Effect of nitrogen addition on soil fauna in a temperate steppe of Inner Mongolia

Nitrogen (N) addition effects on plant and soil biota, and functional processes of terrestrial ecosystems were studied. The response of soil fauna to nitrogen addition has not yet been well studied in comparison to the studies on the response of plant communities. Here, we studied the effects of N addition on soil biota in a semi-arid steppe community in central Inner Mongolia using a field experiment. The experiment consisted of four N addition levels, applied by adding N fertilizers at four rates, including: 0 (control), 25 (low), 50 (medium), and 100 (high) kg·ha-1. After 5 years of treatments, we examined the changes in soil fauna communities and their functional groups in response to different N treatments, and analyzed their relationships with environmental factors. The results showed that N addition at the low and medium levels enhanced the abundance of soil fauna by 75.8% and 76.7%, respectively, in comparison to that of the control, where as N addition at the high rate significantly decreased the diversity of soil faunas. The responses of soil fauna to N addition varied among functional groups. The abundance of predatory and saprophagous fauna decreased by 69.1% and 54.4%, respectively, under the high N treatment; whereas the phytoplasmic and omnivorous functional groups showed no significant changes under different N treatments. The results of RDA analysis also detected a significant negative correlation between the abundance of Mesostigmata nymphs and Laelapidae and N addition rates, and the abundance of Isotomidae and Entomobryidae were indirectly affected by N addition through the effects on plant species richness, soil organic carbon, and litter pH. Our results indicated that low N addition rates may enhance the abundance of soil fauna in the litter layer of steppe ecosystems, whereas a high N addition rate >10 g ·(m2·a)-1 may decrease the diversity of soil fauna.