长期不同植被覆盖对黑土团聚体内有机碳组分的影响

为探究黑土团聚体内土壤有机碳(SOC)的"分馏"特征,揭示不同植被覆盖下土壤团聚体的固碳机制,该文以中国科学院海伦农业生态系统国家野外综合研究站内不同植被覆盖(草地、农田和裸地)长期定位实验的土样为研究对象,利用团聚体湿筛分组、有机碳物理和化学分组相结合的方法,研究了黑土团聚体及其内部的碳密度和腐殖质组分的碳分配特征。研究发现,黑土经过不同植被覆盖31年后,长期草地覆盖使土壤表层SOC、全氮(TN)含量显著增加,农田和无植被覆盖的裸地SOC含量减少,且在裸地显著降低。3种处理中, 2–0.25 mm (含2 mm,下同)粒级团聚体均为优粒级。土壤团聚体的稳定性顺序为草地农田裸地。草地覆盖使土壤大团聚体的比例和有机碳库增加,微团聚体和粉黏粒所占比例和碳库均减少,说明草地覆盖促进了土壤大团聚体形成,土壤固碳能力显著增强。而农田和裸地因外源碳投入少,有机碳含量均是微团聚体大团聚体粉黏粒, SOC主要分布在微团聚体中。不同植被覆盖处理对土壤团聚体内密度组分和腐殖质各组分碳的富集"分馏"作用很明显,与农田和裸地相比,长期草地植被覆盖处理2 mm和2–0.25 mm粒级团聚体中轻组碳含量富集的较多, 2–0.25 mm粒级团聚体中富里酸、胡敏酸和胡敏素的碳富集均最高,而农田和裸地促进了微团聚体内腐殖质碳的富集。草地覆盖显著增加了大团聚体内活性有机碳组分,来源于植物的碳首先进入到大粒径的团聚体中,使土壤团聚结构显著改善,农田和无植被覆盖的裸地土壤中轻组碳含量显著降低,团聚体内有机碳以重组碳和胡敏素为主,稳定化程度更高。     

典型黑土区不同生态系统土壤团聚体有机碳分布特征

基于团聚体分组和闭蓄态微团聚体分离技术,利用典型黑土区27年长期定位试验,研究草地生态系统、农田生态系统和裸地生态系统下土壤团聚体及团聚体内部组分中有机碳的分布,以解析不同生态系统下土壤团聚体和有机碳固持间的关系,揭示黑土有机碳的物理稳定性机制。结果显示:与农田相比,草地土壤有机碳含量显著提高7.6%;裸地土壤有机碳含量显著下降14.1%。草地促进了大团聚体(250μm),尤其2000μm团聚体的形成,提高了土壤团聚体的稳定性;裸地则降低了土壤的团聚化程度及稳定性,大团聚体和微团聚体含量下降,粉粘粒含量相应增加。草地大团聚体中有机碳含量显著高于农田,且内部各组分有机碳含量均有显著提高,其中粗颗粒有机质、闭蓄态微团聚体和粉黏粒有机碳含量增幅分别为600%、54%和65%;裸地增加了粉粘粒结合有机碳含量,降低了大团聚体和微团聚体中有机碳含量,且大团聚体和微团聚体内部各组分有机碳含量均有所下降。3种生态系统类型土壤均以总粉粘粒结合有机碳为主,占土壤总有机碳52%—79%,其作为惰性碳库是黑土有机碳的重要组成部分。黑土有机碳的累积或损失主要表现为活性较强的有机碳库-团聚体中颗粒有机质的增加或减少,与农田相比,草地土壤有机碳的累积主要归因于大团聚体中粗颗粒有机质的增加,为总有机碳增量的3倍;裸地土壤有机碳的损失主要归因于微团聚体中总细颗粒有机质的减少,对总有机碳损失的贡献率为60%。     

岩溶槽谷区植被恢复对土壤团聚体有机碳及碳库管理指数的影响

为揭示岩溶槽谷区植被恢复对土壤结构、土壤有机碳积累和碳库管理水平的影响,该研究选取了弃耕地、林地和草地三种土地利用方式,测定0～20 cm土层土壤团聚体组成、土壤有机碳(SOC)、团聚体有机碳以及土壤易氧化有机碳(EOC)含量。结果表明:(1)与弃耕地相比,林地和草地土壤团聚体平均重量直径(MWD)、几何平均重量直径(MGD)和2～5 mm团聚体含量显著增加,林地和草地土壤团聚体组成以2～5 mm为主,弃耕地以0.5～1 mm和0.25 mm为主,表明退耕还林还草能够促进土壤团聚体形成和稳定。(2)土壤团聚体有机碳含量呈现出林地草地弃耕地,随团聚体粒级增加而增加的趋势;林地和草地以2～5 mm团聚体有机碳贡献率最大,弃耕地则以0.25 mm团聚体贡献为主,表明弃耕地转变为林地和草地后,土壤SOC积累主要归功于2～5 mm有机碳含量的增加,以及团聚体由小粒径向大粒径转变。(3)与弃耕地比较,林地和草地土壤SOC、EOC含量和碳库管理指数(CPMI)均显著提高,其中土壤EOC含量和CPMI变化较为明显;土壤EOC可作为土壤碳库早期变化的有效指标,CPMI能够良好地表征植被恢复对土壤SOC和EOC的影响。     

植被修复初期对贵州高原喀斯特湿地湖滨带碳元素的影响

以贵州草海保护区（海拔约为2171.7 m）为研究对象,采用时空互代法研究不同地貌环境（非喀斯特地貌、喀斯特地貌及植被修复地貌）下,保护区内土壤有机碳（SOC）、易氧化有机碳（ROC）、可溶性有机碳（DOC）含量分布规律及其影响因素,以期为准确估算保护区内碳组分储量及其固碳效益评价提供科学依据。结果表明：（1）不同地貌下有机碳（SOC）含量、易氧化有机碳（ROC）和可溶性有机碳（DOC）与土层深度存在极显著负相关关系,即随土层深度增加而降低;（2）在0-50 cm土层中喀斯特地貌与非喀斯特地貌下土壤团聚体均以>0.2 mm粒级有机碳含量相对较高,<0.125 mm粒级含量最低。（3）植被恢复过程中,土壤碳组分含量显著增加,土壤固碳能力增强提高了土壤有机碳含量,促进了土壤团聚体的形成,同时也提高了土壤团聚体的稳定性。     

植被恢复对退化红壤团聚体稳定性及碳分布的影响

土壤有机碳对土壤团聚体的形成与稳定具有重要影响.研究了植被恢复对侵蚀退化红壤团聚体稳定性的影响,在此基础上探讨了有机碳在团聚体中的分布及有机碳与团聚体稳定性的关系.红壤侵蚀裸地大团聚体水稳定性程度低,植被恢复后大团聚体稳定性显著提高.裸地各级团聚体有机碳含量基本相似,植被恢复后,有机物质输入的增加促进了团聚体的形成,从而改变了土壤团聚体有机碳含量和分配比例.大团聚体对有机碳具有一定的富集作用,有机碳含量高于全土和微团聚体、粉粒及粘粒有机碳含量,大团聚体有机碳恢复速度也快于微团聚体以及粉粒与粘粒有机碳的恢复速度.植被恢复使裸地0～10cm土层大团聚体有机碳占总有机碳的比例从15%左右增加到32%～42%.土壤中增加的有机碳约41%～51%储存在大团聚体中,24%～38%储存在微团聚体中,20%～31%储存在粉粒及粘粒中.团聚体稳定性的增强与土壤有机碳含量密切相关,植被恢复时间越长,土壤有机碳含量越高,土壤团聚体水稳性程度也越好.     

桂北喀斯特山区不同植被类型土壤碳库管理指数的变化特征

以广西北部喀斯特石山地区不同植被类型(青冈栎次生林、灌丛、马尾松林、竹林、草地、农田、裸地)土壤为研究对象,探讨植被类型对活性有机碳库以及土壤碳库管理指数的影响,为喀斯特山区生态环境建设提供依据。结果表明:1)青冈栎次生林土壤碳储量、全氮含量、速效氮含量显著高于其他植被,土壤容重小于除农田以外的其他植被类型; 2)不同植被类型下土壤有机碳(SOC)、活性有机碳(LOC)含量均随土层深度的增加而降低,0～40cm土层SOC含量为1.07～29.76 g·kg~(-1),大小关系表现为青冈栎次生林灌丛马尾松林竹林草地农田裸地; LOC含量为0.58～13.77 g·kg~(-1),表现为青冈栎次生林灌丛马尾松林竹林农田草地裸地; LOC/SOC表现为青冈栎次生林最小,农田最大; 3)土壤碳库管理指数随土层深度的变化为先减小后增加再减小的趋势,0～40 cm土层CPMI平均值表现为青岗栎次生林灌丛马尾松林农田竹林草地裸地; 4)土壤植被类型、土层深度以及二者的交互作用对土壤碳库管理指数及碳库特征具有显著影响。本研究表明,增加植被覆盖以及减少人为活动的干扰,能提高土壤有机碳含量,有利于维持桂北喀斯特山区土壤碳库的稳定性。     

东北黑土水稳性团聚体及其结合碳分布特征

以东北黑土区32对自然和耕作黑土为研究对象,对比研究了两种土壤水稳性团聚体及其结合碳的分布特征.结果表明：自然土壤0～30 cm水稳性大团聚体（>0.25 mm）质量分数及其结合碳均高于微团聚体;随着土层深度的增加,大团聚体及其结合碳逐渐降低,而微团聚体（<0.25 mm）及其结合碳显著增加（P<0.01）.耕作土壤团聚体分布特征与自然土壤相反,与自然土壤相比,耕作土壤大团聚体数量及其结合碳急剧降低,且>1 mm的大团聚体降低幅度远大于其他粒级团聚体.自然土壤大团聚体质量分数与土壤有机碳（SOC）存在明显的正相关关系（P<0.01）,尤其是>1 mm大团聚体;耕作土壤>1 mm大团聚体质量分数与土壤总SOC相关不显著;自然与耕作土壤总SOC与各级团聚体结合碳呈极显著正相关（P<0.01）.与自然土壤相比,耕作土壤大团聚体结合碳明显降低,而微团聚体结合碳增加了37.1%,总SOC含量下降了29.5%,表明大团聚体在总SOC变化中起主要作用.水稳性大团聚体对管理措施响应迅速,可以作为评价农业管理措施转变对土壤肥力和土壤质量影响的指标.     

云雾山草原区不同植被恢复阶段土壤团聚体活性有机碳分布特征

以宁夏云雾山草原自然保护区不同植被群落为研究对象,对0～20 cm土层土壤团聚体活性有机碳分布特征进行分析.结果表明:(1)不同植被群落土壤团聚体活性有机碳含量顺序为退耕草地<百里香群落<铁杆蒿群落<大针茅群落<长芒草群落,与退耕草地相比,封育草地各粒径团聚体活性有机碳含量均显著提高 (P<0.05),表明随着植被的恢复土壤团聚体活性有机碳含量提高并趋于稳定,土壤碳汇效应有可能增强.(2)植被恢复主要影响大团聚体(>0.25 mm)中活性有机碳含量,其中0.5～0.25 mm粒径团聚体中活性有机碳含量最高,微团聚体(<0.25 mm)中活性有机碳含量最低.(3)植被恢复前期(退耕草地-铁杆蒿群落)0～10 cm土层>0.5 mm粒径团聚体中活性有机碳含量较10～20 cm土层有所增加,<0.5 mm粒径团聚体活性有机碳含量变化不大,恢复至后期到长芒草阶段时,0～10 cm土层<0.5 mm粒径团聚体中活性有机碳含量也开始提高, 各粒径0～10 cm土层团聚体活性有机碳含量均比10～20 cm土层有所提高.(4)相关性分析表明,土壤团聚体活性有机碳含量与土壤团聚体总有机碳含量呈极显著线性正相关关系(r=0.9394),团聚体活性有机碳含量可以作为衡量土壤团聚体有机碳动态的一个敏感性指标.     

地膜覆盖对北方旱地土壤水稳性团聚体及有机碳分布的影响

研究不同地膜覆盖时间对北方旱作农田土壤团聚体粒级稳定性和有机碳的影响,可为提升旱作农田生产力和保护农田环境选择合适的管理方式提供科学依据。以辽宁阜新5年秋覆膜(AP)、春覆膜(SP)和不覆膜(CK)的定位试验为研究对象,分析不同覆膜时间对0—10 cm和10—20 cm土层中2 mm、0.25—2 mm、0.053—0.25 mm和0.053 mm粒级的土壤水稳性团聚体的稳定性及有机碳的影响。结果表明,在北方旱作农田,连续5年的地膜覆盖可显著改变0—10 cm土层的土壤各级团聚体的分布、团聚体中有机碳的含量及其对土壤有机碳含量的贡献率,进而增加土壤水稳性团聚体的稳定性,而对10—20 cm土层影响不显著。与不覆膜相比,秋覆膜和春覆膜可显著提高0—10 cm土层2 mm的水稳性团聚体的含量,分别提高了36.3%、47.9%(P0.05),而对微团聚体无显著影响,说明地膜覆盖有利于提高大团聚体数量及稳定性。在0—10 cm土层,粒径2 mm团聚体有机碳含量及储量表现为秋覆膜最高,显著高于春覆膜和不覆膜处理(P0.05)。与裸地不覆膜相比,秋覆膜和春覆膜显著提高2 mm团聚体中有机碳含量对土壤有机碳的贡献率,分别提高了37%和26.1%(P0.05)。而在0—10 cm和10—20cm土层中,微团聚体中有机碳含量对土壤有机碳贡献率没有影响。在辽宁阜新土壤及种植条件下,秋覆膜处理不仅显著提高0—10 cm土壤水稳性大团聚体的含量和稳定性,还可以显著增加水稳性大团聚体有机碳含量及储量,促进有机碳的固存。     

不同施肥处理黑土覆膜后秸秆碳氮在团聚体中的固存特征

地膜覆盖是提高作物产量的重要措施,理解覆膜条件下黑土团聚体中外源碳和氮的固存特征,为深刻认识地膜覆盖措施的可持续应用提供理论依据。选取长期定位试验站(29年)不施肥(CK)、单施化肥(NPK)和有机肥配施化肥(MNPK)3个典型施肥处理,表层土壤(0—20 cm)添加¹³C¹⁵N双标记玉米秸秆后设置裸地和覆膜的田间原位微区培养试验,探讨不同施肥处理结合覆膜黑土团聚体中有机碳和全氮对秸秆来源碳和氮的响应。结果表明,与裸地相比,所有处理覆膜后微团聚体(<0.25 mm)中秸秆来源碳和氮的含量平均降低了26.49%和32.05%。覆膜MNPK与裸地处理相比大团聚体(>0.25 mm)中秸秆来源碳和氮的含量显著降低了35.58%和15.97%,但大团聚体中原土壤有机碳的含量提高了9.16%。在CK和NPK处理微团聚体中,秸秆来源碳占该粒级团聚体有机碳的比例表现为覆膜>裸地,而在MNPK处理各粒级团聚体中则表现为裸地>覆膜。无论覆膜与否,秸秆来源碳对团聚体有机碳和秸秆来源氮对团聚体全氮的贡献率受施肥处理的影响表现为CK>N...     

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...     

Effects of vegetation restoration on soil organic carbon concentration and density in the mid-subtropical region of China

为揭示植被恢复对土壤有机碳(SOC)库的影响机制, 采用空间代替时间的方法, 以湘中丘陵区地域相邻、环境条件基本一致的檵木(Loropetalum chinense)-南烛(Vaccinium bracteatum)-杜鹃(Rhododendron simsii)灌草丛(LVR)、檵木-杉木(Cunninghamia lanceolata)-白栎(Quercus fabri)灌木林(LCQ)、马尾松(Pinus massoniana)-柯(又名石栎)(Lithocarpus glaber)-檵木针阔混交林(PLL)和柯-红淡比(Cleyera japonica)-青冈(Cyclobalanopsis glauca)常绿阔叶林(LAG)作为一个恢复演替序列, 设置固定样地, 采集0-10、10-20、20-30、30-40 cm土层土壤样品, 测定不同恢复阶段SOC含量(C_SOC)和SOC密度(D_SOC), 通过主成分分析方法和逐步回归分析方法分析影响C_SOC、D_SOC变化的主要因子。结果表明: (1)各土层C_SOC、D_SOC随着植被恢复呈增加趋势, 且LAG显著高于其他3个恢复阶段。LAG 0-40 cm土层C_SOC分别比LVR、LCQ、PLL增加12.5、9.3和4.7 g·kg ^-1, 分别提高了248.5%、113.1%和58.5%; D_SOC分别增加67.1、46.1和32.5 t C·hm ^-2, 分别提高了182.0%、79.7%和45.6%。(2) C_SOC、D_SOC与群落植物多样性指数、群落总生物量、地上部分生物量、根系生物量、凋落物层现存量、凋落物层全氮(N)含量、凋落物层全磷(P)含量、土壤全磷(TP)、土壤有效磷(AP)含量、土壤C/N(除C_SOC外)、C/P、N/P、<0.002 mm黏粒百分含量呈显著或极显著正相关关系, 与凋落物层C/N (除D_SOC外)、凋落物层C/P、土壤pH值和土壤容重呈极显著负相关关系, 表明C_SOC、D_SOC随着植被恢复的变化受到植被因子和土壤因子诸多因子的影响。其中, 土壤C/P、土壤pH值和凋落物层C/P对C_SOC、D_SOC影响显著; 此外, <0.002 mm黏粒百分含量也显著影响着D_SOC, 而土壤C/P对C_SOC和D_SOC影响最显著。植被恢复过程中, 凋落物层C/P和土壤C/P、pH值、质地的变化是影响SOC库变化的重要因素。     

宁南山区植被恢复对土壤团聚体水稳定及有机碳粒径分布的影响

土壤团聚作用和土壤有机碳固定之间密切相关.对宁南山区不同植被恢复措施和年限下土壤团聚体粒径分布及稳定性、土壤团聚体中有机碳及其组分分布进行了研究,探讨了有机碳及其组分对植被恢复的响应.结果表明,不同植被恢复措施下,土壤团聚体粒径分布表现为“V”字分布:＞5 mm和＜0.25 mm这两个粒径的团聚体含量最多,5-2 mm、1-0.25 mm团聚体的含量次之,2-1 mm粒径的团聚体含量最少.坡耕地的平均重量直径(MWD)最低,为1.4,其他植被恢复措施下土壤的平均重量直径MWD在1.9-3.1之间.不同的植被恢复措施下,0-20 cm土层和20-40 cm土层全土有机碳含量在7.4-17.7 g/kg之间、微生物碳含量分布在50.3-664.7 mg/kg之间、腐殖质碳含量在0.9-2.5g/kg之间.胡敏酸碳含量分布在0.2-0.6 g/kg,富里酸碳含量在0.6-1.9 g/kg之间.全土有机碳、微生物碳、腐殖质碳、富里酸碳均为坡耕地最低,其他植被恢复措施的有机碳、微生物碳、腐殖质碳、富里酸碳含量分别是坡耕地的1.1-2.3倍、2.0-8.4倍、1.0-2.0倍、1.2-2.4倍.不同粒径团聚体有机碳相比较,大多呈现中间高两边低的变化趋势,最大值出现在中间粒径,即5-2 mm、2-1 mm、1-0.25 mm这3个粒径.逐步回归表明,5-2 mm团聚体和1-0.25 mm团聚体有机碳含量的提高有助于土壤水稳性团聚体的形成.研究结果表明,植被恢复提高了土壤团聚体有机碳含量,在碳形态上,富里酸碳和微生物生物量碳对不同植被恢复措施的敏感度较高,胡敏酸碳含量则相对稳定.     

不同植茶年限土壤团聚体及其有机碳分布特征

作为土壤结构的基本单元和土壤肥力的重要组成部分,土壤团聚体对土壤的物理、化学和生物特性均有重要影响。试验选取了雅安市名山区中峰乡生态茶园区12—15a、20—22a、30—33a和50a的茶园,研究其土壤团聚体及其有机碳总量、储量和活性组分的分布特征,探究植茶年限对土壤团聚体及其有机碳分布的影响。结果表明:(1)研究区土壤以2 mm粒级团聚体为主,约为70%—80%,且在0—20 cm土层植茶20—22a土壤团聚体含量最高;(2)茶园土壤团聚体有机碳含量随团聚体粒级的减小而增加,最大值出现在0.25 mm粒级团聚体,且在植茶50a时达最高值,0—20 cm土层团聚体有机碳含量均高于20—40 cm,土壤团聚体水溶性有机碳和微生物生物量碳随植茶年限的延长呈先增加后降低的变化趋势,植茶30—33a时含量最高,且小粒级团聚体水溶性有机碳含量较高而微生物量碳较低;(3)土壤团聚体对有机碳的贡献率约有70%来自2 mm粒级团聚体,团聚体有机碳储量随植茶年限延长呈增加的趋势,不同植茶年限0—20 cm土层各粒级团聚体有机碳储量均高于20—40 cm土层,且以0.25 mm粒级团聚体有机碳储量最高。研究结果在一定程度上揭示了不同植茶年限土壤团聚体及其有机碳的分布特征,可为改善区域土壤质量及实施退耕还茶工程提供理论指导。     

Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.     

Allocation of mass and stability of soil aggregate in different types of Nei Mongol grasslands

Aims Soil aggregate is an important component of soil structure, playing an important role in the physical and biological protection mechanism of soil organic carbon (SOC) through isolating SOC from microorganisms. As far as we know, there are few studies, however, on exploring the spatial distribution of soil aggregate at the regional scale. Our objective was to investigate the mass allocation and stability of soil aggregate in different types of Nei Mongol grasslands. Methods We have established 78 sites with a size of 10 m × 10 m across the transect of Nei Mongol grasslands and collected soil samples from different soil depth up to 1 m. We used wet sieving method to separate different sizes of aggregate partition and used mean mass diameter (MMD) and geometric mean diameter (GMD) in order to evaluate the stability of soil aggregate. The two-way ANOVA was used to test the difference of mass percentage and stability of soil aggregate in different grassland types and soil depths. In addition, a linear regression analysis was used to analyze the correlations of mass percentage and stability of soil aggregate with both mean annual precipitation (MAP) and mean annual temperature (MAT). Important findings The results showed that the mass percentages of soil aggregate were highest in meadow steppe, while almost equal in typical steppe and desert steppe. However, no significant patterns were found along the soil depth. The mass percentage of soil aggregate fractions were positively correlated with MAP in all soil layers, but negatively correlated with MAT except the layer of 70-100 cm. For the stability of soil aggregate, at 0-10 and 10-20 cm, MMD and GMD of meadow steppe were significantly greater than those of typical and desert steppes, whereas no significant differences among three grassland types were found for other soil layers. Besides, MMD and GMD in meadow steppe and typical steppe gradually decreased along the soil depth.     

Assessing the Impact of Afforestation on Soil Organic C Sequestration by Means of Sequential Density Fractionation

Afforestation is a prevalent practice carried out for soil recovery and carbon sequestration. Improved understanding of the effects of afforestation on soil organic carbon (SOC) content and dynamics is necessary to identify the particular processes of soil organic matter (SOM) formation and/or decomposition that result from afforestation. To elucidate these mechanisms, we have used a sequential density fractionation technique to identify the transfer mechanisms of forest derived C to soil fractions and investigate the impact of afforestation on SOC sequestration. Surface soil samples from continuous maize crop land (C4) and forest land (C3), which had been established 5, 12 and 25 yr, respectively, on the Northeast China Plain were separated into five density fractions. SOC, nitrogen (N) concentration and δ13C data from the three forests and adjacent cropland were compared. Afforestation decreased SOC concentration in the < 2.5 g cm-3 fractions from 5 yr forest sites, but increased SOC content in the < 2.0 g cm-3 fractions from 25 yr forest sites. Afforestation did not affect soil mass distribution, SOC and N proportional weight distributions across the density fractions. The < 1.8 g cm-3 fractions from 12 and 25 yr forests showed higher C/N and lower δ13C as compared to other fractions. Incorporation of forest litter-derived C occurred from low density (< 1.8 g cm-3) fractions to aggregates of higher density (1.8-2.5 g cm-3) through aggregate recombination and C transport in the pore system of the aggregates. Some forest litter-derived C could transfer from the light fractions or directly diffuse and adsorb onto mineral particles. Results from this study indicate that microaggregate protection and association between organic material and minerals provide major contribution to the SOC sequestration in the afforested soil system.     

The impact of agricultural management on soil aggregation and carbon storage is regulated by climatic thresholds across a 3000 km European gradient

Organic carbon and aggregate stability are key features of soil quality and are important to consider when evaluating the potential of agricultural soils as carbon sinks. However, we lack a comprehensive understanding of how soil organic carbon (SOC) and aggregate stability respond to agricultural management across wide environmental gradients. Here, we assessed the impact of climatic factors, soil properties and agricultural management (including land use, crop cover, crop diversity, organic fertilization, and management intensity) on SOC and the mean weight diameter of soil aggregates, commonly used as an indicator for soil aggregate stability, across a 3000 km European gradient. Soil aggregate stability (−56%) and SOC stocks (−35%) in the topsoil (20 cm) were lower in croplands compared with neighboring grassland sites (uncropped sites with perennial vegetation and little or no external inputs). Land use and aridity were strong drivers of soil aggregation explaining 33% and 20% of the variation, respectively. SOC stocks were best explained by calcium content (20% of explained variation) followed by aridity (15%) and mean annual temperature (10%). We also found a threshold-like pattern for SOC stocks and aggregate stability in response to aridity, with lower values at sites with higher aridity. The impact of crop management on aggregate stability and SOC stocks appeared to be regulated by these thresholds, with more pronounced positive effects of crop diversity and more severe negative effects of crop management intensity in nondryland compared with dryland regions. We link the higher sensitivity of SOC stocks and aggregate stability in nondryland regions to a higher climatic potential for aggregate-mediated SOC stabilization. The presented findings are relevant for improving predictions of management effects on soil structure and C storage and highlight the need for site-specific agri-environmental policies to improve soil quality and C sequestration.     

黄土高原天然次生林植被演替过程中土壤团聚体有机碳动态变化

土壤团聚体物理保护是促进有机碳积累主要机制之一。以黄土高原子午岭林区天然次生林植被演替群落为对象,研究从农田、草地（白羊草,Bothriochloa ischaemum）、灌木林（沙棘,Hippophae rhamnoides）、先锋林（山杨,Populus davidiana）到顶级林（辽东栎,Quercus liaotungensis）5个植被演替阶段0-20 cm土壤团聚体稳定性和团聚体有机碳的动态变化,并分析团聚体有机碳的影响因素。结果表明：土壤团聚体稳定性随着植被演替显著提高（P<0.05）,顶级林的团聚体稳定性最高;土壤有机碳含量和各粒径土壤团聚体（> 2 mm、2-0.25 mm、0.25-0.053 mm、<0.053 mm）有机碳含量均随着植被演替而增加。除草地0.25-0.053 mm团聚体有机碳含量最高外,其他演替阶段均为0.25-2 mm粒径最高。根系生物量、凋落物生物量、微生物生物量碳、团聚体稳定性均与团聚体有机碳含量呈显著正相关关系（P<0.05）。总体而言,长期植被演替有助于团聚体稳定性和团聚体有机碳累积。