Mycorrhizal Response to Experimental pH and P Manipulation in Acidic Hardwood Forests

Abstract

Many temperate forests of the Northeastern United States and Europe have received significant anthropogenic acid and nitrogen (N) deposition over the last century. Although temperate hardwood forests are generally thought to be N-limited, anthropogenic deposition increases the possibility of phosphorus (P) limiting productivity in these forest ecosystems. Moreover, inorganic P availability is largely controlled by soil pH and biogeochemical theory suggests that forests with acidic soils (i.e., <pH 5) are particularly vulnerable to P limitation. Results from previous studies in these systems are mixed with evidence both for and against P limitation. We hypothesized that shifts in mycorrhizal colonization and community structure help temperate forest ecosystems overcome an underlying P limitation by accessing mineral and organic P sources that are otherwise unavailable for direct plant uptake. We examined arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) communities and soil microbial activity in an ecosystem-level experiment where soil pH and P availability were manipulated in mixed deciduous forests across eastern Ohio, USA. One year after treatment initiation, AM root biomass was positively correlated with the most available P pool, resin P, while AM colonization was negatively correlated. In total, 15,876 EcM root tips were identified and assigned to 26 genera and 219 operational taxonomic units (97% similarity). Ectomycorrhizal richness and root tip abundance were negatively correlated with the moderately available P pools, while the relative percent of tips colonized by Ascomycetes was positively correlated with soil pH. Canonical correspondence analysis revealed regional, but not treatment, differences in AM communities, while EcM communities had both treatment and regional differences. Our findings highlight the complex interactions between mycorrhizae and the soil environment and further underscore the fact that mycorrhizal communities do not merely reflect the host plant community.

Response of belowground communities to short-term phosphorus addition in a phosphorus-limited woodland

“…This was unexpected given that plants are thought to allocate less carbohydrates to mycorrhizal fungi when nutrients are not limiting (Read 1991). Alternatively, mycorrhizal fungi may respond positively to fertilizer addition if they are nutrient limited (Treseder and Allen 2002) or, more in line with the evidence observed here, if they respond positively to associated changes in soil pH (Kluber et al 2012;Walker et al 2014). Significant indicator OTUs in the unfertilized treatment included fungal genera identified as putative plant pathogens, antagonists of other fungi, and saprotrophs, suggesting that fertilization may have impacted the strength of some biotic interactions, as has been observed for competition in some plant communities (e.g., Coates et al 2013), or may have simply increased the abundance of other fungi relative to those in these functional groups.…”

Soil microbial responses over 2 years following biochar addition to a north temperate forest

“…Despite the changes in soil pH and fungi/bacteria ratio, there was no effect of P treatment on microbial community composition, in contrast to prior studies (Wakelin et al 2009;Kluber et al 2012). It is possible that either P addition did not alter the relative abundances of the observed taxa or that the resolution of the T-RFLP community fingerprinting approach was not sufficient to detect changes in individual functional groups.…”

Seasonal variation in mycorrhizal fungi colonizing roots of Allium tricoccum (wild leek) in a mature mixed hardwood forest

“…Primers were labeled with the fluorochromes 6-carboxyfluorescein (6FAM) (AM1) and 4, 7, 2′ ,4′ ,5′ ,7′-hexachloro-6-carboxyfluorescein (HEX) (NS31). Although previous work has found this primer set specific for AM fungi, as confirmed through cloning and sequencing of PCR amplicon (Burke 2008;Kluber et al 2012), a nonspecific band was observed in the present study at approximately 210 bp along with the specific band representing the 18S rDNA region at 550 bp. It was not possible to prevent amplification of the nonspecific band, despite changes in PCR conditions, including annealing temperature alteration or changes in Taq polymerase.…”

“…Analysis of the root-colonizing AM fungi was thus carried out on gel-extracted PCR products of the putatively specific AM fungal band, but analysis only included those TRFs that could be positively attributed to AM fungi based on predictions of the AM TRFs for recovered clones and those from recent similar studies (Burke 2008;Kluber et al 2012). In this way, the nonspecific A. tricoccum rDNA amplification products would not affect analysis while allowing examination of a sufficiently long PCR product for length polymorphisms.…”