Genotypic variation within a foundation tree species can have ecosystem-level consequences. For example, within naturally hybridizing Populus species, foliar condensed tannin (CT) concentration is genetically based and has a strong negative influence on decomposition and nitrogen (N) mineralization rates. We investigated whether genetic-based changes in foliar CT among naturally occurring Populus hybrids might also affect the abundance and function of ammonia oxidizers, a specific functional group of soil chemoautotrophic microorganisms not trophically linked to plants. Soil ammonia oxidizers, consisting of both bacteria and archaea, perform the first and rate-limiting step of nitrification, a key ecosystem process regulating the availability and retention of soil N. We measured potential nitrification rates and the abundance of both bacterial and archaeal ammonia oxidizers (by real-time PCR amplification of amoA genes) in soil collected from nine forest stands along a naturally occurring Populus hybridization gradient in the Weber River drainage of northern Utah. We also measured the concentration of foliar CT in green leaf tissue collected from the same stands as a surrogate for CT inputs.
We found that foliar CT concentration had a significant, negative effect on both potential nitrification rates (R2 = 0.54) and the abundance of soil ammonia oxidizers (R2 = 0.45). Soil total N, foliar N, and soil pH were not significant predictors of either activity or abundance of soil ammonia oxidizers. These results have two important implications. First, they demonstrate the extended effects of plant genes on a key ecosystem process, nitrification. Second, they provide evidence for indirect genetic linkages among autotrophs across all three domains of life.