Effects of nitrogen inputs and mowing on the abundance and species richness of herbivorous insects in a meadow steppe.

To reveal the effects of nitrogen (N) inputs and mowing on the abundance and richness of insect community in meadow steppe, we investigated the abundance and diversity of herbivorous insects under four treatments in Inner Mongolia meadow steppe in August 2022, including control, N addition, mowing, and combined N addition and mowing. At a long-term control experimental platform, we collected insects using the vacuum sampling method. The results showed that N addition significantly increased the abundance of herbivorous insects, and mowing significantly decreased the abundance of herbivorous insects. Nitrogen addition significantly increased insect abundance in unmown condition but not in the mown condition. The responses of insect abundance at the community level to N addition were mainly driven by the dominant groups, Cicadellidae and Lygaeidae, which was affected by the abundance of Gramineae species. In contrast, their responses to mowing were the opposite. Both N addition and mowing did not affect the diversity of herbivorous insects. Our results indicated that the responses of herbivorous insect abundance to N inputs and mowing were directly regulated by food resources. With increasing food resource availability, the abundance but not the diversity of herbivorous insects increased, with stronger responses of dominant groups than subordinate ones.

Extreme drought exacerbates plant nitrogen­phosphorus imbalance in nitrogen enriched grassland.

The nitrogen­phosphorus (N-P) imbalance induced by N enrichment has received increasing concerns, because N:P ratios play a critical role in driving many fundamental ecological processes. Given the simultaneous occurrence of different global change drivers, it is important to understand whether and how would such N-induced N-P imbalance would be mediated by other global change factors. We examined the interactive effects of N addition (10 g N m-2 yr-1) and extreme drought (-66 % rainfall during the growing season) on species- and community-level N:P ratios in both green and senesced leaves in a temperate grassland of northern China. Extreme drought did not alter soil available N:P ratio under ambient N conditions, but increased that under N enriched conditions. Further, extreme drought did not alter the community-level N:P in both green and senesced leaves under ambient N conditions but significantly enhanced that under N enriched conditions. The drought-induced species turnover made a significant positive contribution to the changes in the community-level N:P ratio under N enriched conditions, but not under ambient N conditions. Our results suggest that the N-induced ecosystem N-P imbalance would be exacerbated by extreme drought event, the frequency of which is predicted to increase across global drylands. Such N-P imbalance would have consequences on litter decomposition, nutrient cycling, and the structures of above- and below-ground food webs. Our findings highlighted the complexity in predicting ecosystem N-P imbalance given the interactions between different global change drivers.