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

Abstract

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 wet‐sieving aggregate fractionation method was coupled with Fourier‐transform mid‐infrared (MidIR) spectroscopy to determine the composition of water‐stable (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.