基于竹阔树种氮素利用分异特性的毛竹扩张机制研究

With fast growth rates and clonal reproduction,Phyllostachys edulis can rapidly invade forest areas,and the expansion of Ph. edulis into its neighboring forests has exerted significant influence on the biodiversity and ecological values of evergreen broad-leaved forests in nature reserves of subtropical China.Our previous studies indicated that,the invasion of Ph. edulis to surrounding evergreen broad-leaved forest reduced soil nitrogen mineralization and nitrification but enhanced soil ammonification at the mixed forests compared to that of evergreen broad-leaved forest.It has been speculated that the alteration of soil nitrogen supply at the interface of bamboo-woodland may be one especially important factor driving Ph. edulis successfully expanding. To test the hypothesis of soil nitrogen transformation on Ph. edulis expansion,Ph. edulis and Castanopsis fargesii (the dominate species of evergreen broad-leaved forests) would be acted as experimental materials to simulate 3 typical stand structure (mono-Ph.edulis ,mixtures and mono-broadleaf) by sand culture in a controlled environment. The growth and root system response of Ph. edulis and C.fargesii to the nitrogen supply (N levels and ammonium:nitrate tration) would be investigated to identify the competitive ability between Ph. edulis and C. fargesii and their N form preference. More importantly,Scanning Ion-selective Electrode Technique (SIET) and 15N Isotope Technique would be further used to analysis the preference and competive uptake on different forms of nitrogen and nitrogen use efficiency of Ph. edulis and C. fargesii.So as to reveal the linkages between soil nitrogen transformation accompanied by Ph.edulis invasion and the successful replacement of surrounding evergreen broadleaved forests by Ph. edulis, and then clarify the underlying regulation mechanism.The research achievements would provide a theoretical reference and technical support for the management of ecosystem on biodiversity and precious broad-leaved forests conservation and for bamboo management(such as fertilization)as well.

毛竹向邻近森林扩张已影响到我国南方自然保护区生物多样性保护和常绿阔叶林生态功能。前期研究表明，毛竹向常绿阔叶林扩张的界面土壤N总矿化和硝化作用明显低于阔叶林，而土壤氨化作用增强，推断土壤N供应变化是驱动毛竹成功扩张的重要因素之一。为验证土壤N转化格局改变对毛竹扩张的调节机制，该项目拟采用可控环境沙培法模拟3种典型林分结构（纯竹、竹阔、纯阔），研究毛竹和阔叶林优势种丝栗栲生长及根系构型对不同N水平和NH4+/NO3-配比变化的响应，以明确毛竹和阔叶树在不同供N条件下的生长竞争能力及喜铵/硝性。重点采用非损伤微测和15N同位素示踪技术剖析毛竹、丝栗栲对NH4+和NO3-的偏向选择及竞争吸收利用能力，以期揭示毛竹入侵常绿阔叶林过程土壤N转化与毛竹替代常绿阔叶林之间的内在联系及其调节机制。研究成果可为自然保护区管理和毛竹林的经营（施肥）提供决策依据和技术支撑。

本研究采用可控环境盆栽法模拟3 种典型林分结构（纯竹、竹阔、纯阔），研究毛竹和阔叶林优势种丝栗栲生长及根系构型对不同N水平和NH4+/NO3-配比变化的响应。结果表明，毛竹是典型的喜铵树种：地上部生长和根系构型各指标随N水平及NH4+/NO3-配比的提高而逐渐增大，并在较高浓度的NH4+-N条件下生长最好；栲树是兼性偏喜铵树种：生长各指标也随着N浓度的增加呈缓慢增加的趋势，并且在较高浓度的NH4+-N或NH4+和NO3-混合N源中较好。在试验处理期间，毛竹、栲树在单独栽培（纯竹、纯阔）和混合栽培（竹阔）条件下生长各指标对不同N 水平和NH4+/NO3-配比具有相似的响应特征，不同栽培方式间差异不显著（P >0.05）。但随N水平及NH4+/NO3-配比的增大，毛竹在相同条件下的生长速度远远大于栲树。此外，两树种生长对环境pH具有较强的适应性及耐酸能力，并且环境土壤pH（3.8-5.8）并不影响其对N的生长响应和形态偏好。竹阔混交林其他几种主要树种的氮形态偏好研究结果表明，油茶也是典型的喜铵树种，刨花楠是中性树种，乌桕是中性稍偏喜硝树种。N形态耐性研究结果表明，毛竹幼苗生长适宜的NH4+-N浓度为8-16 mM，栲树生长的适宜N浓度为2-8 mM；土壤NO3—N (2-40 mM) 对毛竹幼苗生长没有明显影响，但较高的硝浓度（≥16 mM）处理8 W后所有栲树幼苗死亡。. 根尖非损伤微测结果表明，与NO3-相比，两树种更偏向吸收NH4+；并且毛竹比栲树具有更强烈的外源NH4+响应和吸收能力。15N 同位素示踪结果表明，NH4+处理栲树幼苗体内15N积累量是NO3-处理的2.01-4.14倍，各部位积累量为根>叶>茎，铵氮或铵硝混合氮源比硝氮更有利于栲树植株各部位N量积累。 . 毛竹向周边扩张后土壤有效N以铵态N占主导，对喜铵植物有利，而毛竹与相应喜铵树种（如丝栗栲）相比具有更强的外源NH4+响应和吸收能力、更快的生长速度和繁殖能力。因此，毛竹向常绿阔叶林扩张对土壤氮供给的改变是毛竹成功扩张的土壤氮营养调控策略。本研究成果为毛竹入侵常绿阔叶林过程土壤N转化机制提供了理论依据，同时对相应树种的培育和经营（合理施N肥）也具有实践意义。