Changes in nitrogen cycling during the past century in a northern hardwood forest

Abstract

Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.

Fig. 1.

History of terrestrial N availability from tree rings at Mirror Lake. δ¹⁵N in 857 wood segments from 22 trees in the Mirror Lake watershed, with a piecewise linear regression line. Each tree was standardized to a mean δ¹⁵N of 0‰. Partition analysis indicates that the inflection point is at 1929 (95% confidence interval: 1914 to 1937).

Fig. 2.

Geochemical data from surface sediment core of Mirror Lake. Linear regression lines are shown for δ¹⁵N (A and F), δ¹³C (C and H), and C:N (E and J). Seventy-five percent quantile regression for [N] (B and G) and [C] (D and I) was used to minimize the influence of several sediment samples unusually high in mineral material, similar to one that was deposited immediately after a powerful hurricane in 1938. All regressions were performed on samples deposited earlier than 1800. The most recent samples, from 1950 to 2005, are shown in F–J. The vertical dashed line indicates the first record of Euro-American settlement in the Hubbard Brook Valley in 1790.

Fig. 3.

Conceptual diagram of N availability for northern hardwood forests during secondary succession. N availability rises with disturbance, but the recovery of vegetation uptake potential generally decreases N availability soon after. N availability can continue to decline, potentially even below predisturbance N availability, if soil and litter fractions were depleted by disturbance and their recovery immobilizes N. The recovery of immobilization might be followed by an increase in N availability as these pools equilibrate, but there has been little research on these long-term dynamics. Over longer time scales, N availability increases with ecosystem development and increasing base cation limitation. Axes are not to scale.