Differences in Soluble Organic Matter After 23 Years of Contrasting Soil Management

Soluble organic matter (OM) is a small but very mobile and reactive fraction of soil organic matter (SOM). We characterized the properties of soluble OM from 0‐ to 125‐cm soil depth in maize (Zea mays L.)‐based agricultural systems that were managed with conventional tillage (CT) and no‐tillage (NT) for 23 yr. Water‐extractable organic matter (WEOM) and salt‐extractable organic matter (SEOM) were obtained in a sequential extraction procedure using dilute (0.01M CaCl2) and concentrated (0.5 M K2SO4) salt solutions. Subsequently, WEOM and SEOM were characterized using resin fractionation, specific ultraviolet absorption at 254 nm (SUVA254) and humification indices (HI). Concentrations of C and N of SOM and SEOM and C of WEOM declined with increasing soil depth. In contrast to less pronounced differences in C and N of SEOM in CT and NT soils, those of WEOM were greater (52–116% for C and 26–86% for N) in NT than CT soils throughout the soil profile. The proportion of C and N in SOM recovered as WEOM and SEOM increased with soil depth (three to nine times for C and two to five times for N). The proportion of hydrophilic‐C increased with soil depth, but the proportion of humified OM decreased with soil depth. Consistent with reports that indicate CT enhances OM decomposition, our data suggest WEOM and particularly SEOM fractions in CT soils are more decomposed than NT soils down to 125‐cm depth. The relative pool size and properties of WEOM and SEOM demonstrated a higher in situ contribution of SOM to soluble OM pool with increasing soil depth.