Iron reduction and carbon dynamics during redox fluctuations in soil slurries from Luquillo CZO (Puerto Rico)

Abstract:

Iron minerals play an important role in the dynamics of nutrients and soil carbon in tropical ecosystems. Because soils often experience fluctuations between oxic and anoxic conditions (redox fluctuations) the pool of reactive Fe(III) phases is under continuous flux. The oxidation of soluble Fe(II) during oxic conditions can generate Fe(III) phase of variable crystallinity with potentially different susceptibility to Fe(III) reduction during subsequent anoxic periods. We hypothesize that the coupled rates of iron reduction and carbon mineralization will respond to differences in the frequency of redox shifts and the length of the oxic and anoxic periods (Tao(oxic) and Tao(anoxic). To test this, we subjected soil from the upper 15 cm of the Bisley watershed in the Luquillo critical zone observatory (LCZO), Puerto Rico, to five redox fluctuations scenarios. The treatments included either a fixed anoxic period of 6 d with oxic periods of 8, 24, and 72 h (3 treatments); or a fixed anoxic period of 2 d with oxic periods of 8 or 24 h (2 treatments). Results from a preliminary experiment indicated iron reduction rates increased when the oxic period was decreased below 10 h. We are completing analysis of the full dataset from the experiment above and will present Fe(II) concentrations and gas phase concentrations (CO₂ emissions). Results from this experiment will further constrain the type of redox dynamics that influence soil Fe reduction and carbon mineralization.