Primary production in temperate forest ecosystems is widely assumed to be limited by nitrogen, but the assumption is rarely tested. Chronic atmospheric deposition of nitrogen has dramatically altered the N budgets of forests in the northeastern US, while inputs of phosphorus via weathering and atmospheric deposition are presumably unchanged. As N accumulates in the ecosystem, it is possible that P will begin to limit primary production and to control the cycles of other important elements in the ecosystem. Stocks and fluxes of these elements at the reference watershed in the Hubbard Brook Experimental Forest (Woodstock, NH, USA), have been studied continuously for decades, making it possible to detect changes in element cycles over time. We examined a 33-year record of forest floor chemistry and a 39-year record of foliar chemistry to determine whether the ratios of N:P in the canopy and forest floor have changed over the past several decades of nitrogen deposition and forest development.

Results/Conclusions

N:P ratio increased over time in live foliage as well as in the shallow forest floor (O_ie horizon), but there was no detectable change in the bulk forest floor (O_ie plus O_a). The magnitude of the change in the forest floor (from 16 to 20) was similar to that observed in the foliage (from 13 to 17), and the difference in N:P ratio between these stocks can be explained by known nutrient resorption rates. A simple model shows that changes in species composition could not have contributed much to the change in litter layer N:P. Furthermore, the live foliage N:P has crossed the threshold value that some believe separates N-limited systems from P-limited systems, suggesting that the forest is moving towards greater P limitation. Only a factorial experimental fertilization will be able to determine which element controls primary productivity and the cycling of other elements. Such an experiment may be overdue at Hubbard Brook. N and P fertilization experiments will be initiated this year at Hubbard Brook, and also at the nearby Bartlett Experimental Forest and Jeffers Brook, which have higher and lower P availability, respectively.