Effects of high groundwater nitrate inputs on riparian conifer forest growth and associated plant-soil interactions

Increased nitrogen inputs to an ecosystem can initially result in higher rates of plant growth, increased nitrogen concentration in plant tissue, and increased rates of soil processes. Chronic inputs can result in nitrogen saturation, causing the ecosystem to switch from a sink to a source. The potential for nitrogen saturation in riparian zones is not well understood. The objective was to assess the long-term effects of high groundwater nitrate inputs on the growth pattern of eastern white cedar and associated plant-soil interactions. Two forested riparian wetland sites with similar vegetation, soils and hydrology were selected in Southern ON, Canada, an enriched site receiving high groundwater nitrate inputs since the early 1980's from an adjacent cropland and a control site receiving natural groundwater inputs from an upland forest.; Tree cores were extracted and soil, foliage, and leaf litter samples collected from both sites. Litterbag field incubations were conducted to determine decomposition rates and laboratory incubations were carried out to determine denitrification rates. A decline in tree growth due to nitrate loading was not evident. Instead, both sites revealed an increase in growth after the mid to late 1980's, becoming prominent after 1990, suggesting a climate related response. The foliage and litter nitrogen content, C:N ratios of soil organic matter, and decomposition and denitrification rates were not significantly different between sites. Results conclude that plant uptake of nutrients and rates of soil processes have not responded to the high groundwater nitrate inputs. Despite the absence of a response, the enriched site still effectively retained nitrates due to high rates of denitrification.