Preservation of organic carbon promoted by iron redox transformation in a rice-wheat cropping system

Abstract

Mineral protection regulates long-term global preservation of soil organic carbon (SOC), but little research has been conducted on the iron (Fe) oxide availability for carbon binding regulated by seasonal variation of wetting and drying and fertilization in paddy soils. The objective of this study was to explore the effects of five-year rice-wheat rotation and organic fertilization on the production of short-range-ordered (SRO) Fe oxides and SOC sequestration. Five fertilization treatments were examined: (1) no fertilizer control (CK), (2) chemical nitrogen, phosphorus and potassium (NPK), (3) 50% NPK plus manure (NPKM), (4) 100% NPK plus straw (NPKS), and (5) 30% NPK plus manure organic-inorganic compound fertilizer (NPKMOI). Soil samples were collected from 2013 to 2015 after each wheat and rice harvest. A split-plot analysis of variation was used to investigate the respective role of fertilizer treatments (main-plot) and crop seasons (sub-plot) in the transformation of SRO-Fe oxides. Variation partitioning analysis was used to quantify the contribution of SRO-Fe oxides to the accumulation of SOC. Five-year rice-wheat rotation without any fertilization (CK) increased the oxalate extractable Fe oxides (Feo) by 55.06% compared with the initial soil. The Feo concentration was higher (P < 0.05) in the NPKM, NPKS and NPKMOI treatments than in the NPK and CK treatments, and was higher after the rice harvest than after the wheat harvest. Furthermore, the Feo concentration was positively (P < 0.001) correlated with the SOC content and negatively correlated with the specific carbon mineralization rate (SCMR, rate per unit SOC). Moreover, the Feo could explain 52.57% and 40.5% of the variation in the SOC content and the SCMR, respectively. In addition, the partial humification coefficient (PHC, the percentage of SOC change per unit of exogenous carbon input) of the crop straw was 12.92% but amounted to 41.45% to 46.35% for different types of compost. These results indicate that the reductive dissolution and subsequent oxidative precipitation of Fe oxides induced by seasonal alternation of wetting and drying can be further promoted via organic fertilization possibly by forming organo-Fe complexes, which contribute to the accumulation of SRO-Fe oxides and the preservation of organic carbon in paddy soils.