Nitrogen isotope patterns in Alaskan black spruce reflect organic nitrogen sources and the activity of ectomycorrihizal fungi

Abstract

Global patterns in soil, plant, and fungal stable isotopes of N (¹⁵N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial scales, however, it remains difficult to differentiate the underlying causes of plant ¹⁵N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of ¹⁵N values from 31 putatively N -limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant ¹⁵N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and ¹⁵N values to be correlated with the concentration and ¹⁵N values of soil NH₄ + and dissolved organic N (DON) pools, respectively. However, ¹⁵N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the ¹⁵N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the ¹⁵N values within the soil DON pool to determine how choice of soil ¹⁵N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and ¹⁵N values may be useful metrics of changes to these connections.

Citation

Mayor, Jordan R.; Schuur, Edward A.G.; Mack, Michelle C.; Hollingsworth, Terresa N.; Bääth, Erland. 2012. Nitrogen isotope patterns in Alaskan black spruce reflect organic nitrogen sources and the activity of ectomycorrhizal fungi. Ecosystems.15: 819-831.