 d after treatment in . However, concentrations were below the concentration that would affect 25% of the invertebrate species tested. Weight of evidence and toxic exposure ratios were used to characterize the risks associated with herbicide application in northern latitudes and provided both qualitative and quantitative means to communicate the results to the public.

Application Method and Biochar Type Affect Petroleum Hydrocarbon Degradation in Northern Landfarms

To integrate biochar as a practical and successful remedial amendment at northern landfarms, components of its formulation and application must be optimized for site-specific environmental conditions. Different biochar amendments were applied to petroleum hydrocarbon (PHC)-contaminated soil at two northern field sites (Iqaluit and Whitehorse) and in a laboratory study at -5°C to determine the effects of application method (injection or incorporation) and biochar type (wood, fishmeal, bonemeal, and/or compost) on PHC degradation and associated soil properties. Incorporation decreased F2 (equivalent C-C) and F3 (equivalent C-C) PHC concentrations in soil after 31 d, whereas injection did not decrease PHC concentrations until Day 334. Bonemeal-derived biochar selectively stimulated F3-PHC degradation in frozen soil over 90 d under controlled laboratory conditions. In the field, there was little difference in PHC degradation between biochar types and the fertilizer control. Incorporation also increased NO availability, and in field trials, all biochars increased NO availability relative to the fertilizer control, whereas the effects of biochars on NH and PO were variable. Aromatic functional gene abundance was enhanced when treatments were incorporated, compared with when injected. In field trials, 6% Zakus wood plus fertilizer inhibited aliphatic and aromatic gene abundance. Liquid water content increased in incorporated treatments, specifically those amended with fishmeal biochar. Incorporation was the most successful application method for these northern soils, and although biochar amendments are not clearly effective in reducing PHC concentrations, there is evidence to suggest it can beneficially influence soil properties and PHC degradation under specific environmental conditions.