棉花根系分泌物对土壤速效养分和酶活性及微生物数量的影响

采用水培法收集棉花根系分泌物,在耕作1年的土壤中添加棉花根系分泌物,培养10 d后测定土壤中速效养分、酶活性及微生物数量.结果显示,(1)棉花根系分泌物能极显著提高土壤中速效K和速效P含量4.31%～15.03%和5.99%～24.31%(P<0.01);高浓度分泌物处理下速效N含量比对照显著提高11.39%(P<0.05),其它处理影响不显著;各浓度分泌物对土壤有机质含量均无显著影响.(2)各浓度棉花根系分泌物均使土壤中转化酶活性显著提高,且随分泌物浓度的增加而显著增强;低浓度分泌物能显著提高土壤中磷酸酶的活性,所有浓度处理对土壤脲酶活性均无显著影响.(3)中、高浓度的棉花根系分泌物能显著增加土壤中细菌的数量,低浓度的分泌物能显著增加土壤中真菌的数量,而不同浓度处理的土壤中放线菌的数量均无显著的变化.研究表明,棉花根系分泌物可通过促进土壤细菌及土壤真菌的繁殖来增强土壤转化酶和磷酸酶活性,提高土壤速效P、速效K及速效N含量,从而对棉花根际微环境产生深刻影响.     

免耕与留茬对土壤微生物量C、N及酶活性的影响

2005～2008年在内蒙古呼和浩特市清水河县进行定位试验,设免耕留低茬(NL)、免耕留高茬覆盖(NHS)和传统耕翻(T)3种耕作处理方式.结果表明:(1)免耕留高茬覆盖及免耕留低茬长期实施,能显著提高表层土壤有机质、全氮、全钾、碱解氮、速效磷和速效钾含量,且免耕留高茬覆盖处理比传统耕翻分别提高了11%、41%、22%、15%、29%、27%、13%;在测定各个时期内,土壤各营养指标含量整体趋势为NHS＞NL＞T.(2)免耕留高茬覆盖及免耕留低茬耕作方式有利于提高土壤微生物量C、N含量,在各测定时期均以免耕留高茬覆盖处理的土壤微生物量C、N含量最高,传统耕翻最低.与传统耕翻相比,免耕留高茬覆盖处理土壤微生物量C、N含量分别平均提高了69%、43%;测定各个时期,不同处理土壤生物量C、N含量均以7月份含量最高、5月份次之、10月份最低.(3)免耕留高茬覆盖及免耕留低茬处理土壤碱性磷酸酶、蔗糖酶、过氧化氢酶活性和脲酶活性高于传统耕翻,整个测定期内免耕留高茬覆盖处理4种酶平均活性,分别较传统耕翻增加了57%、82%、93%和25%;春季土壤酶活性开始增强,在7月份蔗糖酶、过氧化氢酶和脲酶活性达到最大值,而碱性磷酸酶的峰值出现在6月份.土壤微生物量C、N及土壤酶活性是评价土壤质量的重要因子.     

长期不同施肥对石灰性土壤微生物磷及磷酸酶的影响

采用氯仿熏蒸提取法和磷酸苯二钠比色法分别测定了长期土壤培肥定位试验地所设置的不施肥、单施化肥、玉米秸秆 (低、中、高量 ) 化肥、厩肥 化肥以及休闲 (低量秸秆 化肥 )处理土壤微生物磷和磷酸酶活性。结果表明 ,各施肥处理均不同程度提高了土壤微生物磷 ;除休闲处理外 ,各施肥处理也不同程度提高了磷酸酶活性 ,这有利于为作物生长创造一种良好的土壤条件。石灰性土壤微生物磷与土壤全磷、速效磷之间存在明显的正相关 ,表明要改善石灰性土壤P利用率低的状况 ,提高土壤微生物磷不失为一条行之有效的生物学途径     

混交对亚热带针叶树根际土壤氮矿化和微生物特性的影响

混交阔叶树是退化红壤区针叶林改造的重要措施之一，土壤养分供应和转化是评价混交效应的重要参数，但混交后针叶树根际土壤氮矿化特征及其影响因素还不清楚。选取退化红壤区马尾松（Pinus massoniana Lamb.）纯林、湿地松（Pinus elliottii Engelmann）纯林及其补植木荷（Schima superba Gardn.et Champ.）形成的马-木混交林和湿-木混交林为研究对象，采集4种林分下针叶树根际土壤，测定速效养分含量、氮矿化速率、氮水解酶活性和微生物磷脂脂肪酸含量，探究混交对针叶树根际土壤氮供应及微生物特性的影响，分析土壤氮矿化和微生物特性间的相关性。结果表明：混交显著增加了针叶树根际土壤铵态氮，矿质氮和有效磷含量，而对硝态氮影响不显著。根际土壤氮矿化以硝化作用为主，混交后针叶树根际土壤氨化速率降低了27.0%，硝化速率增加了55.4%，而最终净氮矿化速率增加了24.1%。两个树种间，马尾松根际土壤矿质氮含量和净氮矿化速率显著高于湿地松。针叶树根际土壤真菌、丛枝菌根真菌，总微生物生物量及真菌/细菌比在混交后显著增加，且马尾松根际土壤总微生物和真菌生物量分别比湿地松高18.9%和27.0%。同时，针叶树根际土壤β-N-乙酰氨基葡萄糖苷酶活性在混交后显著增强，且根际土壤硝化速率和净氮矿化速率与微生物指标和酶活性正相关。总的来说，混交阔叶树显著提高了针叶树根际土壤氮供应，以此应对阔叶树混交后带来的养分竞争压力，而马尾松倾向于积极性的应对策略，通过增加土壤氮矿化以适应外界环境改变。     

浙江慈溪旱作农田土壤微生物学性状的时空演变特征

测定了浙江慈溪5个不同利用年限旱作农田土壤（50～700 a）的微生物数量、生物量和酶活性,比较分析了农田土壤微生物学质量与利用年限的相关性,并测定了50 a、100 a和700 a 3个旱作土壤的微生物功能多样性.结果发现：农田土壤在旱作初期（<100 a）,除真菌数量（F）略有上升外,细菌数量（B）、B/F值、微生物生物量C、N及过氧化氢酶、转化酶、脲酶活性全部锐减;耕作100 a之后,仅真菌数量随年限延长而显著降低,细菌数量、B/F值、微生物生物量C、N及过氧化氢酶、转化酶、脲酶活性则全部升高;在50～700 a的整个旱作过程中,土壤微生物生物量C/N值始终随年限延长而显著升高.BIOLOG检测结果显示,旱作农田土壤微生物群落的Shannon、Simpson和McIntosh 3种功能多样性指数在利用年限上的变化规律与细菌数量、微生物生物量及酶活性等完全一致.表明浙江慈溪旱作农田土壤微生物群落结构一直随利用年限的延长而变化,且土壤微生物学质量总体在持续利用100 a左右止降回升.     

种植年限对蔬菜日光温室土壤微生物区系和酶活性的影响

以种植2、4、6、11、13、16、19年的蔬菜日光温室土壤为研究对象,并以露地菜田为对照,测定了土壤微生物区系及酶活性的变化.结果表明: 随着种植年限的增加,土壤中细菌、放线菌和微生物总数均呈现先增加后减少的趋势,在种植11年时达到最大值,分别比对照增加了54.8%、63.7%和55.4%,差异达显著水平;而真菌数量持续上升,种植19年约为对照的2.2倍.微生物生理类群中,纤维素分解菌、自生固氮菌、亚硝酸细菌、反硝化细菌和硫化细菌数量的变化趋势与细菌相似, 种植11年分别为对照的1.5、1.6、1.9、1.4和1.1倍;而氨化细菌数量则呈现先减少后增加的趋势,在种植13年时达到最小值,为对照的56.0%.土壤中脲酶、多酚氧化酶、蔗糖酶、蛋白酶、纤维素酶和碱性磷酸酶活性随种植年限的增加呈现先增强后减弱的趋势,而过氧化氢酶活性较稳定.相关分析表明,细菌、放线菌和微生物总数与各土壤酶均呈显著正相关;而真菌数量与脲酶、蔗糖酶、过氧化氢酶和碱性磷酸酶均呈负相关,其中与过氧化氢酶的相关性达到显著水平.     

鱼蛋白有机液肥对小粉土酶活性和微生物生物量碳、氮的影响

采用恒温土壤培养方法,研究了4种不同鱼蛋白有机液肥施用量[0(对照)、0.5、1.5、2.5 ml·kg-1]条件下小粉土酶活性和微生物生物量碳、氮的变化,及其与土壤养分的相关关系.结果表明:在整个培养过程中,不同鱼蛋白有机液肥施用量处理下土壤pH值变化范围为7.07～7.31,与对照无显著差异;土壤磷酸酶活性显著增强,分别为对照的1.27、1.90、1.96倍;土壤脲酶活性分别比对照提高39.81%、78.06%、173.24%;蛋白酶活性比对照提高56.37%、108.29%、199.98%;土壤微生物生物量碳、氮均随肥料添加量的增加而逐渐增大,分别为对照的1.67、3.95、4.74倍和1.21、2.43、4.06倍.土壤脲酶和蛋白酶活性以及土壤微生物生物量碳、氮在不同施用量处理下达到峰值点的时间不同.土壤磷酸酶、脲酶和蛋白酶活性、土壤微生物生物量碳、氮与土壤养分均呈显著正相关.施用鱼蛋白有机液肥可以显著促进小粉土微生物的生长及酶活性的提高,从而促进土壤有机质的分解转化和速效养分的释放.     

辽东山区不同林龄日本落叶松人工林土壤微生物、养分及酶活性

对辽宁省抚顺市大孤家林场11、20、34和47年生日本落叶松人工纯林表层土壤(0~5 cm)微生物群落结构、养分及酶活性进行了研究.结果表明: 土壤微生物、养分及酶活性各项指标基本呈现11或47年林龄较高,而20或34年林龄较低.随林龄增加,土壤地力呈现衰退趋势,土壤微生物群落结构及酶活性变化对地力衰退呈现响应趋势,不同林龄真菌群落结构的差异较细菌显著.典范对应分析(CCA)表明,土壤养分含量及pH值对微生物群落结构的季节变化没有影响,但对在不同林龄间微生物的变化有影响.土壤全氮、有机碳、C/N、速效氮和pH值对不同林龄细菌分布影响较大,土壤速效磷、全钾、交换性镁离子和pH值对真菌分布影响较大.细菌与真菌群落主要的T RFs片段与氮、磷的相关性都较高,而真菌群落与有机碳、钾的相关性高于细菌群落.11和47年林龄微生物群落与土壤养分、酶活性的相关性高于20与34年.因此,土壤微生物(尤其是土壤真菌)可以敏感地指示土壤肥力的变化.     

加工番茄连作对农田土壤酶活性及微生物区系的影响

通过盆栽试验,研究了连作及活性炭处理对加工番茄土壤微生物数量及土壤酶活性的影响。结果表明:加工番茄连作对其微生物区系及土壤酶活性产生一定影响。随连作年限的增加,过氧化氢酶活性呈上升趋势,而脲酶、多酚氧化酶、蔗糖酶、磷酸酶活性呈下降趋势,通过活性碳处理后,脲酶、土壤蔗糖酶、多酚氧化酶、磷酸酶活性比对照(未活性炭处理)增加,而过氧化氢酶活性降低。随连作年限的增加,连作2a和3a的细菌数量分别比1a降低12.1%和16.8%,真菌数量连作2a和3a分别比种植1a时提高20%和60%,放线菌数量连作2a和3a土壤中分别是种植1a的1.04倍和1.12倍,通过活性碳处理后,细菌的数量在连作2a和3a时分别比对照增加5.8%和1.6%,放线菌和真菌数量比对照分别降低13.0%、23.1%和8.3%、50%。反硫化细菌数量随连作年限延长而增加,硝酸细菌和好氧性自身固氮菌数量减少。可见通过活性炭处理可减轻自毒物质对加工番茄土壤酶和微生物的影响,从而缓解加工番茄连作障碍。     

平朔黄土露天矿区复垦地表层土壤微生物与酶活性分析

矿区是当今世界陆地生态系统被破坏和退化最严重的区域之一,在进行露天煤矿土地复垦与生态重建时不仅要恢复地表植被和生物,还应重视地下土壤微生物生态系统的构建,而国内外相关研究多集中在植被重建及土壤理化性质监测方面,利用土壤微生物及酶活性揭示矿区重构土壤状况的研究尚不多见。通过8个样地24个土壤剖面的采样分析,采用时空替代法和单因素方差分析法对平朔矿区不同复垦年限的排土场和原地貌0—20 cm表层土壤中细菌、真菌、放线菌的数量及蔗糖酶、脲酶和磷酸酶活性及变化进行研究,旨在分析其特征及差异。研究结果表明:(1)从微生物数量及酶活性特征来看,无论复垦年限的时间长短,细菌数量在3类微生物中占有绝对优势,占微生物总数的99.20%以上,其次是放线菌,真菌的数量最少;脲酶活性在3种酶中活性最大,其次是蔗糖酶活性,磷酸酶活性最低。(2)从微生物数量和酶活性的变化状况来看,3类微生物数量和3种酶活性在0—20 cm土层随复垦年限的变化趋势相一致,均随复垦年限的变化先增长后降低,而后又随着复垦年限的增长不断增加。(3)在0—20 cm土层,复垦27年的南排土场的土壤细菌、放线菌和真菌数量在复垦后达到了189.3333×10⁵ cfu/g、0.1312×10⁵ cfu/g和1.1463×10⁵ cfu/g,复垦效果达到原地貌3类土壤微生物数量的65.88%、66.46%和67.74%;蔗糖酶、脲酶和磷酸酶活性分别达到1.9600 mg 100 g^-1 h^-1、6.3600 mg 100 g^-1 h^-1和1.4533 mg 100 g^-1 h^-1,复垦效果分别达到原地貌83.40%、86.30%和86.85%。因此开展矿区土地复垦后土壤微生物及酶活性的相关研究,能够在一定程度上及时反映土地复垦后的土壤质量以及生态系统的恢复状况,以便采取更加合理的复垦方法来提高矿区生态恢复的速度和效果。     

植物、土壤及土壤管理对土壤微生物群落结构的影响

土壤微生物是土壤生态系统的重要组成部分,对土壤微生物群落结构多样性的研究是近年来土壤生态学研究的热点。本文综述了有关植物、土壤类型以及土壤管理措施对土壤微生物群落结构影响的最新研究结果,指出植物的作用因植物群落结构多样性、植物种类、同种植物不同的基因型,甚至同一植物不同根的区域而异;而土壤的作用与土壤质地和有机质含量等因素有关;植物和土壤类型在对土壤微生物群落结构影响上的作用存在互作关系。不同的土壤管理措施对土壤微生物群落结构影响较大,长期连作、大量的外援化学物质的应用降低了土壤微生物的多样性;而施用有机肥、免耕可以增加土壤微生物群落结构多样性,有利于维持土壤生态系统的功能。     

Available soil nitrogen in relation to fractions of soil nitrogen and other soil properties

The available N of 27 soils from England and Wales was assessed from the amounts of N taken up over a 6-month period by perennial ryegrass grown in pots under uniform environmental conditions. Relationships between availability and the distribution of soil N amongst various fractions were then examined using multiple regression. The relationship: available soil N (mg kg^–1 dry soil)=(N_min×0.672)+(N_inc×0.840)+(N_mom×0.227)–5.12 was found to account for 91% of the variance in available soil N, where N_min=mineral N, N_inc=N mineralized on incubation and N_mom=N in macro-organic matter. The N mineralized on incubation appeared to be derived largely from sources other than the macro-organic matter because these two fractions were poorly correlated. When availability was expressed in terms of available organic N as % of soil organic N (N_ao) the closest relationship with other soil characteristics was: N_ao=[N_inc×(1.395–0.0347×CN_mom]+[N_mom×0.1416], where CN_mom=CN ratio of the macro-organic matter. This relationship accounted for 81% of the variance in the availability of the soil organic N.The conclusion that the macro-organic matter may contribute substantially to the available N was confirmed by a subsidiary experiment in which the macro-organic fraction was separated from about 20 kg of a grassland soil. The uptake of N by ryegrass was then assessed on two subsamples of this soil, one without the macro-organic matter and the other with this fraction returned: uptake was appreciably increased by the macro-organic matter.     

Distinct impacts of reductive soil disinfestation and chemical soil disinfestation on soil fungal communities and memberships

Applied Microbiology and Biotechnology - Soil disinfestation is an important agricultural practice to conquer soil-borne diseases and thereby ensure crop productivity. Reductive soil disinfestation...     

The breakdown of malathion in soil and soil components

The disappearance of the organophosphorus insecticide, malathion, from a silt loam soil and from its organic and inorganic components was examined. Half-lives and the time taken for 90% decomposition in nonsterile, sodium azide-treated, and 2.5 Mrad-irradiated soils were similar (3/4–1 1/2 days and 4–6 days, respectively) but breakdown in autoclaved soils was negligible. Decay in nonsterile sand, silt, and clay minus organic matter fractions was 3–6 times slower than that recorded in the original soil. Breakdown of malathion in the clay plus organic matter fraction (organo-mineral complex) was rapid (half-life, 1 day), as was the case in the separated organic matter (half-life, 1 3/4 days). Filter-sterilized organic matter was not as effective in catalyzing the breakdown of malathion (half-life, 4 days), and no loss occurred from any of the autoclaved components. Irradiation doses of 2.5 and 5.0 Mrad had little influence on the ability of soil to degrade malathion. Thereafter, increases up to 20 Mrad had a more drastic, though far from totally inhibitory, effect. Our results suggest that either the colloidal organic matter itself, or a fraction associated with it, is the most important single factor concerned with the rapid breakdown of malathion in the soil studied. Direct microbial metabolism is a slower process and may have a significant role in malathion disappearance in coarsetextured soils low in colloidal organic matter. The catalytic component of the organic matter is suggested to be a stable exoenzyme and is supportive of reports by other workers. The quantitative effect of organo-mineral complex (containing the active degradative ingredient) additions to sand and silt fractions on the rate of subsequent malathion decay is also described.     

The porosity of soil aggregates from bulk soil and from soil adhering to roots

Summary Total porosity and pore-size distribution (p.s.d.) were determined in soil aggregates taken in plots planted with maize and treated with farmyard manure and three rates of compost. Soil aggregates were collected from the soil adherent to the maize roots (root soil aggregates) and from bulk soil (bulk soil aggregates). Mercury intrusion porosimetry was used to evaluate the total porosity and the p.s.d. Treatments did not affect the total porosity of the bulk soil aggregates. The same was observed for the root soil aggregates. However the total porosity of the root soil aggregates was always lower than that of the bulk soil aggregates. The loss of total porosity was found to be due to a decrease in the percentage of larger pores with respect to the total.     

Estimating respiration of roots in soil: Interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO₂ concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO₂ concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material.Root respiration was strongly affected by diurnal fluctuations in temperature (Q₁₀ = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO₂ concentrations (400 to 25000 ppm CO₂; pH around 6). Soil CO₂ was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO₂ concentration to that of incoming air. Irrigation caused a temporary reduction in CO₂ diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions.     

Linking soil bacterial biodiversity and soil carbon stability

Native soil carbon (C) can be lost in response to fresh C inputs, a phenomenon observed for decades yet still not understood. Using dual-stable isotope probing, we show that changes in the diversity and composition of two functional bacterial groups occur with this ‘priming'' effect. A single-substrate pulse suppressed native soil C loss and reduced bacterial diversity, whereas repeated substrate pulses stimulated native soil C loss and increased diversity. Increased diversity after repeated C amendments contrasts with resource competition theory, and may be explained by increased predation as evidenced by a decrease in bacterial 16S rRNA gene copies. Our results suggest that biodiversity and composition of the soil microbial community change in concert with its functioning, with consequences for native soil C stability.Substrate inputs can stimulate decomposition of native soil organic carbon (SOC; Kuzyakov et al., 2000), a phenomenon known as the ‘priming effect'' (Kuzyakov, 2010), and is considered large enough to influence ecosystem C balance (Wieder et al., 2013). Two functionally distinct groups of microorganisms are postulated to mediate priming: one that grows rapidly utilizing labile C, and one that grows slowly, breaking down recalcitrant SOC (Fontaine et al., 2003; Blagodatskaya et al., 2007). However, distinguishing these groups is technically challenging. Here, we used dual-stable isotope probing with ¹³C-glucose and ¹⁸O-water to identify bacteria in these two groups growing in response to single and repeated pulses of glucose. Organisms that utilize labile C for growth assimilate both ¹³C-glucose and ¹⁸O-water into their DNA, whereas organisms that grow using SOC incorporate only ¹⁸O-water. Differential isotope incorporation leads to a range of DNA densities separable through isopycnic centrifugation, which can then be characterized by sequencing (Radajewski et al., 2000).We sequenced fragments of bacterial 16S rRNA genes following single and repeated glucose pulses. We hypothesized that the single pulse of labile C would stimulate growth of opportunistic organisms, thus immobilizing nutrients and suppressing growth and diversity of the SOC-utilizing community, decreasing SOC decomposition (negative priming), a response analogous to that observed in plant communities in response to chronic N additions (Tilman, 1987; Clark and Tilman, 2008). We hypothesized that multiple glucose additions would stimulate growth of a more diverse bacterial community, including more native SOC-utilizing organisms that possess enzymes to decompose recalcitrant compounds, causing positive priming (Fontaine et al., 2003; Kuzyakov, 2010).Soil from a ponderosa pine ecosystem was amended weekly for 7 weeks with 500 μg C-glucose per gram soil (2.65 atom % ¹³C) in 100 μl deionized water or with 100 μl deionized water (n=5). Measurements of δ¹³C–CO₂ and [CO₂] enabled the partitioning of CO₂ into that derived from added glucose or from native SOC (C_SOC):where C_total is CO₂–C from glucose-amended samples, δ_total is the δ¹³C–CO₂ from glucose-amended samples, δ_glucose is the δ¹³C of the added glucose and δ_SOC is the δ¹³C–CO₂ evolved from the non-amended samples. Priming was calculated as the difference between SOC oxidation of the amended and non-amended samples. With this approach, any evolved CO₂ carrying the ¹³C signature of the added glucose is considered respiration of glucose, including ¹³C-labeled biomass and metabolites derived from prior glucose additions. Thus, this approach quantifies priming as the oxidation of SOC present at the beginning of the experiment, consistent with many other studies of priming (Cheng et al., 2003; De Graaff et al., 2010).In a parallel incubation for dual-stable isotope probing, the repeated-pulse samples received unlabeled glucose (500 μg C-glucose per gram soil) for 6 weeks while the non-amended and single-pulse samples received sterile deionized water. In week 7, samples received one of four isotope treatments (n=3): 97 atom % H₂ ¹⁸O (non-amended soil), 99 atom % ¹³C-glucose and 97 atom % H₂ ¹⁸O (single- and repeated-pulse soil), ¹²C-glucose and 97 atom % H₂ ¹⁸O (repeated-pulse soil) or ¹²C-glucose and H₂ ¹⁶O (repeated-pulse soil). After incubating for 7 days, soil was frozen at −40 °C. DNA was extracted, separated through isopycnic centrifugation, and two density ranges were sequenced for the bacterial 16S rRNA gene (Supplementary Figure 1): 1.731–1.746 g ml⁻¹ (hereafter called the SOC-utilizing community) and 1.759–1.774 g ml⁻¹ (hereafter called the glucose-utilizing community).Amplicons of the V3–V6 16S rRNA region were bar coded with broad-coverage fusion PCR primers and pooled before sequencing on a Genome Sequencer FLX instrument. These sequence data have been submitted to the GenBank database under accession number SRP043371. Data were checked for chimeras (Edgar et al., 2011), demultiplexed and quality checked (Caporaso et al., 2010). Taxonomy was assigned to genus at the ⩾80% bootstrap confidence level (Cole et al., 2009).We used the Shannon''s diversity index (H′), commonly used in microbial systems (Fierer and Jackson, 2006), to assess changes in microbial diversity. Analysis of variance was used to compare the amount of DNA within densities between isotope treatments (Supplementary Figure 2) and to test the effects of the treatments on the Shannon''s diversity (Figure 2) and Pielou''s evenness (Supplementary Figure 3) of the active bacterial communities, with post hoc Student''s t-tests, α=0.05. PRIMER 6 and PERMANOVA were used to create the nonmetric multidimensional scaling ordination and to compare bacterial communities between glucose treatments and the two sequenced density ranges.The single pulse of glucose suppressed SOC oxidation, whereas repeated pulses increased SOC oxidation (Figure 1). Few experiments to date have examined priming in response to repeated substrate amendments (Hamer and Marschner, 2005; Qiao et al., 2014), even though in nature soil receives repeated substrate pulses from litterfall and rhizodeposition. Our results demonstrate the dynamic response of SOC decomposition to repeated labile C inputs.Open in a separate windowFigure 1Weekly priming rates calculated as the difference in SOC respired between glucose-amended and non-amended soil (n=5).Dual-stable isotope probing was able to separate the growing bacteria into two groups with distinct DNA densities (P<0.001, PERMANOVA; Figure 3a), indicating differential uptake of ¹³C-glucose and ¹⁸O-water. In response to the initial glucose addition, the diversity of the growing glucose- and SOC-utilizing bacterial communities declined compared with the non-amended community (P<0.001, t-tests; Figure 2), driven by a strong decrease in evenness (Supplementary Figure 3). In the SOC-utilizing community, where DNA was labeled with ¹⁸O only, the relative abundance of Bacillus increased 4.9-fold compared with the non-amended control to constitute 31.6% of the community (Figure 3b). Bacillus survives well under low-nutrient conditions (Panikov, 1995), and is able to synthesize a suite of extracellular enzymes capable of degrading complex substrates (Priest, 1977), traits that are conducive for using SOC for growth. In the glucose-utilizing community, where DNA was labeled with both ¹³C and ¹⁸O, Arthrobacter increased 67.7-fold relative to the non-amended control to constitute 75.5% of the growing bacteria (Figure 3b). In culture experiments, Arthrobacter can rapidly take up and store glucose for later use (Panikov, 1995) and here we find it dominating the high-density DNA fractions, signifying that it is using the labeled glucose to grow. The increased biomass of Arthrobacter may have resulted in greater resource competition, thus reducing the diversity of the growing community, as is frequently found in plant communities (Bakelaar and Odum, 1978; Clark and Tilman, 2008).Open in a separate windowFigure 2Shannon''s diversity index (H′) of the non-amended, single-pulse, and repeated-pulse treatments (n=3) in the SOC- (mid-density) and glucose-utilizing (high-density) communities. Treatments with the same letter are not significantly different from each other (Student''s t, α=0.05).Open in a separate windowFigure 3(a) Nonmetric multidimensional scaling ordination showing differences in growing bacterial communities at the genus taxonomic level in the SOC-utilizing (mid-density; open symbols) and glucose-utilizing (high-density; closed symbols) groups of non-amended (Δ), single-pulse (○) and repeated-pulse (□) treatments (n=3). (b) Pie charts of genera in the SOC- and glucose-utilizing communities of the single- and repeated-pulse treatments (n=3). Genera with relative abundances >5% are listed in the figure legend.After repeated glucose amendments, the diversity of the growing community recovered to non-amendment levels (Figure 2) without strongly dominant organisms (Figure 3b and Supplementary Figure 3). The higher diversity found after repeated glucose pulses may be explained by trophic interactions where predators graze on prey populations that have been enlarged by resource addition, suppressing competition between prey species and causing secondary mobilization of nutrients (Clarholm, 1985). The decrease in total bacterial 16S rRNA gene copies in the repeated-pulse—compared with the single-pulse—treatment (Supplementary Figure 4) supports predation as a potential mechanism explaining the observed diversity increase after repeated glucose pulses.The recovery of diversity after repeated glucose pulses contrasts with resource competition theory (Tilman, 1987). When chronic additions of a limiting resource are applied, species diversity and evenness typically decrease (Bakelaar and Odum, 1978; Clark and Tilman, 2008) because competitive organisms become dominant. We observed this after the single glucose pulse, but not after repeated pulses. This diversity response may be the result of community shifts facilitated by short bacterial life cycles and the tens to hundreds of generations expected during the 7-week incubation (Behera and Wagner, 1974). In contrast, systems on which most ecological theory is based (for example, plants) might achieve perhaps 20 generations in a multi-decadal field experiment (Bakelaar and Odum, 1978; Clark and Tilman, 2008). With more generations, more community dynamics can occur, including increased resource cascades in which extracellular enzymes, metabolites or lysed cells of one functional group increase substrates for another (Blagodatskaya and Kuzyakov, 2008). Our results highlight the opportunity to test ecological theories in microbial ecosystems (Prosser et al., 2007), particularly as the short life cycles of microbes makes them well suited for pursuing ecological questions in an evolutionary framework (Jessup et al., 2004).The priming effect is ubiquitous, yet its drivers remain elusive. Our results suggest that changes in the diversity and composition of the growing bacterial community contribute to priming, and thus that ecosystem properties such as soil C storage may be sensitive to soil microbial biodiversity.     

水土保持林土壤抗蚀性能评价研究

运用土壤有机质含量、水稳性团聚体含量、水稳性团粒平均重量直径、团聚度和分散系数等各项指标,对不同树种组成、不同林龄水土保持林的土壤抗蚀性能进行分析、评价.结果表明,水土保持林对于提高土壤抗蚀性能具有重要作用,这种作用主要针对表层土壤而言;与油松纯林相比,油松阔叶树混交林土壤有机质含量较高,水稳性团聚体含量增加了1.71%～38.53%;且随着林龄的增长,水土保持林土壤抗蚀性能不断增强.     

土壤侵蚀对土壤肥力及土地生物生产力的影响

通过对红壤坡地不同土地利用方式土壤肥力及土地生物生产力的空间分异研究,揭示了土壤侵蚀对土壤肥力和土地生产力的负面影响．即侵蚀导致Ｎ、Ｐ、Ｋ等土壤速效养分含量减少及其在坡面上部的相对贫乏和下部的相对富集;土壤有机质含量降低;土壤机械组成中砂、粉、粘粒比率发生变化,表现为土壤沙化,土地生物生产力下降．