Potential for phosphorus limitation in northern hardwood forest species

Background/Question/Methods

The productivity of many temperate ecosystems has traditionally been considered nitrogen (N) limited. However, atmospheric N deposition has increased N availability in many areas, potentially leading to limitation of productivity by phosphorus (P), especially where soil P sorption is increased by deposition-related acidity. Northern hardwood forests in the eastern US exhibit species-specific influences on N cycling and responses to N additions, suggesting that P limitation may also vary by species and in response to N addition. The objective of this study was to examine how N deposition and tree species affect concentrations of P in plant tissue and indicators of P limitation in soil. N deposition was simulated by adding NH₄NO₃ (50 kg ha^-1 yr^-1) to single-species plots of red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.), eastern hemlock (Tsuga canadensis (L.) Carr.), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britt.), in the Catskill Mountains, New York from 1997 to 2007. Responses were measured by sampling foliage, litter, fine roots, and soil, and by assaying soil enzyme activities.

Results/Conclusions

Consistent species differences were observed in foliar P (F=4.09, p=0.04), litter P (F=9.56, p=0.002), and resorption efficiency (F=6.49, p=0.009), but all were unaffected by N fertilization. Foliar P concentrations were highest in red oak and sugar maple and lowest in hemlock. Fine root P concentrations were positively correlated with foliar P concentrations for sugar maple (r²=0.48, p<0.01) and beech (r²=0.77, p<0.001) but not other species. Soil phosphatase activity, a measure of P demand, was higher in beech and hemlock plots compared to sugar maple and oak plots. Across all species phosphatase activity was negatively correlated with root P concentrations (r²=0.58, p<0.0001) and available soil P (r²=0.50, p<0.0001). Results suggest species exhibit unique influences on cycling of P as well as N. Under some species (sugar maple and red oak), P supplies may meet or exceed plant demand, whereas other species (beech and hemlock), may be more sensitive to P limitation because demand exceeds P supply. The lack of fertilizer effects suggests that either: (1) productivity remains N limited and fertilization rates were not great enough to induce P limitation, or (2) fertilization caused other nutrients (Ca or K) to become limiting.