Earthworms increase mineral soil nitrogen content – a meta-analysis

Soil organisms and their interactions play a key role in various ecosystem processes and functions, such as the provision of nutrients. The main actors in nitrogen transformation processes are microorganisms, but earthworms affect these processes as their activity results in changes of the microhabitat and microbial community. Studies have shown that nitrogen content is higher in earthworm casts than in bulk soil, and that earthworm invasion affects soil mineral nitrogen. However, we still lack a quantitative synthesis of earthworm effects on soil nitrogen in bulk soil that integrates the influence of potential controlling factors (i.e., soil properties, climatic conditions and experimental parameters). Here, we investigated the impact of earthworms on soil ammonium (NH4+), nitrate (NO3-) and total mineral nitrogen (ammonium + nitrate, Nmin) using meta-analytic techniques. Earthworms generally increased NO3- (+ 88%) and Nmin (+ 63%), but did not affect NH4+. We assume that earthworms affect total mineral nitrogen mainly by their impact on NO3-. Endogeic and epigeic earthworms significantly increased NO3- and Nmin, whereas no clear effect of anecic earthworms was found. This result is presumably caused by diverse effects of the different ecological groups on the microbial community composition. Our results for mixed ecological groups (i.e., anecic + endogeic earthworms) reveal potentially antagonistic effects of ecological groups. The impact of earthworm presence on NO3- and Nmin increased when experiments lasted longer than one week. The effect of earthworms on NH4+, NO3- or Nmin was not influenced by earthworm abundance and biomass, soil organic carbon, soil C/N ratio, litter C/N ratio, the initial amount of NH4+, NO3- or Nmin, total soil nitrogen or temperature. However, as data availability or replication across factor categories was low for some of these moderators, the non-significant results should be interpreted with caution. Also, we could not investigate interactions among the controlling factors due to paucity of data. Our study thus reveals important knowledge gaps regarding earthworm effects on soil nitrogen. Overall, our results highlight the importance of earthworms for soil nitrogen cycling and strengthen the call for soil-functional models to incorporate soil faunal effects.