Differential production yet chemical similarity of dissolved organic matter across a chronosequence with contrasting nutrient availability in Hawaii

Dissolved organic matter (DOM) is a critical phase in terrestrial carbon and nutrient cycling forming the basis of many ecosystem functions, yet the primary drivers controlling its flux from organic horizons and resultant chemical composition remain only partially understood. We studied dissolved organic matter production and chemistry from organic soil horizons across a 4.1 My old well-constrained chronosequence in Hawaii. Controlled soil column irrigation and leaching experiments were conducted on field moist organic soil horizons to quantify microbial activity, DOM production and chemistry. Both microbial activity (defined as CO2 production per unit substrate C) and DOM production were found to be lowest in the youngest (0.3 ky) and oldest (4.1 My) sites of the chronosequence, where nutrients (N and P respectively) were most limiting. By contrast, DOM production and microbial activity was greatest at the intermediate-aged (20-350 ky) sites where nutrients were least limiting, unrelated to the mass of organic matter found in the organic horizons. While differences in production rates were found, C-13 NMR spectroscopic results indicated that there was a convergence of chemistry from the solid to the dissolved phase at all sites. In particular, all DOM samples were found to have a high proportion of aromatic acids. With supporting data from a diverse range of ecosystems, we postulate that chemical homogenization of DOM relative to source material is a common feature of many ecosystems due to two microbially mediated processes: (1) similar extracellular enzymatic oxidation conferring solubility to a subset of degradation products; and (2) the rapid selective consumption of the more labile organic compounds in the soil solution.