Earthworm regulation of nitrogen pools and dynamics and marker genes of nitrogen cycling: A meta-analysis

Earthworms, as the ecosystem engineers, both directly and indirectly affect the nitrogen (N) cycle. We aimed to provide a quantitative assessment of the contribution of earthworms to the terrestrial ecosystem N cycle using meta-analysis of 130 publications selected. The natural logarithm of the response ratio (lnRR) was used to indicate the effect size of earthworms on N dynamic variables. The results showed that earthworms significantly affected soil N-cycling microorganisms, including the amoA gene abundance of soil ammonia-oxidizing bacteria (AOB), and significantly promoted soil N cycle processes, including denitrification, mineralization, and plant assimilation. The effects of earthworms on the N cycle were experimental design dependent and affected by factors such as the functional group of earthworm and residue input. The presence of the anecic earthworms decreased the rates of mineralization and nitrification, and increased nitrification and denitrification responses were more pronounced in the presence of the endogeic earthworms than that of the other two functional groups of earthworms. In addition, residue input enhanced the effects of earthworms on the N cycle. The effects of earthworms on nitrous oxide (N2O) emission increased when residues were added. These findings indicate that residue input and introducing suitable functional groups of earthworms into the field can lead to N sustainability without increasing N2O emission. This meta-analysis also provides systematic evidence for the positive effects of earthworms on the plant N pool, N availability (soil ammonium (NH4+) content), and soil microbial biomass N content, showing the potential to alter ecosystem functions and services in relation to N cycling.