Soil Organic Matter Characterization in Bolivia and Ecuador.

Organic matter stabilization is thought to be a process of physical protection and chemical recalcitrance.  The determination of recalcitrant soil organic carbon (SOC) often relies on operational definitions provided by various fractionation techniques, usually particle size or density fractionation.  However, it is unknown if these operational definitions represent true chemical recalcitrance.

Many labs employ density and/or size-based fractionation of SOC as a short-term proxy for determination of ‘labile' and ‘recalcitrant' C.  In practice, ‘labile' C determined from these fractionation procedures is a sensitive indicator of management changes, and so is an attractive parameter for the determination of short-term changes due to management practices.  However, there is evidence that SOC fractionation techniques do not correlate to C chemical stability, and hence recalcitrance, of SOC as determined by NEXAFS and C isotopes.  It has generally been assumed that O-alkyl C is more easily metabolized by soil microbes than aromatic C, and therefore more labile.

We seek to move beyond the operational definitions of labile and recalcitrant C to look at the chemical structural changes in SOC due to conservation agriculture (CA) in the developing world.  This work presents C speciation data from fractionated soil samples from the Andes.  Density fractionation was obtained at the <1.8, 1.8-2.0, and >2.0 g cm^-3 densities in sodium polytungstate (SPT), followed by total C and N content by dry combustion.  Preliminary data on the structural differences between and among fractions and the bulk soil was determined using C k-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.  These data may serve as a proof of concept to determine if density fractionation results represent chemically different SOC pools.  These data will serve as Time Zero reference points against which the influence of conservation agriculture on SOC will be compared, with traditional agricultural practices in each country to serve as a control treatment.