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.