Vertical dynamics of dissolved organic carbon in relation to organic input quality and microaggregate formation in a coarse– textured Ultisol

Against the background of current understanding of dissolved organic carbon (DOC) adsorption onto clay surfaces, it remains unclear if bulk DOC or its fractions contribute to microaggregate formation in the top layers of coarse‐textured soils. We therefore investigated the effects of long‐term inputs of biochemically contrasting organic residues on the chemical characteristics and vertical distribution of DOC in a coarse‐textured Ultisol. During 2007–2008, DOC samples were extracted from soil profiles of a long‐term residue quality field experiment initiated in 1995. In this field experiment, groundnut stover, dipterocarp and tamarind leaf litter, as well as rice straw of contrasting biochemical quality, were applied yearly at 10 Mg ha−1. Groundnut, dipterocarp and tamarind produced large amounts (7.1–11.8 g C m−2) of high‐molecular‐weight (HMW; > 10 kDa) DOC, which was found in high concentrations (30–50 mg C kg−1) in the topsoil (0–15 cm). Rice straw, however, produced large amounts (3.5 g C m−2) of low‐molecular‐weight (LMW; < 1 kDa) DOC during the initial stage of decomposition. Although the HMW DOC was retained in the topsoil (0–15 cm), the LMW DOC was rapidly translocated to lower soil depths (60–80 cm). This translocation was facilitated by the low adsorption potential of the rice straw‐derived LMW DOC on colloidal surfaces of the topsoil. There was a significant positive correlation of C in the HMW DOC with that in fine particles, indicating their contribution to microaggregate formation and thus C accumulation. It was concluded that biochemical quality of residues as a determinant of concentration and chemistry of DOC and its vertical dynamics along the soil profile must be considered for SOC accumulation in coarse‐textured soils. Furthermore, we found reasonable indications that HMW DOC contributes to microaggregate formation in topsoils. Highlights Residue quality determined vertical dynamics of DOC in coarse– textured Ultisols. Lignin‐ and polyphenol‐rich residues produced HMW DOC in topsoil. LMW DOC derived from cellulose‐rich residues was translocated to the subsoil. There were indications that HMW DOC supported microaggregate formation in topsoils.