Pyrolysis Molecular Beam Mass Spectrometry as a Method of Characterizing Soil Organic Matter.

At the molecular level, soil organic matter (SOM) is extremely complex and is composed of thousands of different organic C compounds. Varied chemical and physical fractionation methods have been developed to characterize SOM compounds, but these methods are time consuming and results are method dependent. Advances in spectroscopic analysis have provided the biochemical analysis of SOM without isolating the organic matter. Pyrolysis molecular beam mass spectrometry (py-MBMS) has been recently utilized for the characterization of SOM and has the added advantage of high sample throughput while preserving high molecular weight products compared to other pyrolysis methods. Since this analytical method has only been recently applied to soil there is limited information on how much SOM is reproducibly pyrolyzed in the process and the identity of many of the pyrolysis products still needs to be understood. To better develop this method for future use in SOM characterization we quantified the amount of soil organic matter pyrolyzed in soil samples with varying soil characteristics and determined pyrolysis yields with the addition of soil carbon standards like lignin, amino acids and carbohydrates to chemically characterize their spectral signatures in the soil matrix. We found that between 50 and 70 % of the soil organic matter is pyrolyzed and that the total ion intensity correlates well with the percent soil C. Pyrolysis of the carbon standards by themselves and mixed with soil produced mass/charge peaks at their respective molecular weight in many of the compounds analyzed. We conclude from these preliminary method development results that py-MBMS is well suited to characterizing soil organic matter chemistry and additionally may be able to track the changes that occur from site specific characteristics, but results can be limited in soils with less than 1% C or high clay content.