Print PageWe address whether trait-based mechanisms determine which microbial species respond to N enrichment, and whether the species that proliferate are those that will reduce long-term C sequestration. We hypothesized that taxa that invest in the decomposition of recalcitrant C compounds may have N demands that cannot be met at low N availabilities, owing to evolutionary trade-offs. In addition, we hypothesized that larger, more complex (i.e., more recalcitrant) organic C compounds will be decomposed by a broader phylogenetic range of taxa than are more labile C compounds, and that a narrow phylogenetic range of fungi will proliferate under N enrichment. We tested these predictions in an Alaskan boreal ecosystem, where previous work has found that N additions decrease organic C concentrations in soil. We used DNA sequencing to identify fungal taxa that were positively or negatively affected by N fertilization. In addition, we assessed the relative abundance of recalcitrant- and labile-C users in situ by using nucleotide analog labeling of DNA. Finally, we conducted a database synthesis of 519 bacterial taxa for which substrate use profiles were available, based on GEN III MicroPlate assays on axenic cultures. We calculated the average molar weight of organic C compounds used by each taxon, as an indicator of recalcitrant C use, and then we searched for each taxon in published ocean samplings.
Results/Conclusions
We found that bacterial taxa that used more recalcitrant C compounds were significantly more prevalent in ocean waters with higher nitrate concentrations (independent contrasts, r = 0.680, P < 0.001). Furthermore, we found that larger organic C compounds were used by a broader phylogenetic range (i.e., lower NRI) of bacteria than were smaller compounds. In our Alaskan field sites, fungi that used lignocellulose, a relatively recalcitrant compound, were more phylogenetically dispersed than expected by random chance (net relatedness index or NRI = –1.3). Fungal taxa that occupied N-fertilized plots were more closely related to one another (NRI = 4.4 ±2.28 SE) than were those in the control plots (NRI = -0.35 ±0.40, P = 0.021). Our study suggests a theoretical framework with which to predict how microbes will respond to global change. Specifically, we identified a mechanism (evolutionary trade-offs) that could potentially link sensitivity to environmental conditions with ecological functions of individual species of microbes. Ultimately, N additions may lead to decreases in long-term C sequestration in the boreal forest.
