Black Spruce Forests

  • New paper in Soil Biology and Biochemistry

    Decoupled stoichiometric, isotopic, and fungal responses of an ectomycorrhizal black spruce forest to nitrogen and phosphorus additions

    Jordan R. Mayor, Michelle C. Mack, Edward A.G. Schuur

    ABSTRACT

    Many northern forests are limited by nitrogen (N) availability, slight changes in which can have profound effects on ecosystem function and the activity of ectomycorrhizal (EcM) fungi. Increasing N and phos-phorus (P) availability, an analog to accelerated soil organic matter decomposition in a warming climate, could decrease plant dependency on EcM fungi and increase plant productivity as a result of greater carbon use efficiency. However, the impact of altered N and P availability on the growth and activity of EcM fungi in boreal forests remains poorly understood despite recognition of their importance to host plant nutrition and soil carbon sequestration. To address such uncertainty we examined above and belowground ecosystem properties in a boreal black spruce forest following five years of factorial N and P additions. By combining detailed soil, fungal, and plant d15N measurements with in situ metrics of fungal biomass, growth, and activity, we found both expected and unexpected patterns. Soil nitrate isotope values became 15N enriched in response to both N and P additions; fungal biomass was repressed by N yet both biomass and growth were stimulated by P; and, black spruce dependency on EcM derived N increased slightly when N and P were added alone yet significantly declined when added in combination. These findings contradict predictions that N fertilization would increase plant P demands and P fertil-ization would further exacerbate plant N demands. As a result, the prediction that EcM fungi predictably respond to plant N limitation was not supported. These findings highlight P as an under appreciated mediator of the activity of denitrifying bacteria, EcM fungi, and the dynamics of N cycles in boreal forests. Further, use of d15N values from bulk soils, plants, and fungi to understand how EcM systems respond to changing nutrient availabilities will often require additional ecological information.

    Soil Biology & Biochemistry 88 (2015) 247–256

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    Collecting black spruce foliage for isotopic analysis.

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    Nitrogen Isotope Patterns in Alaskan Black Spruce Reflect Organic Nitrogen Sources and the Activity of Ectomycorrhizal Fungi

    Jordan R. Mayor, Edward A. G. Schuur, Michelle C. Mack, Terresa N. Hollingsworth, and Erland Baath


    Global patterns in soil, plant, and fungal stable isotopes of N (δ15N) 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 δ15N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ15N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ15N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ15N values to be correlated with the concentration and δ15N 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 δ15N values within the soil DON pool to determine how choice of soil δ15N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ15N values may be useful metrics of changes to these connections. 

    Ecosystems (2012) 15: 819–831 PDF