长期施肥条件下黑土有机碳、氮组分的分配与富集特征

研究不同管理措施下黑土有机碳、氮组分的变化特征是深刻认识和理解黑土固碳的基础.本文以黑龙江省农业科学院31年的长期定位试验为基础,采用物理分组法对土壤不同粒径颗粒进行分离,分析6种不同施肥处理31年后,黑土表层(0～20 cm)及亚表层(20～40 cm)土壤有机碳、氮在粗砂粒、细砂粒、粉粒及黏粒中的分配与富集特征.结果表明： 长期施用有机肥可显著提高土壤有机碳、全氮在粗砂粒和黏粒中的分配比例.在表层土壤,有机无机配施（NPKM）处理下粗砂粒有机碳和全氮的分配比例比对照分别提高191.3%和179.3%,单施有机肥（M）处理下黏粒组分的有机碳和全氮的分配比例分别提高45%和47%.亚表层土壤施用有机肥处理各粒级有机碳、氮含量的提高比例低于表层土壤.在表层和亚表层的粉粒组分中,贮存的有机碳占总储量的42%～63%和48%～54%,全氮占总储量的34%～59%和41%～47%.表层土壤施用有机肥可显著增加粗砂粒中有机碳、氮的富集系数,其中有机肥配施化肥（NPKM）处理富集系数最高（2.30和1.88）,而黏粒组分的有机碳、氮富集系数对长期施肥无响应.
     

沙化草地恢复过程中土壤有机碳物理组分和全氮含量的变化

以宁夏中北部盐池县不同恢复阶段的沙化草地(流动沙地、半固定沙地、固定沙地和荒漠草地)土壤为研究对象,分析土壤粗砂粒有机碳、细砂粒有机碳、粘粉粒有机碳、重组有机碳、轻组有机碳和土壤全氮的变异特征,探讨沙化草地恢复过程中土壤有机碳变化机制。结果显示:(1)荒漠草地、固定沙地、半固定沙地0~30cm土层土壤,粗砂粒有机碳、细砂粒有机碳和粘粉粒有机碳含量分别比流动沙地增加了67.7%、69.8%、212.1%和48.8%、35.3%、99.9%以及33.6%、23.0%、48.9%。(2)随着沙化草地不同程度的恢复,轻组有机碳含量、分配比例和重组有机碳含量均表现为流动沙地半固定沙地固定沙地荒漠草地;重组有机碳分配比例随沙化草地恢复程度的升高呈降低趋势。(3)土壤细砂粒、粘粉粒、重组有机碳、轻组有机碳、粗砂粒有机碳、细砂粒有机碳、粘粉粒有机碳含量与土壤有机碳、土壤全氮含量均呈显著正相关关系,与粗砂粒含量均呈显著负相关关系。研究表明,轻组有机碳和粘粉粒有机碳含量对土壤有机碳的影响最大,轻组有机碳、重组有机碳、粗砂粒有机碳和粘粉粒有机碳对土壤全氮的影响最大,表明沙化草地的恢复有利于减小土壤侵蚀,改善土壤结构与质量。     

江西官山常绿阔叶林土壤有机碳组分沿海拔的变化

对江西官山国家级自然保护区不同海拔(400、600、800、1000、1200 m)常绿阔叶林土壤总有机碳、惰性有机碳和活性有机碳进行分析,研究土壤有机碳的海拔分布特征。结果表明: 土壤总有机碳、惰性有机碳及活性有机碳含量在土壤表层最高,随土层加深而逐渐下降。随海拔升高,土壤总有机碳、惰性有机碳、易氧化有机碳、微生物生物量碳及0～20 cm土层土壤颗粒有机碳含量均出现先增后降的趋势, 且在海拔1000 m达到峰值,而土壤水溶性有机碳及20～40 cm土层土壤颗粒有机碳含量无明显变化。在0～10 cm土层,土壤惰性有机碳占总有机碳的比例在海拔800和1200 m显著高于海拔400和1000 m,而土壤活性有机碳占总有机碳的比例在海拔400 m最高;土壤惰性有机碳和活性有机碳占总有机碳的比例在10～40 cm土层随海拔的增加均呈先增加后降低的趋势,峰值分别在1000和600 m处。各组分有机碳与土壤湿度、微生物生物量氮、可溶性有机氮均呈显著正相关,而且活性有机碳与铵态氮呈显著正相关。海拔显著影响常绿阔叶林土壤有机碳组分的分布,惰性有机碳、易氧化有机碳和微生物生物量碳对海拔变化的响应更敏感。高海拔土壤惰性有机碳和活性有机碳在水分和氮素充足条件下易发生分解与转化,降低土壤碳库的稳定性。在全球气温持续升高背景下,要加强高海拔地区森林土壤有机碳的动态变化研究。     

黔中喀斯特坡地浅层裂隙土壤机械组成对降雨的响应

运用野外取样调查结合室内吸管法,分析黔中喀斯特坡地不同土地利用类型浅层裂隙土壤机械组成随降雨的变化特征,以期为该区地下土壤漏失研究提供理论依据和技术支撑.结果表明: 黔中喀斯特坡地浅层裂隙土壤粒径含量总体以粉粒为主,约占57％.在不同降雨量的作用下,坡耕地裂隙土壤颗粒含量变化整体表现为细砂粒含量的减少和粉粒、黏粒含量的增大.草地裂隙土壤颗粒的变化主要是细砂粒含量的减少和黏粒含量的增加.乔木林地和灌木林地裂隙土壤颗粒的变化不规律.降雨量与土壤砂粒含量变化之间存在正相关关系.随着土层深度的增加,各土地利用类型的裂隙对粉粒和极粗砂粒含量的变化有一定影响,其中,粉粒含量随土层深度的增加而减少.     

小兴安岭两种森林类型土壤有机碳库及周转

采用室内培养法测定了不同温度下(8、18、28 ℃)小兴安岭原始阔叶红松林和阔叶次生林土壤有机碳的矿化速率和矿化量,并用三库一级动力学模型对有机碳各库进行拟合.结果表明： 基于单位干土质量的阔叶次生林土壤有机碳矿化速率和累计矿化量均大于原始红松林,但有机碳累计矿化量占总有机碳的比率小于原始红松林.2种森林类型土壤活性碳库和缓效碳库随土层加深而减小,其占总有机碳的比例增加.尽管阔叶次生林土壤活性和缓效碳库均大于原始红松林,但其占总有机碳的比例却小于原始红松林,而土壤惰性碳库及其比例均大于原始红松林,表明阔叶次生林土壤有机碳整体上更稳定.土壤活性碳库平均驻留时间(MRT)为9～24 d,且随土层加深而缩短,而缓效碳库MRT为7～42 a,且随土层加深而延长.土壤活性碳库及其占总有机碳的比例随温度升高而线性增加,缓效碳库则降低;原始红松林土壤活性碳随温度的增速大于阔叶次生林,表明原始红松林土壤有机碳库对温度变化反应更敏感.     

毛乌素沙地沙漠化逆转过程土壤颗粒固碳效应

为揭示毛乌素沙地沙漠化逆转过程中土壤颗粒的固碳效应,选择陕北榆林治沙区从流沙地、半固定沙地到林龄为20～55年生的灌木和20～50年生的乔木固沙林地,采用物理分组法分析了土壤砂粒、粉粒、黏粒结合碳的演变特征和累积速率.结果表明: 对比流沙地,土壤总有机碳及各颗粒碳含量在两种固沙林地均呈显著增加趋势,并以表层0～5 cm土壤碳含量增幅最高.从流沙地到55年生灌木和50年生乔木固沙林地,0～5 cm土层砂粒碳密度增速均为0.05 Mg·hm^-2·a^-1,粉粒碳密度增速分别为0.05和0.08 Mg·hm^-2·a^-1,而黏粒碳密度增速分别为0.02和0.03 Mg·hm^-2·a^-1.0～20 cm土层,两种林地各颗粒碳密度增速平均为0～5 cm土层的2.1倍.按此增速到50～55年生的固沙林地时,两种林地0～20 cm土层的砂粒碳、粉粒碳和黏粒碳密度分别比流沙地平均提高6.7、18.1、4.4倍,并且颗粒碳对总有机碳的累积贡献率平均为粉粒碳(39.7%)≈砂粒碳(34.6%)>黏粒碳(25.6%).综上,毛乌素沙地沙漠化逆转过程土壤颗粒均表现出显著的固碳效应,且以砂粒和粉粒为主要固碳组分.     

宁夏荒漠草原沙漠化过程中土壤粒径分形特征

研究宁夏荒漠草原沙漠化过程中土壤性状、土壤粒径分形维数的变化特征,以及分形维数与土壤性状的关系.结果表明: 草地沙漠化对土壤分形维数(D)影响显著,D值为1.69～2.62.除在10～20 cm土层出现较小波动外,随着沙漠化程度加剧,0～30 cm土层D值整体呈减小趋势.在荒漠草原沙漠化过程中,荒漠草地D值最大,黏粒和粉粒体积百分含量最高,极细砂粒和细砂粒体积百分含量最低;流动沙地D值最小,黏粒和粉粒体积百分含量最小,极细砂粒和细砂粒体积百分含量最高.D与<50 μm和>50 μm粒径的土壤颗粒存在显著正相关和负相关,表明50 μm粒径是决定草地沙漠化过程中土壤分形维数与土壤粒径关系的临界粒径.随着荒漠草原沙漠化加剧,土壤有机质和全氮含量逐渐降低,土壤容重逐渐升高,固定沙地至半固定沙地是荒漠草原沙漠化的质变过程,其中土壤粘粒体积百分含量、粉粒体积百分含量、土壤有机质含量、土壤全氮含量骤减,极细砂粒体积百分含量、细砂粒体积百分含量和土壤容重骤增.分形维数与土壤有机质、土壤全氮和土壤容重显著相关,固定沙地与半固定沙地分形维数的临界值为2.58,因此分形维数2.58可作为荒漠草原沙漠化的退化指标.     

长三角典型水稻土有机碳组分构成及其主控因子

准确把握水稻土有机碳组分构成特征及其主控因子,对定量化评价土壤有机碳质量和未来演变趋势具有重要意义。通过室内土壤呼吸培养实验结合有机碳三库一级动力学方程,模拟得到长三角地区典型水稻土剖面(0—100 cm)各土层有机碳组分含量及其分布特征;并利用主成分分析获取主控因子,建立有机碳组分回归预测模型。结果表明:水稻土活性碳、慢性碳和惰性碳含量随剖面深度增加而降低,上层土壤(0—40 cm)有机碳组分含量下降速度明显快于下层土壤(40—100 cm);水稻土活性碳构成比例不超过5.3%,惰性碳构成比例大于活性碳与慢性碳比例之和,达到60%以上,水稻土有机碳总量变异主要取决于慢性碳和惰性碳组分变异。因此,水稻土固碳重点在于慢性和惰性组分。同时,研究还发现水稻土类型和剖面深度主要在表层对有机碳组分含量和比例构成产生显著影响,土壤有机碳量、全氮和pH是影响水稻土有机碳组分含量分异的主控因子,利用主控因子可较好预测水稻土有机碳组分含量。     

亚热带不同林分土壤表层有机碳组成及其稳定性

在浙江临安玲珑山选取了常绿阔叶林、马尾松林、板栗林和雷竹林4种林分,采用传统的化学方法与固态13C核磁共振(NMR)技术研究其土壤有机碳在不同粒径土壤颗粒中的分布规律和结构特征,探讨林分类别和管理措施对土壤有机碳含量及其结构的影响,为亚热带地区森林固碳和土壤碳库管理提供科学依据。结果显示:(1)土壤表层(0—20 cm)有机碳含量按以下次序递减:雷竹林>常绿阔叶林>马尾松林>板栗林,且板栗林以粉黏粒结合态碳为主,其他林分土壤则以粗砂结合态碳为主;(2)13C NMR结果表明,阔叶林和马尾松林土壤有机碳中烷基碳所占比例最大,而雷竹林和板栗林则是烷氧碳比例最大,表明人工经营措施改变了土壤有机碳的成分组成;(3)随着土壤颗粒变细,有机碳中烷基碳比例增加,烷氧碳比例减少,A/O-A值和疏水碳/亲水碳值逐渐增大,表明颗粒越细,其结合的有机碳结构稳定性越高。     

城市化梯度上亚热带常绿阔叶林土壤有机碳及其组分特征

以珠江三角洲城区(广州)、近郊(鼎湖山)及远郊(怀集)梯度上的亚热带常绿阔叶林为对象,研究不同土层土壤总有机碳、惰性有机碳、活性有机碳(易氧化有机碳、微生物生物量碳和水溶性有机碳)沿城市化梯度的变化,分析城市化对森林土壤有机碳的影响.结果表明: 0～5 cm土层土壤总有机碳和惰性有机碳含量均在城市化梯度上差异不显著,在5～60 cm土层,土壤总有机碳含量在远郊森林显著高于近郊和城区森林,而惰性有机碳含量在近郊森林最高,城区与远郊森林差异不显著.易氧化有机碳含量在近郊森林显著低于远郊(0～60 cm)和城区(0～10 cm)森林,微生物生物量碳含量在城区森林显著低于近郊和远郊森林,而水溶性有机碳含量在近郊森林(0～10 cm)显著低于城区森林.在0～20 cm土层,城区、远郊森林土壤活性有机碳占总有机碳的比例显著高于近郊森林,而惰性有机碳占总有机碳的比例在远郊森林最低;近郊与城区森林惰性有机碳占总有机碳的比例仅在5～10 cm差异显著.城市化增加了土壤活性碳组分,降低稳定性碳组分,不利于总有机碳积累,远郊森林土壤对城市化的响应更敏感.     

石灰性土壤微生物量碳、氮与土壤颗粒组成和氮矿化势的关系

以黄土高原土壤类型和土壤肥力差异较大的25个农田石灰性耕层土壤为供试土样,研究了土壤微生物量碳（BC）、微生物量氮（BN）与土壤氮素矿化势（NO）、全氮（TN）、有机碳（OC）及土壤颗粒组成的关系．结果表明：BC、BN与TN、OC呈极显著正相关（P〈0．01）,表明BC、BN与土壤肥力关系密切,可作为评价土壤质量的生物学指标．BC、BN与NO均呈高度正相关,相关系数分别为0．665和0．741（P〈0．01）．BC、BN、TN、OC、NO与土壤物理性粘粒（〈0．01mm）呈显著或极显著正相关,而与物理性砂粒（〉0．01mm）呈显著或极显著负相关,与物理性粘粒和砂粒比值呈显著或极显著正相关,表明土壤有机质主要通过与土壤物理性粘粒复合而形成有机无机复合体．     

Phosphorus and carbon in soil particle size fractions: A synthesis

Despite the importance of phosphorus (P) as a macronutrient, the factors controlling the pool sizes of organic and inorganic P (OP and IP) in soils are not yet well understood. Therefore, the aim of this study was to gain insights into the pools sizes of OP, IP and organic carbon (OC) in soils and soil particle size fractions. For this purpose, I analyzed the distribution of OP, IP, and OC among particle size fractions depending on geographical location, climate, soil depth, and land use, based on published data. The clay size fraction contained on average 8.8 times more OP than the sand size fraction and 3.9 and 3.2 times more IP and OC, respectively. The OP concentrations of the silt and clay size fraction were both negatively correlated with mean annual temperature (R² = 0.30 and 0.31, respectively, p < 0.001). The OC:OP ratios of the silt and clay size fraction were negatively correlated with latitude (R² = 0.49 and 0.34, respectively, p < 0.001). Yet, the OC:OP ratio of the clay size fraction changed less markedly with latitude than the OC:OP ratio of the silt and the sand size fraction. The OC concentrations of all three particle size fractions were significantly (p < 0.05) lower in soils converted to cropland than in adjacent soils under natural vegetation. In contrast, the OP concentration was only significantly (p < 0.05) decreased in the sand size fraction but not in the other two particle size fractions due to land-use change. Thus, the findings suggest that OP is more persistent in soil than OC, which is most likely due to strong sorptive stabilization of OP compounds to mineral surfaces.

     

若尔盖退化高寒草甸土壤团聚体结合有机碳的变化

选取若尔盖沼泽化草甸及其不同退化程度为研究对象,利用湿筛法进行团聚体分级,并测定各组分有机碳含量,研究了高寒草甸退化对土壤有机碳(SOC,Soil Organic Carbon)、团聚体以及团聚体结合有机碳(OC,Organic Carbon)的影响,旨在从土壤团聚体及其内部组成的角度去解析SOC的变化特征及机制。结果显示：1)退化使大团聚体比例降低且内部组成改变,团聚体稳定性降低。2)退化使各粒级团聚体及大团聚体内部组分结合OC含量均显著降低。3)大团聚体及其内部粗颗粒有机质中OC储量减少是退化中土壤有机碳流失的主要形式,微团聚体、闭蓄态微团聚体和闭蓄态黏粉粒中OC储量随退化增加。4)退化显著降低了高寒草甸SOC含量和储量,表层(0—10 cm)SOC含量变化主要决定于微团聚体和大团聚体OC含量,亚表层(10—20 cm)SOC含量主要受大团聚体OC含量和团聚体平均重量直径(MWD)影响;对于SOC储量,团聚体MWD是表层SOC储量的最重要影响因素,而亚表层SOC储量取决于团聚体组成、土壤理化性质和大团聚体OC含量的综合作用。研究结果表明,改善土壤团聚体组成和稳定性,增加大团聚体有机...     

滇南地区桃树种植模式对土壤有机碳组分及碳库管理指数的影响

经果林种植可改变土壤质量、改善生态环境,同时具有较高的经济效益。合理的种植模式可通过物种间的互补性提高资源利用效率,改善土壤碳库质量并提高综合效益。为探讨桃树种植模式对土壤有机碳组分及碳库管理指数的影响,以云南省开远市不同桃树种植模式(桃树单种-SP和桃树南瓜套种-PP)为研究对象,以毗邻的天然林地(CK)为对照,分析不同种植模式下活性碳库,即高锰酸钾氧化有机碳(POXC)、颗粒有机碳(POC),惰性有机碳库即矿物结合态有机碳(MAOC)在0—40 cm土层的分布情况,明确土壤有机碳组分与土壤理化性质的关系;计算碳库活度指数(CPAI)、碳库指数(CPI)以及碳库管理指数(CPMI),明确不同桃树种植模式的碳库变化情况。结果表明：桃树种植模式和对照的土壤有机碳组分的含量均随着土层深度的增加而减少,平均土壤有机碳(SOC)含量为：14.68 g/kg(CK)>9.57 g/kg(PP)>8.58 g/kg(SP)。平均活性有机碳组分所占比例与POC/MAOC均表现为：SP>CK>PP,PP的活性有机碳比例较低,具有较高的有机碳稳定性。两种桃树种植模式的CPMI在1...     

黑河中游典型土地利用方式下土壤粒径分布及与有机碳的关系

土地利用方式是影响土壤粒径分布、土壤有机碳及其组成含量的重要因素。研究显示:黑河中游典型土地利用类型下,不同土壤粒径分布和土壤总有机碳(TOC)、活性有机碳(AOC)、非活性有机碳(NOC)的含量存在差异。剖面上TOC、AOC、NOC含量较高的旱地、水田、中覆盖度草地与含量较低的戈壁、裸土地、沙地、盐碱地相比,<1μm、1—5μm、5—10μm、10—50μm的粒径含量较高,而50—250μm、250—1000μm的含量较低,这种变化以50μm为分界,分析表明<50μm的粉粒和粘粒可起到固碳作用,而50—250μm、250—1000μm的砂粒起碳损失作用。统计结果表明,以50μm为分界,水田、戈壁、中覆盖度草地剖面上TOC、AOC、NOC与1—5μm、5—10μm、10—50μm呈正相关,与50—250μm、250—1000μm呈负相关。分析发现,粉粒和粘粒与土壤TOC、AOC、NOC的关系较显著,是影响和控制团聚体形成和稳定的重要因素。增加地表植被覆盖度、退化生态系统的植被恢复、农田耕种、防风固沙措施是提高土壤有机碳、粘粒和粉粒含量的方式,也为土壤团聚体的形成和稳定提供基础。     

Carbon storage capacity of semi‐arid grassland soils and sequestration potentials in northern China

Organic carbon (OC) sequestration in degraded semi‐arid environments by improved soil management is assumed to contribute substantially to climate change mitigation. However, information about the soil organic carbon (SOC) sequestration potential in steppe soils and their current saturation status remains unknown. In this study, we estimated the OC storage capacity of semi‐arid grassland soils on the basis of remote, natural steppe fragments in northern China. Based on the maximum OC saturation of silt and clay particles <20 μm, OC sequestration potentials of degraded steppe soils (grazing land, arable land, eroded areas) were estimated. The analysis of natural grassland soils revealed a strong linear regression between the proportion of the fine fraction and its OC content, confirming the importance of silt and clay particles for OC stabilization in steppe soils. This relationship was similar to derived regressions in temperate and tropical soils but on a lower level, probably due to a lower C input and different clay mineralogy. In relation to the estimated OC storage capacity, degraded steppe soils showed a high OC saturation of 78–85% despite massive SOC losses due to unsustainable land use. As a result, the potential of degraded grassland soils to sequester additional OC was generally low. This can be related to a relatively high contribution of labile SOC, which is preferentially lost in the course of soil degradation. Moreover, wind erosion leads to substantial loss of silt and clay particles and consequently results in a direct loss of the ability to stabilize additional OC. Our findings indicate that the SOC loss in semi‐arid environments induced by intensive land use is largely irreversible. Observed SOC increases after improved land management mainly result in an accumulation of labile SOC prone to land use/climate changes and therefore cannot be regarded as contribution to long‐term OC sequestration.     

Adsorption and movement of metalaxyl in soils under unsaturated flow conditions

Adsorption and mobility of the fungicide metalaxyl was studied in three soil types, viz., Hessaraghatta loamy sand (HLS), Bellary clay (BC), and Ooty clay (OC) soils, under laboratory conditions. The K values obtained from the Freundlich adsorption isotherms were 0.04, 10.65 and 13.32 µg/g for HLS, BC and OC soils respectively. Mobility of metalaxyl in these soils was inversely related to K values (r = –0.97). Although, metalaxyl persisted for over 60 days in all the soils, it did not move beyond a depth of 17.5 cm in any of the soils under unsaturated flow conditions. Mobility of metalaxyl was fastest in HLS and slowest in OC soils.     

Sequestration and turnover of plant- and microbially derived sugars in a temperate grassland soil during 7 years exposed to elevated atmospheric pCO2

Temperate grasslands contribute about 20% to the global terrestrial carbon (C) budget with sugars contributing 10–50% to this soil C pool. Whether the observed increase of the atmospheric CO₂ concentration (pCO₂) leads to additional C sequestration into these ecosystems or enhanced mineralization of soil organic matter (SOM) is still unclear. Therefore, the aim of the presented study was to investigate the impact of elevated atmospheric pCO₂ on C sequestration and turnover of plant‐ (arabinose and xylose) and microbially derived (fucose, rhamnose, galactose, mannose) sugars in soil, representing a labile SOM pool. The study was carried out at the Swiss Free Air Carbon Dioxide Enrichment (FACE) experiment near Zurich. For 7 years, Lolium perenne swards were exposed to ambient and elevated pCO₂ (36 and 60 Pa, respectively). The additional CO₂ in the FACE plots was depleted in ¹³C compared with ambient plots, so that ‘new’ (<7 years) C inputs could be determined by means of compound‐specific stable isotope analysis (¹³C : ¹²C). Samples were fractionated into clay, silt, fine sand and coarse sand, which yielded relatively stable and labile SOM pools with different turnover rates. Total sugar sequestration into bulk soil after 7 years of exposure to elevated pCO₂ was about 28% compared with the control plots. In both ambient and elevated plots, total sugar concentrations in particle size fractions increased in the order sand2 for coarse sand, fine sand and silt (about 274%, 17% and 96%, respectively) but about 14% lower for clay compared with the control plots, corroborating that sugars belong to the labile SOM pool. The fraction of newly produced sugars gradually increased by up to 50% in bulk soil samples after 7 years under elevated pCO₂. In the ambient plots, sugars were enriched in ¹³C by up to 10‰ when compared with bulk soil samples from the same plots. The enrichment of ¹³C in plant‐derived sugars was up to 13.4‰ when compared with parent plant material. After 7 years, the δ¹³C values of individual sugars decreased under elevated (¹³C‐depleted) CO₂ in bulk soil and particle size fractions, varying between −13.7‰ and −37.8‰ under elevated pCO₂. In coarse and fine sand, silt and clay fractions newly produced sugars made up 106%, 63%, 60% and 45%, respectively, of the total sugars present after 7 years. Mean residence time (MRT) of the sugars were calculated according to two models revealing a few decades, mean values increasing in the order coarse sand2 led to a net sequestration of about 30% of labile SOM (sugars) while no increase of total organic C was observed at the same plots. The additional labile SOM is gradually incorporated into more stable SOM pools such as silt and clay fractions in the medium term (<7 years). MRT of labile (sugar) SOM under elevated pCO₂ is in the same order of magnitude when compared with studies under ambient pCO₂ though no direct comparison of elevated and ambient plots was possible.