Earthworms facilitate soil mineral associated organic matter formation but increase priming effect depending on litter addition and soil texture

Earthworms affect soil organic carbon (SOC) decomposition and C stabilization into mineral associated organic matter (MAOM) following fresh organic matter input. However, it remains untested how these earthworm-induced C dynamics vary with the rate of fresh organic matter input and soil texture and how they are associated with soil microbial C use efficiency (CUE). Herein, we conducted a 48-day incubation to investigate the impact of earthworms on soil C dynamics following litter input, as well as the relationships of C dynamics with microbial CUE. The experimental set-up consisted of three factors including earthworms (with and without), 13C-labeled grass litter input rate (0, 1 and 6 g C kg−1 soil) and soil texture (grassland soils with either clay or sand addition). Earthworms increased SOC decomposition without litter input by 9 % − 13 %, while amplifying the priming effect (PE) in soil with clay and sand addition at the highest litter addition by 24 % − 139 %, but decreasing the PE in soil with sand and low litter addition by 32 %. In soil with sand addition, earthworms increased MAOM formation efficiency from litter (fraction of added litter C stabilized in MAOM) by 17 % − 23 %, and the litter C sequestration quotient (litter-derived C in MAOM divided by the sum of litter derived C in MAOM and respiration) by 10 % − 27 %. However, earthworm-induced changes in SOC decomposition, PE and MAOM formation were not associated with earthworm-induced changes in microbial CUE. In conclusion, earthworms can facilitate SOC accrual more in soils with sand addition through disproportional amplification of SOC stabilization compared with SOC loss through decomposition. The influence of earthworms on SOC accrual is more likely driven by physicochemical protection of SOC rather than by changes in microbial metabolism.