秸秆与生物质炭施用对土壤温室气体排放的影响差异

采用室内培养试验,向土壤中添加小麦秸秆和不同量生物质炭,同时比较探究秸秆与生物质炭施用对土壤温室气体排放及微生物活性的影响差异。试验共设5个处理:土壤(S)、土壤+1%小麦秸秆(WT)、土壤+1%生物质炭(BC1)、土壤+2%生物质炭(BC2)和土壤+4%生物质炭(BC4)。在培养期内,施秸秆处理土壤CO2排放量比对照处理S显著增加约12.60%~2005.63%,而施生物质炭处理降低约51.49%~97.93%。施秸秆处理的温室气体增温潜势(GWP)是对照处理S的1.12~19.24倍,而施生物质炭处理,即处理BC1、BC2和BC4的GWP分别降低了0.27%~64.06%,15.78%~94.01%和29.43%~92.28%。小麦秸秆施用会明显增加土壤温室气体排放,增加温室效应;而添加生物质炭对土壤CO2、N2O排放表现出一定的抑制作用,并明显减弱温室气体增温潜势,即生物质炭能明显减弱温室效应。添加小麦秸秆促进土壤微生物生物量碳的增加,提高FDA水解酶、脲酶、过氧化氢酶、磷酸酶活性;生物质炭施用一段时间后对土壤过氧化氢酶活性表现为显著激活作用。     

生物质炭与秸秆施用对红壤有机碳组分和微生物活性的影响

采用玉米秸秆、玉米秸秆生物质炭及其两者配合施用于红壤旱地的田间试验,通过有机碳分组、微孔板荧光法及底物诱导呼吸手段,研究不施肥条件下土壤有机碳组分、土壤酶活性及微生物底物利用速率的变化。结果表明:施用9个月后,与对照(不施任何物料)相比,单施秸秆提高土壤易矿化碳含量,对pH、总有机碳含量、惰性碳含量影响较小,而生物质炭及其与秸秆配施显著提高土壤总有机碳和惰性有机碳含量。单施秸秆提高土壤β-葡萄糖苷酶活性,而生物质炭及其与秸秆配施对土壤酶活性无影响。单施秸秆提高土壤微生物对葡萄糖、天冬氨酸和丁香酸的利用速率,提高土壤基础呼吸速率,而生物质炭及其与秸秆配施对土壤呼吸和微生物底物利用速率无影响。生物质炭与秸秆配施对土壤易矿化碳组分和基础呼吸呈显著互作效应。土壤基础呼吸与易矿化碳含量、β-葡萄糖苷酶活性及葡萄糖利用速率呈显著正相关。因此,秸秆炭化相比秸秆直接施用更有利于提高土壤稳定性碳库,降低土壤碳排放。     

油菜秸秆生物质炭对紫色土有机碳矿化和累积效应的影响

采用室内模拟培养试验,以油菜秸秆为对比,分析油菜秸秆生物质炭的施用对紫色土有机碳矿化的影响。在培养试验中,设置0.25%,0.5%和1%3个不同秸秆/土壤质量比的油菜秸秆还田培养处理,依次标记为J_1,J_2和J_3;同时,设置3个不同油菜秸秆生物质炭添加量的培养处理,其生物质炭添加量分别为J_1,J_2和J_3处理中添加的等量油菜秸秆在600℃下制成的生物质炭量,依次标记为S_1,S_2和S_3,所有处理置于25℃下避光培养56d。在培养期内,各处理中土壤有机碳(SOC)矿化速率随时间的变化符合乘幂函数。添加生物质炭处理的SOC累积矿化量、矿化强度和易分解有机碳占总有机碳含量的比例(Ca/CSOC)显著低于与其对应的油菜秸秆处理(P0.05),表明油菜秸秆经过热解制成生物质炭后可显著提高其在土壤中的稳定性。各添加生物质炭处理的SOC累积矿化量、矿化强度、Ca/CSOC和难分解有机碳占总有机碳含量的比例(Cs/CSOC)与对照(无外源碳添加)之间无显著性差异,添加生物质炭几乎不影响紫色土有机碳的分解。另外,S_1处理中外源碳的表观残留率显著高于其它培养处理,而S_2和S_3处理中外源碳的表观残留率则显著低于与其对应的油菜秸秆处理(J_2和J_3)(P0.05)。这表明在低量油菜秸秆还田时,将其制成生物质炭还田比直接还田更利于紫色土有机碳的积累。     

陇东黄土高原油污土壤环境因子对场地生态修复的季节响应

[目的]探讨陇东黄土高原地区油污土壤场地修复过程中土壤环境因子对季节变化的响应机制,为油污土壤植物微生物联合生态修复的推广应用提供参考。[方法]利用陇东适生植物"金盏菊"联合土著石油降解菌剂开展为期285d的场地原位修复试验,采用常规方法测定不同季节土壤理化特性、酶活性和微生物群落特性等环境指标。[结果](1)场地修复过程中土壤TPHs降解率在夏(24.62%±3.96%)秋(29.93%±1.94%)两季明显高于春(3.82%±0.91%)冬(9.24%±0.87%)两季;(2)季节变化和强化处理对油污土壤酶活性和理化特性存在显著交互效应(p0.05)。土壤pH值、含盐率和有机质含量在秋季明显降低(p0.05),而土壤速效养分均在夏秋含量最高(p0.05);(3)春季土壤微生物群落Evenness指数偏低(p0.05),夏季Richness指数明显增加(p0.05),Shannon-Wiener指数在夏秋两季明显高于其他季节(p0.05);(4)NMDS排序结果显示,夏秋两季分异于其他季节主要与土壤微生物群落Richness指数(r2=0.706 3,p=0.002)和速效磷(r2=0.615 7,p=0.005)有关;(5)低温季节影响土壤TPHs降解率的因素主要是土壤理化特性和土壤酶活性的叠加效应(54%),而夏秋两季土壤理化特性、土壤酶活性和土壤微生物特性三者的共同作用可解释土壤TPHs降解率变异的74%。[结论]土壤微生物群落Evenness指数和土壤酶活性偏低是造成春冬两季土壤TPHs降解率偏低,而土壤微生物群落ShannonWiener指数、Richness指数、速效磷含量和生物强化处理有效化水平是陇东地区夏秋两季土壤TPHs降解率明显增加的主要环境因素。     

生物质炭及过氧化钙对旱地红壤酶活性和微生物群落结构的影响

明确生物质炭等改良剂对土壤酶活性及土壤微生物群落结构的影响,对南方红壤旱地改良剂的合理施用及评价不同改良剂对旱地红壤肥力的影响具有重要意义。针对江西旱地红壤进行室内培养试验,试验设置4个处理,即CK、Ca(过氧化钙,1.72 g/kg)、C(生物质炭,21.46 g/kg)、C+Ca(过氧化钙,1.72 g/kg;生物质炭,21.46 g/kg),利用磷脂脂肪酸方法(PLFA),研究改良剂对土壤微生物量和组成以及土壤酶活性、土壤活性有机碳的影响。结果表明:旱地红壤中添加生物质炭和过氧化钙土壤提高可溶性有机碳(DOC)和微生物量碳(MBC)含量;增加蔗糖酶(INV)、淀粉酶(AMY)及脲酶(URE)活性,特别是配施(C+Ca)显著提高土壤活性碳含量及酶活性。PLFA分析表明,土壤微生物总PLFAs量的大小顺序为:C+CaCCaCK;各处理土壤细菌的相对丰度最大,大约占微生物总含量的80%,放线菌次之,大约占微生物总含量的13%～15%,而真菌和丛枝菌根真菌的相对丰度较低。生物质炭和过氧化钙增加了革兰氏阴性菌(GN)/革兰氏阳性菌(GP)值,尤以C+Ca处理增加幅度最大。主成分分析(PCA)表明,添加生物质炭和过氧化钙能够改善土壤微生物群落结构;计算主成分的综合得分,配施(C+Ca)处理的综合得分最高,对土壤微生物群落结构的影响最大。冗余分析(RDA)表明,土壤MBC、DOC和土壤INV酶活性是影响土壤微生物数量和结构的主要因子。因此,施用生物质炭和过氧化钙能够明显提高旱地红壤微生物生物量,改变红壤微生物群落结构以及激发红壤酶活性,且配施效果最显著。     

烟秆生物质炭与化肥配施对植烟土壤有机碳组分及微生物的影响

采用盆栽试验,设置空白对照(CK)、化肥(F)、烟秆生物质炭(B)、烟秆生物质炭配施化肥(BF)4个施肥水平,配施化肥参考烟草专用肥养分配比(N︰P2O5︰K2O=12︰7︰22)与烟秆生物质炭混合施用,探究烟秆生物质炭与化肥配施对植烟土壤有机碳组分及微生物的影响。结果表明：(1)与单施烟秆生物质炭相比,烟秆生物质炭与化肥配施土壤pH显著降低6.59%;土壤有机碳(SOC)和易氧化有机碳(EOC)含量显著增加,但土壤可溶性有机碳(DOC)含量显著降低29.63%,土壤颗粒有机碳(POC)和微生物生物量碳(MBC)影响不显著;(2)与单施化肥相比,烟秆生物质炭与化肥配施使得土壤脱氢酶活性大幅提高近2倍;而与单施烟秆生物质炭相比,土壤蔗糖酶、脱氢酶和过氧化氢酶活性则显著降低;(3)烟秆生物质炭与化肥配施,能够提高土壤细菌群落丰富度及物种多样性;同时,与单施烟秆生物质炭相比,配施化肥使参与土壤有机质分解的土壤绿弯菌门相对丰度提高了16.79%,但土壤细菌α、β多样性显著降低;(4)RDA分析表明,土壤SOC、EOC、POC、MBC是影响土壤有机质分解和参与碳循环细菌分布的主要因素。综上所述,...     

长期连续施用生物质炭对土壤肥力、小麦产量和微生物群落结构的影响

摘 要：基于始于2010年的长期田间试验，研究了每年6 t/hm2 玉米秸秆还田(Straw)，每年2.4 t/hm2 (BC1)、6 t/hm2 (BC3)和12 t/hm2 (BC5)玉米秸秆生物质炭添加对土壤肥力、小麦产量和微生物群落结构的影响。结果表明：与不添加玉米秸秆及其生物质炭的处理（CK）相比，BC5处理显著提高了小麦产量，三个剂量生物质炭处理平均增加小麦产量15%。经过11年的连续添加，土壤容重（BD）从CK的1.06 g/cm3降低至BC5处理的0.73 g/cm3，总碳（TC）从CK的14.9 g/kg 增加至BC5处理的83.8 g/kg。土壤有效氮（AN）、速效钾（AK）和有效磷（AP）均随施用量增加而增加，表现为BC5 > BC3 > BC1 > Straw ≥ CK。本研究中，长期连续施用玉米秸秆及其生物质炭没有显著改变细菌群落多样性，而BC5处理略微增加真菌群落多样性。PICRUSt2功能预测表明生物质炭处理显著增加了碳固定相关功能基因丰度，而降低碳降解相关功能基因丰度。     

棉秆生物质炭添加对棉田土壤理化性质及根际微生物群落结构的影响

以新疆滴灌棉田为研究对象,研究生物质炭施用对棉田土壤理化性质和棉花根际土壤微生物群落特征的影响。试验采集了不施生物质炭(CK)、施生物质炭3 t/hm²(BC1)、施生物质炭6 t/hm²(BC2)和施生物质炭9 t/hm²(BC3)4种处理棉花根际土壤,分析土壤理化性质和根际土壤微生物群落结构和组成的变化。结果表明,生物质炭施用后土壤pH和电导率分别下降了5.58% ~ 9.18%和5.38% ~ 18.04%;与CK相比,生物质炭施用使土壤有机质、全氮、全磷、碱解氮和有效磷含量均显著增加,且BC3增加效果最好,但生物质炭施用导致土壤钾含量显著降低。生物质炭施用显著降低了浮霉菌门(Planctomycetota)、芽单胞菌门(Gemmatimonadota)、厚壁菌门(Firmicutes)和被孢霉门(Mortierellomycota)相对丰度,但增加了子囊菌门(Ascomycota)、斯克尔曼氏菌属(Skermanella)、苔藓杆菌属(Bryobacter)、毛壳菌属(Chaetomium)、头束霉菌属(Cephalotrichum)、金孢属(Chrysosporium)和拟棘壳孢属(Pyrenochaetopsis)的相对丰度。另外,生物质炭施用降低了根际土壤微生物多样性和细菌丰富度,但根际土壤真菌的丰富度提高。生物质炭施用显著影响根际土壤微生物特别是真菌的群落结构,电导率、速效钾和pH是根际土壤微生物群落的主要影响因子。研究表明,生物质炭施用可以改善棉田土壤理化性质进而影响根际土壤微生物群落组成和结构,9 t/hm²为本试验的推荐施用量。     

不同质地潮土施用小麦和玉米秸秆生物质炭对玉米养分吸收和根际土壤胞外酶活性的影响

  【目的】  生物质炭施用于农田土壤中能够改善土壤肥力,并提高作物生产力,而该效应受到土壤条件和生物质炭条件的限制。针对不同土壤条件探究适宜的生物质炭利用方式,对促进农业生产具有重要意义。  【方法】  采用盆栽试验,以壤质和粘质两种质地的潮土为研究对象,分别施用玉米秸秆炭(MBC)和小麦秸秆炭(WBC)两种生物质炭,并以不施用生物质炭的处理为对照(CK)。测定各处理玉米苗期生长、生理抗性和养分吸收差异,并分析各处理根际土壤理化性质和胞外酶等活性。  【结果】  1)与CK相比,壤质潮土中,WBC处理下玉米地上部生物量显著增加了43.7%,总根长显著增加34.3%,而MBC处理没有显著影响。粘质潮土中,WBC和MBC对玉米生物量和根系构型均影响较小。2) WBC和MBC在壤质和粘质潮土中显著降低了苗期玉米叶片中MDA含量,降低幅度在32.7%～55.3%,且两种生物质炭之间没有显著差异;粘质潮土中,MBC处理显著提高了玉米叶片超氧化物歧化酶(SOD)活性,壤质潮土中,WBC和MBC处理对SOD活性均没有显著影响。3)壤质潮土中,生物质炭对苗期玉米地上部氮含量没有显著影响,而对作物全磷和全钾含量有显著促进作用,WBC处理的地上部全磷和全钾含量分别比对照显著提高23.5%和28.7%,且显著高于MBC处理。在粘质潮土中,WBC和MBC处理对地上部全氮和全磷含量均没有显著影响,而MBC处理提高了全钾含量。4)在壤质和粘质潮土中施用生物质炭均改善了根际土壤理化性质。与对照相比,壤质潮土中MBC处理的土壤速效磷含量显著增加了25.4%;粘质潮土中WBC和MBC处理速效磷含量均显著增加了15.03%,并且显著提高了阳离子交换量(CEC)。生物质炭处理提高了根际土壤胞外酶活性,在粘质潮土中WBC和MBC处理的胞外酶活性没有显著差异,而在壤质潮土中WBC处理的酶活性高于MBC处理。  【结论】  施用生物质炭能够调控根际土壤酶活性,提高有效磷含量,改善玉米根系构型,提高苗期玉米养分吸收并增加生物量。生物质炭的施用效果在壤质潮土中比粘质潮土中更好,小麦秸秆炭效应优于玉米秸秆炭。     

生物质炭对土壤有机质活性的影响

为了解施用生物质炭对土壤碳组分的潜在影响,通过室内2年盆栽培养试验研究施用不同用量生物质炭对土壤有机碳积累、有机碳稳定性、微生物量碳和水溶性有机碳的影响,并与施用等碳量的小麦秸秆、酸洗生物质炭(去除生物质炭中的速效养分)及同时施用小麦秸秆与生物质炭的处理进行比较。结果表明,施用生物质炭可显著提高土壤有机碳的积累,增加土壤有机碳的氧化稳定性,降低土壤水溶性有机碳。施用生物质可在短时间内增加微生物量碳,但随着培养时间的增加,其微生物量碳逐渐下降,最终明显低于对照土壤(不施有机物料的处理)。土壤水溶性有机碳的下降可能与生物质炭对其吸附固定有关,而短时间内激发微生物量碳增加可能与施入生物质炭增加了土壤有效养分、改善土壤微生物生长环境有关。研究结果认为,长期单一施用生物质炭可能会引起土壤有机质生物活性的下降,但生物质炭与一般生物质有机肥配合施用可减免这些负影响。     

On the Taxonomy of the Members of ‘Saccharum Complex’

The difficulty and problems encountered in the study of cultivated plants, in general and sugarcane, in particular has been indicated. In order to understand these problems, a brief review on the taxonomy of Saccharum and closely related taxa, namely, Erianthus, Sclerostachya, Narenga and Miscanthus (generally known as ‘Saccharum complex’) has been given. A short account on the important morphological features that are specific to sugarcane has also been stated as classification is commonly based on such morphological characters. A note has been added on the chromosome number, origin, and distribution of the species of ‘Saccharum complex’ members. Taxonomic keys have been devised for identification of the genera of Saccharinae and for the species of Saccharum and Erianthus occurring in India. A new combination, Sclerostachya fallax (Balansa) Amalraj et Balasundaram, has also been proposed.     

Agricultural potential of Mediterranean grain and forage legumes: Key differences between and within Vicia species in terms of phenology, yield, and agronomy give insight into plant adaptation to semi-arid environments

In order to identify key adaptive traits which affect productivity in Mediterranean grain and forage legumes and simultaneously determine the agricultural potential of a wide range of Vicia species, germplasm collected from the wild throughout the eastern Mediterranean was grown under semi-arid conditions in Tel Hadya, Syria (313 mm growing season rainfall). These included species currently in use in Mediterranean agriculture, such as V. sativa L., as well as those more widely used in the past-such as V. ervilia L., but also a broad selection from Section Narbonensis (B. Fedtsch. ex Radzhi) Maxted, including V. narbonensis L, V. johannis Tamamsch., V. hyaeniscyamus Mout., V. serratifolia Jacq., and V. kalakhensis Khattab et al. V. faba, a near relative of the taxa in Section Narbonensis, was included as a domesticated control. Where applicable, a representative range of subspecies was used. Accessions were chosen from a wide range of habitats in terms of latitude (31.02–40.72 decimal^o), longitude (27.1–43.17 decimal^o), altitude (20–1510 m), rainfall (180–1700 mm/yr) and soil depth (5–50 cm) in order to maximise diversity within species. Agricultural potential was determined by measuring seed, hay and biological yield, as well as agronomic traits such as harvest index, standing crop height, and seed size. The comparative influence of phenology and key agronomic traits such as plant habit and seed size on productivity varied tremendously between species, depending on their reproductive strategies. In V. sativa and V. ervilia, the smaller seed species which rely on long vegetative phases and growing seasons to accumulate sufficient biomass to set seed, and in which there was comparatively little agronomic variation, phenology had a large impact on yield. In early emerging taxa such as V. ervilia and V. s. subsp. sativa, with built-in long vegetative phases and growing seasons, seed yield was negatively correlated with flower ing (r = –0.86 to –0.88), whereas the opposite was the case for later emerging taxa such as V. s. subsp. nigra (L.) Ehrh. (r = 0.95). Within V. narbonensis and relatives, the larger seeded Vicia species which rely on more conservative reproductive strategies where high seedling vigour associated with large seeds enables the species to enter reproductive phases relatively early, phenology had a much smaller impact on yield than did variation of key traits such as seed weight, plant habit and pod shattering. Among the undomesticated germplasm harvest indices ranged from 0.09–0.31, hay yields from 0.1–3.4 t/ha, seed yield from 0–2.0 t/ha, and dry matter at maturity from 1.6–6.5 t/ha. Sub-specific taxonomy was crucial in determining agronomic potential. V. narbonensis var. aegyptiaca Kornhuber ex Asch. et Schweinf. showed the most potential, combining an upright habit, large seeds (212 mg) and tendency to retain intact pods after maturity, with the highest yield, harvest index and crop height of all the wild Vicia species. V. sativa subsp. sativa, V. ervilia and V. narbonensis var. narbonensis were less productive, but still showed agricultural potential. The smaller seeded V. narbonensis, var. affinis, var. jordanica H. Schäf. and var. salmonea (Mout.) H. Schäf., and their close relatives V. johannis, V. hyaeniscyamus, V. serratifolia and V. kalakhensis have little to offer Mediterranean agriculture on the basis of poor agronomy.     

Polymorphism of high molecular weight glutenin subunits in three species of Aegilops

A collection of 136 accessions of Aegilops umbellulata (39), Ae. comosa (75) and Ae. markgrafii (22) was analysed for high-molecular-weight (HMW) glutenin subunits composition. The homogeneity of the accessions was studied and 55.1% of the collection was homogeneous for HMW glutenin subunits (29 Ae. umbellulata, 33 Ae. comosa and 14 Ae. markgrafii). The HMW glutenin subunits of Ae. umbellulata are encoded by the Glu-U1 locus; in Ae. comosa results showed that this proteins are encoded at the 1M chromosome, and the locus was named Glu-M1. In Ae. markgrafii it was assumed that HMW glutenin subunits were encoded by an homoeologous locus and it was named Glu-C1. All the accessions of Ae. umbellulata and Ae. markgrafii expressed both, x-type and y-type subunits. Among the Ae. comosa accessions, only one expressed an x-type subunit alone. All the accessions of Ae. umbellulata and some of Ae. comosa had x-type glutenins of higher molecular weights than those commonly present in bread wheat. A total of 8 alleles were detected at the Glu-U1 locus, 11 at the Glu-M1 and 4 at the Glu-C1. The new HMW glutenin variation found in this work suggests their possible utilisation in breeding for wheat quality.     

Effect of agroforestry woody species on earthworm activity and physicochemical properties of worm casts

The effects of five agroforestry woody species (Dactyladenia barteri, Gliricidia sepium, Leucaena leucocephala, Senna siamea andTreculia africana) on the surface aasting activity ofHyperiodrilus africanus were studied in an Alfisol (Oxic Paleustalf) in southwestern Nigeria. Casting activity under the woody species decreased in the following order:Dactyladenia sp. (26.4 Mg ha^-1 year^-1)>Gliricidia sp. (24.4 Mg ha^-1 year^-1)>Treculia sp. (22.9 Mg ha^-1 year^-1)>Leucaena sp. (18.6 Mg ha^-1 year^-1)>Senna sp. (18.3 Mg ha^-1 year^-1). These differences in casting activity were partly explained by microclimatic effects. Irrespective of the woody species, the worm casts were higher in clay and silt contents, bulk density, water-stable aggregates, pH, organic C, exchangeable cations, effective cation exchange capacity, and extractable P levels than the corresponding surface soils. The woody species did affect the physicochemical properties and P sorption of the worm casts. The content of water-stable aggregates of worm casts decreased in the following order:Dactyladenia sp.>Treculia sp.>Senna sp.>Leucaena sp.>Gliricidia sp. Large differences in extractable P levels were observed.Senna sp. was associated with the highest extractable P level (11.5 mg kg^-1) andTreculia sp. the lowest (4.9 mg kg^-1). P sorption was highest on worm casts underDactyladenia sp. and lowest on those underTreculia sp. Without fertilizer application, there were no significant differences in the dry weight of maize grown in the different worm casts. With NPK applications, the dry weight of maize grown in worm casts associated withTreculia sp. was significantly lower than that of maize grown in the other worm casts, mainly due to the low extractable P level. Despite a high organic C and exchangeable K status, maize grown in the worm casts still responded significantly to N and K applications. The N uptake by maize grown in worm casts associated withTreculia sp. was lower than that in the other treatments.     

Phylogenetic relationships of Actinidia and related genera based on micromorphological characters of foliar trichomes

The phylogenetic relationships within the genus Actinidia and among 3 allied genera Clematoclethra, Saurauia, and Sladenia have been very controversial. In order to understand the systematic implication of foliar trichomes in those genera, the micromorphological characteristics of foliar trichomes from 34 taxa were examined by means of light microscopy and scanning electron microscopy (SEM). The observed micromorphological characteristics were classified into eight main types. A phylogenetic analysis of Actinidia and related genera based on 15 micromorphological characters of foliar trichomes was conducted using Wagner parsimony method and Sladenia celastrifolia, Rhododendron hybridum and R. simsii as a complex outgroup. In the resulting phylogenetic tree, Actinidia is shown to be a monophyletic group, and Clematoclethra and Saurauia form another monophyletic group. This study also presents the phylogenetic relationships among 4 sections within the genus Actinidia: the monophyly of sect. Leiocarpae and of sect. Strigosae, and polyphyly of sect. Maculatae and of sect. Stellatae.     

Genetic diversity of rhizobia associated with indigenous legumes in different regions of Flanders (Belgium)

We investigated the diversity of rhizobia isolated from different indigenous legumes in Flanders (Belgium). A total of 3810 bacterial strains were analysed originating from 43 plant species. Based on rep-PCR clustering, 16S rRNA gene and recA gene sequence analysis, these isolates belonged to Bradyrhizobium, Ensifer (Sinorhizobium), Mesorhizobium and Rhizobium. Of the genera encountered, Rhizobium was the most abundant (62%) and especially the species Rhizobiumleguminosarum, followed by Ensifer (19%), Bradyrhizobium (14%) and finally Mesorhizobium (5%). For two rep-clusters only low similarity values with other genera were found for both the 16S rRNA and recA genes, suggesting that these may represent a new genus with close relationship to Rhodopseudomonas and Bradyrhizobium. Primers for the symbiotic genes nodC and nifH were optimized and a phylogenetic sequence analysis revealed the presence of different symbiovars including genistearum, glycinearum, loti, meliloti, officinalis, trifolii and viciae. Moreover, three new nodC types were assigned to strains originating from Ononis, Robinia and Wisteria, respectively. Discriminant and MANOVA analysis confirmed the correlation of symbiosis genes with certain bacterial genera and less with the host plant. Multiple symbiovars can be present within the same host plant, suggesting the promiscuity of these plants. Moreover, the ecoregion did not contribute to the separation of the bacterial endosymbionts. Our results reveal a large diversity of rhizobia associated with indigenous legumes in Flanders. Most of the legumes harboured more than one rhizobial endosymbiont in their root nodules indicating the importance of including sufficient isolates per plant in diversity studies.     

Diversity of native rice (Oryza Poaceae:) species of Costa Rica

We found several populations of wild Oryza species in the lowlands of Costa Rica. The plants showed extensive morphological variation, suggesting the presence of several species. In the morphologic study, 33 traits were scored for plants of all the species. A principal component analysis revealed the significant morphological separation of the different species. The analyses indicated that there are three species, O. grandi­glumis, O. latifolia and O. glumaepatula. Two putative hybrid types were found, both significantly differing in their morphology from the known species and intermediate at several traits. O. grandi­glumis is a new record for Costa Rican flora. Its main population is located in Caño Negro Wildlife Refuge, Los Chiles, Alajuela. O. latifolia is distributed throughout the lowlands of the country and the plants of the Atlantic slope are significantly bigger in general habit than those of the Guanacaste area. During this study a population of O. glumaepatula of hundreds of thousands of plants was discovered in the Medio Queso River wetland, Los Chiles, Alajuela. This population is the most important source of genes for cultivar's improvement from the primary gene pool of rice in Costa Rica. The small ligule and the wide flag leaf characteristic of the two CCDD species separated them from the AA diploid O. glumaepatula. Seed size, ligule size, number of branches in the panicle, plant height and sterile lemma length are all bigger in O. grandi­glumis, and influenced the second factor that separated the CCDD species in two discrete clusters. The species found offer great possibilities for the improvement of rice cultivars and they should be thoroughly studied and appropriately protected.     

Effect of passage through the intestinal tract of detritivore earthworms (Lumbricus spp.) on the number of selected Gram-negative and total bacteria

Laboratory experiments were carried out to investigate the fate of bacteria during and after passage through the intestinal tract of detritivore earthworms. Earthworms (Lumbricus spp.) were fed with cattle dung inoculated 7 days previously with one of five different Gram-negative bacteria. Bacterial concentrations were determined 2 days later in dung and soil, and in gut material from different parts of the earthworm intestinal tract. A high percentage (28–82%) of the total bacteria (epifluorescence direct counts) in the earthworm gut content was culturable. The concentration of total heterotrophic aerobic bacteria did not vary significantly among the five different bacterial additions and the non-inoculated control. In earthworm casts the number of total heterotrophs per gram dry matter (2.1×10⁹) was higher than in soil (1.7×10⁸), but lower than in the dung (1.5×10¹⁰). The test-bacteria, however, showed different survival patterns along the earthworm intestinal tract. The concentrations of Escherichia coli BJ 18 and Pseudomonas putida MM 1 and MM 11 in earthworm casts were lower than in the ingested dung, while concentrations of Enterobacter cloacae A 107 and Aeromonas hydrophila DMU 115 in dung and casts were similar. Ent. cloacae, and to aminor extent E. coli, were reduced in numbers by several orders of magnitude in the pharynx and/or crop. In the hind gut, however, the concentration of Ent. cloacae had increased to the same level as in the ingested dung, while the concentration of E. coli remained low. Our observations indicate that the bacterial flora of ingested food materials changes qualitatively and quantitatively during gut transit.