The effects of mowing and multi-level N fertilization on soil bacterial and fungal communities in a semiarid grassland are year-dependent

The diversity and structure of plant and soil microbial communities are influenced by temporal variability in environmental conditions (e.g., precipitation); however, it is unclear whether the responses of these biotic communities to land use practices (e.g., N fertilization and mowing) also vary over time. Here we investigated how harvesting hay by mowing and applications of N fertilization at different rates (0, 2.5, 5, 10, 20, and 40 g N m−2 yr−1) affected plant, bacterial, and fungal communities by exploring data collected from a field experiment in a semiarid grassland in northern China over two consecutive years of 2017 and 2018. The cumulative precipitation during the growing season differed between the two studied years. The sampling year had more effect on the structure of the plant and soil microbial community than N fertilization and mowing, suggesting that the effects of land use practices varied by year, and were difficult to predict over time. The diversity of bacteria and fungi showed a different response to N fertilization and mowing between the two years. For example, in the wet year, the fungal diversity was up to 11% lower in soil that had been treated using the highest N fertilizer application than in untreated soil, but showed little variation in the dry year. The bacterial diversity was higher for all N application rates in the mown than the unmown land during the wet year, but no difference was observed during the dry year. There were more opportunistic and sensitive taxa for the two years (over 36.9% of top 10% relative abundance of bacterial and fungal taxa) than for N fertilization and mowing (below 33.0% of top 10% relative abundance of bacterial and fungal taxa). The relationships between plant and soil microbial communities differed between the two years, and were much stronger in the dry year than the wet year. We conclude that N fertilization and mowing had varying effects on plant and soil microbial communities in the study area over