Discriminating Inorganic From Total Soil Carbon Using Stoichiometric Relationships From Laser-Induced Breakdown Spectroscopy.

Soil organic carbon (SOC) sequestration in soils is of particular interest for developing National ecosystem services markets.  Semi-arid and arid soils commonly contain significant concentrations of inorganic carbon (IC) in the forms of calcium and magnesium carbonates that interfere with SOC determination.  Determining SOC in calcareous soils requires either 1) soil total C (TC) determination through dry combustion and then subtracting IC determined in a separate analysis or 2) removing IC from a soil sample prior to dry combustion.  Both approaches require additional time, resources, and cost for SOC measurements. It is conjectured that stoichiometric relationships between soil elements could be used to discriminate IC from TC, provided soil elemental data is available.  Samples from two national soil archives (USGS and NRCS) were laboratory analyzed for TC, IC, and total elemental analysis.  Correlations between major soil elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) and TC and IC were examined for stoichiometric relationships.  Additionally, we scanned subsamples of archived soils using a custom laser-induced breakdown spectroscopy (LIBS) instrument for rapid soil elemental analysis.  An emerging total elemental characterization technique, LIBS has the potential to discriminate TC from IC with minimal soil preparation and proper calibration.  Soil samples and associated LIBS data were partitioned into calibration and validation datasets such that validation samples were spatially independent of calibration samples.  The LIBS emission lines for the major soil elements found to have stoichiometric relationships with TC and IC were calibrated to measured elemental concentrations and independently validated for predicting elemental concentration in validation samples.  Partial least squares regression models for predicting TC and IC were also calibrated and independently validated using the same LIBS datasets.  The ability using stoichiometric relationships to distinguish IC from TC could streamline SOC determination, reduce costs, and support the development of ecosystem services markets.