Soil mineral nitrogen content is increased by soil mesofauna and nematodes – a meta-analysis

The world’s soils maintain various ecosystem processes and functions, such as the provision of nitrogen, which is the basis for plant growth. Microorganisms are the key actors in nitrogen transformation processes, but soil fauna can also affect nitrogen cycling , e.g. through food-web interactions or by changing the soil habitat. Several individual studies report increases of soil nitrogen due to soil meso- or microfaunal effects. Recent meta-analyses have shown that nitrogen content and fertility of soils is substantially increased by earthworms. However, we still lack a quantitative synthesis of the influence of smaller soil fauna on soil nitrogen. We present a meta-analysis of effects of soil mesofauna (i.e. enchytraeidae, springtails, mites) and microfauna (i.e. nematodes) on ammonium (NH4+), nitrate (NO3-), and total mineral nitrogen (ammonium + nitrate, Nmin). We furthermore investigated whether the faunal effects depend on functional characteristics (i.e. trophic groups or size classes) or faunal density; or were modulated by experimental conditions or soil characteristics. Our results show that soil meso- and microfauna generally increased NO3- and Nmin concentrations, but did not affect NH4+. Increases of soil nitrogen are found in presence of microbi-detritivores or faunal communities that span various trophic levels. Micro- and mesofauna improve nitrogen availability in the soil in short- to intermediate term of up to two months. Furthermore, the provision of organic material plays an important role, as micro- and mesofauna have a positive effect on soil nitrogen especially when organic material is added and incorporated into the soil. This has important implications for agricultural management with regards to handling of organic residues. No significant moderating influence of micro- and mesofaunal density, litter C:N ratio, soil organic-carbon content, initial amount of NH4+, NO3- or Nmin, or experimental temperature was found. However, data availability or replication across factor categories was low for some of these moderators, and we could not differentiate between size classes (i.e., micro- or mesofauna) for all moderators or investigate interactions among different moderators. Thus, our study reveals important knowledge gaps that should be addressed in future research. Overall, our results underline the role of micro- and mesofauna for soil nitrogen cycling.