Sequential Reaction of Dissolved Organic Matter with Subsurface Soils, Effect of Dry and Wet Cycles.

Organic carbon dynamics in the critical zone are highly controlled by reactions of dissolved organic matter (DOM) with subsurface soils. These reactions includes adsorption, transformation and exchange of both inorganic and organic content. In the natural environment these reactions are coupled to frequent dry/wet cycles.  There is a gap of knowledge regarding the effect of these cycles on DOM-soil reactions.

Bw soil horizon from the Jemez River Basin critical zone observatory were sequentially reacted (four batch steps) with DOM (~50 mg C/L) extracted from mixed conifer forest floor litter layer. Soil DOM pastes were allow to air dry before each step (dried system) and were compared to reaction conducted without drying (undried system). Application of dry step results in lower (10-25%) removal of total organic carbon from DOM solutions. Based on FTIR analysis of supernatant solutions, aliphatic and carbonyl moieties were preferentially removed over polysaccharide for both dried and undried systems. However in dried systems, a higher proportion of carboxylate over amide removal was observed. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) of the fine fraction (<2 mm), revealed significant increase in the carboxyl for all sample reacted with DOM. Soil reacted in the undried system showed slightly higher signals of amide. Drying steps result in elimination of the aromatic signal from NEXAFS spectra. Scanning transmission X-ray microscopy (STXM) showed that the higher carboxylic fraction was spatially associated with calcium and lower carboxylic fraction was associated with iron.  This suggests cation bridging as the main mode of carboxylic interaction with soil. No significant spatial differences were observed for sample reacted without drying steps.

Our results demonstrates that dry/wet cycles can alter DOM reaction by changing both spatial distribution and composition of DOM component in the soil. These changes need to be taken into account for overall carbon modeling in the critical zone.