Is Nitrogen Cycling and Bioavailability Limited By Phosphorus in Northern New England Forest Soils?.

Anthropogenic activities have altered both nitrogen (N) and phosphorus (P) cycles across the globe. One such example is elevated atmospheric N deposition to ecosystems that can shift ecological nutrient limitations from N towards P. While N deposition rates have recently declined in the northeastern U.S., there is limited insight on the interaction between N and P in forest ecosystem response or recovery. This research investigates the relationship between soil N dynamics and P availability in an experimental northeastern U.S. forested watershed. The BBWM is a long-term paired watershed experiment, where the West Bear (WB) watershed has been treated with ammonium sulfate ((NH₄)₂SO₄) since 1989 to study the effects of acidification and N enrichment on forest ecosystem function. The adjacent East Bear (EB) watershed serves as a biogeochemical reference and receives only ambient deposition. Each watershed contains two distinct forest types: northern hardwoods (HW) at lower elevations and softwoods (SW) at higher elevations. To study the interaction of N and P on the O horizons of these forest soils, P additions of 100 kg ha^-1 were applied to soils from both watersheds and forest types. The influences of P additions on N cycling were evaluated using both field experiments and laboratory soil incubations. Field P additions at a rate of 100 kg ha^-1 resulted in an overall 37% decrease in N as ammonium (NH₄-N), the dominant labile inorganic form of N in these soils, in both watersheds.  We attribute this to either enhanced microbial immobilization, or treatment inhibition of mineralization. Noteworthy was that even under ambient N deposition rates, EB displayed parallel, though reduced, responses as were evident in WB. Additions of P had no significant effect on extractable nitrate (NO₃-N) concentrations in either watershed. Ex-situ soil incubations revealed similar outcomes with lower potential net N mineralization (PNNM) rates in P treated soils. A laboratory incubation study with escalating rates of P addition (from 0 kg ha^-1 to 200 kg ha^-1) to O horizon soil materials that had not had prior P additions showed that the increasing rates of P additions to soils resulted in greater soil extractable P concentrations as was the intent. This experiment studied PNNM rates across the P availability response surface and showed that PNNM rates rose with greater P availability, and rose more rapidly in HW compared to SW soils, suggesting P limitations on N cycling. In addition, HW soils appeared to reach saturation at the highest level of P addition (200 kg ha^-1), whereas SW soils did not. Overall, these experiments suggest that P availability limits N dynamics in these watersheds and forest type is important in this response.