北方森林凋落物动态对长期氮沉降的响应

Response of Litter Dynamics of Boreal Forest to Long-term Nitrogen Deposition

森林凋落物是生态系统生产力的重要组成成分,对生态系统物质循环和养分平衡起着促进作用。近些年来日益增加的氮(N)沉降对生态系统稳定构成一定威胁,因此了解大气N沉降增加背景下凋落物动态变化对于预测森林碳循环对气候变化的响应具有重要意义。以连续施N 7年的兴安落叶松林(Larix gmelinii)为研究对象,观测4年(2014-2017)森林凋落物的生产量,旨在探求森林凋落物年际动态变化驱动因子及其对N沉降的响应。以NH4NO3为外施氮源,设置对照(CK:0g·m^-2·a^-1)、低氮(LN:2.5g·m^-2·a^-1)、中氮(MN:5g·m^-2·a^-1)和高氮(-HN:7.5g·m^-2·a^-1)等4种处理,每个处理包括3个重复(n=3)。结果表明,(1)凋落物总量和兴安落叶松凋落叶量的年际动态变化驱动因子为生长季月平均温度,而枝、果实及其它繁殖器官凋落量与年最大风速显著相关。(2)年际凋落量的大小顺序为:2015(3.15±0.31)t·hm^-2·a^-1>2016(3.10±0.25)t·hm^-2·a^-1>2014(2.83±0.31)t·hm^-2·a^-1>2017(1.48±0.25)t·hm-2·-a1,各组分凋落量所占比例大小顺序均为兴安落叶松叶>阔叶>枝>果实及其他繁殖器官;施N处理对总凋落量和兴安落叶松凋落叶产生抑制作用,且凋落量随N浓度增加而逐渐降低,然而不同施N处理对枝、果及其它繁殖器官凋落量作用不明显。(3)N沉降对兴安落叶松凋落叶中C、N、P含量及C/N影响不同:凋落叶C含量整体年际动态变化不明显,且施N对凋落叶中C含量无影响;凋落叶N和P含量在不同年份呈现不同的变化趋势,总体上施氮增加了凋落叶N含量却降低了P含量;凋落叶C/N在各个年份对N添加响应有所不同,主要表现为施N降低了凋落叶C/N。(4)凋落叶N归还量在年际间随着N浓度不同呈现一定的波动,N沉降降低了凋落叶C和P的年际归还量,且表现出N浓度越高,归还量越低的趋势。

Forest litter is an important component of ecosystem productivity which accelerates the ecosystem material cycle and nutrient balance.The increasing nitrogen(N)deposition in recent years poses a certain threat to ecosystem stability.Therefore,understanding the dynamic changes of litter under the background of atmospheric N deposition is of great significance for predicting the response of forest carbon cycle to climate change.In this experiment,we used the Larix gmelinii forest which had a continuous7-year N addition in Greater Khingan Mountains as the research object.The production of forest litter in the past 4 years(2014-2017)was observed,aiming to explore the driving factors of inter-annual dynamic change of forest litter and its response to N addition.The experiment was conducted with NH4 N03 as external N source.Plus,a total of four levels of N addition treatment were set,which were control(CK:0g·m^-2·a^-1)and low N(LN,2.5g·m^-2·a^-1),medium N(MN,5 g·m^-2·a^-1)and high N(HN,7.5 g·m^-2·a^-1)treatment.The results showed that:(1)The inter-annual dynamic changes of total litter and Larix gmelinii litter were driven by the monthly average temperature in the growing season,while the litter of branches,fruits and other reproductive organs was significantly correlated with the annual maximum wind speed.(2)The order of inter-annual litter amount was as follows:2015(3.15±0.31 t·hm^-2·a^-1)>2016(3.10±0.25 t·hm^-2·a^-1)>2014(2.83±0.31 t·hm^-2·a^-1)2017(1.48±0.25 t·hm^-2·a^-1),and the proportion of each component was Larix gmelinii leaf>broad leaf>branch>fruit and other reproductive organs;N addition inhibited total litter and Larix gmelinii leaf litter,and the litter amount decreased gradually with the increase of N concentration.However,the effect of different N addition on litter amount of branches,fruits and other reproductive organs was not obvious.(3)The effects of N addition on C,N,P content and C/N of Larix gmelinii leaves were different:the inter-annual dynamics of litter C content were not obvious,and N addition had no effect on C content in litter leaves.Plus,the N and P contents of litter showed different trends in different years.N addition increased the content of litter N but decreased the content of P.The response of litter C/N to N in different years was mainly reflected by the decrease of litter C/N.And(4)the amount of N-return of litter showed a certain fluctuation with the N concentration during the inter-annual period.N deposition reduced the inter-annual return of litter C and P,and showed an inverse relation between the N concentration and the return trend.