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. Anaerobic decomposition of organic carbon (OC) in submerged ricepaddies is coupled to the reduction of alternative soil electron acceptors,primarily Fe3+. During reductive dissolution of Fe3+ frompedogenic oxides, previously adsorbed native soil organic carbon (SOC) couldbe co-released into solution. Incorporation of crop residues could henceindirectly, i.e. through the stimulation of microbially mediated Fe3+reduction, promote the loss of native SOC via enhanced dissolution andsubsequent mineralisation to CO2 and CH4. Our aim was to estimatethe relevance of such a positive feedback during the degradation of addedOC, and to investigate the impact of irrigation management on this mechanismand on priming effects on native SOC decomposition in general. In a six-weekpot experiment with rice plants, two Bangladeshi soils with contrasting SOCto oxalate-extractable Fe (SOC : Feox) ratios were kept under a regime ofalternate wetting and drying (AWD) or continuous flooding (CF), and wereeither amended with maize shoots or not. The δ13C signatures ofdissolved organic C and emitted CH4 and CO2 were used to infer thedecomposition of added maize shoots (δ13C = −13.0 ‰) versus native SOC (δ13C = −25.4 ‰ and −22.7 ‰). Addition of maizeresidues stimulated the reduction of Fe as well as the dissolution of nativeSOC, and the latter to a larger extent under CF, especially for the soilwith the highest SOC : Feox ratio. Estimated Fe-bound SOC contents denotethat stimulated SOC co-release during Fe reduction could explain thispositive priming effect on SOC dissolution after the addition of maize.However, priming effects on SOC mineralisation to CO2 and CH4 werelower than for SOC dissolution, and were even negative under AWD for onesoil. Enhanced reductive dissolution of Fe-bound SOC upon exogenous OCaddition therefore does not necessarily lead to stimulated SOCmineralisation. In addition, AWD irrigation was found to decrease theabove-mentioned priming effects.

Effect of organic carbon addition on paddy soil organic carbon decomposition under different irrigation regimes