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. delta N-15 in lake sediments is increasingly used as a measure of reactive nitrogen input but problematically, the characteristic depleted delta N-15 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 delta N-15 signal in lake sediment cores. Analyses of snowpack N and delta N-15-NO3 and water chemistry were coupled with bulk sediment delta N-15. Study sites cover a gradient of snowpack delta N-15 (ice sheet: -6 parts per thousand; coast -10 parts per thousand), atmospheric N deposition (ice sheet margin: similar to 0.2 kg ha(-1) yr(-1); coast: 0.4 kg ha(-1) yr(-1)) and limnology. Three Pb-210-dated sediment cores from coastal lakes showed a decline in delta N-15 of ca. -1 parts per thousand from similar to 1860, reflecting the strongly depleted delta N-15 of snowpack N, lower in-lake total N (TN) concentration (similar to 300 mu g N L-1) and a higher TN-load. Coastal lakes have 3.7-7.1x more snowpack input of nitrate than inland sites, while for total deposition the values are 1.7-3.6x 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 delta N-15 is 0.5-2.5 parts per thousand in most inland lakes; n = 6). The primary control of the transfer of the atmospheric delta N-15 deposition signal to lake sediments is the magnitude of external N inputs relative to the in-lake N-pool.