The impact of nitrogen deposition on photobiont‐mycobiont balance of epiphytic lichens in subtropical forests of central China

Excessive nitrogen (N) deposition can impact lichen diversity in forest ecosystems, and this is a particular situation in China. Here, we examined the N uptake, assimilation, and the impact of excessive N deposition on the symbiotic balance of dominant epiphytic lichens in the subtropical forests in the Mts. Shennongjia of central China. The results show that lichen species took up, assimilated and utilized more ammonium than nitrate in a species‐specific way, following the increase of N availability. The photobiont of the lichens decreased with the increase of N concentration following an initial increase, while the mycobiont response to the N addition was not apparent. Considerable variation in response to excessive N deposition exists among the lichen species. Usnea longissima could regulate its N uptake, resulting in a stable photobiont‐mycobiont ratio among N treatments. In contrast, the photobiont‐mycobiont ratio of other four lichens increased initially but decreased when N concentration exceeded a certain level, and N stress may have broken the balance between photobiont and mycobiont of these lichens. Our results suggest that most epiphytic lichens in subtropical forest of central China could uptake and assimilate more ammonium than nitrate and that the balance between photobiont and mycobiont of many epiphytic lichens might change with the increasing N deposition load, which could impact the lichen diversity of this forest ecosystem. We found that the dominant epiphytic lichens uptake and utilization more ammonium than nitrate in a species‐specifically, which suggested that most lichens would be sensitive to nitrogen deposition. Furthermore, we found nitrogen addition would destroy the balance between mycobiont and photobiont, in detail, below the nitrogen threshold of lichens, N addition promotes the N uptake of all lichens and then increases the photobionts and promotes individual growth. When the N addition exceeds the threshold, toxic effects appear in which excess N leads to the imbalance of symbionts, resulting in a decrease in the growth rate of these lichens. Specifically, we found that Usnea longissima can keep a consistent balance between mycobiont and photobiont when nitrogen availability increased, which suggested that nitrogen deposition would change the lichen diversity of the subtropical forests of China.