Spatial dependance of organic carbon–metal relationships: A multi-scale statistical analysis, from horizon to field

Many studies showed that Fe and Al oxides stabilise organic carbon (OC) in soil. However, none of them examined the influence of spatial scale on the relations linking OC to metal oxides in soil. For this purpose we chose a near neutral soil, where these interactions may not be the dominant mechanism leading to OC storage. The aim of the study was to compare OC–metal relationships at two spatial scales: the field scale and pedological pit scale. The conceptual approach included sampling of soil horizons from 27 pits to a depth of 2 m in a 22 ha area. At pit scale, 146 samples were collected from a grid over the surface of a selected profile to obtain a high spatial resolution. Samples were analyzed for OC, Fe, Al, and Si originating from amorphous and crystalline oxides. For both sampling scales, relations between OC and metal oxides were statistically investigated per profile and per single horizon.We observed positive relationships involving amorphous Fe and Al oxides for horizons at pit scale suggesting that at this scale amorphous metal oxides may be involved in processes leading to OC storage in near neutral soils. However, such relationships were no longer observable at field scale. These results show that at larger scales, where the abundance of controls for OC storage is increasing, the empirical relationships established at pit scale are not powerful enough to predict the behaviour of a whole landscape. Mechanistic knowledge established at pit scale may thus not easily be scaled up in soils where interaction with metal oxides is not the only control on OC storage.