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阔叶林和针叶林土壤有机碳氮组分对氮添加的响应(3)

来源:未知 作者:chunt
发布于:2016-10-31 共14116字
  土壤微生物降解过程也是影响土壤活性有机氮含量的重要因子之一,SMBLP代表单位活性有机碳氮组分向微生物生物量碳氮的转化量,可表征微生物的养分利用效率,同时可以指示微生物可利用的基质含量[4].该 研 究 中,氮 添 加 对 土 壤SMB-CLP-C和SMB-NLP-N有显着影响( 见表2) ,说明氮添加可以改变土壤微生物对底物的利用情况,当土壤氮素受限制时,不利于微生物利用底物,氮添加条件下,微生物解除氮的限制,有利于其利用底物[27].其中,除了杉木人工林土壤SMB-NLPⅠ-N随着氮添加水平的增加而降低土壤微生物对底物的利用外,浙江桂天然林和罗浮栲天然林均不同程度地提高了土壤碳氮组分的利用率。可见,输入土壤有机物性质的差异,确实可以通过微生物的利用情况来改变土壤碳氮组分的分配和转化,而氮添加对凋落物分解的作用有所差异[28]. Craine等[29]研究也表明,当土壤中活性有机碳氮组分含量减少时,微生物从惰性有机碳氮库中获取基质的能力提高,从而提高了养分利用效率。这可能是氮添加能发挥调节土壤碳氮组分变化的机理之一,在微生物参与下,不同氮添加水平和土壤原有环境下凋落物归还数量和质量的变化最终调节土壤碳氮组分间的相互转化,影响碳氮循环,重新达到另一种状态,而该模式正期待更多的试验证明。
  
  3. 2不同林分对土壤有机碳氮的影响
  
  不同植被类型通过输入凋落物质量的差异,在氮添加影响土壤碳循环中发挥重要作用[19]. WANG等[30]研究表明,亚热带不同植被类型( 次生阔叶林、杉木林、杉木与桤木混交林、杉木与刺楸混交林) 对土壤活性有机质( 微生物量碳氮、水溶性有机碳氮、轻组有机质) 有显着影响。该研究区内阔叶林凋落物现存量比针叶林多( 见表1) ,从凋落物中淋滤出的可溶性有机质就多[31],结果显示,不同林型下土壤活性和惰性有机碳氮组分含量存在一定差异,但差异不显着,可能与该研究采用酸浸提有机碳氮组分有关,而WANG等[30]只针对土壤活性有机质组分开展研究。凋落物作为外源有机物质,由易分解成分( 如糖类、淀粉、脂肪等) 和难分解成分( 如木质素、多酚等) 组成[32],这些成分与其分解速率密切相关,影响可溶性有机碳的释放。该研究所在区域内阔叶树种---浙江桂天然林和罗浮栲天然林土壤活性有机质中可溶性糖比杉木人工林高,而土壤多酚含量在杉木人工林中最高[33],说明阔叶林土壤活性有机质含量比针叶林高( 见图1、2)。另外,还有研究表明,杉木人工林凋落物养分含量较低、质量较差,不易被微生物分解利用[34],因 而 导 致 杉 木 人 工 林 土 壤w(RP-C) 在w(TC) 中所占比例高于w(LP-C) ,并且w(RP-C) 也高于阔叶林土壤中相应值( 见图1) ; 而这部分惰性有机质中有相当一部分碳主要是以未分解的木质素结构存在于土壤中[35].已有研究[36]表明,针叶树种凋落物中含有较多难分解的、疏水性芳香族化合物,而阔叶树种凋落物中含有较多易变的、亲水性低分子量化合物,这些性质差异的物质在释放和分解过程中也会造成土壤碳氮组分的不同。通常认为,凋落物中较低的CN更有利于凋落物分解释放活性有机质[37],而该研究中,杉木人工林土壤活性有机库中CN最低( 见表1) ,w(RP-C) 却为最高( 见图1) ,这可能是因为针叶林凋落物中木质素含量较高的缘故,木质素会在凋落物的全纤维素类复合物周围形成阻碍分解的屏障[38],从而影响其分解。该研究还发现,土壤w(LPⅠ-C) 与w(SMB-C)、w(LPⅡ-N) 与w(SMB-N)均呈显着正相关(P < 0. 05) ( 见表3) ,说明凋落物进入土壤后,其活性有机质为微生物提供了碳源,同时也增加了土壤微生物数量[31]. ZHOU等[39]研究发现,土壤微生物生物量的增加会加快土壤活性有机库的分解速率,从而有可能推动惰性有机库的分解。正如有研究指出,尽管林分、林分与氮添加的交互影响不显着,但不同条件下,氮添加对土壤碳含量的影响存在差异,应当考虑林分对氮添加影响土壤碳保存的不同响应[40],而且对于微生物分解和利用惰性碳氮来说,在土壤养分供给、满足植物需求方面,活性碳氮组分存在积极作用; 同时在维持与促进土壤碳氮组分稳定、保护生态环境方面活性碳氮组分表现出消极作用,因此,应当在一个系统内综合考虑它们之间复杂的利弊关系,才能客观认识其内在规律、做出前瞻性的预判。
  
  结果显示,w(LP-N) 与w(Ⅱ-DIN)、w(LPⅠ-C)与w(Ⅱ-DIN) 均呈正相关,而w(LP-N) 与w(RIN)、w(LPⅡ-C) 与w(RIC) 均呈负相关( 见表3) ,说明凋落物中易分解物质( 如单糖、淀粉和简单的氨基酸、蛋白质等) 进入土壤后很容易优先被微生物利用、矿化,而凋落物中剩余的较难分解的木质素、纤维素和单宁等在合适条件下才缓慢被微生物利用[41].而无论采用 哪 种 方 法 研 究 土 壤 有 机 质 的 功 能 和 稳 定性[42],不但需要了解不同碳氮组分的状态,更需要研究各组分之间相互转化的机理,而后者对于揭示碳氮组分响应环境变化、回答一些生态问题而言,可能显得更为重要。该研究还发现,土壤w(TC) 与w(DIN)呈显着正相关( 见表3) ,而土壤活性有机质中,3个样地内土壤C/N为11. 40 ~ 11. 74,相对较低,说明土壤有机质腐殖程度很高、有机氮很容易矿化[43].其中,罗浮栲林土壤中活性有机质库Ⅰ的C/N最高,说明其活性有机氮库Ⅰ的矿化程度较高; 另外,浙江桂天然林土壤中活性有机质库Ⅰ的C/N比活性有机质库Ⅱ低,而罗浮栲天然林土壤中活性有机质库Ⅰ的C/N高于活性有机质库Ⅱ,杉木人工林土壤活性有机质库Ⅰ和活性有机质库Ⅱ的C/N差异不显着。虽然浙江桂天然林土壤中w(RP-N) 较低,但在惰性有机质库中C/N最高,说明在一定程度上活性有机质保持较高的稳定性,并且浙江桂天然林土壤的碳氮循环必然在某些方面与同是阔叶林的罗浮栲之间存在差异,说明不同林分土壤活性有机氮库矿化与有机质腐殖程度、凋落物数量、质量和C/N具有差异性外,还可能与针叶林和阔叶林下土壤微生物群落组成有显着差异有关[44],但对于特定微生物功能组是如何影响土壤活性有机库Ⅰ和活性有机库Ⅱ的形成及转化,尚需要进一步研究。
  
  4 结论
  
  a) 在氮添加影响下,不同林分土壤活性有机碳氮组分之间存在差异,其中,氮添加增加了针叶林土壤中的w(LPⅠ-C) 和w(LP-N) ,同时也增加了浙江桂天然林土壤中的w(LPⅡ-C) ,而迅速降低罗浮栲天然林土壤中的w(LP-N) ; 另外,同是阔叶林树种,其土壤有机质也存在差异,说明该试验区土壤有机碳氮组分不仅受外源氮的影响,还受到凋落物数量和质量的影响。
  
  b) 高氮处理可促进微生物对罗浮栲天然林、杉木人工林土壤活性有机碳的利用,但对浙江桂天然林土壤影响不显着,是因为高氮处理条件下浙江桂天然林土壤中惰性有机质库中C/N最高,而且高氮处理对罗浮栲天然林和杉木人工林土壤活性和惰性碳氮组分的影响有所差异,表明不同树种对氮添加的响应不同。
  
  c) 研究显示,仅用酸水解方法获得土壤活性和惰性碳氮组分,并研究土壤碳氮转化及其对氮添加的响应,会忽略一些细微、快速变化的组分特征,而采用更加全面、系统的分级开展研究可能更为有效。
  
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