Contrasting effects of thinning on soil CO2 emission and above- and belowground carbon regime under a subtropical Chinese fir plantation.

Thinning plays a major role in forest soil carbon cycling. However, the mechanisms governing soil C fluxes, i.e., C input through litterfall and fine root (FR) production and C output through soil heterotrophic respiration (Rh), remain unclear. To fill this gap, we quantified the C fluxes in the topsoil layer (0-20 cm) by measuring litterfall, FR production and total soil respiration (Rs) (Ra (autotrophic respiration) and Rh) at three thinning intensities (control; low-intensity thinning: extraction of 30% of individual trees; high-intensity thinning (HIT): extraction of 70% of individual trees) in a 26-year-old Chinese fir plantation in southern China. In the control plots, the total C input (110 g C m-2 year-1) via litterfall (59 g C m-2 year-1) and FR production (51 g C m-2 year-1) was much lower than the C output via Rh (518 g C m-2 year-1). This finding demonstrated that the soil is a C source (407 g C m-2 year-1). Furthermore, the C source increased with increasing thinning intensity, particularly under HIT, due to the decreased litterfall return and increased soil CO2 emissions through Rh; this increase occurred despite the increased C input from FR production. In addition, the C output via Rs significantly increased by 42% under HIT due to the stimulation of Ra and Rh. Consequently, thinning reduced the topsoil C pool by 7-8%. Redundancy analysis indicated that the soil C fluxes following thinning were driven by increased FR mortality, understory plant biomass and diversity, and microbial biomass carbon (MBC). Overall, our results indicate that heavy thinning increases soil C loss by increasing soil CO2 emissions and decreasing litterfall return, even under substantially increased FR production. This finding suggests that thinning practices should consider the trade-off between soil C inputs and outputs to reduce the impact of thinning on forest soil carbon sequestration.

Biogeographic patterns of nutrient resorption from Quercus variabilis Blume leaves across China.

The variation in nutrient resorption has been studied at different taxonomic levels and geographic ranges. However, the variable traits of nutrient resorption at the individual species level across its distribution are poorly understood. We examined the variability and environmental controls of leaf nutrient resorption of Quercus variabilis, a widely distributed species of important ecological and economic value in China. The mean resorption efficiency was highest for phosphorus (P), followed by potassium (K), nitrogen (N), sulphur (S), magnesium (Mg) and carbon (C). Resorption efficiencies and proficiencies were strongly affected by climate and respective nutrients concentrations in soils and green leaves, but had little association with leaf mass per area. Climate factors, especially growing season length, were dominant drivers of nutrient resorption efficiencies, except for C, which was strongly related to green leaf C status. In contrast, green leaf nutritional status was the primary controlling factor of leaf nutrient proficiencies, except for C. Resorption efficiencies of N, P, K and S increased significantly with latitude, and were negatively related to growing season length and mean annual temperature. In turn, N, P, K and S in senesced leaves decreased with latitude, likely due to their efficient resorption response to variation in climate, but increased for Mg and did not change for C. Our results indicate that the nutrient resorption efficiency and proficiency of Q. variabilis differed strongly among nutrients, as well as growing environments. Our findings provide important insights into understanding the nutrient conservation strategy at the individual species level and its possible influence on nutrient cycling.