Abstract. Anaerobic decomposition of organic carbon (OC) in submerged rice
paddies is coupled to the reduction of alternative soil electron acceptors,
primarily Fe3+. During reductive dissolution of Fe3+ from
pedogenic oxides, previously adsorbed native soil organic carbon (SOC) could
be co-released into solution. Incorporation of crop residues could hence
indirectly, i.e. through the stimulation of microbially mediated Fe3+
reduction, promote the loss of native SOC via enhanced dissolution and
subsequent mineralisation to CO2 and CH4. Our aim was to estimate
the relevance of such a positive feedback during the degradation of added
OC, and to investigate the impact of irrigation management on this mechanism
and on priming effects on native SOC decomposition in general. In a six-week
pot experiment with rice plants, two Bangladeshi soils with contrasting SOC
to oxalate-extractable Fe (SOC : Feox) ratios were kept under a regime of
alternate wetting and drying (AWD) or continuous flooding (CF), and were
either amended with maize shoots or not. The δ13C signatures of
dissolved organic C and emitted CH4 and CO2 were used to infer the
decomposition of added maize shoots (δ13C = −13.0 ‰) versus native SOC (δ13C = −25.4 ‰ and −22.7 ‰). Addition of maize
residues stimulated the reduction of Fe as well as the dissolution of native
SOC, and the latter to a larger extent under CF, especially for the soil
with the highest SOC : Feox ratio. Estimated Fe-bound SOC contents denote
that stimulated SOC co-release during Fe reduction could explain this
positive priming effect on SOC dissolution after the addition of maize.
However, priming effects on SOC mineralisation to CO2 and CH4 were
lower than for SOC dissolution, and were even negative under AWD for one
soil. Enhanced reductive dissolution of Fe-bound SOC upon exogenous OC
addition therefore does not necessarily lead to stimulated SOC
mineralisation. In addition, AWD irrigation was found to decrease the
above-mentioned priming effects.