Isotopic evidence for the effects of earthworm and straw amendment on root carbon uptake of upland rice and maize

The role of earthworms in the direct transfer of dissolved soil organic carbon to plants is poorly understood. We quantified this effect by examining the root uptake of carbon from 13C-labeled rice straw in a greenhouse experiment with three non-flooded Philippine soils that greatly differed in texture (from light sandy to loam). Measurements of carbon stable isotope signatures of aerobic rice and maize over a two-month period revealed that earthworms positively affected 13C transfer from rice straw to rice plants but not to maize plants. However, this effect was significant only in loamy and not in sandy soils. Although the direct uptake of dissolved carbon from soil had no impact on rice production, it indicated a widely ignored option to mitigate carbon loss from crop residue decomposition and associated climate risks. Its enhancement by earthworms confirms the important role of this taxon as soil engineers.