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

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.; Ftill-W), no-till spring pea/oilseed-wheat (Pisum sativum L.; Pg/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 < 0.05) under W-W or Pg/O-W than Ftill-W. The absorbance at 254 nm (Abs254), a proxy for DOM aromatic nature, indicated that aromaticity decreased with depth and lower biomass-C inputs (i.e. W-W ≥ Pg/O-W ≥ Ftill-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 < 0.05) and was higher for Ftill-W (2.95) than W-W (2.61) or Pg/O-W (2.28). Overall, DOM became depleted of plant-derived constituents and was enriched by more decomposed, condensed substances in Ftill-W, as compared to W-W or Pg/O-W soils. DOM composition is strongly affected by cropping intensity and such changes are important drivers controlling SOC accretion in arable soils.