Managing wheat-based cropping systems for chemodiverse dissolved organic matter: Bulk- and molecular-level composition analyses

Dissolved organic matter (DOM) is ubiquitous in terrestrial ecosystems and its occurrence within surface and sub-surface soil is now recognized as a primary driver of global carbon-climate feedbacks. The composition of DOM is likely altered by long-term cropping, yet there is limited knowledge about the key agro-environmental factors controlling DOM chemodiversity. Here, we characterized bulk- and molecular-level DOM composition along a gradient of soil organic carbon (SOC) accretion within two semi-arid climates of Montana, USA. A silty-loam Typic Haplustoll (Mollisol) and silty-clay Typic Haplustert (Vertisol) were sampled (0-30 and 50 cm) after a 10-year cycle of conventional till - fallow/winter wheat (F_till-W), no-till spring pea/oilseed-wheat (P_g/O-W), and no-till continuous wheat (W-W), and analyzed using absorbance and fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM concentration decreased with depth (Mollisol and Vertisol) and was significantly greater under W-W or P_g/O-W than F_till-W (Mollisol). Absorbance at 254 nm, a proxy of aromatic nature, indicated aromaticity decreased with depth (Mollisol and Vertisol) and lower aboveground biomass-C inputs, i.e. W-W ≥ P_g/O-W ≥F_till-W (Mollisol). DOM fluorescent humification increased with depth (Mollisol) and was higher for F_till-W than W-W or P_g/O-W (Mollisol). Four individual fluorescing components were determined, including mid-size humic-like, large humic-like, monolignol-like, and amino acid-/tannin-like (Mollisol and Vertisol); with varying fluorescence intensities as a function of cropping system (i.e. W-W = P_g/O-W > F_till-W) (Mollisol) and depth (i.e. 0-10 cm >10-20 cm ≥ 20-30 and 50 cm) (Mollisol and Vertisol). Molecular composition analyses by FT-ICR MS indicated that sub-soil DOM was depleted of plant-derived constituents and was enriched by less complex DOM moieties. After a decade, Mollisol SOC gains were accompanied by higher concentrations of DOM and a compositionally diverse DOM pool; such changes, however, were not observed for the high-clay content Vertisol. Our results provide a comprehensive framework for understanding the distribution of DOM in arable soils and will underpin predictions on how DOM chemodiversity relates to soil health within intensively managed agroecosystems.