黄土高原冬小麦田土壤CH4通量对人工降水的短期响应

为了解黄土高原旱作农田土壤CH4排放对不同降水事件的短期响应过程,分别在冬小麦拔节期和夏闲期进行了人工模拟降水试验,对1~32 mm不同降水量模拟降水后0~72 h土壤CH4排放通量进行了观测.结果表明:模拟降水后旱作农田土壤CH4排放通量变化特征表现出两种不同的模式:低降水量(1、3和8 mm)处理为波动变化,高降水量(16和32 mm)处理呈单峰型变化.降水后72 h土壤CH4累积通量(CH4-C)与降水量(P)呈显著线性正相关(冬小麦拔节期:CH4-C=2.45P-6.09,R2=0.92,P<0.01;夏闲期:CH4-C=2.43P-4.73,R2=0.91,P<0.01).相关分析表明,土壤CH4通量与土壤含水量和土壤微生物生物量碳含量显著相关,而与土壤温度不相关.少量降水(1~8 mm)可以在短期内促进旱作农田土壤对CH4的吸收,加强土壤作为大气CH4汇的强度,然而这种促进作用也会随降水量的增大和降水的下渗而削弱.较大降水(≥16 mm)可以刺激土壤产甲烷菌活性促进CH4释放,在短期内使旱作农田土壤由单一的汇功能转变为汇源双重功能.     

降水格局改变背景下土壤呼吸变化的主要影响因素及其调控过程

全球气候变化带来降水格局的改变。土壤呼吸是土壤碳库向大气释放CO_2的重要途径,其对降水变化的响应对陆地生态系统碳循环和全球气候变化进程有着重要的意义。该研究收集了来自全球各地土壤呼吸对降水变化响应的控制试验结果进行分析,以揭示降水格局变化对土壤呼吸影响的普遍规律和控制机制。结果显示:增加降水促进土壤呼吸2%–135%,减少降水抑制土壤呼吸19%–24%,当降水改变量标准化到所有处理的平均值(当年当地降水量的41%)时,增加降水促进的土壤呼吸量(49%)显著大于减少降雨抑制的土壤呼吸量(21%)。土壤湿度是降水变化下驱动土壤呼吸改变的主要因子,其一方面直接影响土壤呼吸,另一方面通过影响土壤微生物碳库、地上/地下净初级生产力来影响土壤呼吸,总解释度高达98%。同时土壤呼吸对降水变化的响应程度随着环境温度和降水量发生变化。土壤呼吸对降水增加的敏感性随环境温度的升高没有显著变化,但对降水减少的敏感性随着环境温度的升高逐渐增强。随着环境降水量的逐渐增加,土壤呼吸对降水增加和减少的敏感性均呈现下降趋势。说明在未来全球降水格局的改变下,土壤呼吸对降水变化的响应有很大的区域差异,受当地气候条件的影响。     

森林土壤融化期异养呼吸和微生物碳变化特征

采用室内土柱培养的方法,研究在不同湿度(55%和80%WFPS,土壤充水孔隙度)和不同氮素供给(NH_4Cl和KNO_3,4.5 g N/m~2)条件下,外源碳添加(葡萄糖,6.4 g C/m~2)对温带成熟阔叶红松混交林和次生白桦林土壤融化过程微生物呼吸和微生物碳的激发效应。结果表明:在整个融化培养期间,次生白桦林土壤对照CO_2累积排放量显著高于阔叶红松混交林土壤。随着土壤湿度的增加,次生白桦林土壤对照CO_2累积排放量和微生物代谢熵(q_(CO_2))显著降低,而阔叶红松混交林土壤两者显著地增加(P0.05)。两种林分土壤由葡萄糖(Glu)引起的CO_2累积排放量(9.61—13.49 g C/m~2)显著大于实验施加的葡萄糖含碳量(6.4g C/m~2),同时由Glu引起的土壤微生物碳增量为3.65—27.18 g C/m~2,而施加Glu对土壤DOC含量影响较小。因此,这种由施加Glu引起的额外碳释放可能来源于土壤固有有机碳分解。融化培养结束时,阔叶红松混交林土壤未施氮处理由Glu引起的CO_2累积排放量在两种湿度条件下均显著大于次生白桦林土壤(P0.001);随着湿度的增加,两种林分土壤Glu引起的CO_2累积排放量显著增大(P0.001)。单施KNO_3显著地增加两种湿度的次生白桦林土壤Glu引起的CO_2累积排放量(P0.01)。单施KNO_3显著地增加了两种湿度次生白桦林土壤Glu引起的微生物碳(P0.001),单施NH_4Cl显著地增加低湿度阔叶红松混交林土壤Glu引起的微生物碳(P0.001)。结合前期报道的未冻结实验结果,发现冻结过程显著地影响外源Glu对温带森林土壤微生物呼吸和微生物碳的刺激效应(P0.05),并且无论冻结与否,温带森林土壤微生物呼吸和微生物碳对外源Glu的响应均与植被类型、土壤湿度、外源氮供给及其形态存在显著的相关性。     

晋南旱地麦田夏闲期土壤水分和养分变化特征

2009-2011年在晋南旱地冬小麦种植区,研究了传统施肥(CF)、推荐施肥(RF)及垄膜沟播(RFFP)处理结合秸秆覆盖措施对夏闲期(6-9月)2 m土层土壤水分、NO₃^--N,以及0～40 cm土层速效磷、速效钾含量的影响.结果表明: 夏闲期降水可补充旱地麦田2 m土层土壤在冬小麦生长季所消耗的水分,其中94%以上蓄水量集中在0～140 cm土层,休闲效率为6%～27%.夏闲期降水易引起NO₃^--N下移;357～400 mm的降水量可使NO₃^--N淋移到100 cm土层,积累峰值在20～40 cm土层.夏闲期秸秆覆盖或地膜与秸秆配合覆盖可有效提高0～40 cm土层速效磷和速效钾含量,3个夏闲期累计增加量分别为17%～45%和36%～49%.不同处理间以垄膜沟播+沟内覆盖秸秆的二元覆盖模式蓄水培肥效果最佳,3个夏闲期2 m土层土壤累计蓄水215 mm,累计矿化氮90 kg·hm^-2,耕层土壤速效磷和速效钾含量分别累计增加2.7和83 mg·kg^-1,显著高于推荐施肥和传统施肥处理.推荐施肥和传统施肥处理对土壤水分、养分变化的影响无显著差异.     

增加降水对干旱河谷区云南松人工林土壤呼吸的影响

2013年5月至2014年6月,对干旱河谷区云南松(Pinus yunnanensis)人工林进行增加降水试验,试验设置对照(CK,0 mm m~(-2)a~(-1))、增水10%(A1,80 mm m~(-2)a~(-1))、增水20%(A2,160 mm m~(-2)a~(-1))和增水30%(A3,240 mm m~(-2)a~(-1))4个处理水平。采用LI-8100开路式土壤碳通量测量系统测定每月土壤呼吸速率。结果表明,4个处理云南松人工林土壤呼吸速率均呈明显的季节变化,7月最高,2月最低。与CK相比,A1年均土壤呼吸速率无显著性差异(P0.05),A2显著增加了12.88%(P0.05),而A3明显减少了17.71%(P0.05)。3个增水处理均提高了土壤呼吸的温度敏感性,减弱了土壤呼吸与土壤湿度的关系。与土壤温度相比,土壤湿度对土壤呼吸的影响相对较小。增水增加了湿季土壤微生物碳、氮含量,干季对微生物碳含量无影响,但明显降低了微生物氮含量。这说明,降水增加对干旱河谷区云南松人工林土壤呼吸的影响是不尽相同的,适当的增水会促进土壤呼吸,而过量的增水会抑制土壤呼吸。     

蚂蚁筑巢对高檐蒲桃热带森林群落土壤呼吸的影响

蚂蚁筑巢能够改变热带森林土壤微生物与土壤理化性质的状况,从而对土壤呼吸时间动态产生重要影响。本研究以西双版纳高檐蒲桃热带森林群落为研究对象,采用Li-6400-09便携式土壤呼吸测定仪对蚂蚁筑巢地与非筑巢地土壤呼吸进行测定。研究结果表明:(1)高檐蒲桃群落土壤呼吸呈明显的单峰型季节变化趋势,且土壤呼吸速率蚂蚁筑巢地(4.96μmol CO_2m~(-2)s~(-1))高于非筑巢地(4.42μmol CO_2m~(-2)s~(-1))。(2)土壤温度和土壤水分显著影响土壤呼吸的时间动态(P0.01);蚂蚁筑巢显著改变巢内温度与水分(P0.05),进而影响土壤呼吸动态。土壤温度对土壤呼吸动态的贡献:蚁巢(83.8%—91.8%)大于非巢地(81.2%—83.1%),但由于筑巢地土壤湿度低于非巢地,土壤水分对土壤呼吸动态的贡献率表现为蚁巢低于非筑巢地。(3)蚂蚁筑巢显著增加土壤微生物生物量(P0.05),从而对土壤呼吸速率产生极显著的影响(P0.01)。蚂蚁筑巢引起微生物生物量碳的增加能够解释76.9%—71.1%的土壤呼吸变化。(4)蚂蚁筑巢引起土壤理化性质变化对土壤呼吸产生一定的影响。土壤容重与土壤呼吸速率呈显著负相关;土壤呼吸速率与土壤微生物量碳、有机质、易氧化有机碳、全氮、硝氮和铵氮显著正相关(P0.05或P0.01)。因此,蚂蚁筑巢显著改变土壤微生物(如微生物生物量碳)、土壤物理性质(如土壤温度与水分)、土壤化学性质(如碳和氮养分),进而对热带森林土壤呼吸产生重要影响。     

不同灌水施肥策略对土壤微生物量碳氮和酶活性的影响

根区局部灌溉形成一个土壤水分分布不均匀的环境,影响了土壤微生物量和酶活性。为探明这种影响,在2种灌水水平(正常灌水和轻度缺水)和2种有机无机N肥配施(单施无机N肥和有机无机N肥配施)下,以常规灌溉(CI)为对照,研究分根区交替灌溉(AI)和固定部分根区灌溉(FI)对土壤微生物量C(MBC)、微生物量N(MBN)和酶活性的影响。与CI相比,AI提高拔节期土壤MBC和抽雄期土壤脲酶活性,但是降低大喇叭口期土壤MBC以及拔节期和抽雄期土壤MBN和拔节期土壤脲酶活性;FI增加抽雄期土壤转化酶活性,但是降低大喇叭口期土壤MBC和可溶性碳(DOC)以及3个时期土壤MBN。与正常灌水相比,AI下轻度亏水增加拔节期和抽雄期土壤DOC,但是降低大喇叭口期土壤MBC和拔节期土壤MBN。与单施无机N肥相比,AI下有机无机N肥配施增加拔节期土壤DOC、MBN和过氧化氢酶活性,而FI下则降低大喇叭口期土壤MBC和转化酶活性。因此,在轻度缺水和有机无机氮配施条件下,分根区交替灌溉可以提高玉米拔节期土壤微生物量碳和可溶性碳。     

土壤呼吸对降雨响应的研究进展

土壤呼吸是当前区域碳收支及全球变化研究中的一个热点问题。降雨作为一个重要的扰动因子, 对准确估算土壤呼吸具有重要影响, 这在干旱和半干旱地区尤为明显。尽管关于土壤呼吸对降雨响应过程与规律的研究已取得了较大进展, 但是对于其机制的解释仍然存在较大的争议, 集中体现在对“Birch效应” (降雨强烈激发土壤呼吸的现象)的解释上, 即到底是“底物供应改变机制”还是“微生物胁迫机制”在调控该过程。该文综述了土壤呼吸对降雨事件、降雨量及降雨格局的响应过程与规律; 阐述了土壤呼吸各组分对降雨响应的差异, 分析了雨后物理替代与阻滞、底物供应、根系和微生物活性、微生物群落结构与功能等一系列过程引起土壤呼吸改变的机制; 重点阐述了微生物对土壤水分波动的响应与适应机制。在此基础上提出了今后需重点关注的4个方面：1) “底物供应改变机制”与“微生物胁迫机制”的区分; 2)土壤呼吸各组分对降雨响应的差异; 3)不同时空尺度上土壤呼吸对降雨响应的模拟与估算; 4)降雨带来的外援N和H⁺的作用。     

中国西北部草地植被降水利用效率的时空格局

植被降水利用效率(PUE)是评价干旱、半干旱地区植被生产力对降水量时空动态响应特征的重要指标。利用光能利用率CASA(Carnegie-Ames-Stanford Approach)模型估算了2001—2010年中国西北七省草地植被净初级生产力(NPP),结合降水量的空间插值数据,分析了近十年草地植被PUE的空间分布、主要植被类型的PUE,及其时空格局的驱动因素。结果表明:(1)2001—2010年西北七省草地植被的平均PUE为0.68 g C m~(-2)mm~(-1)。在温带草地各类型中,PUE的大小顺序为草甸草原灌丛典型草原荒漠草原荒漠,各类型草地PUE之间差异显著;对于高寒草地而言,高寒草原的PUE显著高于高寒草甸;(2)温带草地PUE的空间分布与年降水量的关系呈抛物线形状(R~2=0.65,P0.001),PUE峰值出现在年降水量P=472.9 mm的地区;荒漠地区植被PUE的空间分布与年降水量的关系同样呈抛物线形状(R~2=0.63,P0.001),PUE峰值出现在年降水量P=263.2mm的地区;对于高寒草地而言,年降水量100 mm以下地区植被PUE变异较大,年降水量大于100 mm的地区植被PUE的空间分布随降水量的变化呈抛物线形状(R~2=0.47,P0.001),PUE峰值出现在P=559.2 mm的地区;(3)不同降水量区域,植被PUE的年际波动与气候因子的关系也有较大差别。在年降水量为200—1000 mm的地区,草地PUE的年际波动与年降水量的变化呈正相关;在年降水量高于1050 mm的地区,草地PUE的年际波动与年均温的相关性较强,相关系数最高可达到0.4。     

黄土旱塬区冬小麦不同施肥处理的土壤呼吸及土壤碳动态

依据黄土旱塬区黑垆土上中国科学院长武站长期定位试验 (始于1984年),于2008年3月到6月,测定了冬小麦连作系统中返青期、拔节期、抽穗期、灌浆期和收获期土壤呼吸日变化、生育期变化以及土壤可溶性有机碳(Dissolved organic C, DOC)和微生物量碳(Soil microbial biomass C, MBC),研究了施肥措施对土壤呼吸、DOC和MBC的影响以及土壤呼吸与碳组分之间的关系.研究涉及6个处理:休闲地(F)、不施肥(CK)、有机肥(M)、氮肥(N)、氮磷肥(NP)和氮磷有机肥(NPM).结果表明,冬小麦连作系统中土壤呼吸的日变化格局呈单峰曲线,最高值出现在12:00左右(拔节期)和14:30左右(成熟期),最小值出现在0:00～3:00之间或6:00左右;冬小麦土壤呼吸速率拔节期最高,其次是灌浆后期,抽穗期最低;不同施肥条件下,各生育期土壤呼吸速率大小顺序:NPM＞M＞NP＞N＞CK＞F.土壤水分亏缺是导致抽穗期和灌浆期土壤呼吸速率降低的重要原因.各施肥处理DOC含量高低顺序为灌浆期＞抽穗期＞成熟期＞返青期＞拔节期;除M,NPM处理MBC含量拔节期＞灌浆期外,各施肥处理MBC含量高低顺序为成熟期＞抽穗期＞灌浆期＞拔节期＞返青期.同一处理不同生育期土壤呼吸速率与DOC,MBC的相关性较低,但同生育期不同施肥处理土壤呼吸与土壤有机碳组分间存在显著的相关性.以F处理土壤呼吸为基础,估算CK、N和NP处理生育期根系对土壤呼吸的平均贡献率依次为36%、45%和54%.     

脉冲式降水对不同类型草地土壤微生物呼吸碳释放量的影响

降水事件引起土壤短时间内释放大量CO_2的现象常称为降水脉冲效应。降水事件发生后,由于水分和养分可获得性快速提升使土壤微生物呼吸速率快速升高至正常水分状况的数倍,从而导致土壤CO_2大量释放并一定程度上影响着生态系统碳循环过程和土壤碳平衡,尤其在干旱或半干旱地区。利用自主研发的能快速测定土壤微生物呼吸速率的装置,对内蒙古三类典型草原(草甸草原、典型草原和荒漠草原)土壤分别开展土壤复湿实验(60%饱和含水量),并采用高频测定(48 h测定288次)。在土壤复湿后在所有温带草地类型中均发生了明显的脉冲效应,降水脉冲过程中单位有机质(土壤有机碳,SOC)最大呼吸速率(R_(SOC-max))整体表现为荒漠草原(1.59 mg C g~(-1) SOC h~(-1))草甸草原(0.73 mg C g~(-1) SOC h~(-1))典型草原(0.50 mg C g~(-1) SOC h~(-1));而脉冲效应的持续时间(Duration)则表现为典型草原(2.5 h)草甸草原(1.5 h)荒漠草原(0.67 h)。在土壤复湿48 h内,单位土壤微生物呼吸累积量(A_(R_(Soil)))的大小规律与单位土壤微生物呼吸速率R_(Soil)一致,均为典型草原草甸草原荒漠草原;然而,如果用土壤有机质进行标准化,单位有机质呼吸累积量A_(R_(SOC))表现为荒漠草原(9.74 mg C g~(-1) SOC)典型草原(6.54 mg C g~(-1) SOC)草甸草原(3.54 mg C g~(-1) SOC),与当地年降雨频率呈负相关关系,表明降水脉冲效应与土壤长期经历的干旱状况存在密切关系。本研究结果不仅证明在干旱半干旱区域降水脉冲效应的普遍性,同时还启发我们应从国家或区域尺度开展研究,以进一步揭示土壤基质含量、土壤干旱状况等对降水脉冲效应的影响。     

Environmental variables controlling soil respiration on diurnal, seasonal and annual time-scales in a mixed mountain forest in Switzerland

Studies on soil respiration in mountain forests are rather scarce compared to their broad distribution. Therefore, we investigated daily, seasonal and annual soil respiration rates in a mixed forest (Lägeren), located at about 700 m in the Swiss Jura mountains, during 2 years (2006 and 2007). Soil respiration (SR) was measured continuously with high temporal resolution (half-hourly) at one single point (SR_automated) and periodically with high spatial resolution (SR_manual) at 16 plots within the study site. Both, SR_automated and SR_manual showed a similar seasonal cycle. SR strongly depended on soil temperature in 2007 (R ² = 0.82–0.92), but less so in 2006 (R ² = 0.56–0.76) when SR was water limited during a summer drought. Including soil moisture improved the fit of the 2006 model significantly (R ² = 0.78–0.97). Total annual SR for the study site was estimated as 869 g C m^?2 year^?1 for 2006 and as 907 g C m^?2 year^?1 for 2007 (uncertainty <10% at the 95% confidence interval, determined by bootstrapping). Selected environmental conditions were assessed in more detail: (1) Rapid, but contrasting changes of SR were found after summer rainfall. Depending on soil moisture at pre-rain conditions, summer rain could either cause a pulse of CO₂ from the soil or an abrupt decrease of SR_automated due to water logging of soil pores. (2) Two contrasting winter seasons resulted in SR being about 60–70% (31.2–44.6 g C m^?2) higher during a mild winter (2007) compared to a harsh winter (2006). (3) Analysing SR for selected periods on a diurnal scale revealed a counter-clockwise hysteresis with soil surface temperatures. This indication of a time-lagged response of SR to temperature was further supported by a very strong relationship (R ² = 0.86–0.90) of SR to soil temperature with a time-lag of 2–4 h.     

Responses of switchgrass soil respiration and its components to precipitation gradient in a mesocosm study

Aims

The objective of this study was to investigate the effects of the precipitation changes on soil, microbial and root respirations of switchgrass soils, and the relationships between soil respiration and plant growth, soil moisture and temperature.

Methods

A mesocosm experiment was conducted with five precipitation treatments over two years in a greenhouse in Nashville, Tennessee. The treatments included ambient precipitation, ?50%, ?33%, +33% and +50% of ambient precipitation. Soil, microbial, and root respirations were quantified during the growing seasons.

Results

Mean soil and root respirations in the +50% treatment were the highest (2.48 and 0.93 μmol CO₂ m^?2 s^?1, respectively) among all treatments. Soil microbial respiration contributed more to soil respiration, and had higher precipitation sensitivity mostly than root respiration. Increases in precipitation mostly enhanced microbial respiration while decreases in precipitation reduced both microbial and root respirations. Across precipitation treatments, soil respiration was significantly influenced by soil moisture, soil temperature, and aboveground biomass.

Conclusions

Our results showed that microbial respiration was more sensitive to precipitation changes, and precipitation regulated the response of soil respiration to soil temperature. The information generated in this study will be useful for model simulation of soil respiration in switchgrass fields under precipitation changes.

     

模拟氮沉降和降雨对华西雨屏区常绿阔叶林土壤呼吸的影响

从2013年12月至2014年11月,通过野外原位试验,对华西雨屏区常绿阔叶林进行了模拟氮沉降和降雨试验,采用LI-8100土壤碳通量分析系统(LI-COR Inc.,USA)测定了对照(CK)、氮沉降(N)、减雨(R)、增雨(W)、氮沉降+减雨(NR)、氮沉降+增雨(NW)6个处理水平的土壤呼吸速率,并通过回归方程分析了温度和湿度与土壤呼吸速率间的关系。结果表明:(1)氮沉降和增雨抑制了常绿阔叶林土壤呼吸速率,减雨促进了常绿阔叶林土壤呼吸速率。(2)减雨使华西雨屏区常绿阔叶林土壤呼吸年通量增加了258 g/m~2,而模拟氮沉降和增雨使华西雨屏区常绿阔叶林土壤呼吸年通量分别减少了321g/m~2和406g/m~2。(3)减雨增加了土壤呼吸的温度敏感性,模拟氮沉降和增雨降低了土壤呼吸的温度敏感性。(4)模拟温度和湿度与土壤呼吸速率间回归方程分析表明,土壤水分对土壤呼吸速率的影响较小。(5)模拟氮沉降和增雨处理减少土壤微生物生物量碳、氮的含量,减雨处理增加了土壤微生物生物量碳、氮的含量。(6)模拟氮沉降和降雨对华西雨屏区土壤CO_2释放的影响未表现出明显的交互作用。     

Bacterial and Heterotrophic Nanoflagellate Densities and C‐biomass Estimates Along an Alaskan Tundra Transect with Prediction of Respiratory CO2 Efflux

Although tundra terrestrial ecology is significantly affected by global warming, we know relatively little about how eukaryotic microbial communities respond and how much microbial respiratory CO₂ may be released due to available organic nutrient sources in the permafrost melt. Prior research has shown a strong positive correlation between bacteria and fungi in some Arctic locales; this research focused on the relationships of terrestrial bacteria and heterotrophic nanoflagellates. The densities and estimated C‐biomass of bacteria and heterotrophic nanoflagellates (a major occurring group of protozoa) were assessed in 14 samples obtained along a 10 km transect in northwest AK during the summer of 2012. Two samples were taken, one at the top and one near the base of seven hummocks along the transect. Densities (no./g soil) of bacteria varied from 2.7–16 × 10⁹, and nanoflagellates 0.7–7.9 × 10⁷. C‐biomass (μg/g soil) of bacteria varied from 358 to 2,114, and nanoflagellates 12–37. Additionally, the rate of respiration was analyzed in the laboratory for each soil sample. A linear relationship between soil respiration and bacterial densities was obtained (20 °C): R_s = 12.32 + 14.07 B_d (p ? 0.01); where R_s is soil respiration (nmol/min/g soil) and B_d = bacterial density (no. × 10⁹/g soil).     

Dynamics of soil respiration in sparse <Emphasis Type="Italic">Ulmus pumila</Emphasis> woodland under semi-arid climate

Sparse Ulmus pumila woodlands play an important role in contributing to ecosystem function in semi-arid grassland of northern China. To understand the key attributes of soil carbon cycling in U. pumila woodland, we studied dynamics of soil respiration in the canopy field (i.e., the projected crown cover area) and the open field at locations differing in distance (i.e., at 1–1.5, 3–4, 10, and >15 m) to tree stems from July through September of 2005, and measured soil biotic factors (e.g., fine root mass, soil microbial biomass, and activity) and abiotic factors [e.g., soil water content (SWC) and organic carbon] in mid-August. Soil respiration was further separated into root component and microbial component at the end of the field measurement in September. Results showed that soil respiration had a significant exponent relationship with soil temperature at 10-cm depth. The temperature sensitivity index of soil respiration, Q ₁₀, was lower than the global average of 2.0, and declined significantly (P < 0.05) with distance. The rate of soil respiration was generally greater in the canopy field than in the open field; monthly mean of soil respiration was 305.5–730.8 mg CO₂ m⁻² h⁻¹ in the canopy field and 299.6–443.1 mg CO₂ m⁻² h⁻¹ in the open field from July through September; basal soil respiration at 10°C declined with distance, and varied from ~250 mg CO₂ m⁻² h⁻¹ near tree stems to <200 mg CO₂ m⁻² h⁻¹ in the open field. Variations in soil respiration with distance were consistent with patterns of SWC, fine root mass, microbial biomass and activities. Regression analysis indicated that soil respiration was tightly coupled with microbial respiration and only weakly related to root respiration. Overall, variations in SWC, soil nutrients, microbial biomass, and microbial activity are largely responsible for the spatial heterogeneity of soil respiration in this semi-arid U. pumila woodland.     

Do changes in flood pulse duration disturb soil carbon dioxide emissions in semi-arid floodplains?

In semi-arid floodplains the average times between floods have been cited to drive metabolic and biogeochemical responses during the subsequent flooding pulse. However, the interaction effects of flood pulse duration and the length of time between floods on the carbon budget are not well understood. Using field experiments, flood pulses—dry cycles were simulated (SF plots—short flood/dry cycles: 15 flood days + 7 dry + 15 flood and LF plots—long flood/dry cycles: 21 flood + 14 dry + 21 flood) in a semi-arid floodplain in Central Spain, in order to study the effects on soil CO₂ emissions. Differences on soil water content among SF, LF and control plots were statistically significant throughout the experiment (p < 0.01). Soil CO₂ emission rates during drying time were significantly related with the duration of previous flooding and inter-flooding intervals (R ² = 0.52–0.64, p = 0.03). During the first stage of desiccation, the high soil water content appears to limit aerobic metabolism. Soil respiration rates similar to those of control plots measurements occurred 1–2 weeks later. Then, soil respiration increased to a maximum rate which was delayed 5–8 weeks, as high soil water content limited microbial activity. While more than 7 days of inundation promoted denitrification, organic nutrients supplied by flood water increased 1% soil respiration during drying. Differences between SF and LF plots in soil CO₂ emissions only appeared after floodplain soil had been subjected to two consecutive flood-dry cycles; 70 days after the second inundation ended, CO₂ fluxes achieved similar values in all treatments. Daily soil CO₂ emission rates during the entire study period (117 days) were comparable, independently of the flood duration and the time between floods (75.76 ± 1.59 and 77.94 ± 0.45 mmol CO₂ m^?2 day^?1, in SF and LF, respectively). Flood disturbance affects site-specific microbial processes, but only during very short time periods. The mechanism by which soil microbial communities cope or adapt to new conditions needs to be reassessed in future research in order to determine the long-term effects of hydrological changes in the soil carbon balance of semi-arid floodplains.     

Supply-side controls on soil respiration among Oregon forests

To test the hypothesis that variation in soil respiration is related to plant production across a diverse forested landscape, we compared annual soil respiration rates with net primary production and the subsequent allocation of carbon to various ecosystem pools, including leaves, fine roots, forests floor, and mineral soil for 36 independent plots arranged as three replicates of four age classes in three climatically distinct forest types. Across all plots, annual soil respiration was not correlated with aboveground net primary production (R²=0.06, P>0.1) but it was moderately correlated with belowground net primary production (R²=0.46, P<0.001). Despite the wide range in temperature and precipitation regimes experienced by these forests, all exhibited similar soil respiration per unit live fine root biomass, with about 5 g of carbon respired each year per 1 g of fine root carbon (R²=0.45, P<0.001). Annual soil respiration was only weakly correlated with dead carbon pools such as forest floor and mineral soil carbon (R²=0.14 and 0.12, respectively). Trends between soil respiration, production, and root mass among age classes within forest type were inconsistent and do not always reflect cross‐site trends. These results are consistent with a growing appreciation that soil respiration is strongly influenced by the supply of carbohydrates to roots and the rhizosphere, and that some regional patterns of soil respiration may depend more on belowground carbon allocation than the abiotic constraints imposed on subsequent metabolism.