N isotope fractionation and measures of organic matter alteration during decomposition

Most soil organic matter (SOM) derives from plant material, but there are substantial gaps in our understanding of its transformation mechanisms. Alterations that occur as SOM decays and is stabilized have proved difficult to study owing largely to its diverse initial chemical composition and stable isotope values. We examined SOM stable isotope ratios in relation to composition using solid-state C-13 Cross Polarization Magic Angle Spin (CPMAS) nuclear magnetic resonance (NMR) in sequentially deeper organic and mineral horizon soil samples collected from an unpolluted ecosystem in southeast Alaska, USA, dominated by C-3 vegetation. We found that, as humified carbon (C not respired as CO, during decomposition) increased in aliphaticity (ratio of unsubstituted aliphatics to carbohydrates), it also increased in N-15 but showed no clear trend in C-13. These results imply that humification (and the concomitant stabilization of soil C) at our site resulted from microbial alteration of organics rather than from accumulation of recalcitrant compounds. The strong, and previously unreported, relation between N-15 and SOM composition found at our study site suggests that degree of SOM humification may be correlated with an increase in delta(15)N SOM values relative to delta(15)N for fresh litter and other source material.