Despite decades of research documenting the quantitative significance of dissolved organic nitrogen (DON) across ecosystems, the drivers controlling its production and consumption are not well understood. As an organic nutrient DON may serve as either an energy or nitrogen source. One hypothesized control on DON concentration in streams is nitrate (NO₃ ⁻) availability. Synoptic surveys of DON and NO₃ ⁻, however, have yielded inconsistent spatial and temporal patterns. Using a nutrient pulse method we experimentally manipulated stream NO₃ ⁻ and measured the response of both the manipulated solute and ambient concentrations of DON in three New Hampshire headwater streams. This direct experimental addition of NO₃ ⁻ often altered ambient DON concentrations in situ, with both increases and decreases observed. The overall relationship between NO₃ ⁻ and DON suggests that DON is primarily used as a nutrient source in these streams, as evidenced by net DON accumulation with added NO₃ ⁻. However, strong underlying seasonal patterns in the response to NO₃ ⁻ addition are also discernable, indicating that the role of DON can switch between serving as a nutrient source to an energy source (as evidenced by net DON reduction with added NO₃ ⁻). We also observed differences in the NO₃ ⁻—DON relationship (net DON accumulation vs. net DON reduction) in two streams less than five miles apart when experiments were conducted within the same month. Based on these results, we expect the role of DON within ecosystems to vary among watersheds and throughout the growing season, alternating between serving as a nutrient and energy source depending on environmental conditions. With the incorporation of a new field-based method we demonstrate that the ambient DON pool can be manipulated in situ. This approach has the potential for furthering our understanding of DON across ecosystems.
