Interpreting Soil Carbon, Nitrogen, and Microbial Responses to Forest Thinning: Multi-site Assessment

Interaction among soil carbon, nitrogen, and microbes remains unconfirmed for managed forests, despite the growing awareness on sustainability of forest ecosystems. This study assessed the post-thinning shifts in soil carbon, nitrogen, and microbes across 13 temperate pine, oak, and larch forests. Each forest had the un-thinned control, light thinning (15-30% basal area reduction), and heavy thinning (30-50% basal area reduction) treatments, where soil (forest floor and mineral soil at 0-10, 10-20, and 20-30 cm depths) carbon and nitrogen contents were monitored 0-1, 3-4, and 6-7 years after thinning. Soil properties, microbial biomass, and enzyme activity were also determined. The effect of thinning at each forest in each period was compared with standardized effect sizes. Effect sizes for soil carbon and nitrogen contents were generally larger under the heavy thinning than under the light thinning, but unrelated to forest type and time after thinning. Effect sizes for soil carbon contents increased with elevating effect sizes for soil nitrogen contents in forest floor (R² = 0.72) and mineral soil at 0-10 cm depth (R² = 0.48). However, differences in soil properties and microbial responses entailed the variation in effect sizes for soil carbon and nitrogen contents. Especially, effect sizes for soil carbon contents were the function of microbial biomass responses to thinning (R² = 0.53), rather than enzyme activity. Given that the microbial biomass responses varied with the post-thinning changes in soil temperature (R² = 0.52), water content (R² = 0.50), and ammonium concentration (R² = 0.65), these factors may mediate the interaction among soil carbon, nitrogen, and microbes over thinned forests. Our findings support the importance of multi-site assessment in clarifying the impacts of forest management on the soil environment.

The Korea Forest Service (2017044B10-1719-BB01), the Ministry of Environment (2014001310008), and the National Institute of Forest Science of South Korea (FM0101-2009-01) provided research grants.