2017 ESA Annual Meeting (August 6 -- 11)

PS 2-29 - Asymbiotic nitrogen fixation in response to canopy versus understory nitrogen addition in forest ecosystems

Monday, August 7, 2017
Exhibit Hall, Oregon Convention Center
Mianhai Zheng1, Wei Zhang1, Yiqi Luo2, Shiqiang Wan3, Shenglei Fu4 and Jiangming Mo1, (1)South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China, (2)Microbiology and Plant Biology, University of Oklahoma, Norman, OK, (3)College of Life Sciences, Hebei University, Hebei, China, (4)College of Environment and Planning, Henan University, Kaifeng, China
Background/Question/Methods

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.

Results/Conclusions

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.