Regional variability in the atmospheric nitrogen deposition signal and its transfer to the sediment record in Greenland lakes

Disruption of the nitrogen cycle is a major component of global environmental change. δ¹⁵N in lake sediments is increasingly used as a measure of reactive nitrogen input but problematically, the characteristic depleted δ¹⁵N signal is not recorded at all sites. We used a regionally replicated sampling strategy along a precipitation and N-deposition gradient in SW Greenland to assess the factors determining the strength of δ¹⁵N signal in lake sediment cores. Analyses of snowpack N and δ¹⁵N-NO₃ and water chemistry were coupled with bulk sediment δ¹⁵N. Study sites cover a gradient of snowpack δ¹⁵N (ice sheet: −6‰; coast -10‰), atmospheric N deposition (ice sheet margin: ∼ 0.2 kg ha⁻¹ yr⁻¹; coast: 0.4 kg ha⁻¹ yr⁻¹) and limnology. Three ²¹⁰Pb-dated sediment cores from coastal lakes showed a decline in δ¹⁵N of ca. -1‰ from ∼ 1860, reflecting the strongly depleted δ¹⁵N of snowpack N, lower in-lake total N (TN) concentration (∼ 300 μg N L⁻¹) and a higher TN-load. Coastal lakes have 3.7–7.1× more snowpack input of nitrate than inland sites, while for total deposition the values are 1.7–3.6× greater for lake and whole catchment deposition. At inland sites and lakes close to the ice-sheet margin, a lower atmospheric N deposition rate and larger in-lake TN pool resulted in greater reliance on N-fixation and recycling (mean sediment δ¹⁵N is 0.5–2.5‰ in most inland lakes; n = 6). The primary control of the transfer of the atmospheric δ¹⁵N deposition signal to lake sediments is the magnitude of external N inputs relative to the in-lake N-pool.