Mobilization and Release of colloidal Carbon from a Soil Column Under Redox Oscillation Condition

Abstract:

Dissolved organic matter (DOM), the most mobile form of carbon (C), strongly influences the cycling, distribution and behavior of C in soil. In wetlands, the reductive dissolution of iron and manganese oxy-hydroxides releases large quantities of DOM into the soil solution. The objective of this study is to quantify the changes in aqueous organic carbon concentration in different sized fractions induced by reduction of iron and increase in pH. Twenty four cm long soil columns were prepared. Columns were run under oxic (as control) and anoxic conditions. Two platinum redox probes were inserted at 10 and 17 cm depths from the soil surface to monitor the redox status of the column. Anoxic and oxic conditions were maintained by flushing with either nitrogen or oxygen gas through the soil. No additional organic sources were added. After 35 days of anoxic environment, column leachate samples were separated by differential centrifugation into five colloidal sized fractions (<450 nm, <220 nm, <100 nm, <50 nm and <2.3 nm). Immediately after the 1^st reduction half cycle, the leachate samples were collected inside the glove box and the soil columns were flushed with oxygen to prepare for 2^nd reduction half cycle. After 1^st reduction half cycle, the pH, ionic strength and aqueous (Fe²⁺) concentration of the column extracts were increased whereas the Eh value was decreased. The range of pH, Eh, ionic strength and concentration of Fe²⁺ was 6.38 to 6.91, -219 to -275 mV, 13.74 to 18.84 mM and 1.8 to 3.41 mg L^-1, respectively. Following the anoxic incubation, the total desorbed C was increased up to 139 mg L^-1. The distribution of C across the five particle size fractions was 3.68-11.73% (> 450 nm), 0.59-5.12% (450-220 nm), 0.45-4.91% (220-100 nm), 0.18-2.91% (100-50 nm), 15.48-35.23% (50 nm – 2.3 nm) and 49.15-63.94% (<2.3 nm). The preliminary results confirmed the release of more nanoparticulate (50-2.3 nm) and truly dissolved (<2.3 nm) organic matter from the anoxic soil column. This study will allow us to systematically evaluate the contribution of colloid mobilization on C release and the distribution of different sized fractions of C under redox dynamic environment.