Management-induced organic carbon accumulation in paddy soils: The role of organo-mineral associations

Iron (Fe) oxides strongly interact with organic matter in soil and play an important role in the stabilization of organic matter. These processes are often influenced by soil cultivation, including tillage, crop rotation and irrigation. We assessed the effect of Fe oxides on organic carbon (OC) accumulation during the development of soils used for paddy rice production in comparison to non-irrigated cropping systems. Soil samples were taken from two chronosequences derived from uniform parent material in the Zhejiang Province (PR China). Bulk soils and soil fractions were analyzed for OC concentrations, soil mineralogy and soil organic matter (SUM) composition was determined by solid-state C-13 NMR spectroscopy. Paddy soils were characterized by increasing OC concentrations, from 18 mg g(-1) to 30 mg g(-1), during 2000 years of rice cultivation, but OC concentrations of non-paddy soils were low in all age classes (11 mg g(-1)). SUM composition revealed from Solid-state C-13 NMR spectroscopy did not change during pedogenesis in either chronosequence. Selective enrichment of lignin-derived compounds, caused by long-term paddy rice management, could not be confirmed by the present study. The management of paddy soils creates an environment of Fe oxide formation which was different to those in non-paddy soils. Paddy soils are dominated by poorly crystalline Fe oxides (Fe-o) and significantly lower content of crystalline Fe oxides (Fe-d-Fe-o). This was in contrast to non-paddy soils, which are characterized by high proportions of crystalline Fe oxides. The paddy-specific Fe oxide composition was effective after only 50 years of soil development and the proportion Fe oxides did not alter during further pedogenesis. This chronosequence study revealed that the potential for OC accumulation was higher in paddy versus non-paddy soils and was already reached at earliest stages of paddy soil development. Changes in paddy soil management associated with redox cycle changes will not only affect Fe oxide composition of paddy soils but most probably also OC storage potential. (C) 2012 Elsevier B.V. All rights reserved.