Deadwood mass and microclimate affect labile soil carbon and nitrogen under thinning of a naturally regenerated oak forest

Context: Labile soil carbon and nitrogen are important indicators for evaluating carbon sequestration and nutrient availability under forest management such as thinning. Aims: This study assessed the effects of thinning on labile soil carbon and nitrogen indices in a naturally regenerated oak forest. Methods: Un-thinned control, 15% thinning, and 30% thinning were set up, and labile soil carbon and nitrogen (microbial biomass, permanganate oxidisable carbon, dissolved carbon and nitrogen, and inorganic nitrogen) and C: N balance indices (C: N ratio and threshold element ratio) were measured after 6 years. Key results: Thinning generally increased labile soil carbon and nitrogen concentrations (P < 0.01); however, their fraction within the total soil carbon and nitrogen decreased under thinning. Redundancy analysis revealed that the remaining deadwood mass was the most influential factor affecting such differences in labile soil carbon and nitrogen indices across the three treatments (P < 0.01), although soil moisture had an effect as well (P < 0.05). Furthermore, the dissolved C: N ratio was higher than threshold element ratio only in 15% and 30% thinnings, indicating that thinning could shift the limiting factor for microbial growth from carbon to nitrogen by adding deadwoods, which provided carbon-rich substrates. Conclusions: Overall, results demonstrate that deadwood retention and soil microclimatic changes could be the significant factors affecting soil carbon and nitrogen storage and the associated microbial affinity in thinned forests. Implications: Therefore, sustainable forestry strategies for thinning residue management should be addressed to reinforce soil carbon and nitrogen conditions and the forest ecosystem functions.