Biogeochemical Cycling of Nitrogen Through a Landscape Rich in Legacy Sediments

Open Access
Author:
Weitzman, Julie Nicole
Graduate Program:
Soil Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 01, 2011
Committee Members:
  • Jason Philip Kaye, Thesis Advisor
Keywords:
  • biogeochemistry
  • nitrogen
  • legacy sediments
Abstract:
Sedimentation rates, anoxic conditions, and eutrophication have all increased in the Chesapeake Bay since the time of European settlement. Legacy sediments, deposited during the historic, post-settlement period marked by deforestation, land clearing, and plowing of uplands and valley slopes, act as a significant non-point source of nitrogen to the Bay. At Big Spring Run in Lancaster, Pennsylvania, these legacy sediments now overlay a buried hydric soil, which affects the contemporary transfer of nitrogen from uplands to streams. Recent research suggests that nitrogen transfer to streams is also affected by soil drying and rewetting. Climate change models predict that the variability and magnitude of precipitation events will increase over time. Such changes could lead to extended periods of droughts, followed by increased precipitation. These dry-rewet cycles can alter the structure and activity of soil microbial communities, impacting nutrient retention and release. This project was undertaken to increase the understanding of nitrogen processing and movement in legacy sediments along a landscape gradient before and after soil drying. Short-term incubations were carried out on field-moist soil as well as on soil that was air-dried, then rewetted prior to incubation. Samples were from three landscape positions (non-legacy uplands, legacy zone, and stream bank) and three layers (top 20 cm, midlayer, and bottom layer) . Respiration rates, net ammonification rates, and net nitrification rates were determined in laboratory incubations. A community-level physiological profile and extracellular enzymes were assayed to determine microbial activity levels. Four key discoveries were found from this work: net nitrification was greatest in the surface soils away from the stream bank; drought induced nitrate pulses were unexpectedly absent; buried hydric soils appeared to have low biological activity and low nitrification potential; and the low activity in the buried layer may have been due to a lack of labile carbon. Given their prevalence in Pennsylvania there was a critical need to understand how nitrogen flows through legacy sediments to improve predictions and management of nitrogen transport from uplands to streams. Future measurements are needed to explore how different hydrological flowpaths impact microbial communities and contaminant pulses in, and between, soil layers, and how such interactions change after the removal of legacy sediments. My data suggest that flowpaths through the buried hydric layer will not have strong nitrogen immobilization, despite high C in that layer.