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Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils

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Abstract

The exudation of carbon (C) by tree roots stimulates microbial activity and the production of extracellular enzymes in the rhizosphere. Here, we investigated whether the strength of rhizosphere processes differed between temperate forest trees that vary in soil organic matter (SOM) chemistry and associate with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. We measured rates of root exudation, microbial and extracellular enzyme activity, and nitrogen (N) availability in samples of rhizosphere and bulk soil influenced by four temperate forest tree species (i.e., to estimate a rhizosphere effect). Although not significantly different between species, root exudation ranged from 0.36 to 1.10 g C m−2 day−1, representing a small but important transfer of C to rhizosphere microbes. The magnitude of the rhizosphere effects could not be easily characterized by mycorrhizal associations or SOM chemistry. Ash had the lowest rhizosphere effects and beech had the highest rhizosphere effects, representing one AM and one ECM species, respectively. Hemlock and sugar maple had equivalent rhizosphere effects on enzyme activity. However, the form of N produced in the rhizosphere varied with mycorrhizal association. Enhanced enzyme activity primarily increased amino acid availability in ECM rhizospheres and increased inorganic N availability in AM rhizospheres. These results show that the exudation of C by roots can enhance extracellular enzyme activity and soil-N cycling. This work suggests that global changes that alter belowground C allocation have the potential to impact the form and amount of N to support primary production in ECM and AM stands.

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Acknowledgments

We would like to thank Colin Averill, Joy Cookingham, Verity Salmon, Poliana Lemos, Marc-Andre Giasson, Jon Karis, Kim Spiller and Winston MacDonald for laboratory assistance. In addition, we thank the Harvard Forest and the NH Department of Resources and Economic Development for granting us access to perform field research. This work was supported by the National Science Foundation (DEB-0743564; DEB-1011479). Also, E.R.B was supported by a Northern Forest Scholar fellowship from the Northeastern States Research Cooperative, a joint program of the University of Vermont, the University of Maine and the Northern Research Station, USDA Forest Service.

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Authors and Affiliations

  1. Department of Geography, Indiana University, Bloomington, IN, USA

    Edward R. Brzostek

  2. Department of Biology, Boston University, Boston, MA, USA

    Alison Greco, John E. Drake & Adrien C. Finzi

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  1. Edward R. Brzostek
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  2. Alison Greco
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Correspondence to Edward R. Brzostek.

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Brzostek, E.R., Greco, A., Drake, J.E. et al. Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils. Biogeochemistry 115, 65–76 (2013). https://doi.org/10.1007/s10533-012-9818-9

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  • DOI: https://doi.org/10.1007/s10533-012-9818-9

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