Human activities have doubled the pre-Industrial supply of reactive nitrogen on Earth, and future rates of increase are expected to accelerate. Yet, little is known about the capacity of the biosphere to buffer elevated nitrogen influx. Past changes in global ecosystems following deglaciation at the end of the Pleistocene provide an opportunity to better understand how nitrogen cycling in the terrestrial biosphere responded to changes in carbon status. We analysed stable nitrogen isotopic values (δ15N) in lacustrine sediments from 86 lakes on six continents. We show that sedimentary δ15N declined from 15,000 yBP to 7056 yBP ± 597 y, a period of increasing atmospheric carbon dioxide concentrations and terrestrial carbon accumulation2. Comparison of the nitrogen isotope record with concomitant carbon accumulation on land and nitrous oxide in the atmosphere suggests reduced nitrogen availability in terrestrial ecosystems during the Pleistocene-Holocene transition. In contrast, we do not observe a consistent change in global sedimentary δ15N during the past 500 years despite the potential effects of changing temperature and nitrogen influx from anthropogenic sources. We propose that the lack of a single response may indicate that modern increases in atmospheric carbon dioxide and net carbon sequestration in the biosphere have the potential to offset recent increased supplies of reactive nitrogen in some ecosystems.