Isotopic evidence for oligotrophication of terrestrial ecosystems

Abstract

Human societies depend on an Earth System that operates within a constrained range of nutrient 68 availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. 69 Examining patterns of foliar N concentrations ([N]) and isotope ratios (δ 15 N) from more than 42,000 samples acquired over years, here we show that foliar [N] declined by 8% and foliar δ 15 N declined by 0.8 -1.9 ‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of MAT and MAP tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75-150 years. 76These patterns are consistent with the proposed consequences of elevated atmospheric CO 2 and longer growing seasons. These declines will limit future terrestrial C uptake and increase nutritional stress for herbivores. 235 much. Preventing these declines in N availability further emphasizes the need to reduce 236 anthropogenic CO 2 emissions.Data and code availability. The datasets generated during and/or analysed during the current study are available in the Dryad repository [link to be generated upon acceptance]. All code used for statistical analyses and figure generation are available on Dryad (XXX).

Variation and evolution of C:N ratio among different organs enable plants to adapt to N‐limited environments

“…Systematic investigation and collaborative analysis of the C:N ratio among different plant organs can reveal the adaptation strategies of the a plant continuum, which means that a plant is made up interrelated organs (Zhang, He, et al, ). Although the TRY plant trait database (https://www.try-db.org) provides a huge number of records of plant elements, it is more suitable for analyzing the characteristics of a single plant organ (Butler et al, ; Craine et al, ). In this study, we systematically examined variation in the C:N ratio of different organs across taxa and forest types.…”

“…Plant C:N ratio also differs among environments. N is considered as the most limiting element in terrestrial ecosystems (Craine et al, 2018). The degree of N limitation varies from region to region (LeBauer & Treseder, 2008) because of topography, climate, and nutrient supply (Chadwick, Derry, Vitousek, Huebert, & Hedin, 1999).…”

Hotspots of nitrogen deposition in the world's urban areas: a global data synthesis

“…Indeed, many terrestrial ecosystems are facing N limitation due in part to warming temperatures and declining N availability relative to increasing atmospheric concentrations of carbon dioxide (Craine et al . ). However, the importance of the urban N cycle is gaining recognition, given that an increasing majority of people live in cities (UN ) and the water they drink and air they breathe directly affect their health (Kampa and Castanas ; Schwarzenbach et al .…”

“…Most research on atmospheric N deposition has been conducted in rural areas and has demonstrated declining deposition rates in places like the northeastern US, Europe, and the central Indo-Pacific (Ackerman et al 2018). Indeed, many ter-restrial ecosystems are facing N limitation due in part to warming temperatures and declining N availability relative to increasing atmospheric concentrations of carbon dioxide (Craine et al 2018). However, the importance of the urban N cycle is gaining recognition, given that an increasing majority of people live in cities (UN 2015) and the water they drink and air they breathe directly affect their health (Kampa and Castanas 2008;Schwarzenbach et al 2010).…”

Forest N Dynamics after 25 years of Whole Watershed N Enrichment: The Bear Brook Watershed in Maine

“…There is also evidence of increased C flow to soils increasing soil C availability and potentially stimulating microbial N immobilization in temperate forests (Groffman et al, 2018). The increased biological N uptake, combined with reduced atmospheric N inputs, have reduced the available N in soils, and there is accumulating evidence for the oligotrophication of these temperate forests (McLauchlan et al, 2017; Craine et al, 2018; Groffman et al, 2018). The results reported here also suggest that these forests may be transitioning to greater N limitation over time, and long‐term studies like BBWM are essential to understand how these changes are altering forest productivity and forest health.…”