Soft X-ray spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions.
There is a growing acceptance that associations with soil minerals may be the most important overarching stabilization mechanism for soil organic matter. However, direct investigation of organo-mineral associations has been hampered by a lack of methods that can simultaneously characterize organic matter (OM) and soil minerals. In this study, STXM-NEXAFS spectroscopy at the C 1s, Ca 2p, Fe 2p, Al 1s, and Si 1s edges was used to investigate C associations with Ca, Fe, Al, and Si species in soil clay fractions from an upland pasture hillslope. Bulk techniques including C and N NEXAFS, Fe K-edge EXAFS spectroscopy, and XRD were applied to provide additional information. Results demonstrated that C was associated with Ca, Fe, Al, and Si with no separate phase in soil clay particles. In soil clay particles, the pervasive C forms were aromatic C, carboxyl C, and polysaccharides with the relative abundance of carboxyl C and polysaccharides varying spatially at the submicrometer scale. Only limited regions in the soil clay particles had aliphatic C. Good C-Ca spatial correlations were found for soil clay particles with no CaCO3, suggesting a strong role of Ca in organo-mineral assemblage formation. Fe EXAFS showed that about 50% of the total Fe in soils was contained in Fe oxides, whereas Fe-bearing aluminosilicates (vermiculite and Illite) accounted for another 50%. Fe oxides in the soil were mainly crystalline goethite and hematite, with lesser amounts of poorly crystalline ferrihydrite. XRD revealed that soil clay aluminosilicates were hydroxy-interlayered vermiculite, Illite, and kaolinite. C showed similar correlation with Fe to Al and Si, implying a similar association of Fe oxides and aluminosilicates with organic matter in organo-mineral associations. These direct microscopic determinations can help improve understanding of organo-mineral interactions in soils.
Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware , Newark, Delaware 19711, United States.
Chunmei
C
Chen
James J
JJ
Dynes
Jian
J
Wang
Chithra
C
Karunakaran
Donald L
DL
Sparks
D000538:Aluminum Silicates / Q000737:chemistry*; D002244:Carbon / Q000032:analysis; D002412:Cations / Q000032:analysis; D000077215:Clay; D004602:Elements; D008853:Microscopy / Q000379:methods*; D008903:Minerals / Q000737:chemistry*; D009930:Organic Chemicals / Q000737:chemistry*; D052638:Particulate Matter / Q000032:analysis; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis; D013057:Spectrum Analysis / Q000379:methods*; D056928:X-Ray Absorption Spectroscopy; D014965:X-Rays
D000538:Aluminum Silicates; D002412:Cations; D004602:Elements; D008903:Minerals; D009930:Organic Chemicals; D052638:Particulate Matter; D012987:Soil; D012989:Soil Pollutants; D002244:Carbon; D000077215:Clay