Level of soil moisture determines the ability of Eisenia fetida to reincorporate carbon from decomposed rice straw into the soil

Burning is the most common practice for rice straw disposal. Due to associated negative environmental and climatic effects, development of viable alternatives, preferably based on the natural functions of soil biota are needed. In the conditions of non-tropical rice-growing systems, where periods of flooding are very short, such an approach seems to be particularly promising. We carried out a mesocosm experiment to assess the possibility of using the model earthworm species Eisenia fetida (Savigny 1826) to decompose rice residues and control associated CO2 and CH4 emissions from paddy soils at different soil moisture levels. We filled 96 mesocosms with three types (32 each) of rice paddy soils collected in key regions of rice production in Russia: Krasnodarsky Krai (the Sea of Azov lowland, Calcic Phaeozems), the Republic of Kalmykia (the Volga river valley, Haplic Phaeozems) and Primorsky Krai (Khanka lake lowland, the Russian Far East, Umbric, Histic Fluvisols). We added 2.5 g dry rice straw in each mesocosm. The experiment had a full factorial design including three categorical factors: soil type (n = 3), soil moisture level (12, 25, 50 and 75% soil water holding capacity) and E. fetida earthworm addition (none and 4 individuals per mesocosm). The integral emission of CO2 across the observation period of 10 days significantly differed between moisture classes with the highest values at 25% (p < 0.05). Earthworm amendment had no effect on CO2 flux in all moisture treatments besides 75%, where it was positive. The detectable CH4 emissions were observed only at soil moisture levels of 50 and 75%. Earthworms strongly positively affected this parameter at 75% soil moisture level (p < 0.05). Carbon content after the experiment was significantly higher in the earthworm-inoculated microcosms only at the 25% moisture level. We conclude that E. fetida may positively contribute to carbon sequestration during rice straw degradation in the studied rice paddy soil types only under certain levels of substrate moisture (25% in our case). This highlights the importance of soil encountering abiotic conditions when developing climate-friendly systems for rice straw decomposition and carbon immobilization. It also suggests the potential of using E. fetida as a viable agent of biological rice straw recycling during the drained stages in non-tropical rice paddies or in artificial confinements.