Variations in iron-bound organic carbon in soils along an altitude gradient and influencing factors in a subtropical mountain ecosystem of southern China

Purpose

The main factors governing iron-bound organic carbon (Fe-bound OC) formation under different environmental conditions have been less well identified. In this study, we investigated the distribution of Fe-bound OC and the influencing factors in soils of a typical subtropical mountain ecosystem.

Materials and methods

Soil samples at different altitudes (402–1653 m) and depths (0–20 cm, 20–40 cm, and 40–60 cm) in the Nanling Mountains of southern China were collected. The influence of soil property indicators and microbial diversity characteristics on the formation of Fe-bound OC was explored.

Results

The Fe-bound OC content in the soils ranged from 1.40 ± 1.32 g/kg to 15.09 ± 3.57 g/kg, accounting for an average of 23.49 ± 13.03% of the total organic carbon (TOC), and generally increased with altitude and exhibited significantly higher values at the soil depth of 0–20 cm, while its proportion to TOC (f_Fe-OC) gradually increased as soil depth increased. The more significant indicators in explaining the variation of Fe-bound OC include TOC, particulate organic carbon (POC), free iron oxides (Fe_d), amorphous iron oxides (Fe_o), complexed iron oxides (Fe_p), Fe_o/Fe_d, pH, Ca, sand and silt. Fe_o/Fe_d was strongly linearly correlated with Fe-bound OC, thus can be a good predictor of Fe-bound OC content. RDA analysis showed that Fe-bound OC/f_Fe-OC had a relatively closer correlation with bacterial phyla including Proteobacteria, Actinobacteriota, Chloroflexi and Firmicutes.

Conclusions

The attitude gradient and soil depth significantly influence the content of Fe-bound OC. The distribution of Fe-bound OC exhibited a certain correlation with both biotic and abiotic factors.