Effects of soluble organic carbon addition on CH4 and CO2 emissions from paddy soils regulated by iron reduction processes

An incubation experiment with the addition of glucose was conducted to evaluate the effects of carbon and iron (Fe(III)) reduction on methane (CH4) and carbon dioxide (CO2) emissions from paddy soils. Soils of a rice–rapeseed (Brassica napus) rotation and rice–fallow/flooded rotation were collected from Qianjiang (QR and QF, respectively) and Xianning (XR and XF). Incubation was conducted under flooding at 25°C ± 1°C with or without (CK) glucose over 40 days. With glucose addition, cumulative CH4-C emissions from QR, QF, XR and XF soils were 5.31, 35.26, 13.92 and 27.58 mg kg–1, respectively, and cumulative CO2-C emissions were 594.33, 620.49, 549.42 and 792.46 mg kg–1. Compared with CK, glucose addition significantly (P < 0.05) increased cumulative CH4 fluxes in QR and QF soils 11.07-fold and 1.39-fold, respectively, and cumulative CO2 fluxes 0.41-fold and 0.44-fold, whereas the effects of glucose addition on CH4 and CO2 fluxes in XR and XF soils were negligible. In addition, the soil Fe(II)/(Fe(II) + Fe(III)) fraction correlated positively with CH4 fluxes during the major emission period (P < 0.05), and the Fe(II) production rate was positively correlated with the CO2 flux during the whole incubation period. Furthermore, Fe(III) reduction strongly competed with CH4 emission, especially in XR and XF soils, which derived from quaternary red clay. The results suggest that Fe(III) reduction plays a key role in mediating the carbon cycle of paddy soils.