How do earthworms affect decomposition of residues with different quality apart from fragmentation and incorporation?

Abstract

Earthworms especially contribute to decomposition by fragmenting, incorporating, and mixing residues into the soil. However, residues are already mechanically fragmented and incorporated into the soil by combine harvesters in most arable cropping systems of China, which may diminish the effects of earthworms. How earthworms affect the decomposition of residues with different quality, apart from fragmentation and incorporation, is still not clear. Thus, the aim of the present 60-day soil microcosm incubation experiment was to assess how earthworms affect the decomposition of residues of different quality, aside from fragmentation and incorporation. Four residues with different quality (high quality: rapeseed cake; medium quality: corn leaf; low quality: rice straw and corn stalk) were fragmented to 1-mm pieces using a laboratory blender and then thoroughly mixed with soil. The decomposition rates of different residues with or without earthworms (Metaphire guillelmi) were determined by measuring the CO2 emission. We hypothesized that earthworms accelerate high-quality residue decomposition more than low-quality residues. Results showed that earthworms increased cumulative CO2 emission for each residue type separately. Carbon derived from the residues was calculated and the decomposition kinetics was fitted with the first-order exponential model. During the initial 20 days of the incubation, the decomposition rate constant (k) of low-quality residues was significantly decreased by earthworm addition and significantly increased during the later period (20–60 days) of the incubation. However, no significant difference was detected between earthworm presence and absence treatments on the k value with high- and medium-quality residue incorporation. In general, earthworm addition increased inorganic nitrogen contents in most residue treatments during the entire incubation period, except for the rapeseed cake-treated soil. Our results indicated that earthworms have no effect on the decomposition of high-quality residues, but have a positive effect on the later stages of low-quality residue decomposition.