Soil Organic Matter Functional Group Composition in Relation to Organic Carbon, Nitrogen, and Phosphorus Fractions in Organically Managed Tomato Fields

The objectives of this study were to examine soil organic matter (SOM) functional group composition and its relationship to labile SOM fractions with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We analyzed soils from 13 organically managed tomato (Solanum lycopersicum) fields in northern California for labile organic C, N, and P fractions and by DRIFTS for bands representing organic functional groups, including aliphatic C-H (2924, 2850, 1470, 1405, 1390 cm(-1)), aromatic C=C (1650 cm(-1)) and C-H (920, 840 cm(-1)), polysaccharide and phenol C-O (1270, 1110, 1080 cm-1), and amine and amide N-H (3400, 1575 cm(-1)). Significant differences in relative band intensities occurred among the 13 organic tomato fields, in particular a relative increase in absorbance of bands representing aliphatic C-H positively associated with soil organic carbon (SOC), as well as permanganate-oxidizable carbon (POXC), extractable organic carbon (EOC) and nitrogen (EON), and potentially mineralizable N (PMN). In comparison, organic P fractions like sodium bicarbonate extractable (NaHCO3-P-o) and sodium hydroxide extractable organic P (NaOH-P-o) were poorly associated with SOC and functional groups represented by bands, including aliphatic C-H. This could reflect limitations of DRIFTS, but is consistent with hypotheses of greater decoupling of C and P vs. C and N in soils. This study implicates relative differences in organic functional groups with differences in SOC and labile SOM fractions, and in agreement with previous studies, identifies absorbance of infrared bands representing aliphatic C-H functional groups in these systems as a potential indicator of SOM transformations related to changes in its labile fractions.