Extracellular enzyme activity in the mycorrhizospheres of a boreal fire chronosequence

Tracy B Gartner,Robert L Sinsabaugh,Kathleen K Treseder,Glenna M Malcolm

doi:10.1016/j.pedobi.2011.12.003

Abstract

Abstract Saprotrophic microbes are typically credited with producing extracellular enzymes that recycle organic matter, though roots and mycorrhizal fungi also can contribute and may compete with the saprotrophs. We examined extracellular enzyme activity associated with the mycorrhizospheres of arbuscular mycorrhizal, ectomycorrhizal, dual-colonized (arbuscular and ectomycorrhizal), and ericoid mycorrhizal plants in a fire chronosequence in Alaska. Bulk soil and soil from beneath host plants were gathered in July 2004 and assayed for five enzymes that target organic C, P, and N substrates. Compared to bulk soil, activities of the C-targeting enzymes β-1,4-glucosidase and peroxidase were lower in arbuscular mycorrhizospheres and ericoid mycorrhizospheres, respectively. Moreover, extracellular enzyme activity varied among mycorrhizosphere types. Specifically, N-targeting leucine aminopeptidase was highest in arbuscular mycorrhizospheres, followed by ericoid and ectomycorrhizal/dual-colonized mycorrhizospheres; β-1,4-glucosidase had the reverse pattern. In addition, enzymatic stoichiometry suggested that extracellular enzyme producers invested more in C-acquisition than in N-acquisition in recent fire scars compared to mature forests. These data extend previous findings that roots and mycorrhizal fungi compete with saprotrophs by showing that the strength of this effect varies by mycorrhizal host. As a result the community composition of mycorrhizal host plants might mediate enzymatic activity in boreal soils.

Highlights

Mycorrhizospheres of plants consist of plant roots and their associated mycorrhizal structures, plus the surrounding soil and microbes directly influenced by them (Rambelli 1973)
We examined extracellular enzyme activity associated with the mycorrhizospheres of arbuscular mycorrhizal, ectomycorrhizal, and ericoid plants in a fire chronosequence of upland boreal forests of Alaska
Compared to bulk soil, enzyme production was reduced in the mycorrhizospheres of arbuscular mycorrhizal plants for ␤-1,4-glucosidase, and in the mycorrhizospheres of ericoid plants for peroxidase

Summary

Introduction

Mycorrhizospheres of plants consist of plant roots and their associated mycorrhizal structures, plus the surrounding soil and microbes directly influenced by them (Rambelli 1973). Numerous studies have established that ecological processes such as decomposition, N mineralization, and microbial community composition can be altered by the presence of plant roots; this phenomenon is referred to as the “rhizosphere effect” (e.g., Hiltner 1904; Katznelson 1946; Cheng et al 2003). Fungi and bacteria conduct nutrient transformations by secreting extracellular enzymes that break down soil organic matter (SOM) and release C, N, and P. Mycorrhizal fungi rely primarily on host plant C, so their contribution to SOM degradation is generally considered modest relative to that of the asymbiotic microbial community (Dighton 2003). Mycorrhizal fungi may directly contribute to SOM decomposition (Talbot et al 2008)

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Discussion

Conclusion