Effect of rice straw amendment on dissolved organic and inorganic carbon and cationic nutrients in percolating water from a flooded paddy soil: A microcosm experiment using 13C-enriched rice straw

A flooded soil column experiment was conducted in which 13C-enriched rice straw (RS; 7.0 g kg−1 dry weight soil) was added to evaluate the effects of RS application on the leaching of cationic nutrients (Ca, Mg, and Fe), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and CH4 from the plow layer of rice paddies by water percolation. The origins of these nutrients in rice paddy soil were studied by comparing their content in the soil before and after the percolation experiment. The proportion of total cationic nutrients in percolating water to the content of exchangeable Ca, Mg and amorphous Fe in the soil during the 111 d experiment increased from 21% to 46% for Ca, 34–61% for Mg, and 4.5–14% for Fe as a result of RS application at 25 °C. The decrease in exchangeable Ca, Mg and amorphous Fe after the experiment were comparable to the total amounts detected in the percolating water, suggesting that these cationic nutrients in the water were derived from exchangeable Ca, Mg and amorphous Fe in the soil. The application of RS increased DIC and CH4 leaching more than twofold compared to a control experiment in which 13C-enriched RS was not applied. The contribution of RS carbon to DIC, DOC and CH4 in percolating water was 80, 23 and 70%, respectively. The application of RS was shown to suppress the decomposition of soil organic matter, based upon measurement of a low 12C content of DIC in the percolating water.