Depth and Tillage Dependent Stoichiometry of C, N, S, and H in Ukrainian Mollisol

ABSTRACT Tillage profoundly alters the vertical organization and coupling of biophilic elements in soils, yet depth‐resolved responses of carbon (C), nitrogen (N), sulfur (S), and their stoichiometric relationships remain poorly constrained in Mollisols. This study quantified the long‐term effects of conventional tillage (CTu), deep minimum tillage (DRTu), and shallow minimum tillage (RTu) on the distribution of total carbon (TC), organic carbon (SOC), inorganic carbon (SIC), organic nitrogen (SON), organic sulfur (SOS), organic hydrogen (SOH) in soil, and their stoichiometric ratios across a 0–100 cm profile of a Ukrainian Mollisol. Elemental composition was determined by CHNS‐O elemental analysis following carbonate removal, and stoichiometric ratios were evaluated on a molar basis. Conventional tillage homogenized SOC and SON within the plow layer (0–30 cm) but resulted in the widest molar C:N, C:S, and C:H ratios. This stoichiometric expansion signifies a functional shift in SOM architecture: the preferential mineralization of labile, heteroatom‐rich organic fractions and the subsequent dominance of element‐depleted, chemically processed residues stabilized as mineral‐associated organic matter (MAOM). Conversely, shallow minimum tillage promoted strong vertical stratification (0–5 cm), maintaining the narrowest stoichiometric ratios. These narrow ratios, coupled with significantly higher SOH content, indicate the preservation of “molecularly fresh” plant‐derived and microbially processed organic matter, characterized by high functional group density and aliphatic structures that are otherwise lost under intensive inversion. Deep minimum tillage exhibited a hybrid response, combining surface accumulation with enhanced retention of SOC, SON, SOS, and SOH in the 20–40 cm layer and pronounced narrowing of C:S and N:S ratios in deeper horizons (> 60 cm), indicative of effective co‐stabilization of carbon and sulfur at depth. Across all tillage systems, increasing depth was associated with convergence toward narrower C:N:S ratios, reflecting progressive microbial processing and increasing mineral association of organic matter. Our results demonstrate that tillage intensity acts as a selective pressure, shifting the Mollisol profile from a biophilic, element‐rich state under conservation management to a more recalcitrant, element‐depleted state under intensive inversion.