Phosphorus Limits Nitrogen Dynamics in the O Horizon of a Forested Watershed in Maine, USA

Anthropogenic emissions have altered the N status of forests in the northeastern United States, highlighting the importance of our imperfect understanding of the interaction between nitrogen (N) and phosphorus (P) in response to atmospheric N deposition. We investigated soil N and P dynamics at the Bear Brook Watershed in Maine (BBWM), a long‐term paired watershed experiment established to study ecosystem response to experimental N enrichment. One watershed (West Bear [WB]) was treated bimonthly for 25 yr with ammonium sulfate, and the reference East Bear (EB) received only ambient N deposition. We conducted field and laboratory studies to examine the interaction between N and P in O horizon soils in both EB and WB. In the “Field Addition” study, an addition of 100 kg P ha −1 resulted in an overall 38% decrease in extractable ammonium (NH 4 + –N) due to increased immobilization. Even under ambient N deposition rates, EB displayed parallel, although lower, evidence of P limitation compared with WB. In contrast, in the “Laboratory Addition” study, soils incubated in the laboratory at rates of 0 to 200 kg P ha −1 showed increases in potential net N mineralization (PNNM) and NH 4 + –N concentrations in response to P additions. There was no effect of N status in the Laboratory Addition study, but hardwood soils showed a faster response and greater rates of PNNM compared with softwood soils. Despite contrasting responses, both the field and laboratory studies suggested that N dynamics at BBWM were P limited and that N status and forest type influenced the N and P interaction. Core Ideas N transformations in Maine's forest soils are P limited. P limitation is greater in N‐enriched soils. Hardwood soils show greater response to P additions than softwood soils.