Soil Fe minerals
Two assays were used to quantify redox-sensitive soil Fe pools (Liptzin & Silver, 2009; Huang & Hall, 2017). First, we used a hydrochloric acid (HCl) extraction to solubilize reactive Fe(II) and total solubilize reactive FeHCl [Fe(II) + Fe(III)] from short-range order and organo-Fe complexes; 3.75 g fresh soil was extracted in 30 mL 0.5 M HCl solution. The concentrations of Fe(II) and Fe(III) were analyzed colorimetrically. Second, we used citrate-ascorbate to extract chelatable short-range order (oxy) hydroxides and organo-Fe complexes (Feca). This pool is generally defined as reducible amorphous or poorly crystalline Fe (Hall & Silver, 2015). To estimate Feca, 1.25 g fresh soil subsamples were added to the freshly prepared citrate-ascorbate solution in a 1:30 ratio, vortexed, and then shaken in the dark for 16 h. Then, the samples were centrifuged at 10,000 r/min for 10 min and the supernatant solution was measured using ICP-OES. Higher Fe(II)HCl: FeHCland Feca can be good indicators of reducing conditions in humid tropical forest soils (Liptzin & Silver, 2015). To determine total free Fe (Fecd), Fe was measured in citrate-dithionite extractions at a 1:60 dry soil-to-solution ratio.