Dynamics of Aggregate Stability Influenced by Soil Management and Crop Residues

Abstract

The type of tillage and crop systems used can either degrade or cause a recovery of the structure of agricultural soils. The objective of this study was to determine the structural stability of the soil using mean weight diameter (MWD) of soil aggregates in three different periods of a succession of crops consisting of beans/cover plants/maize under no tillage (NT) and conventional tillage (CT) management systems. Soils were sampled at 0‐ to 5‐cm and 5‐ to 15‐cm depths in three periods (P1, P2, P3): 1) November 2002 (spring/summer), 2) April 2003 (beginning of autumn), and 3) December 2003 (end of spring/beginning of summer). Aggregate stability was determined by wet sieving. The effects of the tillage systems, vegetal residues, and sampling depths on the structural stability of the aggregates were assessed and then related to organic matter (OM) contents. Aggregate stability showed temporal variation as a function of OM contents and sampling period. No tillage led to high MWD values in all study periods. The lowest MWD values and OM contents were observed 4 months after the management of the residues of cover plants. This finding is consistent with the fact that at the time of the samplings, most of the OM had already mineralized. The residues of sunn‐hemp, millet, and spontaneous vegetation showed similar effects on soil aggregate stability.     

淡色生赤壳菌YLZ42发酵液稳定性及活性物质的初步研究

从广西玉林土壤中分离获得一株淡色生赤壳菌YLZ42,其发酵产物对蜡样芽孢杆菌等具有良好的抑制作用。以蜡样芽孢杆菌为指示菌对该菌株发酵产物的活性特性进行了研究,结果表明该菌发酵液的抑菌活性在40~60℃温度范围内比较稳定,60℃加热1 h,活性损失小于7%;在p H 2~7范围内抑菌活性稳定,p H大于7时,活性损失较大,在p H 8条件下处理12 h活性损失已达到64%;发酵液适宜在酸性条件下保存,p H 4的发酵液放置15 d后活性损失小于5%;活性物质对紫外线不敏感,经紫外照射24 h后抑菌活性基本不变。然后采用萃取、薄层制备、HPLC制备等提取分离方法得到了抑菌活性物质的纯品,分子量为513,与相关文献中报道的活性代谢产物的分子量均不同。     

基于综合法的重力坝整体稳定性安全度分析

采用ABAQUS有限元软件,利用先降强后超载的综合法、依据塑性应变能和屈服区贯通两种突变性判据对重力坝进行整体稳定性分析。分析表明：采用综合法时,初始强度折减系数为1.3分析重力坝整体稳定性比较适合;初始强度折减系数较大,综合安全系数也较大。     

Competitive Strategies and Growth of Neighbouring Bromus valdivianus Phil. and Lolium perenne L. Plants Under Water Restriction

Often perennial pastures have to tolerate soil water restriction during summer, which can affect the relative abundance of the most desired species. In the south of Chile, Bromus valdivianus and Lolium perenne are preferred species. The competitiveness of B. valdivianus and L. perenne was evaluated when sown in pots in a glasshouse as monocultures or a mix (50/50 %) with restricted water availability. The water restriction treatments were as follows: 80–85 % field capacity (FC), 45–50 % FC and 20–25 % FC, maintained for 1196 growing degree days. For both species, dry matter (DM) accumulated per plant and per pot decreased similarly with increasing water restriction, but at tiller level, resource allocation differed, for B. valdivianus root growth was accentuated over aerial development, but this was less so for L. perenne. The foliage mass per tiller for B. valdivianus decreased relatively more than that for L. perenne with increasing water restriction. As monocultures, B. valdivianus produced larger tillers than L. perenne, such that B. valdivianus tillers had 2.2 times greater lamina mass, 3.6 times more leaf area and 2.5 times greater root mass than those of L. perenne. However, L. perenne produced a larger number of smaller tillers that enabled foliage, root and total mass at plant level, to be similar to that of B. valdivianus. Within the mixed pasture, L. perenne tiller density increased compared with when grown as a monoculture, but not for B. valdivianus. The results of the study suggested that L. perenne and B. valdivianus have differences in growth strategies that allow them to survive under environmental stress and competition. Competitiveness increased for L. perenne aboveground without water restriction and that for B. valdivianus increased belowground as water restriction was increased.     

Hiper Strut新型悬架对整车操纵稳定性的影响评价

为研究Hiper Strut（High Performance Strut）新型悬架对整车操纵稳定性的影响,将一款国产SUV所使用的麦弗逊前悬架替换为适用于该车的Hiper Strut前悬架.利用Adams/Car建立使用Hiper Strut前悬架的整车仿真模型,并进行相关操纵稳定性仿真试验.仿真结果表明,使用该新型悬架的整车在操纵稳定性评价中得分较高,说明Hiper Strut悬架的结构设计有助于提高整车操纵稳定性.所建模型及仿真分析对Hiper Strut新型悬架的深入研究和推广应用具有一定的实际参考价值.     

Quality of Mince from Alaska Pollock (Theragra chalcogramma) Frames

The quality and frozen stability of minces produced from Alaska pollock (Theragra chalcogramma) frames were compared against commercially produced fillet mince. Fresh frame mince had a higher defect level, lower nutritional content, and darker color. Cooked frame mince had a softer texture than commercial mince. After 6 mo frozen storage, frame mince had deteriorated to where it had unacceptable color, flavor and cooked texture. Rancidity increased more rapidly in frame mince than commercial fillet mince. Blending frame mince at levels of 10% to 20% with commercial mince produced acceptable fresh quality but marginal frozen storage stability.     

Soil structural stability and erosion rates influenced by agricultural management practices in a semi‐arid Mediterranean agro‐ecosystem

Unsuitable agricultural practices can cause loss in soil quality and erodibility to thus increase or trigger desertification under Mediterranean conditions. A field experiment was performed at the El Teularet‐Sierra de Enguera Experimental Station (eastern Spain) to assess the influence during a 5‐yr period of different agricultural practices on physical and chemical indicators of soil quality (total and water‐soluble carbohydrates, glomalin‐related soil proteins (GRSP), total organic carbon, aggregate stability (AS), vegetation cover and soil erosion). The management practices included residual herbicide use, ploughing, ploughing + oats, addition of oat straw mulch and a control (land abandonment). Adjacent soil under natural vegetation was used as a reference for local, high‐quality soil and as a control for comparison with the agricultural soils under different management practices. Oat straw mulching led to higher levels of water‐soluble carbohydrates, GRSP and AS and lower soil erosion rates, resulting in values similar to those in the soil under native vegetation. The lowest levels of carbohydrates and GRSP were for the plots that were treated with herbicide or were ploughed. The maintenance of and increases in stable aggregates promoted by the different agricultural management practices over the years were attributed to increases in labile organic fractions such as carbohydrates and to the GRSP content. The results demonstrate that land abandonment (control plot) or the use of a cover (plants or straw) contributes to increases in soil quality and reduces the risk of erosion. The research also shows that sustainable agricultural management allows soil to recover and that the use of straw mulching is the most effective management strategy.     

南蝠捕食行为及其对稻田虫害发生的影响

为了解南蝠(Ia io)在维持稻田生态系统稳定性中的作用,采用粪便分析和直观法,研究了南蝠的捕食行为及其对稻田虫害发生的影响。结果表明:南蝠是控制农田害虫的有效天敌之一,在其捕食区域内稻田病虫害较轻,2～3km半径区域水稻的平均产量比6～7km半径区域增产98.2kg/667m2;南蝠的食物组成除鸟类外,还有鞘翅目、鳞翅目、半翅目、直翅目、膜翅目和蜱螨等昆虫/生物,多数种类昆虫均是水稻害虫或病害的传播者。南蝠在维护稻田生态系统稳定性和提高农作物产量方面具有重要的作用。     

青贮玉米有氧暴露过程中营养品质及细菌群落多样性

青贮饲料营养品质的优劣很大程度上由其微生物组成决定,有氧暴露会引起微生物群落多样性发生变化。为探明有氧暴露后青贮玉米细菌多样性分布及其对青贮品质的影响,本试验对有氧暴露后0 h, 1 h, 8 h, 24 h, 36 h, 48 h, 72 h, 96 h, 120 h青贮玉米(Zea mays L.)的营养品质和细菌群落结构测定与分析。结果表明：青贮玉米在有氧暴露过程中可溶性糖、粗蛋白和干物质含量降低,中性洗涤纤维和酸性洗涤纤维含量上升,钙和磷含量变化不明显;有氧暴露过程中细菌多样性呈先升高后降低的趋势,在有氧暴露36 h时细菌多样性最高;有氧暴露不同时间,优势细菌门分别是厚壁菌门和放线菌门,优势细菌属为乳酸菌属;有氧暴露过程中厚壁菌门与可溶性糖和蛋白质含量呈极显著正相关关系,与中性洗涤纤维和酸性洗涤纤维含量呈极显著负相关关系;变形菌门和酸杆菌门细菌与可溶性糖含量呈负相关关系。综上所述,与生产实践相结合青贮玉米应该在有氧暴露8 h内完成饲喂。     

多元胡麻轮作模式对土壤团聚体特征及氮素含量的影响

为探索西北旱区利于维护农田土壤良好的理化性质的种植制度,采用大田试验,以休闲处理为对照,研究了100% Flax、50% Flax（Ⅰ）、50% Flax（Ⅱ）和25% Flax 4个胡麻种植频率,100% Flax（Cont F）,50% Flax（Ⅰ）：WFPF、FPFW、PFWF、FWFP,50% Flax（Ⅱ）：FWPF、WPFF、PFFW、FFWP,25% Flax：WPWF、PWFW、WFWP、FWPW,其中F为胡麻,P为马铃薯,W为小麦,共13个种植模式下农田土壤团聚体稳定性及其氮素含量的分布状况。结果表明：土壤团聚体平均重量直径（MWD）和几何平均直径（GMD）与分形维数（D）、团聚体破坏率（PAD）之间呈极显著负相关关系,与0.5~1.0 mm粒径中NH₄⁺-N含量和>2 mm粒径中NO₃^--N含量呈显著正相关关系,团聚体稳定性的增大显著提高了土壤中硝铵态氮含量。50% Flax（Ⅰ）胡麻种植频率以及马铃薯茬口显著增加了>0.25 mm粒径大团聚体含量、MWD和GMD,显著降低了土壤的D、PAD和土壤可蚀性（K）;不同轮作模式下FWFP处理显著提高0.25~2.00 mm粒径水稳性大团聚体含量,显著增加土壤团聚体MWD和GMD和抗侵蚀能力,降低分形维数和团聚体破坏率,使其土壤团聚体结构越稳定。由此表明,FWFP轮作模式可显著提高土壤水稳性大团聚体含量,促进土壤大团聚体的形成,增加土壤团聚体稳定性,对我国北方胡麻轮作可持续发展具有重要指导意义。