Nitrogen and carbon isotopic dynamics of subarctic soils and plants in southern Yukon Territory and its implications for paleoecological and paleodietary studies

Abstract

We examine here the carbon and nitrogen isotopic compositions of bulk soils (8 topsoil and 7 subsoils, including two soil profiles) and five different plant parts of 79 C3 plants from two main functional groups: herbs and shrubs/subshrubs, from 18 different locations in grasslands of southern Yukon Territory, Canada (eastern shoreline of Kluane Lake and Whitehorse area). The Kluane Lake region in particular has been identified previously as an analogue for Late Pleistocene eastern Beringia. All topsoils have higher average total nitrogen δ15N and organic carbon δ13C than plants from the same sites with a positive shift occurring with depth in two soil profiles analyzed. All plants analyzed have an average whole plant δ13C of −27.5 ± 1.2 ‰ and foliar δ13C of –28.0 ± 1.3 ‰, and average whole plant δ15N of −0.3 ± 2.2 ‰ and foliar δ15N of –0.6 ± 2.7 ‰. Plants analyzed here showed relatively smaller variability in δ13C than δ15N. Their average δ13C after suitable corrections for the Suess effect should be suitable as baseline for interpreting diets of Late Pleistocene herbivores that lived in eastern Beringia. Water availability, nitrogen availability, spacial differences and intra-plant variability are important controls on δ15N of herbaceous plants in the study area. The wider range of δ15N, the more numerous factors that affect nitrogen isotopic composition and their likely differences in the past, however, limit use of the modern N isotopic baseline for vegetation in paleodietary models for such ecosystems. That said, the positive correlation between foliar δ15N and N content shown for the modern plants could support use of plant δ15N as an index for plant N content and therefore forage quality. The modern N isotopic baseline cannot be applied directly to the past, but it is prerequisite to future efforts to detect shifts in N cycling and forage quality since the Late Pleistocene through comparison with fossil plants from the same region.

Nitrogen isotopes suggest a change in nitrogen dynamics between the Late Pleistocene and modern time in Yukon, Canada

“…Tahmasebi et al [ 9 ] described collection and analysis of modern plants from east of Kluane Lake and the Whitehorse area of Yukon Territory ( Fig 1 ). Modern ground squirrel bones were also collected from these areas (3 bones from Kluane Lake in 2013; 11 individuals from Erik Nielsen Whitehorse International Airport in 2014).…”

“…A change in N dynamics and availability should be traceable using the δ 15 N of plants [ 9 ] and animals. Higher plant δ 15 N generally reflects higher N availability and a more open N cycle [ 10 ].…”

“…In this study, we combine the stable carbon- and nitrogen-isotope compositions of modern [ 9 ] and fossil plants and animals to test for changes in N-isotope dynamics in Yukon Territory, northwest North America, between the Late Pleistocene and present time. Our study was focused in the Yukon, including portions that were not glaciated during the Pleistocene.…”

“…We measured the stable carbon- and nitrogen-isotope compositions of modern Yukon arctic ground squirrel ( Urocitellus parryii ) bone collagen and structural carbonate, and compared these results to isotopic data for modern Yukon plants collected earlier by Tahmasebi et al [ 9 ]. From this comparison, we have established the C- and N-isotope discrimination between arctic ground squirrel bone collagen and diet.…”

“…Higher plant δ 15 N reflects higher N availability and a more open N cycle in terrestrial ecosystems [ 8 , 10 ], and provides an index of N availability [ 20 ]. The main differences between more and less open N cycles are illustrated in Supporting Information S1 Fig [ 9 ].…”