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

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From: Biogeosciences(Vol. 18, Issue 18)
Publisher: Copernicus GmbH
Document Type: Brief article
Length: 366 words

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Abstract :

Anaerobic decomposition of organic carbon (OC) in submerged rice paddies is coupled to the reduction of alternative soil electron acceptors, primarily Fe.sup.3+ . During reductive dissolution of Fe.sup.3+ 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 Fe.sup.3+ reduction, promote the loss of native SOC via enhanced dissolution and subsequent mineralisation to CO.sub.2 and CH.sub.4 . 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 : Fe.sub.ox) 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 [delta].sup.13 C signatures of dissolved organic C and emitted CH.sub.4 and CO.sub.2 were used to infer the decomposition of added maize shoots ([delta].sup.13 C = -13.0 0/00) versus native SOC ([delta].sup.13 C = -25.4 0/00 and -22.7 0/00). 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 : Fe.sub.ox 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 CO.sub.2 and CH.sub.4 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.

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Gale Document Number: GALE|A675548399