Multi-site assessment of soil nitrogen stocks across temperate forests under different thinning intensities, recovery times, and site conditions

Increasing research interests have been paid to understand the factors controlling soil nitrogen (N) stocks under diverse environmental conditions and forest thinning regimes. This study investigated soil N stocks across 13 temperate forests, each of which received three thinning intensities (unthinned control, 15–30 %, and 30–50 % basal area removals) under varying pre-treatment conditions (altitude, slope, soil pH, soil moisture, stand age, stand density, diameter at breast height, and tree height). The total N stored in the forest floor (L, F, and H layers) and mineral soils (0–10, 10–20, and 20–30 cm) was determined 1, 4, and 7 years after thinning. Given the various site conditions and thinning regimes, a standardized effect size was used to analyze the influences of thinning on N stocks. The N stocks (Mg N ha−1) of the forest floor and at 0–10, 10–20, and 20–30 cm mineral soil depths were 0.02–0.46, 0.32–3.21, 0.29–3.03, and 0.25–2.54 across all studied forests, respectively. The averaged effect sizes indicated decrease in forest floor N stocks and increase in mineral soil N stocks under thinning due to the reduced litterfall and eventual input of thinning residues. Thinning intensity negatively affected the effect sizes for the N stocks (P