The influence of residue quality on soil organic C (SOC) retention has been called into question. A field experiment in Northeast Thailand, in which contrasting quality organic residues were applied yearly for 13 yr, was used to determine quantities, locations, and stability of SOC in the soil matrix and identify residue quality parameters affecting SOC stabilization in a tropical sandy‐textured soil. Total organic C (TOC) content was highest in intermediate‐quality tamarind (Tamarindus indica L.) at 3.58 g kg−1 (intermediate N, lignin, and polyphenol contents), followed by groundnut (Arachis hypogaea L.) stover at 2.63 g kg−1 (high N), dipterocarp (Dipterocarpus tuberculatus Roxb.) at 2.63 g kg−1 (low N, high lignin and polyphenols), and rice (Oryza sativa L.) straw at 1.77 g kg−1 (high cellulose). Microaggregates (Mi) (0.053–0.25 mm) stored the highest C content (34–49% of TOC), with tamarind having the highest C content. Carbon in large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), and free organic matter (>0.053 mm) was significantly positively correlated with C, lignin, and polyphenols. Carbon in microaggregates and fine particles (<0.053 mm) was significantly negatively correlated with C/N ratio. Soil fraction C was negatively correlated with residue cellulose contents. Protected C lost through mineralization in Mi was lower in tamarind (7% Mi‐C) followed by groundnut (9.5%), dipterocarp (17.7%), and rice straw (18.6%). It was significantly positively correlated with cellulose and C/N ratios but negatively correlated with N contents. Possible mechanisms of aggregate formation are based on microbial synthesis of both persistent (humic substances) and transient (polysaccharides) binding agents as influenced by residue quality. The results showed clearly that residue quality plays an important role in SOC accumulation in tropical sandy soils.

The objectives of this study were to investigate decomposition patterns and soil organic matter (SOM) accumulation of incorporated residues (10 Mg ha -1 year -1 ) of different quality, and identify microbiological parameters sensitive to changes in SOM dynamics, in a 13-year-old field experiment on a sandy soil in Northeast Thailand. Mass loss was fastest in groundnut stover (high N), followed by rice straw (high cellulose) and tamarind (intermediate quality), and slowest in dipterocarp (high lignin and polyphenol) following a double exponential pattern. The decomposition rate k 1 (fast pool) was positively correlated with cellulose (r = 0.70*) while k 2 (slow pool) was negatively related to lignin (r = -0.85***) and polyphenol (r = -0.81

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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.