We are interested in the response of terrestrial ecosystems to environmental change, and their role in the Earth’s climate system. This is important because as atmospheric CO2 concentrations increase, and temperature and precipitation patterns are altered across much of Earth, we lack the ability to predict if terrestrial ecosystems will be a source or sink of C to the atmosphere.  Our research focuses on C cycling in plants and soils, with expertise in the application of isotopes as tracers of processes.

Our overarching research question is: what is the fate of C in terrestrial ecosystems? This topic spans temporal scales such as whether newly assimilated C is quickly returned to the atmosphere by plant metabolism or sequestered as soil organic matter for centuries to millennia. This topic also covers spatial scales from the study of microbial processes to plant C allocation to landscape-scale climatic controls on ecosystem function.

We combine field measurements, laboratory work, and computational analyses of large continuous datasets. Currently, there are three major themes in our research:

Plant carbon allocation    •   Belowground CO2 fluxes    •   Linking water and carbon

A methodological tool that is used in most of our research is the application of stable and radioactive isotopic tracers, especially radiocarbon (14C) measured by accelerator mass spectrometry (AMS). Radiocarbon is a novel and powerful tool to study terrestrial C cycling on timescales of hours to millennia. It can be used to determine the age of C, the mean residence time or turnover of C pools, and it can also be used as a source tracer. We use natural abundance, bomb spike, and tracer levels of 14C in our research.  We have a new state-of-the-art AMS in the Arizona Climate and Ecosystems (ACE) Isotope Lab at NAU.

We have worked in the Owens Valley and on Santa Cruz Island in California, the Kohala on the Big Island of Hawaii, at Harvard Forest in Massachusetts, Bartlett Experimental Forest in the White Mountains of New Hampshire, and Howland Forest in Maine.  We have ongoing projects at the Rocky Mountain Biological Laboratory in Colorado, at the Sevilleta LTER in New Mexico, and in the coast redwood forests of Northern California. We are currently working on establishing field sites in the diverse ecosystems of Northern Arizona.

Plant carbon allocation

Carbon enters terrestrial ecosystems through one well-understood pathway, photosynthesis. Once within the plant, C may be allocated to above- and below-ground structures, and to growth, metabolism, protection, or storage. However, very little is known as to how plants allocate C, and consequently model representations of these processes are simplistic.

We investigate the fate of newly assimilated C in different ecosystems to understand how new C is allocated to plant metabolism. To do this, we use pulse-chase labeling methods with isotopic tracers (low-levels of 14C measured by AMS and 13C).  See Carbone et al. (2007) and Carbone & Trumbore (2007).

We also study the availability, distribution, and ecological role of nonstructural carbon (NSC; sugars, starch, and lipids) pools in mature trees.  To do this, we measure NSC concentrations in plant tissues and use bomb spike 14C approaches to determine its age (a proxy for the mean residence time).  See  Richardson et al. (2013), Carbone et al. (2013)Richardson et al. (2015), Furze et al. (2019), and Furze et al. (2020).

Ongoing work is focusing on a collaborative project at the Sevilleta LTER in New Mexico looking at the age of C stored within and used by mature piñon pine trees under large scale experimental drought. And a new project in California coast redwoods, looking at the role of nonstructural C in resprouting and resilience in response to the extreme fires that occurred in 2020. Both projects are funded by NSF.

Linking water and carbon

Understanding interactions between C and water cycling is increasingly important as precipitation and temperature patterns change, and C cycling is limited by water across ~40% of the Earth’s surface. We have had various projects that that range from semiarid, subalpine, to tropical forest ecosystems. These studies address basic questions of how C and water cycles (including cloud shading and water inputs, snow, and rainfall) are fundamentally linked, and the implications for the fate of terrestrial C with climate change.  See Carbone et al. (2011), Carbone et al. (2013) and Marin-Spiotta et al. (2011).

Ongoing work on this topic focuses on more than a decade of measurements at the Rocky Mountain Biological Laboratory and newly established field sites on Snodgrass Mountain near Crested Butte, Colorado.  This is the headwaters of the Colorado River and a collaboration with an intensive science effort by Lawrence Berkeley National Laboratory and funded by the Department of Energy. 

Belowground CO2 fluxes

Terrestrial ecosystem respiration is the largest flux of CO2 to the atmosphere, integrating above- and below-ground, plant, microbial, and abiotic sources. Flux measurements alone (e.g., from eddy covariance towers or soil chambers) cannot distinguish the contributions of CO2 from these different sources. The development of process-based models that can predict how plants, microbes, and abiotic sources respond to changing environmental conditions requires that field experiments be able to separate them.

The majority of ecosystem respiration comes from belowground, called soil respiration. Current models do not adequately represent the mechanisms causing variation in soil respiration primarily because of methodological difficulties associated with measuring soil respiration at high temporal frequencies, and separately attributing flux variations to different biotic and abiotic processes.  Our research addresses these two primary experimental challenges by combining high-time resolution flux measurements and 14C source partitioning techniques Carbone et al. (2008), Carbone et al. (2011), Carbone et al. (2013).  We apply the information to improve models of soil and ecosystem respiration Carbone et al. (2016).  All our past automated soil respiration data is available in COSORE.

We are seeking a PhD student or postdoc interested in synthesizing arid ecosystem soil respiration fluxes and establishing new measurements in Northern Arizona.