Bulk optical characterization of dissolved organic matter from semiarid wheat-based cropping systems

Abstract Dissolved organic matter (DOM) plays a critical role in the cycling of nutrients and long-term agricultural sustainability. The composition of DOM in soil is likely altered due to management, yet there is limited knowledge on the effect of long-term cropping on DOM chemical character. Here, we characterized water extractable DOM composition along a gradient of soil organic carbon (SOC) affected by differing cropping and tillage intensity in a semiarid climate of the northern Great Plains, USA. Soil samples (0–10, 10–20, 20–30 cm) were collected from conventional till-fallow winter wheat ( Triticum aestivum L.; F till -W), no-till spring pea/oilseed-wheat ( Pisum sativum L.; P g /O-W), and no-till continuous wheat (W-W) fields, and analyzed using UV/Vis absorbance and excitation-emission matrix fluorescence spectroscopy. The concentration of DOM decreased with depth and was significantly greater ( P g /O-W than F till -W. The absorbance at 254 nm (Abs 254 ), a proxy for DOM aromatic nature, indicated that aromaticity decreased with depth and lower biomass-C inputs (i.e. W-W ≥ P g /O-W ≥ F till -W). Multidimensional parallel factor (PARAFAC) analysis revealed humic-like (C1, C2), monolignol-like (C3), and protein/tannin-like (C4) components with varying fluorescence intensities as a function of cropping system and soil depth. DOM humification, indicated by the humification index (HIX), increased significantly with depth ( P till -W (2.95) than W-W (2.61) or P g /O-W (2.28). Overall, DOM became depleted of plant-derived constituents and was enriched by more decomposed, condensed substances in F till -W, as compared to W-W or P g /O-W soils. DOM composition is strongly affected by cropping intensity and such changes are important drivers controlling SOC accretion in arable soils.