Short-term machinery impact on microbial activity and diversity in a compacted forest soil

Forest soils are complex ecosystems including a huge biodiversity including different microbial communities that are responsible for soil nutrient cycling and organic matter decomposition. One of the main threats to forest soil health is soil compaction caused by forest exploitation activities and, in particular, by wood extraction operations. These latter may change the physical, chemical and, in turn, biological properties of soil, eventually impacting tree growth and regeneration. However, there is a significant lack of knowledge in understanding the response of soil microbial communities to soil compaction. Furthermore, most of previous studies did not properly frame the short-term response of soil microbiota to compaction, which could serve as an early indicator of soil health. This study aims to investigate and monitor the short-term response of forest soil to severe compaction stress integrating soil physico-chemical analysis with biological and biochemical analysis. To investigate the early response of soil microbial communities to compaction over time, forest soil was analyzed 8 and 12 months after the repeated passage of a tractor pulling some logs simulating skidding operations in a heavily trafficked area for wood extraction. Despite the initial strong increase in bulk density (up to 42 %), the soil almost recovered from compaction after just one year. Most of the soil chemical parameters (i.e. pH, C, and N content) were not affected by soil compaction, while the analysis of enzymatic activity showed a change of some functions upon soil compaction in the first 8 months, followed by a substantial recovery after one year. The microbial communities showed different responses to compaction, highlighting a greater bacterial community resilience in the short term compared to the fungal community, which showed persistent and significant shifts between compacted and not-compacted soil. We also identified some indicator species that may be useful for monitoring early changes in microbial communities and activities following soil compaction.