Carbon and nitrogen dynamics in agricultural soils and shallow groundwater at two southern Ontario sites

The purpose of this research was to investigate carbon and nitrogen dynamics in the soil and shallow groundwater zones in agricultural regions. Three related field studies were conducted at two Southern Ontario sites.;The soil zone study monitored extractable and soil microbial biomass carbon and nitrogen over the winter at both sites. The most notable result of the study was an overwinter accumulation of extractable soil nitrogen, mostly in the organic form. At both sites, the level of extractable nitrogen increased until mid-February, when continuous ground ice was formed, and the levels declined. The levels of soil extractable nitrogen at both sites had declined to pre-accumulation levels before the major leaching event associated with spring thaw, and the overwinter accumulation did not appear to contribute to leaching of excessive nitrogen below the soil zone during spring thaw. It was not determined if minor leaching events associated with high nitrogen concentrations occurred during the winter.;Although freezing and thawing may be injurious to part of the soil microbial biomass, the overwinter changes in the levels of soil extractable nitrogen, and estimates of the rate of turnover of the soil microbial biomass based on the monitoring study data, suggested that the microbial biomass was active throughout the winter. Thus the hypothesis that a sustained collapse of the soil microbial biomass due to freezing and thawing might provide a reservoir of carbon and nitrogen available for leaching in spring thaw was not borne out.;Using the natural abundance of ;In the third field investigation a detailed groundwater monitoring study was conducted at both sites to investigate the validity of the existing conceptual model of groundwater contamination by non-point agricultural nitrogen. Groundwater nitrogen concentrations exhibited considerable spatial and temporal variability at both sites and very high concentrations of nitrogen were confined to long and thin solute 'enclaves'. Water extractable nitrate levels as high as the concentrations observed in the groundwater 'enclaves' were measured in the soil zone monitoring study only immediately after fertilizer application. Thus it seems that the enclaves of N-rich water may be derived from direct leaching of fertilizer N.;A conceptual model postulated that the enclaves were artifacts of infiltration through preferential zones of flow in the unsaturated zone. Regardless of its origin, the observed spatial and temporal variability of groundwater nitrogen suggested that the use of one or even a few groundwater installations to assess non-point source contamination may not be adequate.