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Title: Soil organic matter functional group composition in relation to organic C, N and P fractions in organically managed tomato fields characterized by mid-infrared spectroscopy

Author
item ANDREW, MARGENOT - University Of California
item Calderon, Francisco
item PARIKH, SANJAI - University Of California
item JACKSON, LOUISE - University Of California

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2015
Publication Date: 4/24/2015
Publication URL: http://handle.nal.usda.gov/10113/61923
Citation: Andrew, M., Calderon, F.J., Parikh, S., Jackson, L.E. 2015. Soil organic matter functional group composition in relation to organic C, N and P fractions in organically managed tomato fields characterized by mid-infrared spectroscopy. Soil Science Society of America Journal. doi:10.2136/sssaj2015.02.0070.

Interpretive Summary: This is a research paper investigating the use of infrared spectroscopy for the study of soil organic matter. Methods of soil and spectral processing for improving the reliability of infrared absorbance with soil nutrient pools were evaluated using soils from 13 organically managed tomato fields in the Sacramento Valley, CA. Enhancement of organic bond absorbances was achieved by subtracting mineral absorbances from infrared spectra of soils, using backgrounds obtained by different treatments. This work demonstrates that spectral subtractions can improve relationships of infrared absorbance with soil nutrient pools, and can be used to provide insight on soil organic matter chemistry to help link differences in nutrient pools with differences in soil carbon content that may reflect management.

Technical Abstract: The chemical composition of soil organic matter (SOM) can be an important determinant of the nutrient supply capacity of soils. This is especially relevant for low-input or organic agricultural systems dependent on SOM mineralization for crop nutrient supply. The objectives of this study were first to improve a method for characterization of SOM composition, and then to examine relationships of soil nutrient pools with markers of SOM composition. We investigated SOM composition of soils from 13 organic tomato fields in northern California with Fourier-transform infrared (FTIR) spectroscopy. Soils were analyzed for C and N pools, scanned to obtain their mid-infrared spectra, and ashed or oxidized with sodium hypochlorite in order to enhance the organic features through spectral subtractions. Hypochlorite oxidation caused fewer changes in mineral spectral features than ashing and yielded subtraction spectra of SOM with fewer artifacts of subtraction. Subtraction spectra obtained by either ashing (AS) or oxidation (OS) showed enhanced organic features at 3400-2800 and 1700-1000 cm-1. Absorbance intensity in these regions was associated with total soil C (TSC), permanganate-oxidizable carbon (POXC), and microbial biomass C (MBC) and N (MBN). Band areas representing chemically labile (2930, 1420 and 1150 cm-1) and refractory (1620, 840, 802 and 637 cm-1) SOM moieties showed significant differences among fields, and had much stronger correlations with labile soil C and N pools in OS compared to bulk soil spectra (BS). With increasing TSC, band areas increased for chemically labile moieties and decreased for chemically refractory moieties. Principal component analysis of band areas showed that differences in aliphatic and aromatic band areas in OS accounted for the majority of variation (56%) in markers of SOM composition across plots and increases in aliphatic markers were associated with increasing TSC. Likewise, the ratio of band areas at 1620 to 2930 cm-1 exponentially decreased with increasing TSC (R2 = 0.56, p < 0.0001) and POXC (R2 = 0.62, p < 0.0001). Thus, FTIR band areas at 2930 and 1620 cm-1 were confirmed as useful indicators of chemically labile and refractory SOM components in bulk soil, respectively, but even more so with the subtractive method, which also identified band areas at 1420 and 1150 cm-1, and at 802, 840 and 637 cm-1, as indicators of chemically labile and refractory SOM components, respectively. These FTIR markers could be used in rapid and low cost assessments of nutrient cycling capacity and soil productivity.