Functional responses of ectomycorrhizal fungal communities to long-term fertilization of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) stands in central British Columbia

Abstract

Repeated fertilization aimed at maintaining optimal tree nutrition can produce substantial increases in timber volumes and shorten rotation times in lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) stands in British Columbia. Intensive N enrichment of soil can alter belowground microbial communities, especially ectomycorrhizal fungi, with reduced ability to access organic nutrients; however, it is not known whether long-term application of balanced fertilizers has the same effect. Between 1996 and 2007, three P. contorta stands in central British Columbia, Canada, were exposed to three fertilizer treatments. Three 0.164ha plots per site received no (controls), periodic (200kgha−1N, plus P, K, S, Mg, and B every 6 years), or annual (same nutrients adjusted to maintain foliar N concentration at 1.3% and other nutrients in balance) fertilization. Ectomycorrhizal fungal communities were characterized from seven samples per plot by morphotyping, followed by direct sequencing of fungal ribosomal DNA. Activities of eight hydrolytic or oxidative enzymes were quantified in the mycorrhizospheres of the four dominant types of ectomycorrhizae per plot. Increased frequency of fertilization reduced taxonomic richness of the ectomycorrhizal fungal community on roots, and reduced the relative abundance of mycorrhizae formed by Suillus spp. and Cortinarius spp. The proportions of ectomycorrhizae formed by Piloderma spp. and Cenococcum spp. were not affected, and others were too spatially heterogeneous or too rare to evaluate. The activities of cellobiohydrolase, β-glucosidase, xylosidase, leucine amino peptidase and laccase associated with Piloderma and Cenococcum mycorrhizae increased under annual fertilization for at least one site. Enzyme activities of Cenococcum mycorrhizae were positively correlated with total soil N (P<0.05), whereas those of Piloderma mycorrhizae were negatively correlated with pH (P<0.05). In spite of treatment and site effects, the dominant influence on the enzyme profile of an ectomycorrhizal root was the fungal symbiont. We conclude that the shifts in species composition with fertilization did not reduce the ability of the ectomycorrhizal fungal community to degrade soil organic matter; hence, we do not expect this ecosystem service to be compromised by long-term optimal nutrition fertilization, especially if applied on a periodic basis.