Effects of compost amendments and experimental drought on grassland soil microbial communities

Prolonged drought is a major stressor for grassland ecosystems. In addition to decreasing plant productivity, it can affect soil microbial activities and thus destabilize nutrient cycling and carbon (C) sequestration. Soil organic amendments (OAs), such as compost, can be used to enhance soil fertility and mitigate drought effects. In this study, we evaluated the responses of fungal and bacterial communities to a 3-year-long experimental drought and compost treatment across four soil depths in two Swedish grasslands and at an upper and a lower topographic position. Results showed that while drought reduced soil moisture and compost amendment increased C content in the topsoil, the effects on microbial abundance and community composition within this time frame were weak, and detectable only in the topsoil. Fungal abundance increased with compost addition, which also affected community composition, while fungal communities were resistant to drought. Bacterial communities were not significantly affected by any of the treatments. This suggests that microbial ecosystem functions were resistant to the experimentally reduced precipitation. Overall, variation between sampling sites was more important for microbial community composition than treatments, highlighting the need for a better understanding of small-spatial-scale environmental controls on soil microbial and plant communities and their ecosystem functions.