土壤剖面微生物数据可作为温带草原退化的特征指标

正我国温带草原分布广泛,是欧亚大陆温带草原植被类型的重要组成部分,具有重要的生态和经济价值[1],而其退化已成为我国草原牧业和生态可持续发展的主要瓶颈[2]。土壤微生物在土壤形成和演化过程中起着不可替代的作用,是土壤中物质转化和养分循环的驱动力。土壤微生物生物量和土壤微生物胞外酶活性则是土壤生态系统结构和功能演替的特征指示物[3]。目前,对草地退化的研究大部分集中于退化现状及其驱动力,植物群落结构及其稳定性,生物量变化及其土壤肥力水平,而相关的土壤微生物特     

土壤活性有机质及其与土壤质量的关系

活性有机质是土壤的重要组成部分 ,主要包括溶解性有机碳、微生物生物量、轻组有机质。它在土壤中具有重要作用 :(1)可以表征土壤物质循环特征、评价土壤质量 ,可以作为土壤潜在生产力以及由土壤管理措施引起土壤有机质变化的早期指标 ;(2 )在养分周转中起重要作用 ,是植物的养分库 ,可以提供植物所需要的养分如氮、磷、硫等 ;(3)能稳定土壤结构 ,对维持团粒结构稳定性有重要作用。从土壤养分、土壤物理、化学性质方面讨论了活性有机质与土壤质量的关系。土壤中的溶解性有机碳、微生物生物量碳氮含量与土壤有机碳、全氮和碱解氮等物质的含量呈正相关。活性有机质受土壤质地、含水量、温度等因素影响 ,与土壤酸碱度、阳离子交换量等也有关。土壤微生物生物量碳和微生物量 C/有机碳比与土壤粘粒、粉粒含量呈正相关、与砂粒含量呈负相关     

喀斯特地区土壤表层CO2释放通量的影响因素Ⅰ:规律

测定了贵州喀斯特地区土壤表层CO2释放通量,同时还测定了土壤微生物生物量碳以及土壤可溶性有机质含量和土壤湿度。研究表明,贵州喀斯特地区全年土壤表层CO2释放通量与温度变化呈正相关关系,与土壤微生物生物量碳呈负相关关系;当温度＞20℃时,土壤表层CO2释放通量与土壤湿度呈正相关,与土壤可溶性有机碳含量呈负相关。     

鸡粪对铜污染土壤微生物生物量碳的影响

采用室内恒温好气培养法,研究了2．0％、4．0％、6．0％添加量鸡粪对铜污染土壤微生物生物量碳的影响。结果表明：向土壤中施入2．0％、4．0％、6．0％的鸡粪后,有机碳总量比对照分别增加了0．85、1．49、2．04倍,而可浸提有机碳含量分别增加了3．13、5．70、8．23倍,鸡粪的添加促进了土壤可浸提有机碳含量的增加,土壤可浸提有机碳含量和鸡粪添加量呈正相关（r=0．994^＊＊,n=12）。各处理土壤有机碳和可浸提有机碳含量随培养时间逐渐下降,90d后有机碳含量趋于稳定;土壤可浸提有机碳含量120d后趋于稳定。鸡粪显著促进了土壤微生物生物量碳含量和土壤微生物商的增加（P〈0．01）,与对照相比,2．0％、4．0％和6．0％鸡粪处理土壤微生物生物量碳含量分别增加2．88、5．19、6．98倍,土壤微生物生物量碳（SMBC）含量和土壤微生物商（SMQ）都与鸡粪的添加量呈正相关（rSMBC=0．998^＊＊,rSMQ=0．858,n=12）。在培养过程中士壤微生物生物量碳与土壤微生物商都呈现先下降再升高再下降的趋势。在培养试验的早期,鸡粪的添加有利于缓解铜污染土壤微生物生物量碳的下降,添加鸡粪的处理在培养的第10d后土壤微生物生物量碳含量开始上升,而对照则在30d以后才开始上升;添加鸡粪的处理在培养前期微生物生物量碳的最大下降幅度分别为：2．0％处理（52．74％）、4．0％处理（45．92％）、6．0％处理（55．52％）,而对照的下降幅度为92．02％。培养后期添加鸡粪处理土壤微生物生物量碳仍保持较高的水平。     

大豆土微生物生物量碳的测定方法

微生物生物量碳在土壤养分转化、贮存、释放中起重要作用,其研究方法一直受到同外学者的重视。我国在这方面的研究刚刚起步。本文就大豆土的微生物生物量碳（BC）的测定方法进行了实验,并对实验条件、进程进行了讨论,以提出一个适宜于大豆上微生物BC的测定方法。1材料与方法1．1供试土壤中层黑土土壤来自长春农牧大学试验田和解放军总后嫩江基地。有正茬、重茬三年、重茬2年、迎茬、重茬7年8个土样。1．2土壤微生物生物量联的测定采用Vance等（1987）的FE怯（。j。湿上用氯仿熏蒸24h后,以0．SVK2SO4抽提,同时设对照。抽提的有机碳…     

祁连山3种典型生态系统土壤微生物活性和生物量碳氮含量

 测定分析了祁连山高寒草甸、山地森林和干草原土壤中微生物活性、生物量碳氮含量。结果显示：就土壤微生物生物量碳含量,森林比干草原和高寒草甸中分别高60%和120%以上,干草原比高寒草甸中高40%以上(p<0.05)。就土壤微生物生物量氮含量,0～5 cm土层,森林比高寒草甸和干草原中分别高64%和111%以上,高寒草甸比干草原中高29%;5～15 cm土层,森林比干草原和高寒草甸中分别高7%和191%以上,干草原比高寒草甸中高171% 以上(p<0.05)。森林和干草原中土壤微生物生物量碳比例比高寒草甸中高32%以上,0～5和5～15 cm土层,森林和干草原中土壤微生物生物量氮比例比高寒草甸中高150%以上（p<0.05）。就土壤微生物活性,0～5和5～15 cm土层,森林和高寒草甸比干草原中高26%以上;15～35 cm土层,森林比干草原和高寒草甸中高28%以上 (p<0.05)。土壤微生物生物量碳氮含量与有机碳含量及微生物生物量氮含量和比例与微生物生物量碳含量和比例呈现正相关（r²>0.30,p<0.000 1）。土壤微生物生物量氮含量、微生物生物量碳氮含量比例、微生物活性与土壤pH值呈显著负相关,土壤微生物生物量碳氮含量及其比例、微生物活性与土壤湿度呈正相关。说明祁连山3种生态系统土壤中微生物生物量和活性受气候要素、植被、有机碳、pH值和湿度等因素 的共同影响。     

内蒙古温带草原土壤微生物生物量碳的空间分布及驱动因素

【目的】探索内蒙古温带草原土壤微生物生物量碳的空间分布特征以及驱动因素。【方法】在内蒙古自治区境内沿着年均温、年降水梯度选择17个草原样点,在土壤剖面上分0-10 cm、10-20 cm、20-40 cm、40-60 cm、60-100 cm五层,分别采集土壤样品,测定土壤微生物生物量碳以及主要的环境和生物影响因子,分析不同草地类型以及不同土壤深度土壤微生物生物量碳的差异,探索非生物因子和生物因子对土壤微生物量碳的影响。【结果】草甸草原土壤微生物量碳最高,典型草原次之,荒漠草原最低。在0-10 cm土壤中,草地类型间的微生物量碳变异系数高于草甸草原和典型草原,低于荒漠草原;在0-100 cm土壤中,草甸草原样点间的微生物量碳的变异系数低于典型草原和荒漠草原。土壤微生物量碳与年降水、土壤含水量、粘粒含量、土壤养分元素、地上生物量、地下生物量呈显著正相关,与年均温和土壤p H值呈显著负相关关系。随着土壤深度的增加,土壤微生物量碳显著减少,非生物因子与微生物量碳的相关性减弱,草地类型间以及同一草地类型不同样点间的变异系数增加。0-10 cm土壤微生物量碳与10-40 cm土壤微生物量碳的相关指数高于0.5,与40-100 cm的土壤微生物量碳的相关指数小于0.3。【结论】内蒙古温带草原土壤微生物量碳的垂直分布呈现一定的规律性,且非生物因子对微生物量碳的影响也呈现垂直减弱的规律。     

石羊河下游退耕地土壤微生物变化及土壤酶活性

采用时空替代法,对石羊河下游不同年限(1,2,3,4,5,8,15,24、31 a)退耕地土壤微生物(细菌、真菌和放线菌)数量、生物量(碳、氮和磷)及土壤酶(过氧化氢酶、蔗糖酶、脲酶和磷酸酶)活性变化及三者的相关性进行了测定和分析。结果表明,在退耕1—31 a的9个样地样方中土壤三大类微生物数量以细菌最高,放线菌次之,真菌最低。总体来看,三大土壤微生物数量的加权平均值最大值均在退耕后的前8 a。土壤微生物生物量碳在退耕初期随着退耕年限的增加而减小,退耕4 a后逐渐增大,退耕24 a期间达到了加权平均值的最大,最后趋于稳定;土壤微生物生物量氮在退耕初期随着退耕年限的增加而增加,退耕4 a加权平均值的最大值出现,随后逐渐减小的趋势,并且不同退耕年限土壤微生物生物量氮差异显著;土壤微生物生物量磷在退耕初期随着退耕年限的增加而增加,退耕8 a前后加权平均值达到最大值,随后逐渐减小,最终趋于稳定。土壤酶活性总趋势随着退耕地自然演替时间的增加呈波动式下降。不同土壤层次(0—10 cm,10—20 cm,20—30 cm及30—40 cm),土壤微生物数量、生物量及土壤酶活性随土层深度显著降低,并且表层土壤微生物生物量及土壤酶活性占有较大比例。土壤微生物及土壤酶活性的变化是一个极其缓慢的互动过程,存在着互相回馈的响应,特别是真菌与放线菌、微生物量氮及蔗糖酶,放线菌与过氧化氢酶、蔗糖酶,微生物量碳与磷酸酶,微生物量氮与脲酶,微生物量磷与蔗糖酶均存在极显著的相关性。总体来看在退耕年限4—5 a前,有利于土壤发育,退耕后期土壤肥力呈下降的趋势。     

广西十万大山地区不同植被类型土壤微生物特征

为研究广西十万大山地区热带不同植被类型土壤微生物特征及其与土壤养分之间的关系,对次生阔叶林、马尾松林、灌草丛和撂荒地的土壤理化性质、微生物数量特征及微生物生物量碳氮磷进行了测定。结果表明:相同土层的土壤微生物总数大小依次为:次生阔叶林马尾松林灌草丛撂荒地,并随土壤深度增加而减少。土壤微生物生物量碳氮磷随土壤深度的增加而逐渐降低,在不同植被类型的土壤中差异显著。次生阔叶林、马尾松林、灌草丛的土壤微生物生物量与土壤养分呈极显著相关,而撂荒地的相关性明显低于其他3种植被类型,并且其土壤微生物生物量磷与全氮、速效氮和速效钾含量无相关性。由此可见,土壤微生物数量和微生物生物量均可作为评价十万大山森林生态系统土壤肥力的指标;可采用植被恢复手段促进土壤微生物群落的发育、改良土壤特性以促进该区域退化生态系统的恢复。     

武夷山不同海拔高度土壤活性有机碳变化

采用连续熏蒸 培养法,测定了福建武夷山自然保护区不同海拔高度具有代表性的中亚热带常绿阔叶林、针叶林、亚高山矮林以及高山草甸土壤中有效碳含量,分析了土壤有效碳（LOC）与微生物量碳（MBC）、土壤总有机碳（TOC）、细根生物量（FRB）和土壤全氮（TN）之间的关系.结果表明：土壤有效碳占总有机碳的3.40%～7.46%;微生物量碳只是土壤有效碳中的一部分,占土壤有效碳26.87%～80.38%; 不同林分土壤有效碳含量随海拔增高而显著增大,随土层深度的增加而降低;土壤有效碳与微生物量碳、土壤总有机碳、细根生物量、土壤全氮之间呈极显著的相关关系.高海拔土壤有效碳含量显著高于低海拔土壤.     

Drying–Rewetting Cycles Affect Fungal and Bacterial Growth Differently in an Arable Soil

Drying and rewetting is a frequent physiological stress for soil microbial communities; a stress that is predicted to grow more influential with future climate change. We investigated the effect of repeated drying–rewetting cycles on bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, on the biomass concentration and composition (PLFA), and on the soil respiration. Using different plant material amendments, we generated soils with different initial fungal:bacterial compositions that we exposed to 6–10 repetitions of a drying–rewetting cycle. Drying–rewetting decreased bacterial growth while fungal growth remained unaffected, resulting in an elevated fungal:bacterial growth ratio. This effect was found irrespective of the initial fungal:bacterial biomass ratio. Many drying–rewetting cycles did not, however, affect the fungal:bacterial growth ratio compared to few cycles. The biomass response of the microbial community differed from the growth response, with fungal and total biomass only being slightly negatively affected by the repeated drying–rewetting. The discrepancy between growth- and biomass-based assessments underscores that microbial responses to perturbations might previously have been misrepresented with biomass-based assessments. In light of this, many aspects of environmental microbial ecology may need to be revisited with attention to what measure of the microbial community is relevant to study.     

围封对退化高寒草甸土壤微生物群落多样性及土壤化学计量特征的影响

围封对植被处于近自然恢复状态的退化草地有一定的修复作用,开展轻度退化草地围封过程中生物与非生物因素的协同互作研究是完整地认识草地生态系统结构和功能的基础.本试验对围栏封育10年的轻度退化草地的土壤化学计量特征进行了研究,同时采用高通量基因测序技术并结合Biolog-Eco方法,调查了土壤微生物多样性和功能的变化.结果表明:轻度退化草地实施围封后,土壤铵态氮含量显著升高,全钾含量显著降低,土壤有机碳、全氮、全磷、硝态氮、速效磷和速效钾则无明显变化.高寒草甸土壤微生物碳和氮在轻度退化和围栏封育草地间差异不显著;围栏封育后草地土壤微生物碳氮比显著高于轻度退化草地.随培养时间的延长,高寒草甸不同土层土壤微生物碳代谢强度均显著升高,土壤微生物碳代谢指数在轻度退化和围栏封育草地间差异不显著.高寒草甸土壤细菌OTUs显著高于真菌,轻度退化与围栏封育草地土壤微生物相似度为27.0%～32.7%.围封后,土壤真菌子囊菌门、接合菌门和球壶菌门相对丰富度显著升高,担子菌门显著降低,土壤细菌酸杆菌门显著低于轻度退化草地.土壤真菌和细菌群落组成在不同土层间差异较大,在轻度退化和围栏封育草地间仅有表层土壤真菌群落组成表现出较大差异.土壤细菌多样性受土壤全氮和速效钾影响较大,真菌多样性受地上生物量影响较大.土壤微生物对碳源利用能力主要受土壤速效钾影响.综上,长期围封禁牧对轻度退化草地土壤养分和土壤微生物无明显影响,且会造成牧草资源浪费,适度放牧可以保持草地资源的可持续利用.     

亚热带树种转换对林地土壤微生物群落结构和功能的影响

通过分析杉木采伐迹地营造阔叶树种尾巨桉和固氮树种黑木相思人工林后土壤微生物群落组成和酶活性,探讨造林树种转换对于改善杉木林地土壤微生物特性的影响.结果表明: 树种转换对土壤微生物群落组成和酶活性的影响主要局限于0～10 cm土壤层.杉木转换为固氮树种黑木相思后,显著提高了0～10 cm土壤层总脂肪酸含量、真菌、革兰氏阳性细菌、革兰氏阴性细菌和放线菌生物量.主成分分析表明,黑木相思人工林土壤微生物群落组成与杉木和尾巨桉人工林具有显著差异,土壤中革兰氏阳性细菌、阴性细菌和放线菌丰度显著提高.在0～10 cm土壤层,黑木相思人工林土壤纤维素水解酶、乙酰氨基-葡萄糖苷酶和酸性磷酸酶活性均显著高于杉木和尾巨桉人工林.研究表明,杉木转变为固氮树种黑木相思后会显著提高微生物生物量和酶活性,有助于土壤有机质的恢复,加快养分循环过程.     

Soil biological activity and their seasonal variations in response to long-term application of organic and inorganic fertilizers

The objectives of this study were to explore the effects of long-term and continued application of fertilizers and manures on microbial biomass, soil biological activity and their seasonal variations in surface and subsurface soils in relation to soil fertility. For this, soils were sampled in spring, summer and autumn from Shenyang Long-term Experimental Station, northeastern China. The results showed that soil total nitrogen (N), organic carbon (C), basal respiration, microbial biomass and enzymatic activity increased in manure-amended surface soils, but decreased with soil depth. Long-term application of inorganic fertilizers significantly decreased soil pH value, sucrase activity and microbial biomass C, but increased soil metabolic quotient (qCO₂). However, no significant effect of inorganic fertilizers on soil total N, urease activity and microbial biomass N was observed in comparison with CK0 (neither tillage nor fertilization) and CK (no fertilizers). There was no significant difference between CK0 and CK in soil total N, organic C and microbial activity in surface soil layer (0–20 cm), but these parameters in subsurface soil layer (20–40 cm) were higher in CK than in CK0. Moreover, seasonal changes were observed in terms of soil nutrient contents, enzymatic activity, microbial biomass and soil respiration. There were significant correlations between soil microbial biomass C and N, between organic C and sucrase activity and between total N and urease activity, respectively. It is recommended that combined use of organic manure with inorganic fertilizers should be considered to maintain higher microbial biomass, soil biological activity and soil fertility. Considering considerably high nutrients reserve and microbial activity in subsurface layers of soil and wind-erosion-caused nutrient loss in spring in north China, we also propose that low tillage should be considered to make use of nutrients in soils.     

Influence of wheel traffic and tillage on microbial biomass, residue decomposition and extractable nutrients in a Coastal Plain Soil

The interactive effects of tillage and compaction from wheel traffic were tested on active bacterial and fungal biomass and organic matter decomposition in the planting row at the surface and within the plow layer of a Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudult). This experiment was arranged in a split plot design with four replications. Main plots were compaction: 1) compaction from wheel traffic and 2) no compaction from wheel traffic; subplots were tilalge system: 1) conventional tillage and 2) no-tillage. Despite a significant increase in bulk density, compaction from wheel traffic and tillage system had no consistent effects on active bacterial or active fungal biomass either in the top 7.5 cm of soil or in the 15–20 cm depth of soil. Active bacteria and fungal biomass at both depths were usually lower in the winter months than the spring, summer or autumn months. Organic matter decomposition, nutrient mineralization and nutrient availability did not differ among soils that received tillage or compaction from wheel traffic. Organic matter decomposition was greater in all treatments when decomposition bags were buried at 15–20 cm than when they were placed on the surface of the soil. The soil that was sampled was an extremely sandy soil so there was probably not a significant effect of compaction on soil aeration and structure.Mention of trade names or commercial products does not constitute endorsement or recommendation of use.Mention of trade names or commercial products does not constitute endorsement or recommendation of use.     

Effect of long‐term mineral fertilisation on soil microbial abundance,community structure and diversity in a Typic Hapludoll under intensive farming systems

Fertiliser application can not only influence plant communities, but also the soil microbial community dynamics, and consequently soil quality. Specifically, mineral fertilisation can directly or indirectly affect soil chemical properties, microbial abundance and, the structure and diversity of soil microbial communities. We investigated the impact of six different mineral fertiliser regimes in a maize/soybean rotation system: control (CK, without fertilisation), PS (application of phosphorus plus sulphur), NS (application of nitrogen plus S), NP (application of N plus P), NPS (application of N, P plus S) and NPSm (application of N, P, S plus micronutrients). Soil samples were collected at the physiological maturity stage of maize and soybean in March of 2013 and 2014, respectively. Overall, mineral fertilisation resulted in significantly decreased soil pH and increased total organic carbon compared with the control (CK). The analysis of terminal restriction fragment length polymorphism (T‐RFLP) revealed that mineral fertilisers caused a shift in the composition of both bacterial and fungal communities. In 2013, the highest value of Shannon diversity of bacterial terminal restriction fragments (TRFs) was found in control soils. In 2014, NPSm treated soils showed the lowest values of diversity for both bacterial and fungal TRFs. In both crop growing seasons, the analysis of phospholipid fatty acid (PLFA) detected the lowest value of total microbial biomass under CK. As PLFA analysis can be used to evaluate total microbial community, this result suggests that fertilisation increased total microbial biomass. When the bacterial and fungal abundance were examined using real time polymerase chain reaction, the results revealed that mineral fertilisation led to decreased bacterial abundance (16S rRNA), while fungal abundance (18S rRNA) was found to be increased in both crop growing seasons. Our results show that mineral fertiliser application has a significant impact on soil properties, bacterial and fungal abundance and microbial diversity. However, further studies are needed to better understand the mechanisms involved in the changes to microbial communities as a consequence of mineral fertilisation.     

Metabolic Status of Bacteria and Fungi in the Rhizosphere of Ponderosa Pine Seedlings

We determined the quantity and metabolic status of bacteria and fungi in rhizosphere and nonrhizosphere soil from microcosms containing ponderosa pine seedlings. Rhizosphere soil was sampled adjacent to coarse, fine, or young roots. The biovolume and metabolic status of bacterial and fungal cells was determined microscopically and converted to total and active biomass values. Cells were considered active if they possessed the ability to reduce the artificial electron acceptor 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT) to visible intracellular deposits of INT formazan. A colorimetric assay of INT formazan production was also used to assess dehydrogenase activity. INT-active microorganisms made up 44 to 55% of the microbial biomass in the soils studied. The proportion of fungal biomass that exhibited INT-reducing activity (40 to 50%) was higher than previous estimates of the active proportion of soil fungi determined by using fluorescein diacetate. Comparison between soils from different root zones revealed that the highest total and INT-active fungal biomass was adjacent to fine mycorrhizal roots, whereas the highest total and active bacterial biomass was adjacent to the young growing root tips. These observations suggest that fungi are enhanced adjacent to the fine roots compared with the nonrhizosphere soil, whereas bacteria are more responsive than fungi to labile carbon inputs in the young root zone. Colorimetric dehydrogenase assays detected gross differences between bulk and rhizosphere soil activity but were unable to detect more subtle differences due to root types. Determination of total and INT-active biomass has increased our understanding of the role of spatial compartmentalization of bacteria and fungi in rhizosphere carbon flow.     

长期施肥对红壤水稻土磷脂脂肪酸特性和酶活性的影响

对中国科学院红壤生态实验站长期定位试验中不同施肥处理红壤水稻土磷脂脂肪酸(PLFA)特性及酶活性进行了分析.结果表明:不同施肥处理的土壤酶活性、养分、微生物生物量及微生物群落多样性差异较大;施肥处理增加了PLFA的种类和微生物量;施肥土壤的真菌PLFA量大于不施肥土壤,细菌PLFA量小于不施肥土壤,说明真菌较细菌更能适应养分贫瘠的条件.NPK平衡施肥和施有机肥处理的PLFA总量均高于施无机氮肥和未施肥处理,两者分别比未施肥处理高222%和79%,表明NPK平衡施肥和施有机肥更有利于作物生长.施肥还可增加土壤酶活性,其中,土壤脲酶和磷酸酶活性可以作为衡量土壤肥力水平的指标.     

Soil Microbial Community Structure and Metabolic Activity of Pinus elliottii Plantations across Different Stand Ages in a Subtropical Area

Soil microbes play an essential role in the forest ecosystem as an active component. This study examined the hypothesis that soil microbial community structure and metabolic activity would vary with the increasing stand ages in long-term pure plantations of Pinus elliottii. The phospholipid fatty acids (PLFA) combined with community level physiological profiles (CLPP) method was used to assess these characteristics in the rhizospheric soils of P. elliottii. We found that the soil microbial communities were significantly different among different stand ages of P. elliottii plantations. The PLFA analysis indicated that the bacterial biomass was higher than the actinomycic and fungal biomass in all stand ages. However, the bacterial biomass decreased with the increasing stand ages, while the fungal biomass increased. The four maximum biomarker concentrations in rhizospheric soils of P. elliottii for all stand ages were 18:1ω9c, 16:1ω7c, 18:3ω6c (6,9,12) and cy19:0, representing measures of fungal and gram negative bacterial biomass. In addition, CLPP analysis revealed that the utilization rate of amino acids, polymers, phenolic acids, and carbohydrates of soil microbial community gradually decreased with increasing stand ages, though this pattern was not observed for carboxylic acids and amines. Microbial community diversity, as determined by the Simpson index, Shannon-Wiener index, Richness index and McIntosh index, significantly decreased as stand age increased. Overall, both the PLFA and CLPP illustrated that the long-term pure plantation pattern exacerbated the microecological imbalance previously described in the rhizospheric soils of P. elliottii, and markedly decreased the soil microbial community diversity and metabolic activity. Based on the correlation analysis, we concluded that the soil nutrient and C/N ratio most significantly contributed to the variation of soil microbial community structure and metabolic activity in different stand ages of P. elliottii plantations.     

Distribution of soil carbon and microbial biomass in arable soils under different tillage regimes

We have measured total soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial lipid contents (as indices of microbial biomass and community structure), and their distributions to 60 cm depth in soils from replicated medium-term (2003?C2008) experimental arable plots subject to different tillage regimes in Scotland. The treatments were zero tillage (ZT), minimum tillage (MT; cultivation to 7 cm), the conventional tillage (CT) practice of ploughing to 20 cm, and deep ploughing (DP) to 40 cm depth. In the 0?C30 cm depth range, SOC content (corrected for bulk density differences between tillage treatments) was greatest under ZT and MT, but over 0?C60 cm depth the SOC contents of these treatments were similar to the CT and DP treatments. DOC concentrations declined with increasing depth in ZT and MT above 20 cm, but there were no significant differences with depth in the CT and DP treatments. Beneath 20 cm, there was little change in DOC concentration with depth for all treatments, although for the MT treatment, there was less DOC beneath the depth of cultivation. The total microbial biomass decreased with increasing depth over the 0?C60 cm range in the ZT and MT treatments, whereas it decreased with depth only below 30?C40 cm in the CT and DP treatments. The microbial biomass was significantly different only between 0?C5 cm in the ZT, CT and DP treatments, but not for other depths between all treatments. The bacterial biomass was greater in the ZT treatment than in MT, CT and DP near the soil surface, but not significantly different over the whole profile (0?C60 cm). The fungal biomass decreased with depth in the ZT and MT treatments over the whole 0?C60 cm depth range, whereas it decreased with depth only below 20 cm in the CT and DP treatments.