Nitrogen addition to a mixed pine-broadleaved forest under elevated atmospheric CO2 exacerbates phosphorus and potassium deficiencies

Abstract

Forests are vital ecosystems because they capture and store carbon from our atmosphere; this is essential for maintaining the balance between carbon uptake and release in the global carbon cycle. However, human activities—mainly burning fossil fuels—are releasing carbon dioxide (CO2) into our atmosphere at a rate we are not confident forests can keep pace with. As we face climate change, it is critical to understand exactly how forests respond to elevated CO2 to inform best forest management practices.

Since trees utilize CO2 to photosynthesize, it is generally thought that increased amounts of CO2 will increase biomass, which will create a positive feedback loop for carbon sequestration. However, there are likely limitations—namely of nutrients—to production that prevent this from being the case. This study aims to better understand such tree nutrient limitations and responses to climate change conditions through the most realistically simulated means possible.

This project uses data from the Duke Forest Free-Air CO2 Enrichment (FACE) experiment to investigate the effect of CO2-fumigation and nitrogen (N)-fertilization on nutrient concentrations in trees in their natural ecosystems. The experiment began in 1993 in the Blackwood Division of the Duke Forest in Durham, North Carolina on a 90-ha loblolly pine plantation with relatively acidic soils of low fertility. The split-plot randomized block experimental design had two main levels: first, four of the eight 30mdiameter plots were fumigated with elevated CO2; next all eight plots were divided in half and only one half was fertilized with N. This created four distinct treatment groups: Ambient Control, Ambient Fertilized, Elevated Control, and Elevated Fertilized—each represented by four “half-plots.” Starting in 2010, loblolly pine and broadleaved foliage, wood, and roots were harvested and analyzed for the six main plant nutrients: carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). We hypothesized that low concentrations of any other main plant nutrients—not just N—would mute trees’ response to elevated CO2. That is, fertilizing with only N would not be sufficient to alleviate nutrient limitations.

Data from the Duke FACE experiment were analyzed for a signature of any nutrient limitations under elevated CO2. Linear mixed models with random effects were created for each main plant element (C, N, P, K, Ca, and Mg) in each tree component. These components included foliage, branch wood, stem wood, and roots for loblolly pine, sweetgum, a collection of other broadleaved species, small understory specimens, and vines. The statistical models were used to assess whether nutrient concentrations were significantly affected by elevated CO2 and N-fertilization.

Overall, elevated CO2 and N-fertilization did not significantly affect any nutrient concentrations across all species in all aboveground components. We then compared Duke FACE loblolly pine foliar nutrient ratios to reference values—or values that are considered adequate for normal tree growth—and found that the system was P- and K-limited (P:N and K:N were 24% and 26% below adequate). In fact, adding N not only failed to alleviate P- and K-limitations, but significantly exacerbated them.

Interestingly, we found a strong response to elevated CO2 and N-fertilization in the roots, especially in loblolly pine. Under N-fertilization, [P] was almost 16% higher and [K] was 29% higher under elevated CO2 than under ambient CO2 in pine roots, suggesting increased root production and exploration for the elements that limited tree production. The additional C supplied by elevated CO2 and exacerbated nutrient limitation by N-fertilization created a high demand for P and K to support increased biomass production. In response, root biomass increased, and P and K were locked belowground and used locally.

The Duke FACE experiment is special because it allows us to understand how trees will respond to future climate change conditions before they happen. With this vital information, we can implement preemptive forest management practices that support continued production and carbon sequestration under elevated CO2. The results of this study underscore that elevated CO2 will only increase tree production when all main plant nutrients are adequately available. Thus, balanced fertilizers, rather than only N-fertilizers, should be applied to forests to ensure that our global carbon cycle is maintained during a most imperative time.