I study how changing environmental conditions affect the cycling and availability of nutrients to terrestrial plant communities. I aim to understand how these nutrient dynamics feed back to the fates of terrestrial carbon. I currently focus on how temperature affects microbial-community and ecosystem nitrogen (N) and carbon (C) dynamics across natural environmental gradients.
Temperature Effects on Soil Nutrient Availability
Using a natural elevation gradient on the Island of Hawaii, I have investigated how mean annual temperature (MAT) affects the cycling and availability of N to plants. I am also working to quantify the effect of mean annual temperature on gross N cycling dynamics and associated soil microbial communities in a temperate forest MAT gradient at the Hubbard Brook Experimental Forest in New Hampshire, USA. Finally, I am working to determine the effect of elevated N and phosphorus (P) deposition on soil microbial N cycling in long-term N and P fertilization gradient plots in New Hampshire, USA.
Temperature-Driven Forest Nutrient Limitation
Plant roots forage for nutrients, but may modify how much effort is put towards accessing high-nutrient patches depending on environmental conditions such as temperature and background nutrient availability. We tested whether primary production is nitrogen (N) or phosphorus (P) limited at different mean annual temperature (MAT) across a natural MAT gradient on the island of Hawaii (Litton et al. 2011). We levereaged fine root ingrowth into fertilized root-ingrowth cores as an index for plant nutrient demand. We also analyzed the effects of temperature and ambient soil N availability on rates of arbuscular mycorrhizal colonization. Together, our results suggest that as MAT and N bioavailability increase colinearly, plant roots forage for a combination of nitrogen and phosphorus, rather than one nutrient alone. Plants also have significantly higher associations with arbuscular mycorrhizal fungi as MAT increases. Together, these results suggest that as temperatures increase, N bioavailability increases, and plants respond with increase foraging for both N and P together. Further, plants increase their ability to access both N and P at higher MAT through elevated rates of mycorrhizal colonization (Pierre et al. in review) .
Functional Genes as Drivers of forest Nutrient Cycling
To better understand the molecular controls on N bioavailability with increasing MAT, I quantified the abundance and expression of the microbial gene amoA, which controls the transcription of rate-limiting nitrification enzymes. Our results suggest that temperate regulates the abundance of this gene, thereby contributing directly to nitrogen available to plants in forests (Pierre et al. 2017).
Urban Afforestation & Soil Nitrous Oxide Flux
In urban systems, forest nitrogen processing may function differently than in natural forests. These differences may be important for understanding how urban forest creation will affect greenhouse gas fluxes from cities. In New York City, I studied the effects of different urban afforestation strategies on the production of nitrous oxide, a potent greenhouse gas (Pierre et al. 2016). We found that changing the types of woody plants in an urban forest may differently stimulate nitrous oxide production. Ultimately, these management choices may alter urban greenhouse gas mitigation efforts.