Abstract
The impact of atmospheric nitrogen deposition on carbon exchange between forest and atmosphere is one of the research hotspots of global change ecology, past researchers have extensively studied the impacts on leaf level, while the impacts on crown CO2 exchange are still unclear. Therefore, we explored the impacts of different nitrogen addition levels on crown CO2 exchange of Fraxinus mandshurica saplings and their responses to the changes of major meteorological factors (photosynthetically active radiation, PAR; vapor pressure deficiency, VPD; and air temperature, Tair) with a novel automated chamber system. There are four levels of nitrogen addition treatments: control (no nitrogen addition, CK), 23 (low nitrogen addition, LN), 46 (medium nitrogen addition, MN), and 69 kgN·hm−2·a−1 (high nitrogen addition, HN). Our results showed that all nitrogen addition treatments increased daily average and accumulated gross primary production (GPP), crown respiration (R), and net crown CO2 exchange (Ne), especially at medium and high nitrogen levels. Similarly, maximum net photosynthetic rate (Nemax) and apparent quantum efficiency (α) were promoted. The change of Ne with PAR, Tair, and VPD showed that nitrogen addition postponed the appearance of photosynthesis midday depression. In addition, the monthly accumulation of R with all nitrogen addition treatments showed an increasing trend (June to July), and then decreased (July to September) during the growing season, while the Ne and GPP decreased gradually with seasonal vegetation senescence. Finally, the crown shifted from carbon sink to carbon source at the end of the growing season, however, the change under high nitrogen treatment occurred 3 days later. The crown CO2 exchange measurements provide a new perspective to better understand the response of forest ecosystem CO2 exchange to elevated nitrogen deposition and provide a basis for related carbon model parameter correction under the influence of nitrogen deposition.
Highlights
Our results provided strong evidence that nitrogen addition enhanced the net crown carbon exchange (Ne), especially MN and HN treatments, which increased by 52.31 and 55.68% compared with the control, respectively, while the LN was only by 11.01%
We found α in nitrogen addition treatments were higher than that in control, there was no uniform rule for α under different treatments, which ranged from 0.032 to
We found crown carbon fixation increased in the early and middle of the growing season (Figure 5), which probably because that temperature was suitable for plant growth, thereby increasing photosynthesis, maintaining and building their own material increased, so did leaf, branch, and trunk respiration, resulting in an increase in overall crown respiration, which reached a peak at the end of July and early August
Summary
The atmospheric nitrogen deposition rate has risen dramatically due to rapid agricultural, industrial, and urban development. The monitoring study found that the nitrogen deposition of European forests was 25–60 kg N·hm−2 ·a−1 [1], and that of Los Angeles forests was 35–50 kg N·hm−2 ·a−1 [2], the annual bulk nitrogen deposition in China has increased from 13.2 kg N·hm−2 ·a−1 in the 1980s to 21.1 kg N·hm−2 ·a−1 in the 2000s [3]. Forests are the most important part of terrestrial ecosystems and the direct carriers of nitrogen deposition. Increasing enhanced atmospheric nitrogen deposition increases the availability of nitrogen in forest ecosystems and affects the carbon cycle by influencing the photosynthesis and respiration of plant’s canopy leaves [4,5,6,7]
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