Stoichiometry of dissolved organic matter controls the origin of polysaccharides in dissolved organic carbon in sandy soils receiving contrasting-quality plant residues
Authors: Poosathit Ratanaporn , Vityakon Patma 
Published: March 2024

Chemical quality of plant residues, which are the source of dissolved organic C (DOC), determine the molecular structure, notably functional group polysaccharides in DOC, and the origins of DOC-polysaccharide, which transitioned from plant- to microbial-derived as decomposition progressed. The microbial-derived DOC-polysaccharides contribute to soil organic C (SOC) stabilization, but what factor associated with the residue chemical quality regulates the origin of the DOC-polysaccharides remains unknown. Balanced DOC and dissolved N (DN) (DOC-and-DN) stoichiometry of microbial substrates as indicated by DOC-to-DN ratio enhances microbial C use efficiency (CUE). The CUE indicates the production of microbial-derived DOC compounds. We hypothesized that DOC-and-DN stoichiometry in soils receiving plant residues was a key factor controlling the origin of DOC-polysaccharides. The objectives of this study were to determine 1) DOC-to-DN ratio during decomposition, 2) relationships of DOC-to-DN ratio and microbial metabolic quotient (qCO2 - inverse of CUE), and 3) relationships of qCO2 and DOC-to-DN ratio with DOC-polysaccharides. This study employed data from year 13 of a long-term field experiment on the effect of annual application of varying quality residues on decomposition processes in sandy soils. During the early stage of decomposition (week 0-2), DOC-to-DN ratio of N-rich groundnut stover (GN) residue decreased, in contrast to low-N rice straw (RS), and dipterocarp leaf litter (DP) and medium-N tamarind leaf+petiole litter (TM). During the intermediate stage (week 2-8), GN and RS had increasing ratios, as opposed to DP and TM. Groundnut-treated soil had lower average ratio (6.8&#177;2.6) than TM, RS and DP (10.7&#177;4.4, 12.1&#177;5.4, 14.1&#177;7.2, respectively). Positive influences of the ratio on qCO2 in the three medium-to-low-N soils (R2 = 0.60** - 0.74**) indicated that the ratio had significant control on the CUE. The qCO2, in turn, had significant influence (non-linear relationship) on DOC-polysaccharides (R2 = 0.30*).&#160; In the early stage (high qCO2), it decreased (indicating the increase of CUE) corresponding to the decreases in DOC-polysaccharides indicating that these were plant-derived. The qCO2 decreased further beyond the threshold of 0.00026 mg CO2-C kg-1 microbial biomass C h-1 whichmarked the beginning of the intermediate stage (low qCO2), corresponding to an increase in microbial-derived DOC-polysaccharides.&#160; This increase signified the change in their origin. Negative influence of DOC-to-DN ratio on DOC-polysaccharides (R2 = 0.36*) during the intermediate stage showed that the increased ratios caused the decreases in DOC-polysaccharides. Our results identified DOC-and-DN stoichiometry in residues-treated soils as a prominent factor controlling the origin of the DOC-polysaccharides. Imbalanced DOC-and-DN stoichiometry in low-N residues, i.e., higher DOC-to-DN ratios than microbial biomass C-to-N ratios, brought about the decrease in microbial-derived DOC-polysaccharides during the later stage. Soil management via organic inputs requires careful consideration of changes in DOC-and-DN stoichiometry which can affect SOC accumulation.