施氮对森林生态系统AM真菌群落组成及多样性的影响

Can understory nitrogen addition overestimate the effects of nitrogen deposition on arbuscular mycorrhizal fungal community?

丛枝菌根(AM)真菌能够和绝大多数陆生植物形成互惠共生体,具有重要的生态功能。在氮(N)沉降日益严重的背景下,越来越多的土壤生态学家开始关注N沉降对AM真菌群落的影响,然而已有研究大多数集中在草地生态系统,对森林生态系统的关注相对较少,而在森林生态系统开展的模拟研究又多采用林下施N的方式,忽略了冠层发生的一系列生态过程,可能无法准确反映真实情形。依托鸡公山野外控制试验平台,采用高通量测序技术就不同施N方式(林下vs林冠)及速率对AM真菌alpha多样性和群落组成的影响进行了连续4 a的监测。试验综合考虑植被、坡向和坡度等因素,采用完全随机区组设计,包括4个区组(重复),每个区组随机设置5个样方,对应5个不同处理:对照(CK)、林冠施N 25 kg hm^-2 a^-1(CN25)和50 kg hm^-2 a^-1(CN50)、林下施N 25 kg hm^-2 a^-1(UN25)和50 kg hm^-2 a^-1(UN50)。结果发现,在目前的N素添加水平和时间尺度上,施N方式和施N速率对AM真菌的alpha多样性都没有显著影响,二者之间也无交互作用。然而,经过一年的试验处理,施N方式对AM真菌群落组成产生了轻微的影响,而施N速率有极显著的影响,且二者之间存在显著交互作用。当施N速率为25 kg hm^-2 a^-1时,林冠施N和对照相比差异不显著,而林下施N处理AM真菌群落组成与对照相比差异极显著,与林冠施N相比,差异也极显著;当施N速率为50 kg hm^-2 a^-1时,林冠施N与对照处理群落组成有略微差异(P=0.080),林下施N与林冠施N及对照处理相比AM真菌群落组成均没有显著变化。在接下来的三年中,施N方式和施N速率对AM真菌的群落组成都没有显著影响,二者之间也无显著交互作用。这说明在特定的施N速率和处理时间下,林下施N可能会高估自然N沉降对AM真菌群落组成的影响。随着处理时间的延长,不同处理下AM真菌群落有趋同的趋势,可能是因为AM真菌群落对N沉降产生了适应性。本研究评估了施N方式对森林生态系统AM真菌群落组成与结构的影响,在未来的研究中需要设定更多的N素梯度和更长的时间跨度,才能够更全面的认识N沉降的生态效应。

Arbuscular mycorrhizal(AM)fungi can form symbiosis with most terrestrial plants and provide important ecological services.Under the intensified nitrogen(N)deposition,more and more soil ecologists pay attention to the impacts of N deposition on AM fungal community.However,most relevant studies were carried out in grassland ecosystems,while studies on forest ecosystem were very limited.Moreover,most simulation studies in forest ecosystems applied N to the forest floor,ignoring the canopy processes,may not accurately reflect the natural situations.We conducted a field experiment on the influence of N addition mode(understory vs canopy)and rate on AM fungi in a mixed deciduous forest of China.The experiment had a fully randomized block design with four blocks(replicates)and each block included five plots.Within each block,each plot was randomly assigned with one of the five treatments:ambient(CK),canopy addition of N at 25(CN25)or 50 kg hm^-2 a^-1(CN50),understory addition of N at 25(UN25)or 50 kg hm^-2 a^-1(UN50).AM fungal alpha diversity indices and community composition were determined by high-throughput sequencing.The results showed that under the experimental conditions,AM fungal richness and Shannon diversity indices were not significantly altered by N addition mode,rate or their interactions.However,after one year of experimental treatment,N application mode showed a marginal effect on AM fungal community composition,while N application rate showed an extremely significant effect,and the treatment interaction was also significant.At N addition rate of 25 kg hm^-2 a^-1,the difference between canopy N addition and control was not significant,while the AM fungal community composition was significantly changed by understory N addition.At N application rate of 50 kg hm-2.a-1,canopy addition of N slightly altered AM fungal community composition,but understory of N application did not change AM fungal community composition.In the next three years,N addition mode,rate or their interactions all showed no significant effects on AM fungal community composition.Overall,the results indicated that understory application of N may overestimate the effect of N deposition on the AM fungal community under natural conditions at certain N application rate and time scale.AM fungal communities under different treatments tend to converge over time,suggesting that AM fungal community may have adapted to N deposition over time.This study evaluated the effects of different N application modes on AM fungal community in forest ecosystem.We could have a more comprehensive understanding of the ecological impacts of N deposition by considering broader N gradients and longer observation period in future research.