Isotopic evidence for increased carbon and nitrogen exchanges between peatland plants and their symbiotic microbes with rising atmospheric CO 2 concentrations since 15000 cal. yr BP.

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO 2 this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over the longer-term (decadal to century) feedbacks between climate, CO 2 and plant N uptake. The symbiosis between plants and microbes can help plants with mycorrhizal N uptake or biological N 2 fixation - the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present results for plant N isotope composition (δ 15 N) in a peat core that dates to 15000 cal. yr BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO 2 concentrations in the past. We found that an increase in atmospheric CO 2 concentration happened with a decrease in δ 15 N values of both Sphagnum moss and Ericaceae over this time period when constrained for climatic factors. A modern experiment demonstrated that δ 15 N of Sphagnum mosses decreased with increasing N 2 fixation rates. These findings suggested that N 2 fixation in Sphagnum moss by symbiosis with cyanobacteria and N uptake in Ericaceae by symbiosis with mycorrhizal fungi both likely increased with rising atmospheric CO 2 concentrations, highlighting a longer-term feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome.