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凋落物亦称枯落物,是生物组分产生并且归还到地面的新陈代谢产物,其发生在植物生长发育过程中[1]。它是降解者的物质和能量来源,影响陆地生态系统净生产力,在维持土壤肥力,对物质循环和能量流动有不可或缺的作用[2-3]。目前有关凋落物分解的研究主要集中于2个方面,一是关于凋落物所处的外在分解环境[4-5],二是凋落物自身的质量,即关于凋落物自身的化学性质和养分含量[6-7]。氮作为植物生长所必要的重要元素之一,同样也是构成凋落物质量的重要指标,凋落物以及所在环境中氮元素有效性的高低同样对凋落物的分解具有重要作用。Vivanco等[8]和项文化等[9]的研究结果表明施用外源氮可以促进凋落物的分解。已有研究证明,凋落物的分解速率与其自身所含的初始C/N比值存在显著的线性关系[10,11]。大量的研究结果表明,杉木凋落物在分解初期阶段普遍存在着氮富集现象,杉木新鲜凋落物中的氮养分含量较低,难以维持分解者的生长发育,从而阻碍了杉木凋落物的分解,其分解速率常与氮以及其相关的凋落物质量指标,如C/N和木质素/N比值等指标具有紧密相关性,而通过外源性施氮处理,则有利于杉木凋落物的分解[12]。因此,大多数学者提议将凋落物本身的C/N比值作为衡量其质量和预测其分解速率的重要指标[13],并推测凋落物初始养分中含有某个C/N比临界值,如果高于这一C/N比值则不利于微生物对凋落物的分解和利用,导致养分被强烈固持于凋落物中[14-15],反之则有利于凋落物的分解。
杉木(Cunninghamia lanceolata)是我国南方主要的速生针叶用材树种之一,具有生长快,材质好,病害少等优点[16]。随着社会的发展,林业集约生产水平逐渐提高,杉木速生、丰产的优点得到充分地体现,但是由于人工林连栽造成的地力衰退现象已经成为影响杉木人工林可持续经营的主要因素[17]。除了采取人为措施恢复地力外,土壤的培肥能力也是关键因素之一,凋落物的分解作为土壤养分的重要来源之一,对土壤培肥功能的研究至关重要,因此,加强杉木凋落物分解特性的研究,对于维持与提高杉木人工林地力具有重要意义。本研究以福建省南平市顺昌洋口国有林场南山管护站杉木大径材林分为试验地,研究林下植被保留和林下植被去除2种林下植被管理措施下不同C/N比的杉木凋落叶分解情况,通过外源氮添加调控杉木凋落叶的C/N比值,并置于上述2种林分中自然分解,观测其分解过程中碳、氮、磷、钾、钙、镁和锰的养分动态和释放规律以及其相关性。以期进一步论证杉木凋落物分解的内在C/N比值机理,从而为促进杉木人工林的养分循环,改善土壤养分状况,缓解杉木人工林地力下降速度、维持杉木人工林长期生产力提供科学理论依据。
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2018年11月在2种林分下分别设置3个20 m×20 m样地,对每块标准样地内的杉木进行每木调查,采用围径尺测量胸径,红外线测高测距仪测量树高,并在样地的4个角均打下水泥桩以作标记。不同林分类型样地基本情况见表1。
林分类型 样地 坡向 坡位 坡度/° 密度/(株·hm−2) 平均树高/m 平均胸径/cm 林下植被保留 1 西南 中 31 950 22.68±2.70 31.86±4.29 2 西南 中 32 1050 22.30±4.21 31.27±3.22 3 西南 中 34 875 23.50±2.30 32.54±4.22 林下植被去除 1 南 中 35 1050 20.58±1.02 26.86±5.11 2 南 中 36 975 20.30±2.15 27.26±3.17 3 南 中 35 920 19.98±3.73 27.45±4.299 注:表中数据为平均值±标准误差,下同。 Table 1. General information of sample plots with different understory vegetation management measures
为保证凋落叶养分含量基本一致采用异地分解法。试验材料取自杉木中龄林尚未分解的新鲜杉木凋落枝,将杉木凋落叶从收集的杉木凋落枝剥离后,带回试验室自然风干后备用。经测定,杉木凋落叶初始全氮含量为8.10 mg·g−1,全碳含量为482.54 mg·g−1,全磷含量为0.33 mg·g−1,C/N比值为60.86,C/P比值为1393.19,N/P比值为23.86。杉木凋落叶初始钾含量为2.89 g·kg−1,初始钙含量为4.23 g·kg−1,初始镁含量为1.33 g·kg−1,初始锰含量为1.21 g·kg−1。
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试验共设计了4种C/N处理:C/N比值60.86(CK)、C/N比值40.57(N1)、C/N比值30.45(N2)、C/N比值20.25(N3),每个处理重复25次。选取NaNO3作为外源氮源,分别将质量浓度为2 g·L−1、4 g·L−1和6 g·L−1的NaNO3溶液均匀喷施于叶面,少量多次,待其风干后测定C/N比值含量,直至各处理样品的杉木凋落叶C/N比值达到试验要求。准确称取各样品所需的10±0.005 g经风干后的杉木凋落叶,装入网眼0.5 mm、大小为20 cm×20 cm的尼龙分解袋中,于2018年12月,分别放置于林下植被保留和林下植被去除共2种杉木人工林的3个预设样地内。放置时去除地表凋落物层,每个分解袋之间间隔50 cm,贴上标签,用竹钉固定四角在地表,保证分解袋紧贴表层土壤,让凋落叶自然分解。每种处理分解袋纵向放置,处理间设置3 m的缓冲带。
分解袋放置60 d、120 d、180 d、240 d和300 d后分别取样,每个样地每个处理取4袋,取样时戴一次性手套,每个分解袋均单独装入自封袋中,用于计算杉木凋落叶残留率。每次取样时,去除各分解袋上新鲜凋落物和林下植被去除林分中新生长的林下植被。取回的样品清除杂质,分别称质量后装入信封,做好标记,于烘箱中80 ℃烘干至恒重,然后称重,用粉碎机研磨后,用于营养元素含量测定。
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根据参考文献[18],凋落叶全氮和全碳采用全自动碳氮分析仪(Elemental Analyzer Vario ELIII,德国),全磷采用碱熔−钼锑抗比色法,全钾采用碱熔−火焰光度法,全钙和全镁含量采用EDTA络合滴定法,全锰含量测定采用原子吸收分光光度法。
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养分释放率的计算公式为:
式中:Et为养分释放率(%),Wt为分解t时间后剩余的凋落叶干质量(g),Ct为凋落叶分解t时间后剩余的凋落叶养分含量(g·kg−1),Wo为凋落叶分解初始干质量(g),Co为凋落叶分解初始养分含量(g·kg−1)。
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初始数据采用Office 2019进行整理和统计,采用SPSS 26.0进行独立样本t检验、单因素方差分析、双因素方差分析、线性回归分析和相关分析,单因素分析选用Duncan差异显著性检验,相关性分析选用Person相关分析。
Effects of Different Exogenous C/N Ratios on Nutrient Release Rate during Litter Decomposition in Cunninghamia lanceolata Forests with Different Understory Vegetation Management Measures
doi: 10.12172/202109260002
- Received Date: 2021-09-26
- Available Online: 2021-10-25
- Publish Date: 2021-12-10
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Key words:
- Cunninghamia lanceolata /
- Understory vegetation management /
- C/N ratio /
- Nutrient release
Abstract: The aim of this study was to study the nutrient elements dynamics during litter decomposition in Cunninghamia lanceolata forests with different C/N ratios, so as to provide reference for scientific management of Cunninghamia lanceolata forest. The decomposition bag method was used to observe Cunninghamia lanceolata litter decomposition for 300 days. Four kinds of exogenous C/N treatments were set up: C/N ratio 60.86 (CK), C/N ratio 40.57 (N1), C/N ratio 30.45 (N2), C/N ratio 20.25 (N3). The nutrient contents in the decomposition process of Cunninghamia lanceolata litter in different stands were determined and analyzed. The results were as follows: (1) During the decomposition of Chinese fir litter, the C/N and C/P ratios were lower than the initial value, while the N/P ratio was higher than the initial value; (2) The concentrations of N, P, Ca and Mn were higher in CK treatment, and there were significant differences in the concentrations of P, Ca and Mn among different treatments; (3) During the litter decomposition, C, K, Ca and Mg all showed nutrient release, N showed nutrient enrichment, and P and Mn showed enrichment phenomenon during 180-300 days of decomposition, but on the whole showed nutrient release; (4) Decomposition time had great influence on seven elements (N, C, P, K, Ca, Mg, Mn) and three element ratios (C/N, C/P, N/P). There was a significant negative correlation between N and stand, a significant positive correlation between C/N and stand, a very significant negative correlation between different C/N treatments and Mn, and no significant correlation between other elements and stand and C/N treatments. (5) The retention of understory vegetation was useful for the release of C, K and Mg, while removal of understory vegetation is useful for the enrichment of N, Ca, P and Mn. During the 300-day litter decomposition process, exogenous nitrogen had a low promoting and high inhibiting effect on Cunninghamia lanceolata litter decomposition of , and with the extension of decomposition time, the promoting effect weakened and the inhibiting effect enhanced. Cunninghamia lanceolata litter decomposition required a low C/N ratio and a large demand for phosphorus. The research results could provide scientific basis for the management of Cunninghamia lanceolata forest.