Centennial-scale reductions in nitrogen availability in temperate forests of the United States

K K Mclauchlan,P E Higuera,J J Battles,S S Perakis,L M Gerhart,J M Craine,B E Mcneil,M C Mack,D M Nelson,N Pederson,A J Elmore

doi:10.1038/s41598-017-08170-z

Abstract

Forests cover 30% of the terrestrial Earth surface and are a major component of the global carbon (C) cycle. Humans have doubled the amount of global reactive nitrogen (N), increasing deposition of N onto forests worldwide. However, other global changes—especially climate change and elevated atmospheric carbon dioxide concentrations—are increasing demand for N, the element limiting primary productivity in temperate forests, which could be reducing N availability. To determine the long-term, integrated effects of global changes on forest N cycling, we measured stable N isotopes in wood, a proxy for N supply relative to demand, on large spatial and temporal scales across the continental U.S.A. Here, we show that forest N availability has generally declined across much of the U.S. since at least 1850 C.E. with cool, wet forests demonstrating the greatest declines. Across sites, recent trajectories of N availability were independent of recent atmospheric N deposition rates, implying a minor role for modern N deposition on the trajectory of N status of North American forests. Our results demonstrate that current trends of global changes are likely to be consistent with forest oligotrophication into the foreseeable future, further constraining forest C fixation and potentially storage.

Highlights

N deposition rates to forests are generally elevated over pre-Industrial levels due to widespread increases in supplies of reactive N from human activities, potentially increasing forest N availability[1]
A large portion of the climate space in North American forests is represented by our sampled sites, which range in mean total annual precipitation (MAP) by over 2400 mm, mean annual temperature (MAT) by 19 °C and N deposition by 14.5 kg ha−1 y−1 (Supplementary Fig. 1)
Trajectories of wood δ15N have been declining from 1850 C.E. to present, demonstrated when site-level patterns are summarized in a composite curve (Fig. 2)

Summary

Introduction

N deposition rates to forests are generally elevated over pre-Industrial levels due to widespread increases in supplies of reactive N from human activities, potentially increasing forest N availability[1]. In support of the declining N hypothesis, observations of leaf N concentrations in European forests have shown multi-decadal declines[8], forests of the eastern U.S have demonstrated increased demand for N relative to supply[9], and streamwater nitrate export has been declining for decades at sites in the U.S.10 and Europe[11]. Despite these indications, there is no coherent picture of the long-term changes in N availability of forested ecosystems at broad scales. Our sample reflects much of the variation in forests across the U.S and applies to a wide variety of forest types

Methods

Results

Conclusion