Mechanisms controlling soil organic carbon composition pertaining to microbial decomposition of biochemically contrasting organic residues: Evidence from midDRIFTS peak area analysis

We used midDRIFTS peak area analysis to derive mechanisms underlying SOC composition in a sandy soil amended with four organic residues. We relied on a field experiment in Northeast Thailand where organic residues of contrasting biochemical quality (g kg−1) (rice straw (RS, 5 nitrogen (N), 29 lignin (L), 7 polyphenols (PP), 507 cellulose (CL)), groundnut stover (GN, 23 N, 68 L, 13 PP, 178 CL), dipterocarp (DP, 6 N, 176 L, 65 PP, 306 CL) and tamarind (TM, 14 N, 88 L, 32 PP, 143 CL) were applied yearly since the experiment started in 1995. At the end of year 13 (2008), samples of the bulk soil (0–15 cm) were obtained to evaluate the long-term effect of residue inputs on SOC composition. In addition, short-term microbial decomposition of the same residues in litter bags was assessed at intervals during a 26 week period to relate the biochemical quality of decomposing residues to SOC composition in bulk soils. All samples were subjected to midDRIFTS peak area analysis to categorize the abundance (“low” versus “high”) of the most prominent labile and stable organic functional groups. The spectral information was used to examine four mechanisms (i.e., physico-chemical protection, C loss, regulatory N effect and SOC stabilization) that are responsible for SOC composition. A conceptual model for residue decomposition was formulated based on an established approach to classifying residues according to their N, L and PP contents, which we extended by adding the CL content and modifying the threshold levels of these four biochemical properties. Combining mechanistic evaluation of SOC composition together with the conceptual model of decomposition controlled by organic residue quality provided deeper insights into the microbially mediated regulation of SOC stabilization and composition. We conclude that this approach will have important implications for soil organic matter management aimed at supplying N and sequestering C in sandy soils.