Organic carbon accumulation on soil mineral surfaces in paddy soils derived from tidal wetlands

• No additional formation of mineral surfaces during 2000 years of soil development
• Paddy soils are characterized by high OC coverage on mineral surfaces.
• Non-paddy soils, in which decalcification was retarded, showed lower OC coverage.
• Iron oxides and SOM protect each other in organo-mineral associations.
• OC coverage by complex association between clay minerals, iron oxides and SOM

We studied organic carbon (OC) accumulation in organo-mineral associations during soil development on calcareous parent material. Two chronosequences in the Zhejiang Province, PR China, were investigated; one under paddy cultivation with a maximum soil age of 2000 years, and the other under upland crops where the oldest soil was 700 years old. Bulk soils and soil fractions of the uppermost A horizons were analyzed for OC concentrations, radiocarbon (14C) contents, total pedogenic iron oxide concentration and oxalate extractable proportions of iron (FeOX) oxides. The specific surface area of soil minerals was measured with the Brunauer–Emmett–Teller (BET-N2) method on four conditions: untreated, after organic matter removal, after iron oxide removal and after removal of both. Initial soil formation on calcareous marine sediments includes soil decalcification and OC accumulation. Paddy soils are characterized by an accelerated decalcification, higher contents of OC and FeOX oxides, and a pronounced accumulation of modern OC. The mineral constitution of the soil material indicated already a certain degree of weathering since the earliest stages of pedogenesis and remained unchanged in paddy and non-paddy soils. The study provides no evidence of formation of new clay-sized minerals during soil development, which could supply new surfaces for OC accumulation. However, the study revealed higher OC coverage on mineral surfaces in decalcified paddy soils. Therefore, we assume the specific surface area and the specific affinity of FeOX oxides for OC storage to play an important role for OC accumulation in organo-mineral associations. In contrast, the surface area of minerals in non-paddy soils, in which decalcification and the proportion of FeOX oxides were much lower, showed significantly lower OC coverage. Selective removal of SOM or iron oxides clearly showed that iron oxides and SOM protect each other in organo-mineral associations primarily in paddy fine clay-sized fraction. Thus, we explained the higher OC coverage on mineral surfaces by complex association between clay minerals, iron oxides and SOM in paddy soils.