Different effects of canopy and understory nitrogen addition on soil organic carbon and its related processes in a subtropical forest

Purpose The response of forest soil organic carbon (SOC) to atmospheric nitrogen (N) deposition is generally studied via understory N addition, whereas canopy processes are often ignored. This implies potential bias in the response to N addition. Experiments incorporating both understory and canopy N addition in forests can explore the consistency of N addition approaches. Materials and methods An experiment with canopy addition of 50 kg N ha −1 year −1 , understory addition of 50 kg N ha −1 year −1 , and blank control (0 kg N ha −1 year −1 ) was designed in a subtropical evergreen broadleaved forest in China. After 5 years of N addition, elemental stoichiometry (C, N) and stable isotopes ( 13 C and 15 N) of the plants (litter, fine roots, leaves) and soil samples were determined. We measured concentrations of total organic C and different fractions (microbial biomass, recalcitrant, readily oxidizable, and dissolved C), total N, nitrate and ammonium N, and soil acidity (pH) in surface soil samples (0–5 cm) in the forest. Results The changes in C, N concentrations, C/N ratios, and δ 15 N of litter and understory plant materials were different between the two N addition approaches. The contribution of litter C input and the mean residence time of soil organic C increased by 23% and decreased 1.8 years, respectively, due to understory N addition but was unaffected by canopy N addition. The concentrations of total organic C, microbial biomass C, and recalcitrant organic C were significantly different between the two approaches (lower microbial biomass C as a result of understory N addition than in control treatment). The significantly different C concentrations in macroaggregates and clay-silts, and the mean residence time between the approaches showed the different effects on the stability of soil organic C. Conclusions The approach of canopy N addition is necessary to improve our understanding of the unbiased impact of atmospheric N deposition on soil organic C and its related processes, e.g., aboveground C input, SOC fractions, and soil stability in subtropical forests. Long-term experiments incorporating both understory and canopy N addition simulating atmospheric N deposition are required to further understand the influence of enhanced N deposition on forest soil processes.