The Inhibitory Effects of Nitrogen Deposition on Asymbiotic Nitrogen Fixation are Divergent Between a Tropical and a Temperate Forest
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Asymbiotic nitrogen (N) fixation (ANF) is an important source of N in pristine forests and is predicted to decrease with N deposition. Previous studies revealing N fixation in response to N deposition have mostly applied understory N addition approaches, neglecting the key processes (for example, N retention and uptake) occurring in forest canopy. This study evaluated the effects of N deposition on N fixation in the soil, forest floor, mosses, and canopy leaves in a temperate forest (in central China) and a tropical forest (in southern China) with different treatments: control, understory N addition, and canopy N addition. Results showed that total ANF rates were higher in the temperate forest (2.57 ± 0.19 mg N m−2 d−1) than in the tropical forest (1.34 ± 0.09 mg N m−2 d−1). N addition inhibited the soil, forest floor, moss, and foliar N fixation in the temperate forest, whereas it inhibited only the soil N fixation in the tropical forest. Compared to canopy N addition, understory N addition overestimated the inhibitory effects of N deposition on total ANF slightly in the tropical forest (by 35%) but severely in the temperate forest (by 375–472%) due to neglecting canopy retention of N. In summary, our findings indicate that ANF has different rates and sensitivities to N addition between tropical and temperate forests and that understory N addition overestimates the N deposition effects on ANF in forests, particularly in the temperate forest. These findings are important for our accurate understanding and estimate of terrestrial N fixation under N deposition scenarios.
Keywordsasymbiotic nitrogen fixation canopy nitrogen addition canopy nitrogen retention nitrogenase activity nitrogen deposition nitrogen fixation rates understory nitrogen addition
We appreciate three anonymous reviewers for their constructive comments on the manuscript. We acknowledge financial supports from the National Natural Science Foundation of China (Nos. 31670488, 31370011, 31770523), the Natural Science Foundation of Guangdong Province (No. 2017A030313168), the National Postdoctoral Program for Innovative Talents (BX20180312), China Postdoctoral Science Foundation (2018M640836), and the UCAS Joint PhD Training Program. We thank Ms. Xiaoping Pan and Mr. Peng Mao for their assistance in laboratory and field experiments. The authors declare no conflict of interests.
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