Is Nitrogen Cycling and Bioavailablity 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. Cool, temperate forest ecosystems of eastern North America often have limited external P inputs; therefore P retention relies heavily on internal cycling between the forest floor, microbial biomass, and plants. In addition new P is released for biological use from geochemical sinks by the weathering of primary minerals. 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 were applied to soils in both the EB and WB watersheds. The influences of P additions on N cycling were evaluated using both field experiments and laboratory soil incubations. The influence of watershed (i.e., long-term, whole watershed treatment versus the reference) and forest type framed the experimental design. In addition, a batch soil mineral weathering experiment studied P supply from mineral weathering in these soils. Field and laboratory studies suggested there were P limitations on N dynamics at BBWM. Field P additions at a rate of 100 kg ha -1 resulted in an overall 37% increase in N immobilization as ammonium (NH4-N), the dominant labile inorganic form of N in these soils, in both watersheds. Noteworthy was that even under ambient N deposition rates, EB displayed parallel, though reduced, signs of P limitation 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 lower potential net N mineralization (PNNM) rates in P treated soils, likely a result of increased immobilization of N by microbes. 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. In addition, HW soils appeared to reach saturation at the highest level of P addition (200 kg ha-1), whereas SW soils did not. There was no significant difference in P weathering rates from the mineral horizons between watersheds or forest types. The weathering experiment included the mineral soil horizons, and showed that the degree of P weathering was strongly influenced by horizon with the C horizon having a greater rate of P weathering than the B horizon, attributable to less weathered minerals in the parent material. Overall, these experiments strongly suggest that P availability limits N dynamics in these watersheds, and P limitations are even more evident under elevated N availability. These results are relevant to changing forest N dynamics that can result from environmental factors such as atmospheric N deposition or warming climatic trends that have the potential to accelerate N dynamics. The results suggest important influences of forest composition on the interaction between N and P in these soils, with much of this influence attributed to soil microbial communities in these studies. Understanding these complex interactions among essential nutrients is made even more important as we increasingly rely on managed forested landscapes for the ecosystem services they provide to society. Yet it is these same forested landscapes that are continually exposed to changing rates of atmospheric N and sulfur (S) deposition, climatic shifts in temperature, moisture, and seasonality, as well as other stressors such as pest and pathogens