Phosphorus Enhances Uptake of Dissolved Organic Matter in Boreal Streams

Retention of carbon (C), either by physical mechanisms or microbial uptake, is a key driver of the transformation and storage of C and nutrients within ecosystems. Both the molecular composition and nutrient content of organic matter influence the rate at which it is retained in streams, but the relative influence of these characteristics remains unclear. We estimated the effects of nutrient content and molecular composition of dissolved organic C (DOC) on uptake in boreal streams by measuring rates of C retention, in situ, following introduction of leachates derived from alder, poplar, and spruce trees subject to long-term fertilization with nitrogen (N) or phosphorus (P). Leachate C:N varied approximately twofold, and C:P varied nearly 20-fold across species and nutrient treatments. Uptake of DOC was greatest for leachates derived from trees that had been fertilized with P, a finding consistent with P-limitation of uptake and/or preferential sorption of P-containing molecules. Optical measures indicated that leachates derived from the three tree species varied in molecular composition, but uptake of DOC did not differ across species, suggesting weak constraints on retention imposed by molecular composition relative to nutrient limitation. Observed coupling between P and C cycles highlights the potential for increased P availability to enhance DOC retention in headwater streams.