Soil organic carbon associated with iron oxides in terrestrial ecosystems: Content, distribution and control

Soils are the largest terrestrial organic carbon pool, containing more organic carbon than vegetation and the atmosphere combined. Therefore, a small change in soil organic carbon (SOC) can have a dramatic influence on the concentration of atmospheric carbon dioxide. Mineral-association (e.g., interaction with iron- (Fe-) oxides) has been demonstrated to be a key mechanism underlying long-term SOC persistence. Moreover, increasing evidence suggests that Fe oxides are more widely distributed than aluminum oxides (particularly in tropical and subtropical regions) in all kinds of soils worldwide and have the advantages of a higher surface area and a larger adsorption capacity. However, the contribution of Fe-bound organic carbon to the total SOC pool (f_Fe-OC) in terrestrial ecosystems and its geographical pattern and underlying mechanisms are not fully understood. We hypothesized that (1) soil depth was the primary driver of the f_Fe-OC in terrestrial ecosystems, with higher values observed in subsoil than in topsoil, and that (2) the soils in oxygen-deficient ecosystems would have a higher f_Fe-OC than those in aerobic ecosystems. Here, we compiled 351 observations of f_Fe-OC across terrestrial ecosystems from 24 published articles to assess the f_Fe-OC content across different soil depths, ecosystem types and climatic zones using Kruskal-Wallis and Wilcoxon rank-sum tests. We also used a partial least squares (PLS) model to distinguish the relative importance of climatic, pedological, and mineral factors in explaining the geographical distribution of f_Fe-OC in bulk soil, topsoil, and subsoil, respectively. We found that the f_Fe-OC was, on average, 21.9% across terrestrial ecosystems but varied greatly at different soil depths, with average values of 15.4% in topsoil and 37.5% in subsoil. The f_Fe-OC varied greatly in different terrestrial ecosystems, following the order of wetlands (24.5%) > grasslands (16.2%) > forests (14.9%) > croplands (14.8%), with higher values observed in oxygen-deficient ecosystems (24.2%) than in aerobic ecosystems (15.7%). Moreover, the f_Fe-OC across climatic zones showed the order of tropical and subtropical (23.7%) > boreal (21.9%) > temperate (20.2%) > plateau (16.6%). Soil depth was a primary control of the f_Fe-OC in terrestrial ecosystems, and the underlying mechanisms were much different: The f_Fe-OC in topsoil was strongly controlled by the concentrations of amorphous and complex Fe oxides, but that in subsoil was primarily controlled by soil pH, texture (clay, silt and sand contents) and bulk density. Therefore, the f_Fe-OC in topsoil could be the result of the direct accumulation of Fe oxides in different forms, but that in subsoil was mainly derived from the complexation and coprecipitation of Fe oxides and SOC. Our findings provide fundamental support for the mineral protection of Fe oxides in long-term SOC persistence in terrestrial ecosystems and distinguish the differential mechanisms underlying Fe oxide-associated SOC between topsoil and subsoil. Future research should focus on the coupling of Fe oxides and other metals (e.g., aluminum, calcium and manganese) in controlling SOC stabilization under the fluctuation of environmental change.