Earthworms offset straw-induced increase of greenhouse gas emission in upland rice production.

Increasing water scarcity and rapid socio-economic development are driving farmers in Asia to transform traditionally flooded rice cropping systems into non-flooded crop production. The management of earthworms in non-flooded rice fields appears to be a promising strategy to support residue recycling and mitigate greenhouse gas (GHG) emissions triggered by residue amendment. We conducted a field experiment on non-flooded rainfed rice fields, with and without residue amendment. In-situ mesocosms were inoculated with endogeic earthworms (Metaphire sp.), with either low (ET1: 150 individuals m), or high density (ET2: 450 individuals m), and a control (ET0: no earthworms). We measured GHG emissions (methane (CH); nitrous oxide (NO); carbon dioxide (CO)) twice a week during the cropping season with static chambers. Effects of earthworms on yield and root growth were additionally assessed. Earthworms offset the enormous increase of CH emissions induced by straw amendment (from 4.6 ± 5 to 75.3 ± 46 kg CH-C ha in ET0). Earthworm activity significantly reduced CH release, particularly at ET2, by more than one-third (to 22 ± 15 kg CH-C ha). In contrast, earthworm inoculation did not affect NO emission. Straw amendment more than doubled the global warming potential (GWP). Earthworms reduced GWP by 39% at low (ET1) and 55% at high densities (ET2). Earthworm activity reduced root mass density under conditions of straw amendment but did not affect yield. Earthworms can significantly reduce detrimental effects of rice crop residue amendment on GHG release under upland rice production. Organic carbon (C) might be preserved in earthworm casts and thereby limit C availability for CH production. At the same time, earthworm activity might increase methanotrophic CH consumption, due to improved soil aeration or less root exudates. Consequently, earthworms have a strong potential for regulating ecosystem functions related to rice straw decomposition, nutrient allocation and thus GHG reduction.