The presence of earthworm Pontoscolex corethrurus rather than organic matter sources indirectly controls N2O flux in tropical plantation soils

Abstract Both organic matter status and earthworm activity may affect the microbial-mediated N2O production in tropical plantations which contribute greatly to the global N2O emission. The representative pantropical earthworm Pontoscolex corethrurus was likely to be increasingly dominant in degraded lands due to intensified disturbances such as understory removal and soil erosion. We noticed that the relatively fertile surface soil plays key roles in fueling both microbes and earthworms. Therefore, we hypothesized that N2O production was stimulated most by earthworm P. corethrurus in less-disturbed tropical plantations where the surface soil layer was conserved. To determine the effect of organic matter sources and P. corethrurus activity on N2O flux in tropical plantation soils, a field mesocosm experiment including four organic treatments with and without earthworms was then conducted. In plant-free mesocosms, mineral soils (60–80 cm layer) received four organic matter treatments: no organic matter (CK), non-fragmented fallen leaves (NL), partially fragmented leaves (PL) and surface soil containing decomposed litter (0–5 cm, SS), which were used to simulate different levels of disturbance-mediated resource depletion. We found that although the population of P. corethrurus declined with the intensity of food resource limitation, mostly P. corethrurus stimulated N2O flux, but occasionally reduced N2O flux in the organic matter-depleted control soils. In contrast, organic matter sources did not exert significant effects on N2O flux. This study indicates that, although the disturbance-induced changes in organic matter supply altered earthworm population size, the presence of P. corethrurus rather than organic matter sources is the determinant of N2O flux in the studied tropical soils. Although the occasionally negative earthworm effects on N2O flux brings uncertainty, the population size-independent positive earthworm effect on N2O flux may simplify the modeling of earthworm contributions to greenhouse gas fluxes.