Plant Isotopes as Indicators of N Cycling Processes in Agricultural Fields

When nitrogen (N) availability exceeds biological demand, excess N, especially nitrate (N03~), may subsequently pollute ground and surface water. Agricultural practices in Southern Ontario typically supplement soils with organic and inorganic nutrients to aid in crop development, and employ various management techniques to limit nutrient loss. Excess N has several potential fates, which are controlled by the net effects of numerous N cycling reactions in the soil that are often difficult to measure directly. N cycling in soils is controlled in large part by soil moisture, as it affects microbial activity and soil redox conditions. Stable isotope geochemistry is a powerful tool that provides information on N sources and processes. This study uses crop N and carbon (C) isotope ratios to provide insights into the net effects of soil N cycling and N fate. This research was conducted at the Strawberry Creek Watershed (SCW), an agricultural research watershed located between Kitchener-Waterloo and Guelph, Ontario. A subsequent lab-based grow experiment was conducted at Wilfrid Laurier University in Waterloo, Ontario. The SCW exhibits elevated N03" concentrations in groundwater, tile discharge, and the stream itself. Previous isotopic work revealed that this N03" is largely derived from chemical fertilizer and manure applications. Field-scale hydrological processes lead to areas where the fate of applied N differs, which has an isotopic effect on the residual N that is available to plants. Results of this study indicate significant patterns in the isotopic signature of plant tissue, in both temporal and spatial scales. At the plot-scale where soil conditions are similar, there is little to no variation in foliar isotope values, but at the field-scale there appears to be a large amount of variability related to soil moisture and N loss. However, that variability is reduced when soil moisture values are grouped. There exists a significant difference between crop productivity and soil moisture, as the medium moisture range (%VWC 30-40) produced corn yields almost 10% greater than wet (%VWC >40) and dry (%VWC <30) soil moisture areas. This relationship can potentially provide insight into ideal conditions for N uptake availability. The simulated crop growing experiment provided a simplified and controlled system for investigating the relationship between soil and plants. As the soil from all the barrels came from the same field location, and all N inputs were quantifiable, the differences in foliar 6N provided meaningful insight into N-cycling. The combination of enriched foliar 6N and soil N20 production in the relatively wet barrels provides reasonable conclusions about the value of using plant isotopes in the investigation of soil processes. This investigation is critical in furthering the efficiency of N application on crops; as well as, the subsequent decrease in the ecological impacts of farming. Reducing agricultural N leaching to ground and surface water requires a better understanding of N fate in the soil zone, and will result in more effective agricultural nutrient management.