29. Ecosystem Response to Climate Change in the Western United States: Linking Carbon, Nitrogen, and Water Cycling Through Isotopic Studies of Soils

Climate change in the Western United States will simultaneously affect the water, carbon, and nutrient cycles of ecosystems, with concomitant changes in biological resources and the ecosystem services on which society depends. Of particular importance in the region is the seasonality and amount of precipitation, which is expected to shift toward drought conditions in the Western United States over the next 50 years (Milly and others, 2005).  As a result, a decline in water available for plant growth and nutrient-cycling in soils will likely lead to dramatic shifts in ecosystems and ecosystem services involving water and carbon.

Much research has been devoted to understanding how variations in climate affect the cycling of soil carbon and nitrogen, the most essential nutrient in Western U.S. soils (Luo and others, 2006). Changes in water, carbon, and nitrogen involve dynamic processes that are inexorably linked through atmosphere - terrestrial ecosystem processes.   An effective metric for dynamic cycling is the use of residence time, or turnover time, because water, carbon, and nitrogen are cycled through plants, then biota, and finally through various forms in soils. Very short residence times are indicative of active, available forms. Long residence times are suggestive of inactive or unavailable forms; yet such reservoirs are important because they potentially become active when environmental conditions change. 

While water, carbon, and nitrogen cycling have been studied independently, few if any studies have quantitatively linked water, carbon and nutrients through their active cycling.  Carbon, water, and nutrient information is captured by soil in two useful ways: (1) Soils are dynamic and responsive to short-term variations in climate. For example, evapotranspiration of water and respiration of carbon from soil have diurnal patterns that closely mimic changes in moisture and photosynthetic uptake, and (2) soils also contain less dynamic, more stable forms that reflect longer-term “legacies” of water, carbon, and nutrient cycling (Trumbore, 2006; Maher and others, 2006; Dijkstra and others, 2008).  By quantitatively linking residence times of water, carbon, and nitrogen to climate, therefore, it may be possible to forecast the vulnerability of these constituents to changes in climate.

We seek a postdoctoral fellow to advance new and emerging isotopic techniques in the geological, pedological, and microbial sciences. A strong background combining biogeochemistry with earth surface dynamics as part of broad studies of biogeochemistry at the landscape scale is preferred.  The successful candidate will explore and examine the potential for linking water, carbon, and nitrogen cycling in soils through the use of isotopic techniques. USGS has over 1500 archival collections of soil profiles collected in the 1900s from the western U.S. in a variety of soil types, landforms, geomorphic ages, and climatic settings that can provide a powerful starting point for evaluating the utility of different isotopic methods. Approaches for determining residence and turnover times of water (using U/Sr isotopes), carbon (¹⁴C, ¹³C) and nitrogen (¹⁵N) on decadal timescales are of particular importance for capturing changes over historic time periods.  In later stages of this research, the results from this work will allow for forecasting ecosystem vulnerability to global change phenomenon. 

Some of the hypotheses to consider include:

1. In the Western United States, the cycling of water, carbon, and nitrogen are quantitatively linked through the availability of water for plant uptake and soil leaching. 
2. Residence times of water, carbon and nitrogen can be coupled to water availability on landscape scales using historic climate records and isotopic measures of residence times
3. A predictive understanding of the fate of water, carbon, and nitrogen cycling under changing climate can be gained from landscape and soil assessments.

References

Dijkstra, P., Laviolette, C. M., and others, 2008, N-15 enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function: Ecology Letters, v. 11, no. 4, p. 389–397.

Luo, Y. Q., Field, C. B., and others, 2006: Does nitrogen constrain carbon cycling, or does carbon input stimulate nitrogen cycling?: Ecology, v. 87, no. 1, p. 3–4.

Maher, K., DePaolo, D. J., and others, 2006, U-Sr isotopic speedometer: Fluid flow and chemical weathering rates in aquifers: Geochimica et Cosmochimica Acta, v. 70, no. 17, p. 4417–4435.

Milly, P. C. D., Dunne, K. A., and others, 2005, Global pattern of trends in streamflow and water availability in a changing climate: Nature, v. 438, no. 7066, p. 347–350.

Trumbore, S. E., 2006, Carbon respired by terrestrial ecosystems—Recent progress and challenges: Global Change Biology, v. 12, p. 141–153.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Soil science, geochemistry/biogeochemistry, stable isotope science, hydrology, soil microbiology

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Chemist, Research Soil Scientist, Research Hydrologist, Research Biologist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Jennifer Harden, (650) 329-4949, jharden@usgs.gov; Mark Waldrop, (650) 329-5005, mwaldrop@usgs.gov; Katharine Maher (Stanford University), (650) 329-4978, kmaher@usgs.gov; Carol Kendall, (650) 329-4576, ckendall@usgs.gov; Jayne Belnap, (435) 719-2333, jayne_belnap@usgs.gov

Human Resources Office contact: Candace Azevedo, (916) 278-9393, caazevedo@usgs.gov

Summary of Opportunities