Using Fourier-Transform Mid-Infrared Spectroscopy to Distinguish Soil Organic Matter Composition Dynamics in Aggregate Fractions of Two Agroecosystems

The relationship between soil organic C (SOC) content and its composition as impacted by management is not well understood and may influence long-term storage of SOC. To better understand the potential for SOC storage in specific aggregate pools (e.g., physically protected intra-aggregate C), a wetsieving aggregate fractionation method was coupled with Fourier-transform mid-infrared (MidIR) spectroscopy to determine the composition of waterstable (macroaggregates, microaggregates, and silt + clay) and intra-aggregate (particulate organic matter [POM], microaggregates, and silt + clay) fractions under an integrated crop-livestock (ICL) system and continuous cotton (Gossypium hirsutum L.) production. These agroecosystems were located in the semiarid Texas High Plains on a clay loam soil. The ICL system included a paddock of grazed WW-B. Dahl Old World bluestem [Bothriochloa bladhii (Retz) S.T. Blake] and a no-till crop rotation of wheat (Triticum aestivum L.)-fallow-rye (Secale cereale L.)-cotton for livestock grazing and cotton production. Distance-based redundancy analysis (dbRDA) of the MidIR spectra distinguished the intra-aggregate POM from the other fractions. The intra-aggregate POM fraction was correlated with increased pseudo-absorbance (PA) in regions associated with carboxylates, phenolic C-O, aliphatic C-H, and quartz, while macro-and microaggregates demonstrated higher PA at 1570 and 1700 to 1765cm(-1). Differences attributed to management were identifiable in intra-aggregate POM and both silt + clay fractions. Within the intra-aggregate POM, dbRDA showed that bluestem was characterized by higher PA at 2800 to 3000 and 3300 to 3450 cm(-1) relative to the rest of the rotations. In the silt + clay fractions, more labile functional groups were found following rye and cotton planting.