Extracellular enzyme activity in the mycorrhizospheres of a boreal fire chronosequence

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.

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). Frequently, roots decrease turnover rates of litter C and increase turnover rates of soil organic C (Cheng and Kuzyakov 2005). Traditionally, mycorrhizal fungi have not been explicitly incorporated within the rhizosphere concept (Timonen and Marschner 2006), even though mycorrhizal fungi colonize the roots of most terrestrial plants (Newman and Reddell, 1987, Allen et al., 1995). The most common mycorrhizal groups are arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and ericoid mycorrhizal fungi. They differ in their morphology, taxonomy, and physiology (Smith and Read 2008), so mycorrhizosphere effects on nutrient cycling could likewise vary depending on the mycorrhizal group involved (Linderman 1988). Nevertheless, few field studies have contrasted mycorrhizosphere effects associated with these three major groups.

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). However, there is accumulating evidence that mycorrhizal fungi produce a greater variety of enzymes than originally thought, including enzymes that target recalcitrant organic C (reviewed in Cairney and Burke, 1998, Burke and Cairney, 2002, Read and Perez-Moreno, 2003). Thus, mycorrhizal fungi may also directly contribute to SOM decomposition (Talbot et al. 2008).

The extent to which plants and their mycorrhizal fungi contribute to extracellular enzyme production and decomposition in natural systems, however, remains unclear. Plant roots can directly release extracellular phosphatases for P acquisition (Duff et al. 1994) and proteases for N acquisition (Godlewski and Adamczyk 2007), but are not generally considered important producers of extracellular enzymes targeting lignocellulosic compounds. Culture-based studies have indicated that the enzyme capacity of mycorrhizal fungi is rarely as high as that of fungal saprotrophs. Specifically, white-rot fungi frequently display higher capacities than ericoid fungi, followed by ectomycorrhizal fungi, and then arbuscular mycorrhizal fungi (Read and Perez-Moreno 2003).

Fires are becoming more frequent in boreal forests, with potential consequences for nutrient dynamics. For instance, annual totals of burned area in Alaska increased 50% in the last decade, ostensibly due to a warmer, drier climate in this region (Kasischke et al. 2010). Fires can alter the nutrient availability of soils, directly via volatilization and mineralization of nutrients, and indirectly via changes in plant community composition and litter quality (Kasischke and Stocks 2000). Specifically, N availability can either increase or decrease following a fire, depending on fire severity and changes in plant community composition; organic material is often burned off and re-accumulates over time; and P availability tends to be greatest in young burn scars (Van Cleve et al., 1983, Van Cleve et al., 1993, Van Cleve et al., 1996, Smith et al., 2000, Treseder et al., 2004, Harden et al., 2006, O’Neill et al., 2006).

These shifts in nutrient availability during ecosystem recovery might alter the nutrient status of plants, mycorrhizal fungi, and other organisms. According to economic principles, plants and mycorrhizal fungi are expected to invest more in the production of a particular extracellular enzyme when the nutrient targeted by that enzyme is limiting to growth (Allison et al. 2010). Furthermore, enzymatic stoichiometry indicates the relative degree to which organisms are investing in acquisition of various nutrients. Previous studies have indicated that ratios of β-1,4-glucosidase:N-acetyl-glucosaminidase assess acquisition of C versus N (Sinsabaugh et al., 2008, Sinsabaugh et al., 2009). As ecosystems recover from fire, we might expect that ratios of β-1,4-glucosidase:N-acetyl-glucosaminidase should be relatively high in fairly young sites, since soil organic C is scarce; and also relatively low where N availability is high. Any shifts as ecosystems recover from fire may have consequences for large-scale nutrient dynamics, since younger ecosystems are becoming more common in the boreal landscape owing to shortening fire-return intervals (Kasischke et al. 2010).

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. Our hypothesis focused on the activities of enzymes in soil associated with the different mycorrhizal plants. Based on laboratory findings (e.g., Read and Perez-Moreno 2003), we hypothesized that enzyme activity would increase in the order of arbuscular mycorrhizal mycorrhizospheres < ectomycorrhizal (or dual-colonized arbuscular mycorrhizal + ectomycorrhizal) mycorrhizospheres < ericoid mycorrhizospheres. In addition, we hypothesized that investment in C versus N acquisition should vary with site age as described above.