水位和施氮对若尔盖高寒湿地土壤非共生固氮的影响

Effects of water table and nitrogen addition on the rate of non-symbiotic nitrogen fixation in an alpine wetland of Zoige

高寒湿地中土壤微生物固氮是氮元素进入生态系统的主要途径之一,环境因子变化对土壤固氮功能的影响仍不明晰。在四川若尔盖高寒湿地搭建了由27个生态模拟箱组成的中宇宙实验系统,通过控制水位和模拟氮沉降,研究水位变化和施氮对土壤非共生固氮的影响。实验设计了3水位水平×3施氮水平共9个处理,测定了生态模拟箱中表层土壤的非共生固氮速率,土壤碳、氮含量,以及地上植物生物量和植物氮含量,比较不同水位和施氮处理下非共生固氮速率的变化规律并分析其与土壤和植物碳、氮含量的关系。研究发现:生态模拟箱中土壤非共生固氮速率范围是0.003—7.35μg N g^-1 d^-1,从不淹水到淹水的处理土壤非共生固氮速率提高约2倍。施氮处理中固氮速率随土壤含水量升高而增强的敏感性高于施氮对照处理,且施氮处理下的生态模拟箱中土壤有机碳含量显著升高,据此推测施氮可能使淹水的生态模拟箱中的浮游植物提高生产力而释放可利用有机碳,从而间接促进土壤非共生固氮。本研究获得以下结论:(1)若尔盖高寒湿地中土壤水位是限制固氮速率的重要因子;(2)施氮背景下土壤含水量对非共生固氮的促进效应更明显。

Non-symbiotic N2 fixation represents one of the main avenues of nitrogen(N)input into ecosystems,and thus plays a critical role in the N cycling in alpine wetlands.However,it remains unclear how environmental changes may influence N fixers and their N-fixing ability.We conducted a mesocosm experiment in the Zoige wetland to examine the responses of soil non-symbiotic N2 fixation in alpine wetlands to rising water table and N addition.The mesocosm consisted of 27 plots filled with in situ peat cores sampled from the Zoige wetland.We established nine treatment combinations,i.e.,three levels of water table crossed by three levels of N addition,with three replicates for each combination.We measured the rate of non-symbiotic N2 fixation,soil carbon and N contents of the top soil,aboveground biomass and plant N contents for each plot.The rate of non-symbiotic N2 fixation ranged from 0.003 to 7.35μg N g^-1 d^-1,being higher in flooded than well-drained soils.High soil water content promoted N2 fixation,especially under nitrogen addition.The possible explanation for stronger water effects under N addition may be that soil organic carbon,which is required by non-symbiotic N fixers,can be rapidly released by phytoplankton after N addition in flooded plots.This is evidenced by higher soil organic carbon under nitrogen addition.We concluded that water table is the main factor affecting non-symbiotic N2 fixation of the top soil in Zoige Wetland,and that N addition enhances the extent to which high soil water content can promote non-symbiotic N2 fixation.