Distribution of 15N in plants and soils across a Pinus contorta/Artemisia tridentata ecotone

Background/Question/Methods

The distribution of ¹⁵N in soil reflects the sources, sinks, and fractionation processes associated with biotic and abiotic functions. Soil ¹⁵N signatures tend to increase with depth, while plant ¹⁵N is more variable. As climate change, management, and environmental factors impact the distribution of ecosystems, plants will encounter, and possibly alter, different soil nutrient conditions.  Our study seeks to characterize nutrient distribution across a lodgepole pine (Pinus contorta)/ big sagebrush (Artemisia tridentata) ecotone, as well as test if nutrient distribution is correlated to plant ¹⁵N signatures. We collected leaf and soil samples across the ecotone boundary at three sites in Wyoming and Colorado, USA, and analyzed them for %N and ¹⁵N signatures. Leaf samples represented three functional types: grasses, shrubs, and trees. Soil samples represented depth intervals of 0-10cm, 10-30cm, 30-60cm, and 60-100cm. We asked the following questions: Do N and ¹⁵N concentrations and distributions in the soil differ between the two ecotones? Do foliar %N and δ¹⁵N differ within and among functional types between the two ecotones?

Results/Conclusions

The patterns in δ¹⁵N through soil depths corresponded with the generally observed trend of an increase with depth, likely reflecting the influence of older pools and fractionation associated with plant uptake.  Surface soil values were ~3.5‰, increasing to ~7.1‰ at depth.   In general, woody plant functional types had higher ¹⁵N values than grass or forb functional types.  δ¹⁵N signatures were highest in trees (~3.7‰), followed by shrubs (~1.5‰), and grasses (~0.5‰). Many processes affect plant δ¹⁵N, including the signature of the source pool, fractionation on uptake, within-plant fractionation following uptake.  To reliably interpret these results, further study is necessary.  Predictions of future climate change in this region suggest that winter precipitation will decrease, potentially leading to a shift in the depth distribution of soil resources to surface horizons, and changes in plant functional types, including shifting ecotones.  The ability to quantify changes in nutrient distribution in these areas will assist us in understanding the dynamic relationship between soil resource distribution and plant functional type dominance.