果园土壤有机碳及呼吸速率对豆科和禾本科草类的差异反应

在行间长期(连续6年)种植豆科植物毛苕子和禾本科植物黑麦草的苹果园,调查土壤不同深度的有机碳组分和表层土壤呼吸速率日变化。结果表明:间作2种植物后,行间土壤0—20,20—40cm土层总有机碳(TOC)、轻质有机碳(LFOC)、可溶性有机碳(DOC)、颗粒有机碳(POC)、易氧化有机碳(ROC)和微生物生物量碳(MBC)含量均显著提高;在40—60cm土层,仅DOC含量明显提高;0—20cm土层的TOC和全部有机碳组分及20—40cm土层的TOC、DOC、POC和ROC的含量在种植毛苕子后均明显高于种植黑麦草。TOC与LFOC、TOC与POC、DOC与LFOC、DOC与MBC、DOC与ROC、MBC与ROC、ROC与POC、LFOC与ROC之间均呈显著正相关,土壤呼吸速率与LFOC、DOC、MBC和ROC之间也呈显著正相关。种植2种草类均显著提高土壤呼吸速率,并使呼吸速率日变化峰型更突出,其中毛苕子的作用效果比黑麦草更显著。     

不同复垦年限煤矸山重构土壤有机碳及其组分差异

土壤有机碳含量是土壤肥力状况的重要标志之一,其活性组分对田间管理措施反映敏感。因此,分析煤矸山复垦重构土壤有机碳含量及其组分差异对于揭示土壤碳库变化、指导复垦地田间管理措施的实施有重要意义。本研究以山西省霍州曹村煤矸山复垦后5a(R-5a)、7a(R-7a)和9a(R-9a)的果园为对象,通过与当地原地貌果园(CK)对比,分析了3种复垦样地土壤总有机碳(TOC)及其组分可溶性有机碳(DOC)、微生物生物量碳(MBC)、轻组有机碳(LFOC)和重组有机碳(HFOC)的差异,以及与土壤环境因子间的关系。结果表明:①随复垦年限的增加,3种复垦样地土壤TOC、LFOC和HFOC含量均呈先增后减趋势,DOC含量呈增加趋势,MBC含量呈先减后增趋势;但与CK相比,3种复垦样地土壤TOC、DOC、MBC、LFOC和HFOC含量均明显偏低。②DOC/TOC和MBC/TOC在R-7a样地中最低,LFOC/TOC随复垦年限的增加呈增加趋势,HFOC/TOC呈减少趋势,表明土壤中更多的有机碳从稳定态转变为活性态。③土壤全氮、全磷、碱解氮、碳氮比、黏粒含量、pH和含水量均不同程度影响有机碳含量,其中全氮、全磷、黏粒含量和土壤pH为关键因子。     

华北平原农业土地利用方式对土壤水稳性团聚体分布特征及其有机碳含量的影响

以在华北平原具有典型代表的禹城市为研究区域,通过对该地区主要农业土地利用类型的土壤进行采样和分析,系统研究和比较了不同农业土地利用方式下土壤团聚体分布特征及其有机碳的含量.研究结果表明,不同农业土地利用方式对土壤水稳性团聚体分布和组成有着显著(p＜0.05)影响.与传统粮田比较,果园和苜蓿地土地利用方式明显提高了土壤中＞2mm和＞0.25 mm水稳性团聚体的比例,同时也降低了0.5～1 mm和0.25～0.5 mm粒级的水稳性团聚体比例.果园土壤平均重量直径(MWD)和几何平均直径(GMD)较传统粮田分别提高了70.5%和108.4%.设施农业用地降低了土壤较大粒级水稳性团聚体的比例和团聚体的稳定性.果园表层土壤总有机碳(TOC)、轻组分有机碳(LFOC)和颗粒有机碳(POC)较传统粮田分别提高了54.8%、136.4%和124.6%.设施农业用地则显著(p＜0.05)降低了土壤TOC、LFOC和POC的含量.不同粒级团聚体中土壤有机碳含量皆以果园为最高,苜蓿地次之,设施农业用地土壤不同粒级团聚体中有机碳平均含量为最小,这种差异在＞2mm和0.5～1 mm的团聚体中表现最为明显.相关和回归分析表明,＞0.25 mm团聚体含量、MWD和GMD与土壤TOC、LFOC和POC皆呈现显著正相关性,其中与POC之间的相关关系达到极显著水平(p＜0.01),其相关系数,分别为0.923**、0.912**和0.897**.     

小兴安岭森林沼泽湿地土地利用变化对土壤活性碳组分的影响

[目的]以小兴安岭森林沼泽为研究对象,研究湿地经过人类活动开垦为农田,和排水造林,以及弃耕地的土壤活性碳组分,为深入了解土壤活性碳组分动态变化及其可能影响全球变暖的机制研究提供科学依据。[方法]选择小兴安岭森林沼泽湿地4种土地利用方式(天然沼泽、排水湿地、弃耕地、农田)为研究对象,在野外调查和室内分析的基础上,对比分析土壤有机碳(SOC)、溶解性有机碳(DOC)、微生物碳(MBC)、易氧化碳(EOC)、轻组有机碳(LFOC)、颗粒有机碳(POC)的含量变化和比例关系,并用相关分析法分析土壤有机碳各活性组分之间的关系。[结果]不同土地利用方式土壤有机碳含量随土层深度增加而降低,总体上,在土壤剖面上,天然沼泽的SOC含量大于其他土地利用方式,其他3种土地利用方式之间差异不显著(p0.05)。不同土地利用方式下土壤活性碳组分(DOC,MBC,EOC,POC,LFOC)含量在垂直分布上均呈现出随土壤深度的增加而降低的趋势,土壤活性碳组分含量顺序总体上呈:天然沼泽排水湿地弃耕地农田。4种土地利用类型土壤DOC占SOC的比例,在垂直剖面上,无明显规律变化。土壤SOC含量与DOC,MBC,EOC,POC,LFOC之间的相关性均达到显著水平(p0.05)。土壤DOC和MBC之间呈显著性正相关(p0.05),LFOC和POC之间呈极显著性正相关(p0.05),EOC与其他活性碳组分相关性均不显著(p0.05)。[结论]土地利用变化会对小兴安岭森林沼泽土壤活性碳组分产生影响,应该合理开发小兴安岭森林沼泽湿地。     

长期不同施肥管理对稻田土壤有机碳库特征的影响

以19年的长期定位施肥土壤为材料,研究不同施肥管理:不施肥(CK),施无机肥(N、NP、NK、NPK),施有机肥(OM),有机-无机配施(F、F')下稻田耕层土壤有机碳库组分及含量变化.结果表明:不同施肥方式对土壤有机碳及其组分有显著影响,土壤总有机碳(TOC)变化趋势为有机无机配施(平均12.34g/kg)>单施有机肥(平均12.15 g/kg)>无肥(平均10.56 g/kg)>化肥(平均9.78 g/kg);有机肥和化肥配施土壤徽生物量碳(SMBC)、水溶性有机碳(WSOC)、轻组有机碳(LFOC)及SMl3(2/TOC、WSOC/TOC、LFOC/TOC均显著高于单施化肥土壤的.与不施肥相比,化肥、有机肥的施用均显著增加了土壤重组有机碳(HFOC)和HFOC/TOC,其中,化肥的施用更有利于土壤重组有机碳(LFOC)的积累.单施有机肥或有机无机配施显著增加了较大粒级(>0.25 mm)水稳性团聚体及其TOC含量,而单施化肥则显著增加了较小粒级(<0.25 mm)及其TOC含量.因此,长期施用有机肥,特别是有机肥与无机肥配施能提高土壤活性碳含量和土壤团聚体稳定性,从而保持和提高了土壤质量和持续生产力.     

岩溶区不同土地利用方式对土壤有机碳及其组分的影响

选择重庆市中梁山岩溶槽谷中5种土地利用方式(弃耕地、草地、菜地、橘园地和林地)为研究对象,在野外调查和室内分析的基础上,采用方差分析法对比分析了土壤总有机碳、溶解性有机碳、易氧化性碳、轻组有机碳、颗粒有机碳及矿物结合态有机碳的含量变化,并用相关分析法分析了土壤有机碳各组分之间的关系。结果表明:不同土地利用方式土壤总有机碳(TOC)含量随土层深度增加而降低,上下层土壤TOC含量在2.69~13.88g/kg之间,不同植被覆盖类型、耕作方式和施肥是影响土壤TOC分布的重要因素。不同土地利用方式下有机碳各组分(DOC、EOC、LFOC、POC和MOC)含量在垂直分布上均呈现出随土壤深度的增加而降低的趋势,但不同土地利用方式之间的差异较大,主要与植被类型、施肥管理、耕作方式以及人为干扰有关。弃耕地含量均比较低,林地和草地受人为干扰较少含量较高,菜地土壤受施肥和翻耕影响,上下层土壤含量差异不大,橘园地上下层土壤5种有机碳组分含量差异均是最大的。弃耕地受之前耕作影响,弃耕时间短,有机质输入量少,POC/MOC值相对较低,菜地受施肥和翻耕影响上下层土壤POC/MOC值相对比较稳定,草地、橘园地和林地0—20cm土壤POC/MOC值均高于20—40cm,下层土壤有机碳比较稳定。0—20cm和20—40cm土壤有机碳及组分之间存在相关关系,0—20cm土壤TOC与LFOC和MOC之间呈显著正相关,LFOC与POC之间也呈现显著正相关;20—40cm土层,土壤TOC与EOC、LFOC和MOC之间呈显著正相关性,MOC与TOC以及EOC和LFOC均呈现正相关性,LFOC与POC之间关系由表土层的显著正相关转变为极显著正相关。下层土壤比上层土壤有机碳稳定性强,尤其是土壤TOC、LFOC和MOC能够敏感地反映土壤碳库的变化,可以作为土壤有机碳稳定性的敏感性指标。     

高潜水位煤矿区完整复垦周期的土壤碳演变特征

在济宁高潜水位矿区的充填复垦区内,分析完整复垦周期的土壤碳演变特征。通过从塌陷地(M1)、复垦年限为0,3年的复垦耕地(M2、M3)以及正常耕地(M4)采集土样并检测土壤全碳(TC)、有机碳(SOC)、有机碳密度(SOCD)、有机碳组分(WDOC、POC、MOC、LFOC、HFOC、MBC)及稳定性同位素(δ13C),研究不同复垦周期阶段下土壤碳空间特征的变化情况。研究表明:(1)塌陷地的TC、SOC、SOCD和各有机碳组分含量较低。经过复垦措施期后,SOCD在复垦当年就恢复到正常耕地的水平,TC和SOC含量随着复垦年限不断增加。WDOC、POC、LFOC、MBC等组分恢复速率较快,MOC和HFOC恢复周期较长。(2)TC、SOC、SOCD和各有机碳组分的恢复速率随着土层深度的增加逐渐减缓。(3)复垦土壤的δ13 C均值在不同深度间的差异较小,而标准差和变异系数偏大,说明复垦土壤层次不明显且同一层次间土壤来源混乱。从整个复垦周期来看,复垦工作虽具有"碳汇"作用,但会对土壤层次造成破坏。建议进行塌陷地治理时,尽量采用对土层扰动较小的复垦方式,对表土剥离和回填工序进行优化和监督,从而达到更好的恢复效果。     

土壤活性有机碳不同组分对保护性耕作的响应

为分析保护性耕作措施对旱作农田土壤活性有机碳的影响以及对土壤有机碳的敏感响应指标进行归纳总结,探讨适合西南丘陵区紫色土农田生态系统在保护性耕作下的有机碳分组方法和活性碳有效指标提供科学依据,进而为土壤固碳减排、提高土壤质量奠定理论基础。在重庆北碚西南大学试验农场对平作(traditional tillage,T)、垄作(ridge tillage,R)、平作+秸秆覆盖(traditional tillage+straw mulching,TS)和垄作+秸秆覆盖(ridge tillage+straw mulching,RS)4种处理下、西南紫色土丘陵区"旱三熟"种植模式下的土壤总有机碳(Total soil organic carbon,TOC)、颗粒有机碳(Particulate organic carbon,POC)、水溶性有机碳(Dissolved organic carbon,DOC)、易氧化有机碳(Readily oxidized organic carbon,ROC)、可矿化有机碳(Mineralized organic carbon,MOC)以及微生物量有机碳(Microbial biomass carbon,MBC)进行了测定和分析。结果表明,秸秆覆盖不仅增加了表层土壤的TOC,也有利于20cm以上的耕层TOC的增加,而垄作对TOC的影响很小。秸秆覆盖处理下各土层POC均显著高于传统耕作,而单纯的垄作处理对5cm以下土层的POC没有影响,却显著地降低了0-5cm土层POC含量。DOC含量随土层的加深逐渐降低,且不同处理的差异主要出现在表层0-5cm土壤中,5cm以下土壤的DOC含量不易受到耕作措施的影响。TS、RS与对照相比均显著提高0-5cm土层的ROC含量,R与T之间差异不显著,秸秆覆盖只对土壤表层ROC影响很大。T、R、TS、RS处理全年日均土壤呼吸分别为1.84,1.65,2.22,2.21μmol/(m2·s),年土壤呼吸总量分别为696.31,624.41,840.12,836.33g/m2。与对照相比,垄作对土壤呼吸的影响不大,秸秆覆盖加速了土壤呼吸速率。秸秆覆盖处理显著提高了MBC含量,而垄作处理的影响不大。MBC与土壤呼吸速率呈显著正相关,R2介于0.756~0.919之间,P0.05。通过对土壤总有机碳及各组分之间的相关分析分析结果显示,TOC、POC、DOC和ROC在0-5cm土层表现显著的正相关,其他土层没有明显的规律。TOC、POC、DOC、ROC和MBC对耕作措施的响应变化基本一致,表现为对垄作处理的响应不显著,而受秸秆覆盖的影响很大,其中颗粒有机碳占总有机碳的35.74%~49.66%,二者显著正相关,可以作为反映土壤有机碳变化的敏感指标。     

不同利用方式黑钙土有机碳组分剖面分布特征

为研究土地利用方式对土壤碳库的影响,以东北黑钙土区的天然草地、人工林地和耕地为研究对象,采集0—100 cm土体中不同土层样品(A、AB、B_k、BC和C层),测定不同利用方式土壤有机碳(SOC)、水溶性有机碳(WSOC)、轻组有机碳(LFOC)和重组有机碳(HFOC)含量。结果表明:3种利用方式SOC、LFOC和HFOC主要分布在A层,但草地WSOC含量在B_k层最多(0.27 g/kg)。由A~C层,3种土地利用方式土壤有机碳及组分有机碳含量总体均呈减少趋势,但减少程度明显不同,天然草地缓慢减少,人工林地急剧减少,耕地逐渐减少。草地转换为林地和开垦为耕地后,均造成土壤有机碳及组分有机碳含量减少,WSOC减少34%和48%,LFOC减少20%和37%,HFOC减少7%和5%,SOC减少10%和16%。草地转换为林地和开垦为耕地后,WSOC/SOC、LFOC/SOC显著降低,但HFOC/SOC却提高,说明草地被开垦后活性有机碳含量快速下降。土地利用方式和土层对SOC、WSOC和LFOC具有显著影响,且对SOC和HFOC叠加效应较强。同时,土壤理化性质也在一定程度上影响着SOC、WSOC、LFOC和HFOC。应制定合理土地利用管理政策,保护自然草地免遭破坏,减少土壤有机碳流失,发挥草地生态系统碳固存的重要作用。     

行间生草对核桃园土壤养分、有机碳组分及酶活性的影响

探讨行间生草对核桃园土壤养分、有机碳组分及酶活性的影响,为核桃园的土壤管理提供理论依据和基本资料。以汾阳核桃园土壤为研究对象,设置大豆、毛苕子、箭筈豌豆、黑麦草、自然生草、清耕6个处理,每个处理重复3次,研究核桃园不同土壤深度(0～60 cm)的土壤养分、有机碳组分、酶活性的变化特征。结果表明,随着土层深度的增加,土壤养分含量、酶活性、有机碳组分含量呈降低的趋势,生草对土壤表层(0～20 cm)的影响最大。在0～20 cm土层,与清耕相比,黑麦草处理显著增加土壤有效磷、速效钾的含量,分别为清耕的1.18、1.15倍;豆科植物(箭筈豌豆、毛苕子、大豆)处理下的土壤全氮、碱解氮含量分别提高了17.0%～19.3%、9.7%～11.7%。种植黑麦草的土壤总有机碳(TOC)、轻质有机碳(LFOC)、可溶性有机碳(DOC)、颗粒有机碳(POC)、易氧化有机碳(ROC)含量分别提高了21.3%、46.0%、40.1%、12.8%、37.0%;在20～40 cm土层,种植黑麦草的TOC、LFOC、DOC、POC、ROC含量分别提高了16.1%、46.0%、22.7%、8.2%、25.3%。豆科植物(大豆)处理显著提高土壤脲酶的活性(P<0.05),提高了16.0%;黑麦草处理显著提高了土壤的蔗糖酶、磷酸酶、过氧化氢酶的活性(P<0.05),分别提高了10.2%、32.5%、121.4%。因此,要根据土壤肥力状况,选择不同的生草进行种植。     

肥料减施条件下水稻土壤有机碳组分对紫云英—稻草协同利用的响应

研究紫云英与稻草不同利用模式在化肥减量条件下稻田表层土壤有机碳及其活性组分的差异,为合理利用有机物料调控土壤肥力提供参考。采用湖南省南县连续5年(2011—2015年)大田定位试验的方法,研究了减量施肥下洞庭湖地区紫潮泥双季稻田0-20cm表层土壤总有机碳(TOC)、微生物量碳(MBC)、可溶性有机碳(DOC)、易氧化有机碳(EOC)和轻组有机碳(LFOC)对紫云英与稻草不同利用模式的响应。结果表明:与不施肥(CK)相比,单施化肥(F100)TOC、MBC、DOC、EOC和LFOC的含量均有显著上升,增幅分别为12.2%,19.5%,18.6%,11.3%和100.9%。化肥减量20%条件下紫云英与稻草利用模式更有助于有机碳组分的积累,其中晚稻留高桩还田冬种紫云英(F80+HR+A)处理效果最为突出,与CK相比,增幅分别为18.3%,47.2%,24.1%,20.0%和204.0%。减量施肥下紫云英单独利用相比稻草单独还田更益于MBC、DOC与EOC的积累,增幅分别为9.8%,4.0%和0.6%,土壤LFOC则相反,下降了34.8%。敏感指数分析表明土壤有机碳及其活性组分中LFOC对土壤质量变化最为敏感,其次是MBC,二者可作为衡量土壤质量状况的良好指标。F80+HR+A各有机碳组分敏感性均显著高于F100处理。相关性分析表明水稻产量、土壤有机碳及其组分之间均呈极显著相关(p0.01)。综上所述,化肥减量条件下不同紫云英与稻草利用模式对紫潮泥稻田土壤有机碳及其活性组分含量的影响存在明显差异,稻草还田能够显著提高土壤总有机碳和轻组有机碳的含量,紫云英的利用则有利于土壤微生物量碳、可溶性有机碳和易氧化有机碳的积累,而紫云英与稻草协同利用模式,尤其是晚稻留高桩还田冬种紫云英更有助于土壤有机碳及其活性组分的积累。     

碳氮添加对雨养农田土壤全氮、有机碳及其组分的影响

为探明碳氮添加4年后,土壤全氮、有机碳及其组分(可溶性有机碳、微生物量碳、轻组和重组有机碳)的变化特征,依托布设于甘肃省定西市安定区李家堡镇的不同碳源配施氮素田间定位试验,涉及秸秆、生物质炭、氮素3个因素,秸秆设置为不施、施用秸秆2水平;生物质炭为不施和施用生物质炭2个水平;氮素设置为不施氮、施纯氮50 kg/hm^2、施纯氮100 kg/hm^2 3个水平,共9个处理。结果表明:不同处理下土壤全氮、有机碳及其组分的含量均随土层的加深而降低。添加生物质炭对土壤全氮、有机碳及其组分均具有不同程度的提升效应。添加秸秆对土壤全氮、有机碳和可溶性有机碳、微生物量碳、轻组有机碳均具有显著提升效应,仅在0-5 cm土层对重组有机碳有显著提高。添加氮素可显著提升土壤全氮、有机碳和可溶性有机碳、微生物量碳、轻组有机碳含量。较其他处理,添加生物质炭对土壤全氮、有机碳和重组有机碳的提升效应最高,添加秸秆对可溶性有机碳、微生物量碳、轻组有机碳的提升效果最优。从提升土壤质量的角度出发,推荐秸秆配施氮素模式,该模式下土壤碳素有效性高、易于被微生物利用,有利于作物生长。从提高土壤固碳角度考虑,推荐生物质炭配施氮素模式,该模式有利于碳的封存。     

长期施肥对土壤活性有机碳指标的影响

以棕壤长期肥料定位试验地土壤为试材,研究和对比了不同施肥处理耕层土壤各项活性有机碳指标对长期施肥的响应。研究结果表明,长期施用化肥、有机肥以及有机肥配施化肥均显著改变了土壤总有机碳(TOC)含量。与此同时,土壤的轻组有机碳(LFOC)、易氧化有机碳(ROC)和微生物量碳(MBC)含量亦对长期施肥产生与TOC基本一致的响应。相关分析结果表明,土壤的LFOC、ROC和MBC可以作为长期施肥对土壤TOC影响的评价指标,且三者的指示灵敏度依次为MBC>LFOC>ROC。     

黄土高原半干旱区不同土地利用下的土壤有机碳、碳组分和土壤营养

Cropland (CP), native grassland (NG) and two shrub land treatments which were converted from cropland in 1985:seabuckthorn (Hippophae rhamnoides L. ) (ST), and branchytamarisk (Tamarix ramosissima) (BT) were investigated to evaluate effects of land use conversion on soil organic carbon (SOC) and soil nutrients in the semi-arid region of the Loess Plateau of China. Total organic carbon (TOC), light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), total N (TN), nitrate nitrogen (NO₃^--N) and nitrite nitrogen (NO₂^--N), ammonium nitrogen (NH₄⁺-N), total P, and available P (AP) were measured. The results showed that SOC in NG, ST and BT were 12.7%, 27.7% and 34.8% higher than that of the cropland, respectively. LFOC, light fraction (LF) dry matter, ratio of TOC to TN (C/N) and the ratio of TOC to AP (C/P) were higher in the shrub land or native grassland than in the cropland. Cropland had the highest TN, the sum of NO₃^--N and NO₂^--N, TP and AP due to the use of chemical fertilizers. TOC significantly correlated with LFOC, HFOC and C/N. LFOC significantly correlated with dry matter of the LF and C/N. TN, the sum of NO₃^--N and NO₂^--N and AP were significantly negatively correlated with TOC and LFOC. Therefore, land use conversion from cropland to shrub land, or maybe grassland, contributed to SOC sequestration and improved soil nutrients stabilization.     

地膜覆盖对黄土高原旱作春玉米田土壤碳氮组分的影响

基于2年田间试验,研究了地膜覆盖对旱作春玉米田土壤有机碳、全氮及其组分的影响,试验包括地膜覆盖玉米田、无覆盖玉米田和裸地休闲3个处理,分层测定了0—40cm土层有机碳、全氮、颗粒有机碳氮、潜在矿化碳氮和微生物量碳氮含量。结果表明:在0—40cm土层,各处理间土壤有机碳和全氮含量均无显著差异。与不覆盖相比,地膜覆盖处理0—40cm土层颗粒有机碳氮及其所占比例分别降低了29.0%,33.3%,29.9%,35.7%;0—10cm土层潜在可矿化碳及其所占比例分别降低了17.8%和16.1%,潜在可矿化氮和微生物量碳及其所占比例无显著差异,但在0—10cm土层地膜覆盖微生物量氮含量及其所占比例分别较不覆盖处理提高了10.6%和10.5%(p0.05)。与裸地休闲相比,无覆盖处理0—40cm土层潜在可矿化碳氮分别提高了12.8%和14.7%,地膜覆盖处理则分别提高了7.8%和6.5%(p0.05),但种植玉米降低了微生物量碳氮含量及其所占比例。在0—40cm土层覆盖与否对潜在可矿化碳氮和微生物量碳氮影响不显著。总体来看,地膜覆盖能够在一定程度上提高表土微生物量氮组分及其所占比例,但显著降低了中活性碳氮组分含量及其比例,不利于长期的土壤碳氮固定。     

Organic carbon and its fractions in paddy soil as affected by different nutrient and water regimes

As an essential indicator of soil quality, soil organic carbon (SOC) and its fractions play an important role in many soil chemical, physical, and biological properties. A 4-year field experiment was conducted to determine the effects of different nutrient and water regimes on paddy soil organic carbon quality by measuring the major SOC fractions. Four nutrient regimes were compared: (i) control; (ii) chemical fertilizers only (CF), (iii) combined application of chemical fertilizers with farmyard manure (FYM) (CM), and (iv) combined application of chemical fertilizers and wheat straw (CS). Two soil water regimes included continuous waterlogging (CWL) and alternate wetting and drying (AWD). The total organic carbon (TOC) and total nitrogen (TN) in paddy soil were 40–60% and 37–67% higher in the combined organic sources and chemical fertilizers treatment against the sole chemical fertilizers treatment (CF), especially under continuous waterlogging (CWL). By fractionalizing SOC, it was observed that, under the water regimes of CWL, easily oxidizable carbon (EOC), particulate organic carbon (POC), light fraction organic carbon (LFOC), microbial biomass carbon (MBC), and mineralizable organic carbon (MNC) in the organically treated paddy soil were significantly (P<0.05) lower, as compared with alternate wetting and drying (AWD). Especially for CM treatment, EOC, POC, LFOC, MBC, and MNC in the paddy soils under the regime of CWL were 23.5%, 32.7%, 16.3%, 56.8% and 25.1% lower than those by AWD, respectively. The proportions of EOC, POC, LFOC, MBC and MNC as a percent of TOC in the CWL were lower than those in the AWD, especially for the CM treatment. In the water regime of CWL, no significant differences were seen in the corresponding proportion of all the investigated organic fractions to soil total organic carbon (TOC) among the three fertilization treatments, whereas in the AWD, the corresponding proportions of different carbon fractions to TOC in the organic fertilizer treatments were significantly (P<0.05) higher than those in the chemical fertilizer treatment. Under continuous waterlogging, the proportion of soil water stable aggregate >250 μm (WSA) decreased by 42–45% and clay dispersion ratio (R_CD) increased by 12–38%, as compared to the water regimes of AWD, when FYM or wheat straw was incorporated into paddy soil. Correlation analysis showed that, under the water regimes of AWD, WSA was significantly and positively related to EOC, LFOC and POC with the coefficients (r) of 0.822, 0.889, 0.912 (P<0.01), respectively. R_CD was negatively correlated to EOC, LFOC and POC with the r=−0.796, −0.854, and −0.897 (P<0.01), respectively, under AWD. Under the water regimes of CWL, there were no significant (P<0.05) correlations between WSA as well as R_CD and any organic carbon fraction except POC.     

Effect of tillage and rotation on organic carbon forms of chernozemic soils in Saskatchewan

The effects of selected tillage and rotation systems on soil organic carbon and its fractions were studied on Chernozemic soils in south‐western and east‐central Saskatchewan. After practicing a no‐till fallow unfertilized‐wheat rotation for 7 years on an Orthic Brown Chernozem in south‐western Saskatchewan, total soil organic carbon (TOC) in the 0 – 5 cm and 5 – 10 cm layers was slightly lower than the tillage fallow‐unfertilized wheat comparable treatment. However, light fraction of organic carbon (LFOC) was similar in the two treatments. Comparison of the tillage fallow‐unfertilized wheat to a treatment involving conversion to a fertilized continuous cropping system for 10 years showed TOC increased slightly in the two depths and LFOC increased by 24 % and 29 % in the 0 – 5 cm and 5 – 10 cm layer, respectively, of the continuous cropping treatment. Microbial biomass carbon (MB‐C) was increased significantly at the 5 – 10 cm depth. After conversion of fallow‐wheat to alfalfa as perennial forage for 10 years, TOC increased by 80 % and 27 %, LFOC by 245 % and 286 %, and HFOC by 63 % and 20 % at 0 – 5 cm and 5 – 10 cm depths, respectively, compared to the tilled cereal‐fallow system. Meanwhile, water soluble organic carbon (WSOC) was not affected but MB‐C increased significantly. In an Orthic Black Chernozem in east‐central Saskatchewan, the depletion and restoration of organic carbon was observed when native sod was changed into cropland and then back to grassland. For example, the TOC of cropland under cereal‐fallow rotation for 62 years decreased by 42 % and 33 % at 0 – 5 cm and 5 – 10 cm depths, respectively, compared to native sod. The LFOC decreased by 79 % and 74 % in the layers, and reductions in WSOC and MB‐C were even greater. After cropland was re‐seeded to grassland for 12 years, the concentration of total organic carbon was increased by 16 % and 22 % while the mass of organic carbon was the same as the cropland in the two layers. The LFOC and MB‐C amounts in the grass seed‐down were double that of the cropped land, but the amounts of TOC, LFOC, and MB‐C in grass seed‐down were still significantly lower than the native sod.     

Soil carbon pool and effects of soil fertility in seeded alfalfa fields on the semi-arid Loess Plateau in China

The dynamics of the soil organic carbon pool and soil fertility were studied in soils with different number of growing years of alfalfa (Medicago sativa L.) in the semiarid Loess Plateau of China. The soil water content and soil water potential decreased and the depth of desiccated layers grew with the number of growing years of alfalfa. The soil organic C (SOC) cannot be enhanced on short timescales in these unfertilized and mowed-alfalfa grasslands in the topsoil, but the light fraction of organic C (LFOC), soil microbial biomass C (MBC) and microbial biomass N (MBN) all increased with the number of growing years. When alfalfa had been growing for more than 13 yr, the soil MBC increased slowly, suggesting that the MBC value is likely to reach a constant level. SOC, soil total P (STP), available P (AvaiP) and the ratio of SOC to soil total N (C/N) all decreased monotonically with the growing years of alfalfa up to 13 yr and then increased. SOC was significantly positively correlated with STP, AvaiP, soil total C (STC) and soil total N (STN) (R=0.627**, 0.691**, 0.497*, 0.546*, respectively). MBC and LFOC were significantly positively correlated with the number of growing years of alfalfa (R=0.873*** and 0.521*, respectively), and LFOC was more sensitive to vegetation components, degree of cover and landform than to the number of years of growth. SOC showed a significant negative correlation with LFOC/SOC and MBC/SOC (R=−0.689**, −0.693**, respectively). A significant positive correlation exists between MBC and soil inorganic C (SIC). LFOC, MBC, LFOC/SOC and MBC/SOC were all significantly positively correlated with each other. Therefore, practices that involve water-harvesting technologies and add residues and phosphate fertilizer to soils should be promoted to improve soil nutrients and hydration and to postpone the degradation of alfalfa grasslands under long-term alfalfa production.