Asymbiotic nitrogen (N) fixation is an important N source in forests and predicted to decrease with increasing N deposition rates. Previous studies revealing aysmbiotic N fixation associated with N deposition mostly applied understory N addition approaches, neglecting the realistic N deposition that goes through forest canopy before falls into the soils. Thus, it is unclear how canopy N retention may influence the effects of N deposition on asymbiotic N fixation. In this study, we evaluated the effects of N deposition on N fixation in canopy (moss and canopy leaves) and understory (soil and forest floor) compartments in a temperate forest (in central China) and a tropical forest (in southern China) using five N addition treatments: control (C), understory N addition at 25 kg N ha-1 yr-1 (UN25), at 50 kg N ha-1 yr-1 (UN50), canopy N addition at 25 kg N ha-1 yr-1 (CN25), and at 50 kg N ha-1 yr-1(CN50). Our goal was to investigate whether traditionally understory N addition approaches could realistically reflect the effects of N deposition on asymbiotic N fixation in forest ecosystems.
In the tropical forest, UN50 suppressed N fixation in the soil. In the temperate forest, UN25 and UN50 suppressed N fixation in the soil and forest floor, whereas CN25 and CN50 suppressed N fixation in the moss and canopy leaves. Compared with CN50, UN50 overestimated the effects on the soil N fixation by 0.4−0.8 fold in the tropical forest. Compared with CN25 and CN50, UN25 and UN50 overestimated the effects on the soil and forest floor by 2.0−8.8 folds, but underestimated the effects on the moss and canopy leaves by 0.8−1.3 folds in the temperate forest. The effects of N deposition on total N fixation were overestimated using understory N addition approaches, which were weak in the tropical forest (by 0.4 fold) but strong in the temperate forest (by 3.7−4.6 folds) due to the effects of canopy N retention. Thus, our results suggest that understory N addition approaches can be applied in tropical forests but not in temperate forests. Previous studies may have overestimated the effects of N deposition on total N fixation by ~4 folds in temperate forests due to neglecting canopy N retention, which will lead to inaccurate evaluation and projection of N cycles in temperate regions.