Location: Commodity Utilization Research
Title: Analysis of carbon functional groups in mobile humic acid and recalcitrant calcium humate extracted from eight US soils Authors
Submitted to: Pedosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 16, 2012
Publication Date: October 28, 2013
Citation: He, Z., Cao, X., Mao, J., Ohno, T., Waldrip, H.M. 2013. Analysis of carbon functional groups in mobile humic acid and recalcitrant calcium humate extracted from eight US soils. Pedosphere. 23(6):705-716. Interpretive Summary: Soil organic matter is involved in mineral weathering, trace metal mobility and toxicity, and other ecosystem processes. Soil organic matter is formed from decomposed plant and microbial biomass, and can be separated into different fractions. One of these approaches involves the sequential extraction of mobile humic acid (MHA) and recalcitrant Ca humate (CaHA) fractions. In this work, we sequentially extracted MHA and CaHA fractions from eight soils collected from six U.S. states, representing a variety of soils and ecoregions. The C functional groups of these fractions were characterized by solid state 13C NMR spectroscopic techniques. The relationships between soil properties, C functional groups identified by solid state 13C NMR spectroscopy, and ultrahigh resolution mass spectrometry were statistically analyzed. To our knowledge, this type of correlation analysis has not been reported in the literature. This work obtained basic structural information of the humic fractions of these soils, which would be useful in understanding differences in nutrient cycling and in promoting data comparison between these soils. Further data comparison provided insight on the internal bonding/association of the C functional groups of soil humic fractions.
Technical Abstract: Solid state 13C nuclear magnetic resonance (NMR) spectroscopy is a common tool to study the structure of soil humic fractions; however, knowledge regarding carbon structural relationships in humic fractions is limited. In this study, mobile humic acid (MHA) and recalcitrant calcium humate (CaHA) fractions were extracted from eight soils collected from six U.S. states and representing a variety of soils and ecoregions, were characterized by this spectroscopic technique and analyzed for statistical significance at p=0.05. We found that the abundances of COO and N-C=O functional groups in the MHA fractions was negatively correlated to soil sand content, but was positively correlated to silt, total N and soil organic carbon contents. In contrast, the abundances of the COO and N-C=O functional groups of was only positively correlated to the content of clay in the CaHA fractions, indicating that the two humic fractions were associated with different soil components. The two 13C NMR peaks representing alkyls and OCH3/NCH were negatively correlated to the peaks representing aromatics, aromatic C-O and N-C=O/COO. Comparison of the sets of data from 13C NMR spectroscopy and ultrahigh resolution mass spectrometry revealed that the aromatic components identified by the two methods were highly consistent. The comparison further revealed that protein in MHA was associated with, or bound to, the nonpolar alkyl groups, but a component competitively against (or complementary to) aromatic groups in the MHA composition. These observations provided insight on the internal correlations of the functional groups of soil humic fractions.