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Abstract:
Paddy soils are subjected to periodically changing redox conditions. In order to understand better the redox
control on long-term carbon turnover, we assessed carbon mineralization and dissolved organic carbon
(DOC) of paddy topsoils sampled along a chronosequence spanning 2000 years of rice cultivation. Non-paddy
soils were used as references. We exposed soils to alternating redox conditions for 12weeks in incubation
experiments. Carbon mineralization of paddy soils was independent of redox conditions. Anoxic conditions
caused increasing DOC concentrations for paddy soils, probably because of desorption induced by increasing
pH. We assume desorption released older, previously stabilized carbon, which then was respired by a microbial
community well adapted to anoxic conditions. This assumption is supported by the 14C signatures of respired
CO2, indicating larger mineralization of older carbon under anoxic than under oxic conditions. The increasing
DOC concentrations under anoxic conditions did not result in an equivalent increase in carbon mineralization,
possibly because of little reducible iron oxide. Therefore, net DOC and CO2 production were not positively
related under anoxic conditions. The overall 20–75% smaller carbon mineralization of paddy soils than of
non-paddy soils resulted from less respiration under oxic conditions. We conclude that carbon accumulation in
paddy as well as in other wetland soils results from a microbial community well adapted to anoxic conditions,
but less efficient in mineralizing carbon during transient oxic periods. Carbon accumulation might be even larger when mineralization under anoxic conditions is restricted by a lack of alternative electron acceptors.
