Dynamics of soil iron-bound organic carbon under throughfall in a subtropical evergreen broad-leaved forest.

Throughfall could alter the distribution characteristics and formation mechanisms of soil iron-bound organic carbon (Fe-OC) by its hydrological regulating effects (reducing rainfall volume, slowing down rainfall intensity, and prolonging rainfall duration) and leaching of organic matter from the canopy. We investigated the impacts of forest throughfall on the contents of three forms of iron oxides and Fe-OC in soil through field in-situ controlled experiment in a typical subtropical evergreen broadleaf forest in Sanming, Fujian Province. Compared with the control (canopy gap rainfall), throughfall significantly increased the contents of free iron oxide (Fed) and complexed iron oxide (Fep) in soil by 3.6% and 6.3%, respectively. The increase in Fed occurred mainly in the topsoil (0-20 cm), whereas the enhancement of Fep occurred in both surface and deep (20-40 cm) soil layers. The amorphous iron oxide (Feo) content was not affected by throughfall. Throughfall significantly increased soil Fe-OC content (increased by 24.9%), the contribution ratio of Fe-OC to soil organic carbon (increased by 19.7%), and the carbon-to-iron molar ratio. These effects were consistent across soil layers but showed temporal variations. Throughfall enhanced the accumulation of Fed and Fep, and strengthened the stability and retention functions of soil Fe-OC, by mitigating leaching through canopy interception and introducing canopy-derived organic matter.