Does crop rotation affect soil organic matter stratification in tillage systems?

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Tillage practices are known to influence soil stratification, but crop rotation can strongly affect fluxes of energy and matter, and thus play a role in determining soil organic matter (SOM) fate. Using a high-resolution stratified soil sampling at 2.5 cm intervals from 0 to 30 cm depth, we evaluated the half century impacts of tillage and crop rotation on SOM stratification, and how specific soil organic functional groups drive SOM accrual in typical USA Midwest agroecosystems. Three levels of tillage intensity—no-tillage, chisel, and moldboard, and three crop rotations—continuous-corn, 2-year corn-soybean, and 3-year corn-forage-forage—were evaluated at two sites with contrasting soil characteristics (silt-loam versus clay-loam). Soil organic matter concentration was determined by loss on ignition and soil organic functional group abundances were estimated by diffuse reflectance infrared Fourier transform spectroscopy (mid-DRIFTS). No-tillage systems had a more stratified distribution of SOM than in more intensive tillage systems, with greater SOM concentrations in the uppermost layers of silt-loam (0−10 cm, 9.4–20% change to chisel/moldboard) and clay-loam soils (0−30 cm, 12–15% change). Under no-tillage, crop rotation with forage maintained or increased SOM accrual (0–17.5/20 cm, -0.9–22% change to other rotations), but including soybean in the rotation diminished SOM accrual when compared to continuous-corn (-8 to -12 % change) or corn-forage rotation (-11 to -18 % change). Abating tillage increased abundance of aliphatic and phenol in the uppermost soil layers, and these organic functional groups were primarily driving SOM accrual, while SOM was inversely related to more recalcitrant functional groups (aromatic, carbonyl, and carboxylate). Soil organic functional groups (65.4 % of R2) and soil type, sampling depth, and management (34.6 % of R2) accurately predicted SOM concentrations (R2 = 93.4) underscoring their importance as a pathway to SOM accrual. Minimizing tillage intensity and rotating perennial forages in corn-based agricultural systems may lead to greater organic resource abundance that drive SOM accrual. This may present additional benefits to soil health, plant roots, and soil organisms in conservative agricultural systems.