碳氮添加对不同湿度条件下温带森林土壤融化过程中氧化亚氮排放的影响

采用室内土柱培养方法,研究不同土壤湿度(55%和80%土壤充水孔隙度,WFPS)条件下外源碳(葡萄糖,6.4 g C·m~(-2))和2种形态氮(NH_4Cl和KNO_3,4.5 g N·m~(-2))的添加对温带成熟阔叶红松混交林和次生白桦林土壤冻结后融化过程中氧化亚氮(N_2O)排放量的影响。结果表明:冻结过程会激发2种林分土壤融化初期N_2O的排放。随着土壤湿度的增加,2种林分土壤大量消耗硝态氮,反硝化作用强烈,导致融化初期N_2O激发效应的强度大,持续时间长,尤其是白桦林土壤。单施葡萄糖后,2种林分土壤大量消耗铵态氮和硝态氮,进而显著促进2种林分土壤融化初期N_2O的激发排放;随着土壤湿度的增加,葡萄糖对2种林分土壤N_2O累积排放量的促进作用减弱,这可能与高湿度条件下,冻结后融化过程中土壤释放大量溶解性有机碳(DOC)有关。低湿度条件下,2种林分土壤融化过程N_2O排放是铵态氮限制性的,即硝化潜势占主要优势,尤其是白桦林土壤;高湿度条件下,白桦林土壤具有很强的反硝化潜势,并且随着葡萄糖的施加,这种反硝化潜势加强。逐步回归分析显示:2种林分土壤冻结后融化过程N_2O累积排放量受到土壤pH、WFPS及水浸提DOC含量的影响,共同解释其66%的变化,并且与土壤水浸提溶解性有机氮含量呈显著的正相关;阔叶红松混交林土壤冻结后融化过程N_2O累积排放量与微生物生物氮呈显著负相关。综上可推测,温带森林土壤融化过程中N_2O的排放主要依赖于冻结处理后的土壤pH、WFPS以及溶解性有机质释放量的变化。     

冻融条件和土壤湿度对森林土壤渗漏液溶解性有机质含量与光谱结构特征的影响

土壤溶解性有机质(DOM)含量及其稳定性影响土壤碳氮循环关键过程,目前气候变化下森林土壤DOM含量及其光谱结构特征仍不明确.本研究利用长白山阔叶红松混交林和次生白桦林表层土壤进行室内冻融模拟试验,结合三维荧光光谱-平行因子分析方法,研究冻融强度和冻融循环次数及其交互作用对不同湿度温带森林土壤渗漏液DOM含量、组分和光谱结构特征的影响.结果表明: 森林土壤渗漏液DOM含量及其组分因林分类型、土壤湿度、冻融强度、冻融循环次数不同而存在差异.2种林分土壤渗漏液DOM含量均在中湿度下最低,并受高强度冻融影响显著,且随冻融循环次数增加呈现先增加后降低的趋势.可鉴别DOM的3个荧光组分:胡敏酸类DOM、富里酸类DOM和蛋白类DOM;阔叶红松混交林土壤渗漏液DOM组分以富里酸类物质为主,腐殖化程度较高;而次生白桦林土壤渗漏液DOM组分以胡敏酸类物质为主,3组分受冻融强度显著影响,稳定性较低.经冗余分析(RDA)发现,林分在很大程度上决定森林土壤DOM属性变化,次生白桦林土壤渗漏液DOM含量及其3组分荧光强度大于阔叶红松混交林;土壤湿度显著影响DOM芳香性,2种林分土壤渗漏液DOM芳香性均呈中湿度>高湿度>低湿度的趋势;随冻融强度增加,阔叶红松混交林土壤渗漏液DOM芳香性显著降低;多次冻融循环显著提高2种林分土壤渗漏液DOM腐殖化程度.因此,不同冻融作用下,低湿度温带森林土壤渗漏液DOM含量及其生物有效性呈现增加的趋势,尤其是次生白桦林土壤,可能会增加春季冻融期温带森林土壤溶解性有机质淋溶损失.这些结果可为深入研究野外冻融期温带森林土壤溶解性有机质周转机制提供参考.     

温带森林不同演替阶段下的土壤CO2排放通量昼间变化

采用时空替代法,在长白山北坡分别选取了红松针阔叶混交林演替序列的5个不同阶段:草地、灌木林(幼龄林)地、白桦林地、阔叶杂木林地和红松阔叶林地,进行土壤CO_2排放通量昼间变化野外同步观测研究,旨在揭示温带森林不同演替阶段下的土壤呼吸CO_2排放过程的差异,探究其与温度、湿度、土壤理化性质等环境因子的关系。结果表明:(1)温带森林不同演替阶段下的土壤CO_2排放通量具有统一性,均为大气CO_2的源,这种统一性确保了小的时段(如昼间)观测能通过换算,实现CO_2排放量的估算。(2)CO_2排放通量的昼间排放都呈现出明显的单峰型,峰值在13:00—15:00左右,草地和灌木林地的峰值大概在13:00左右,明显提前于白桦林地、阔叶杂木林地和红松阔叶林地(14:00—15:00左右)。红松阔叶林地的土壤呼吸有明显的滞后性特征,峰值在15:00左右,比其他几个样地明显推迟。(3)土壤CO_2排放通量平均值由低到高排列依次为草地(2.760μmol m~(-2)s~(-1))、灌木林地(2.854μmol m~(-2)s~(-1))、白桦林地(3.048μmol m~(-2)s~(-1))、阔叶杂木林地(3.696μmol m~(-2)s~(-1))、红松阔叶林地(4.61μmol m~(-2)s~(-1))。随着温带森林演替的正向进行,土壤CO_2排放通量依次增大,次序为草地灌木林地白桦林地阔叶杂木林地红松阔叶林地。(4)环境因子中,0—5 cm土壤温度与土壤CO_2排放通量相关系数最高,土壤温度监测对土壤CO_2排放量的估算作用明显。     

DOM对米槠次生林不同土层土壤微生物呼吸及其熵值的影响

可溶性有机质(Dissolved organic matter,DOM)作为土壤可溶性有机碳的重要来源,进入土壤之后通过改变土壤微生物数量和活性影响土壤矿化。DOM输入对土壤微生物呼吸和熵值的研究多集中在表层土壤,但对深层土壤微生物呼吸和熵值的影响关注较少。通过室内培养实验(120 d)研究米槠(Castanopsis carlesii)鲜叶DOM添加对表层土壤(0—10 cm)和深层土壤(40—60 cm)微生物呼吸及其土壤代谢熵和微生物熵的影响,为揭示DOM输入对亚热带森林土壤碳过程的影响提供理论依据。结果表明,在培养第1天,添加DOM的表层和深层土壤CO_2瞬时排放速率均显著高于对照(P0.001),分别是对照(不添加DOM)的3.58倍和6.93倍,之后显著下降。就累积排放量而言,无论是DOM添加处理还是对照,表层土壤显著大于深层土壤;在米槠鲜叶DOM添加后,表层土壤累积排放量显著大于对照的表层土壤(P0.001),但DOM添加处理深层土壤累积排放量与对照的深层土壤无明显差异。就微生物生物量碳而言,表层土壤微生物生物量碳含量在培养期间显著大于深层土壤。在整个添加DOM培养期间,表层土壤微生物生物量碳含量显著大于表层对照土壤,深层土壤微生物生物量碳含量显著大于深层对照土壤(第3天除外)。培养结束时(120 d),米槠鲜叶DOM添加处理下,表层土壤和深层土壤有机碳含量与第3天相比分别减少26%和19%。米槠鲜叶DOM添加处理后的深层土壤代谢熵(qCO_2)显著低于对照的深层土壤和DOM添加处理的表层土壤qCO_2(P0.001),说明外源DOM进入深层土壤后提高了土壤微生物对碳的利用效率。米槠鲜叶DOM添加处理后的深层土壤微生物熵是培养第3天的1.58倍,显著大于培养初期(P0.05),而DOM添加处理的表层土壤、对照的表层土壤与深层土壤的微生物熵分别是培养第3天的68%、79%和21%,说明DOM添加提高了深层土壤质量。     

冻融期去根处理对小兴安岭6种林型土壤微生物量的影响

春季冻融期,在小兴安岭的阔叶红松(Pinus koraiensis)林、谷地云冷杉(Picea koraiensis-Abies nephrolepis)林、阔叶红松择伐林、白桦(Betula platyphylla)次生林、红松人工林、兴安落叶松(Larix gmelinii)人工林的去根处理样地和对照样地进行野外取土实验,分析了根去除对上述林型土壤微生物量的影响以及与土壤环境因子的关系。结果表明:冻融循环期间对照样地和去根处理样地的林型、土壤层次、取样时间均显著地影响土壤微生物量碳(MBC)(P0.05),对照样地中各林型的土壤微生物量氮(MBN)差异显著,而去根处理样地中各林型的MBN没有显著差异(P0.05);冻融循环期间去根处理显著地减少了大部分林型及土层(谷地云冷杉林0—10 cm及择伐林外)的MBC,而去根处理对大部分林型及土层(阔叶红松林0—10 cm,谷地云冷杉林和择伐林的10—20 cm除外)的MBN没有显著影响。说明在小兴安岭春季冻融期根系对土壤微生物量的影响不可忽视。     

川西亚高山森林土壤呼吸和微生物生物量碳氮对施氮的响应

随着全球大气氮沉降的明显增加,将有可能显著影响我国西部地区受氮限制的亚高山森林生态系统。土壤微生物是生态系统的重要组成部分,是土壤物质循环和能量流动的重要参与者。由于生态系统类型、土壤养分、氮沉降背景值等的差异,土壤呼吸和土壤生物量碳氮对施氮的响应存在许多不确定性。而施氮会不会促进亚高山森林生态系统中土壤呼吸和微生物对土壤碳氮的固定?基于此假设,选择了川西60年生的四川红杉(Larix mastersiana)亚高山针叶林为研究对象,通过4个水平的土壤施氮控制试验(CK:0 g m~(-2) a~(-1)、N1:2 g m~(-2)a~(-1)、N2:5 g m~(-2) a~(-1)、N3:10 g m~(-2)a~(-1)),监测了土壤呼吸及土壤微生物生物量碳氮在一个生长季的动态情况。结果表明:施氮对土壤呼吸各指标和土壤微生物碳氮都有极显著的影响,施氮能促进土壤全呼吸、自养呼吸、异养呼吸通量和土壤微生物生物量碳氮的增长,施氮使土壤呼吸通量提高了11%—15%,土壤微生物量碳提高了5%—9%,土壤微生物量氮提高了23%—34%。在中氮水平下(5 g m~(-2) a~(-1))对土壤呼吸的促进最显著。相关分析发现,土壤呼吸与微生物生物量碳氮和微生物代谢商极呈显著正相关,微生物量碳氮与土壤温度呈极显著的正相关,与土壤湿度呈极显著负相关。通过一般线性回归拟合土壤呼吸速率与土壤10 cm温湿度的关系,发现土壤呼吸速率与土壤温度呈极显著的正相关,与土壤湿度极显著负相关(P0.001),中氮水平下土壤温度敏感性系数Q_(10)值(7.10)明显高于对照(4.26)。     

长白山白桦林不同演替阶段土壤有机碳组分的变化

为了解长白山天然针阔混交林群落恢复演替土壤碳储量的变化,采用空间代替时间的方法,选取白桦幼龄林、白桦中龄林、白桦成熟林、阔叶红松成熟林和阔叶红松过熟林5个不同演替序列,研究其土壤总有机碳(SOC)、易氧化有机碳(ROC)、微生物生物量碳(MBC)及颗粒有机碳(POC)含量。结果表明:随着白桦林从早期到晚期的演替,SOC、MBC、ROC、POC以及土壤全氮、全磷和碳氮比(C/N)均呈现先逐渐增加后保持稳定的规律。随着土层深度的增加,SOC、MBC、ROC和POC含量均显著降低(P0.05),5个演替序列内ROC/SOC和POC/SOC的变化范围分别为12.91%~47.95%和14.21%~69.46%。相关分析表明:MBC、ROC和POC含量与土壤总有机碳(SOC)含量呈极显著正相关(P0.01),SOC、MBC、ROC和POC含量与全氮、全磷及碳氮比呈极显著正相关(P0.01)。研究结果为了解白桦林在演替过程中土壤有机碳的稳定性变化和固碳潜力提供数据支持。     

浙江天童地区常绿阔叶林退化对土壤养分库和碳库的影响

为了解常绿阔叶林退化对土壤碳库和养分库的影响,采用空间代替时间的研究方法,以常绿阔叶林顶级群落为参照,选择了次生常绿阔叶幼年林、次生针阔混交林、次生针叶林、灌丛和灌草丛代表不同的退化类型,分别对其土壤氮磷养分库、碳库进行了调查和分析。结果表明：土壤氮库贮量从大到小依次为,成熟常绿阔叶林、次生常绿阔叶幼年林、灌丛、次生针叶林、灌草丛和次生针阔混交林;土壤总磷含量也是在成熟林最高,次生针阔混交林和次生针叶林的总磷含量显著高于次生常绿阔叶幼年林和灌丛;土壤有机碳含量从高到低依次为：成熟常绿阔叶林,次生针叶林、次生常绿阔叶幼年林、灌丛、灌草丛和次生针阔混交林;土壤铵态氮在成熟林、灌丛和灌草丛的库容量最大,其次分别为次生幼年常绿阔叶林、次生针阔混交林,最小的为次生针叶林;硝态氮则在灌草丛的库容量最大,其次分别为次生针叶林、次生针阔混交林和成熟林针叶林,最小的为次生常绿阔叶幼年林和灌丛。统计显示,常绿阔叶林退化不仅导致土壤有机碳库含量的显著下降,也使得土壤氮磷养分库含量显著下降。可以认为,砍伐导致的大量生物量输出和森林管理措施的影响,植物种类组成的改变,土壤物理性质的改变以及养分和有机碳的主要生物化学转化环节发生改变是导致此类变化的主要因素,常绿阔叶林顶极群落土壤是该地区土壤的最大养分库和碳库。     

鼎湖山针阔叶混交林凋落物CO2释放对模拟酸雨的响应

该研究2011年1月开始在鼎湖山针阔叶混交林(混交林)进行模拟酸雨实验,设置4个不同处理水平,即对照(CK)(pH为4.5左右的天然湖水)、T_1(pH=4.0)、T_2(pH=3.25)和T_3(pH=2.5)。2013年1—12月对不同酸雨强度处理下的森林凋落物CO_2释放速率进行为期1 a的连续观测,探讨酸雨对混交林凋落物C排放的影响。结果表明:凋落物CO2释放通量在对照样方为(1 507.41±155.19) g CO_2·m~(-2)·a~(-1),其中湿季和旱季分别占年通量的68.7%和31.3%。模拟酸雨抑制了森林凋落物CO_2释放,与CK相比,T_2和T_3处理下的CO_2释放通量分别显著降低15.4%和42.7%(P0.05);且这种抑制作用具有季节差异性,处理间的显著差异只出现在湿季。凋落物CO_2释放速率与土壤温度和土壤湿度分别呈显著指数相关和显著直线相关,同时,酸雨处理降低了凋落物CO_2释放的温度敏感性。混交林凋落物CO_2释放在模拟酸雨下的抑制效应与土壤累积酸化而导致的土壤微生物活性变化有关,表现为模拟酸雨作用下土壤pH值和微生物量碳显著下降。上述结果说明酸雨是影响混交林土壤碳循环的重要因子之一。     

阔叶红松林生态化学计量学特征及其对纬度梯度的响应

为研究红松叶片的养分及生态化学计量学特征的空间分布及其影响因素,依据我国温带阔叶红松林的分布特点,沿纬度梯度,选取长白山(42°27'N)、张广才岭(44°16'N)和小兴安岭(48°05'N)等3个地区的典型阔叶红松老龄林,测定红松叶片碳(C)、氮(N)、磷(P)含量,及表层土壤(0—15 cm)、中层土壤(15—30 cm)的有机碳(SOC)、全氮(TN)和全磷(TP)含量,分析了其分布特征及其相互关系。结果表明:1)红松叶片C、N、P含量显著高于土壤。表层土壤C、N、P含量变化范围分别为27.6—87.4,2.0—7.2 mg/g和0.26—0.92 mg/g;中层土壤为8.1—59.7,0.7—4.6 mg/g和0.2—0.82 mg/g;而叶片为495.5—507.4,12.7—172.5 mg/g和1.1—2.1 mg/g。2)土壤中的SOC、C/N、C/P均随纬度升高极显著增加,而叶片各元素计量特征随纬度的变化不显著。3)叶片N、P含量分别与土壤N、P含量显著正相关,同时,叶片N与土壤C/N、P与土壤N/P显著相关。相比较而言,红松叶片N、P含量较低,这可能说明阔叶红松林土壤N、P供应不足,而叶片N/P仅为9.9,说明东北阔叶红松林N限制更加明显。本研究为阐明东北温带阔叶红松林的养分供应状况和限制因素,为阔叶红松林区提高红松生产力的管理措施的提出奠定了基础。     

Response of soil carbon dioxide fluxes,soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

Biochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO₂) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO₂ fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO₂ fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO₂ fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO₂ fluxes and MBC content, respectively. Both soil CO₂ fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO₂ fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO₂ flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.     

Changes in soil carbon and nitrogen properties and microbial communities in relation to growth of Pinus radiata and Nothofagus fusca trees after 6 years at ambient and elevated atmospheric CO2

Increasing atmospheric CO₂ concentration can influence the growth and chemical composition of many plant species, and thereby affect soil organic matter pools and nutrient fluxes. Here, we examine the effects of ambient (initially 362 μL L^?1) and elevated (654 μL L^?1) CO₂ in open‐top chambers on the growth after 6 years of two temperate evergreen forest species: an exotic, Pinus radiata D. Don, and a native, Nothofagus fusca (Hook. F.) Oerst. (red beech). We also examine associated effects on selected carbon (C) and nitrogen (N) properties in litter and mineral soil, and on microbial properties in rhizosphere and hyphosphere soil. The soil was a weakly developed sand that had a low initial C concentration of about 1.0 g kg^?1 at both 0–100 and 100–300 mm depths; in the N. fusca system, it was initially overlaid with about 50 mm of forest floor litter (predominantly FH material) taken from a Nothofagus forest. A slow‐release fertilizer was added during the early stages of plant growth; subsequent foliage analyses indicated that N was not limiting. After 6 years, stem diameters, foliage N concentrations and C/N ratios of both species were indistinguishable (P>0.10) in the two CO₂ treatments. Although total C contents in mineral soil at 0–100 mm depth had increased significantly (P<0.001) after 6 years growth of P. radiata, averaging 80±0.20 g m^?2 yr^?1, they were not significantly influenced by elevated CO₂. However, CO₂‐C production in litter, and CO₂‐C production, microbial C, and microbial C/N ratios in mineral soil (0–100 mm depth) under P. radiata were significantly higher under elevated than ambient CO₂. CO₂‐C production, microbial C, and numbers of bacteria (but not fungi) were also significantly higher under elevated CO₂ in hyphosphere soil, but not in rhizosphere soil. Under N. fusca, some incorporation of the overlaid litter into the mineral soil had probably occurred; except for CO₂‐C production and microbial C in hyphosphere soil, none of the biochemical properties or microbial counts increased significantly under elevated CO₂. Net mineral‐N production, and generally the potential utilization of different substrates by microbial communities, were not significantly influenced by elevated CO₂ under either tree species. Physiological profiles of the microbial communities did, however, differ significantly between rhizosphere and hyphosphere samples and between samples under P. radiata and N. fusca. Overall, results support the concept that a major effect on soil properties after prolonged exposure of trees to elevated CO₂ is an increase in the amounts, and mineralization rate, of labile organic components.     

Effects of forest degradation on microbial communities and soil carbon cycling: A global meta‐analysis

Aim

The aim was to explore how conversions of primary or secondary forests to plantations or agricultural systems influence soil microbial communities and soil carbon (C) cycling.

Location

Global.

Time period

1993–2017.

Major taxa studied

Soil microbes.

Methods

A meta‐analysis was conducted to examine effects of forest degradation on soil properties and microbial attributes related to microbial biomass, activity, community composition and diversity based on 408 cases from 119 studies in the world.

Results

Forest degradation decreased the ratios of K‐strategists to r‐strategists (i.e., ratios of fungi to bacteria, Acidobacteria to Proteobacteria, Actinobacteria to Bacteroidetes and Acidobacteria + Actinobacteria to Proteobacteria + Bacteroidetes). The response ratios (RRs) of the K‐strategist to r‐strategist ratios to forest degradation decreased and increased with increased RRs of soil pH and soil C to nitrogen ratio (C:N), respectively. Forest degradation increased the bacterial alpha‐diversity indexes, of which the RRs increased and decreased as the RRs of soil pH and soil C:N increased, respectively. The overall RRs across all the forest degradation types ranked as microbial C (?40.4%) > soil C (?33.3%) > microbial respiration (?18.9%) > microbial C to soil C ratio (qMBC; ?15.9%), leading to the RRs of microbial respiration rate per unit microbial C (qCO₂) and soil C decomposition rate (respiration rate per unit soil C), on average, increasing by +43.2 and +25.0%, respectively. Variances of the RRs of qMBC and qCO₂ were significantly explained by the soil C, soil C:N and mean annual precipitation.

Main conclusions

Forest degradation consistently shifted soil microbial community compositions from K‐strategist dominated to r‐strategist dominated, altered soil properties and stimulated microbial activity and soil C decomposition. These results are important for modelling the soil C cycling under projected global land‐use changes and provide supportive evidence for applying the macroecology theory on ecosystem succession and disturbance in soil microbial ecology.     

亚热带次级森林演替过程中模拟氮磷沉降对土壤微生物生物量及土壤养分的影响

氮（N）沉降深刻影响着森林生态系统的生物多样性、生产力和稳定性。亚热带地区森林土壤磷（P）的有效性较低,N沉降将更突显P的限制作用。N、P输入对亚热带次级森林土壤的影响是否依赖于森林演替阶段知之甚少。选取两种不同演替年龄阶段（年轻林：<40 a;老年林：>85 a）的亚热带常绿阔叶林,设置模拟N和/或P沉降（10 g m^-2 a^-1）4个处理（Ctrl、N、P、NP）,连续处理4.5年后采集表层、次表层和下底层（0-15、15-30、30-60 cm）土壤样品,综合分析了土壤微生物生物量碳（MBC）氮（MBN）和多种土壤养分含量。结果表明,MBC、MBN及土壤养分含量均随土壤深度增加而降低。N添加对两种演替阶段森林土壤中MBC和MBN均无显著影响。施P相关处理（P和NP）对年轻林表层土壤MBC和MBN无显著影响,但显著增加了老年林表层土壤MBC和MBN（P<0.05）,表明老年林可能比年轻林更易受P限制。N添加显著增加了两种演替森林表层土壤可溶性有机氮（DON）、氨态氮（NH₄⁺-N）和硝态氮（NO₃^--N）的含量（P<0.05）;P相关处理（P和NP）显著增加两种演替阶段表层和次表层土壤速效磷（AP）以及表层土壤全磷（TP）的含量（P<0.05）。土壤MBC和MBN与土壤中各养分指标（可溶性有机碳DOC、DON、NH₄⁺-N、NO₃^--N、AP、全碳TC、全氮TN和TP）呈显著正相关关系,土壤TC、TN和DOC是影响土壤微生物生物量的主要因子。研究可为评估和揭示未来全球环境变化背景下不同演替林龄亚热带森林的土肥潜力及土壤质量的演变提供一定的科学理论依据。     

Effects of elevated CO2 and nitrogen addition on soil organic carbon fractions in a subtropical forest

Aims

The aim of this study was to investigate the effects of elevated CO₂ concentration and nitrogen addition on soil organic carbon fractions in subtropical forests where the ambient N deposition was high.

Methods

Seedlings of typical subtropical forest ecosystems were transplanted in ten open-top chambers and grown under CO₂ and nitrogen treatments. The treatments included: 1) elevated CO₂ (700?μmol?mol^-1); 2) N addition of 100?kg NH₄NO₃ ha^-1?yr^-1; 3) combined elevated CO₂ and N addition; and 4) control. We measured soil total organic carbon (TOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC).

Results

Results showed that elevated CO₂ alone did not significantly affect soil TOC, POC and ROC after 4?years of treatment, but increased soil MBC and soil respiration compared to the control. N addition alone had no significant effect neither on soil TOC, POC and ROC, but decreased MBC and soil respiration over time. However, the elevated CO₂ and N addition together significantly increased soil POC and ROC, and had no significant effect on soil MBC.

Conclusions

This study indicated that even in N-rich subtropical forest ecosystems, inputs of N are still needed in order to sustain soil C accumulation under elevated CO₂.     

Quantitative Isolation of Microbial DNA from Different Types of Soils of Natural and Agricultural Ecosystems

A novel procedure was developed for direct quantitative isolation of microbial DNA from soil. This technique was used to evaluate microbial DNA pools in soils of contrasting types (chernozems and brown forest soils) under different anthropogenic loads. A strong correlation was found between microbial biomass and DNA contents in soils of different types (R ²= 0.799). The ratio of soil CO₂ emission rate to the amount of extractable DNA in the soil was shown to reflect the physiological state of the soil microbial community; this ratio can be used as an ecophysiological parameter similarly to the metabolic quotient qCO₂.     

Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland

Microbial responses to three years of CO₂ enrichment (600 μL L^–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO₂. Microbial C:N ratios significantly decreased under elevated CO₂ (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO₂ (qCO₂ = basal respiration/microbial C) did not exhibit any significant differences between CO₂ treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO₂ and phosphorus (P) fertilizer (1 g P m^–2 y^–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO₂. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO₂ on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO₂ enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO₂ must be studied in natural, undisturbed systems.     

Long‐term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha^?1 year^?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.