Effects of soil bulk density on seminal and lateral roots of young maize plants (Zea mays L.)

It is well established that increasing soil bulk density (SBD) above some threshold value reduces plant root growth and thus may reduce water and nutrient acquisition. However, formation and elongation of maize seminal roots and first order lateral (FOL) roots in various soil layers under the influence of SBD has not been documented. Two studies were conducted on a loamy sand soil at SBD ranging from 1.25 g cm^–3 to 1.66 g cm^–3. Rhizotrons with a soil layer 7 mm thick were used and pre‐germinated plants were grown for 15 days. Over the range of SBD tested, the shoot growth was not influenced whereas total root length was reduced by 30 % with increasing SBD. Absolute growth rate of seminal roots was highest in the top soil layer and decreased with increasing distance from the surface. Increasing SBD amplified this effect by 20 % and 50 % for the top soil layer and lower soil layers, respectively. At the end of the experiment, total seminal roots attributed to approximately 15 % of the total plant root length. Increasing SBD reduced seminal root growth in the lowest soil layer only, whereas FOL root length decreased with SBD in all but the uppermost soil layer. For FOL, there was a positive interaction of SBD with distance from the soil surface. Both, increasing SBD and soil depth reduced root length by a reduction of number of FOL roots formed while the length of individual FOL roots was not influenced. Hence, increasing SBD may reduce spatial access to nutrients and water by (i) reducing seminal root development in deeper soil layers, aggravated by (ii) the reduction of the number of FOL roots that originate from these seminal roots.     

不同灌溉策略下冬小麦根系的分布与水分养分的空间有效性

通过田间试验研究了少量多次和少次多量的灌溉方式下冬小麦根系的分布与水分养分的空间有效性。结果表明 :少量多次的灌溉方式降低了冬小麦返青后表层根系的生长 ,减少了拔节后该层根系的衰退。在少次多量的灌溉方式下返青期不灌水促进了表层根系的生长 ,然而拔节后该层根系衰退较多 ,但中层 ( 3 0～ 60cm)根系生长高于少量多次的灌溉方式。不同灌溉策略下根系分布的差异并不影响冬小麦对土壤水分和养分的吸收 ,由于播前土体内蓄水不足 ,三种灌溉方式下 0～ 90cm土壤可用水在收获后均消耗殆尽。灌溉促进了表层硝态氮的吸收和向下迁移 ,但两种灌溉方式下硝态氮在土体内的迁移均未超出 60cm土体 ,仍在根层之内。而不同的灌溉方式对冬小麦全生育期内土体速效磷钾的分布没有影响。扬花前两种灌溉方式下冬小麦的生长发育和养分的吸收并无差异 ,扬花后少次多量的灌溉方式由于水分供应不足 ,影响了灌浆 ,降低了千粒重 ,进而影响了产量 ,同时土壤水分缺乏也减少了该时期养分的吸收。而在少量多次的灌溉方式下 ,扬花后灌水不仅可以促进冬小麦灌浆 ,提高千粒重 ,而且增加了对养分的吸收。     

农田耕作措施对近地表风沙运动的影响

我国北方传统耕作模式的犁耕、耙磨及磨平整地措施对农田近地表风沙活动影响显著。通过对春季半干旱沙区农田风沙活动的观测,本文对犁耕、耙磨磨平两种处理下近地表0~60 cm内输沙率和风沙流结构随田块长度的变化规律进行了研究。结果表明,耙磨磨平处理破坏了犁耕在农田地表形成的非可蚀性土块结构,使近地表0~60 cm内输沙率增大,在强沙尘事件中增加幅度可达数10倍。犁耕地表上输沙率随田块长度的增加呈指数关系递增,而耙磨磨平后风沙流发育的饱和路径长度减小,在强沙尘事件中近地表输沙率随田块长度的增加而递减。此外,上风向处犁耕地表低层输沙比例(Q0-10/Q0-60)小于耙磨磨平地表,且随田块长度的增加逐渐增大,而耙磨磨平后风沙流结构随田块长度的变化不大。     

强制补饲对早期断奶仔猪腹泻及消化道的影响

选用二元杂交仔猪10窝,随机分为对照组和试验组,8日龄时开始补饲,对照组自由采食,试验组在自由采食基础上,增加强制补饲至21日龄.结果表明,强制补饲可明显增加28日龄断奶仔猪断奶前的饲粮采食量:试验组(600.6±29.97)g,对照组(202.6±16.59)g(P<0.01);断奶前后日增重显著提高(P<0.01);断奶后仔猪腹泻频率显著降低:试验组(14.78±2.13)%,对照组(45.75±6.26)%(P<0.01);28日龄胃内pH值低于4,显著低于对照组(P<0.05);肠道内大肠杆菌数量显著低于对照组(P<0.01);35日龄时,试验组胃质量显著大于对照组(P<0.05),胃和十二指肠黏膜绒毛显著高于对照组(P<0.05).     

培养基质对丛枝菌根(AM)真菌生长发育的影响

温室条件下，以盆栽培养方法研究不同培养基质对丛枝菌根(Arbuscular Mycorrhizae,AM)真菌Glomus mosseae生长发育的影响。结果表明：不同理化性质的基质对菌根共生体生长发育的影响不同，综合考虑菌根长度、根外菌丝量及孢子数3项指标,以沙土混合物(体积比3∶1)对G. mosseae菌剂的生长发育最为有利。宿主植物菌根长度及根中的可溶性糖浓度与根外孢子数有正相关关系，而宿主植物中磷浓度与菌根真菌的生长发育也有类似的关系。说明培养基质的养分状况、水分状况、通气状况等诸多因素都会影响菌根共生体的建立和发展。宿主植物的菌根长度、根中可溶性糖浓度以及宿主植物磷浓度对菌根真菌的生长发育有显著影响。因此，工厂化AM菌剂生产中，应以沙土混合物(体积比3∶1)为生产G. mosseae菌剂的培养基质。     

黄土丘陵区不同树龄旱作枣园细根空间分布特征

以黄土丘陵区2、6、10、15龄旱作枣林(Ziziphus jujube cv.Junzao)为研究对象,采用根钻法,距树干0.5、1、1.5 m处、分层(0.2 m)钻取土样,分析了旱作枣林细根随树龄的变化特征。结果表明:随着枣林树龄增大,枣林细根根长密度增加,比根长减小;2龄枣树细根主要分布于径向1.5 m以内和垂向1.6 m以上,10、15龄枣树细根分布超过径向1.5 m和垂向3 m以上,并在株间形成根系高密度区,6龄枣树细根径向分布范围大于2龄,垂向分布与10龄和15龄接近;不同树龄枣林细根根长密度均随土层深度增加而减小,且主要集中在0~0.6 m土层中;随着树龄增加,细根根长密度径向分布无差异(10、15龄)。研究表明:2、6龄枣林应靠近树干地表处施肥,而理论上成熟期10、15龄枣林可在林内任意位置施肥;同时为防止枣林减产和退化,需增加枣林管理措施以有效降低枣树自身奢侈性耗水和非生产性耗水。     

Increased plant density with reduced nitrogen input can improve nitrogen use efficiency in winter wheat while maintaining grain yield

ABSTRACT

Plant density and nitrogen (N) input level have notable effects on root development, distribution in the soil profile, and in turn, N-uptake of winter wheat. Our study objectives were to identify whether a high yield can be maintained with a reduced N input by increasing plant density. Field studies were conducted during four successive seasons (2014–2015, 2015–2016, 2016–2017, and 2017–2018) using a widely planted cultivar, Tainong18. Two regimes of N fertilization (180 kg ha^?1 and 240 kg ha^?1) and three planting densities (135, 270, and 405 plants per m²) were used. Higher plant density led to increased root length density (RLD) and enhanced N uptake from the whole soil profile. The RLD in the soil profile at 0–1.2 m, 0–0.4 m, and 0.4–0.8 m decreased while in the 0.8–1.2 m layer it increased in response to reduced N input. The combined effects of higher plant density and lower N input resulted in reduced N uptake, a lower nitrogen nutrition index (NNI), unchanged grain yield, and improved N use efficiency. In conclusion, it is possible and sustainable to maintain a high wheat yield with reduced N input by increasing plant density.     

Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability

The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg⁻¹ P as Ca₃(PO₄)₂ (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis, of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role in the better growth of the canola compared to the wheat genotypes.     

Eubacterial communities in different soil macroaggregate environments and cropping systems

Different positions within soil macroaggregates, and macroaggregates of different sizes, have different chemical and physical properties which could affect microbial growth and interactions among taxa. The hypothesis that these soil aggregate fractions contain different eubacterial communities was tested using terminal restriction fragment length polymorphism (T-RFLP) of the 16S ribosomal gene. Communities were characterized from two field experiments, located at the Kellogg Biological Station (KBS), MI, USA and the Ohio Agricultural Research and Development Center (OARDC), Wooster, OH, USA. Three soil management regimes at each site were sampled and management was found to significantly affect T-RFLP profiles. The soil aggregate erosion (SAE) method was used to isolate aggregate regions (external and internal regions). Differences between eubacterial T-RFLP profiles of aggregate exteriors and interiors were marginally significant at KBS (accounting for 12.5% of total profile variance), and not significant at OARDC. There were no significant differences among macroaggregate size classes at either site. These results are in general agreement with previous studies using molecular methods to examine microbial communities among different soil macroaggregate size fractions, although further study of communities within different aggregate regions is warranted. Analysis of individual macroaggregates revealed large inter-aggregate variability in community structure. Hence the tertiary components of soil structure, e.g. arrangement of aggregates in relation to shoot residue, roots, macropores, etc., may be more important than aggregate size or intra-aggregate regions in the determination of the types of microbial communities present in aggregates. Direct microscopic counts were also used to examine the bacterial population size in aggregate regions at KBS. The proportion of bacterial cells with biovolumes >0.18 μm³ was higher in aggregate interiors than in exteriors, indicating potentially higher activity in that environment. This proportion was significantly related to percent C of the samples, while total bacterial cell counts were not.     

含关节间隙的Delta机器人弹性动力学与振动特性分析

针对经济实用型并联机器人关节间隙对动平台位置精度与系统振动特性影响的问题,以Delta机器人为研究对象,利用数理统计原理对含关节间隙的Delta机器人支链进行了运动学分析,结合Lankarani-Nikravesh碰撞接触力模型与具有动态修正系数的Coulomb摩擦模型对关节间隙广义碰撞力进行了研究。利用空间有限元理论与拉格朗日方程,充分考虑主、从动臂的空间动力特性与运动协调关系,建立了Delta机器人弹性动力学模型,在定义杆件虚长度的基础上,将关节间隙产生的广义碰撞力结合到弹性动力学模型中,建立了含关节间隙的Delta机器人弹性动力学模型。借助FARO激光跟踪仪对间隙弹性动力学模型进行了验证分析,利用脉冲锤击法与Workbench软件仿真对Delta机器人的振动特性进行了研究分析。试验结果表明,考虑关节间隙时动平台中心点的运动轨迹较不考虑关节间隙时更靠近试验运行结果,验证了间隙弹性动力学模型的合理性与正确性,并且,系统前两阶非零固有频率的理论值与试验值的相对误差分别为3.544%和12.026%,两者相当接近。另外,由仿真结果可以发现3组从动臂是Delta机器人整机系统中最薄弱的环节。该研究可为经济实用型并联机器人的位置误差补偿与系统减振优化提供参考。