Impacts of earthworm activity on the fate of straw carbon in soil: a microcosm experiment.

Earthworms not only facilitate carbon (C) stabilization, but also accelerate organic matter mineralization by enhancing microbial respiration. However, the fate (mineralization vs stabilization) of newly added C by straw returning in arable lands with earthworm activity is still unclear. In the present 40 days incubation study, we incorporated artificially 13C-labeled straw into soil with and without presence of earthworms (Metaphire guillelmi). Flux measurements of CO2 from soil (mineralization) were taken regularly, while straw-derived C remaining in the soil (stabilization) was measured at the end of the incubation. There was no significant difference of the cumulative CO2 emission between earthworm presence and absence treatment. However, earthworm presence significantly decreased straw-derived cumulative CO2-C emission when compared with the treatment without earthworm. Besides, earthworm incubation led to a significantly low light fraction organic carbon (LFOC) content and straw-derived LFOC proportion. Relative to the non-earthworm treatment, straw-derived C content significantly decreased in micro-aggregates (< 0.25 mm), but increased in large macro-aggregates (> 2 mm) in the earthworm treatment. In total, only 3.8% of added straw C was assimilated by earthworm within 40 days, while most of the straw C remained in the soil. Earthworms decreased straw-derived CO2-C emission from 10.0 to 8.1% when compared with the non-earthworm treatment. In the present short period incubation experiment, compared with the soil without earthworms, the presence of Metaphire guillelmi (1) resulted a higher soil CO2 emissions, which may mainly evolved from the older SOC, and (2) stabilized more residue-derived C in the soil aggregates. We therefore propose that Metaphire guillelmi may increase soil organic carbon pool turnover rates in the short term after straw returning by replacement of older SOC with newly added straw C.