SGER: 13C and 15N Isotope Fractionation and Measures of Organic Matter Humification During Decomposition Across Ecosystems

University Corporation At Monterey Bay, Seaside CA

Investigators

Abstract

In a recently completed study in SE Alaska we found a strong and unexpected correlation between 15 N and aliphaticity in soils. Using Nuclear Magnet Resonance (NMR), we are developing a relatively inexpensive and simple technique for estimating humification in soils. The unusually high C content of these Alaskan mineral soils (up to 18%), we believe, facilitated obtaining reliable NMR spectra and stable isotope values. We propose here to extend this previous work to forest soil types across a broader range of climatic zones, all of which tend to be lower C content (<7%) and drier than these Alaskan soils. We will determine the extent to which 15 N and 13 C values correspond to degree of soil organic matter humification (using NMR to estimate aliphaticity) in 4 contrasting ecosystems, in particular those in which illuviation is less pronounced than in the Alaskan soils, N is less limiting (open N systems), and microbial processes are faster. To do this will require exploring a variety of techniques to obtain reliable NMR spectra in the presence of paramagnetics (especially Fe). Based on the widely reported and consistent increase in 15 N values with depth (and an associated lack of consistent trend with depth reported for 13 C), our expectation is that 15 N but not 13 C values will correspond to degree of humification across ecosystems worldwide. If we successfully obtain reliable NMR spectra for the soils we study, and if this 15 N-aliphaticity pattern holds elsewhere, we will have developed a comparatively cheap proxy for degree of SOM humification, which should allow controls on this important process to be studied much more efficiently than now.