A root observation and sampling chamber (rhizotron) for pot studies

Summary A plant root observation chamber (rhizotron) was designed to examine soil-grown roots under a stereomicroscope and to sample roots and soil during the growth period of a pot study. The mini-rhizotrons are inexpensive to construct and are suitable for replicated, multitreatment experiments. Illustrative data on root hair and lateral development are presented for seedlings of four crop species.Vermont Agric. Exp. Stn. Journal Article No 572.     

Measurement of soil respiration using closed chamber method: An IRGA technique

A closed chamber method (CC-method) using an infra-red gas analyzer (IRGA) for measuring soil respiration was examined. Two major factors which potentially cause errors: (i) volume of air sampled from the chamber; and (ii) measuring period of time, were examined in laboratory experiments. Field measurements were also conducted with both the CC-method and the open-flow IRGA method (OF-method) throughout a year. The results of laboratory experiments showed that (i) sampling volume of air should be less than 0.2% of the volume of the chamber; and (ii) the air within the chamber should be sampled several times within 20 min. Field measurements showed that soil respiration rates measured by the CC-method were not significantly different from those by the OF-method. The results of this study indicate that the CC-method is as effective for the measurement of the soil respiration rates as the OF-method.     

辣椒疫霉菌侵染模型和侵染条件定量研究

在生长箱内控制条件下分析测定了土壤温度、水分含量对辣椒疫病死苗率的影响.结果表明:土壤温度和水分状况是决定辣椒疫病菌侵染的重要因子,病菌侵染的最适土壤温度为22 ℃～28 ℃,最适土壤含水量为40%,土壤过于干燥和过饱和都不利于病菌侵染发病;辣椒疫病死苗率与土壤温度、水分含量及其互作可用数学模式描述.田间调查发现,辣椒疫病田间流行趋势可用Gompertz模型描述,发病率与初始发病率、土壤温度、水分含量以及空气温度密切相关.建立了田间辣椒疫病发病率预测模型.     

蚯蚓和三叉苦对亚热带人工林土壤N2O和CH4通量的短期效应

在广东鹤山大叶相思(Acacia auriculaeformis)人工林内设置外来蚯蚓西土寒宪蚓(Ocnerodrilus occidentalis)和乡土植物三叉苦(Evodia lepta)野外控制实验,用静态箱-气相色谱法对土壤N2O和CH4通量进行15 d的原位测定,研究蚯蚓和三叉苦对土壤N2O和CH4通量的影响。结果表明,三叉苦并未明显增加土壤N2O和CH4的通量,而假植物(模拟三叉苦的物理效应)则显著促进了土壤N2O的释放通量。整个实验阶段,蚯蚓效应分别使无植物对照和三叉苦处理土壤N2O通量增加了26.7%和66.3%,而在种假植物条件下,添加蚯蚓使土壤N2O通量降低了39.7%;同时,蚯蚓效应使对照处理土壤CH4吸收通量增加了10.3%,使假植物处理土壤CH4吸收通量降低了90.6%,而使三叉苦处理土壤CH4释放通量增加了301.8%。可见,蚯蚓能够促进人工林土壤N2O释放;同时促进人工林土壤从CH4“汇”向“源”转变。三叉苦的物理过程促进土壤N2O的释放,而三叉苦的生物过程抑制土壤N2O的排放。如何减缓人工林中土壤N2O和CH4的排放,必须综合考虑植物物理过程、生物过程以及蚯蚓对土壤N2O和CH4排放过程影响的独立效应和交互效应。     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.     

植物根系构型原位观测识别技术研究进展

根系是植物从土壤中获取养分和水分并输送给植物体的重要器官,由于土壤的不透明属性阻碍了根系的观测,缺乏有效的原位根系观测方法已经构成了对植物根系深入研究的技术瓶颈.本文首先从根系研究的技术需求出发,对破坏性根系研究法、非破坏性根系二维观测法(微根管法)和根系三维构型原位观测方法如X射线计算机层析成像( X-ray computed tomography,XCT)、核磁共振成像法(magnetic resonance imaging,MRI)等进行了综述,并总结了各自的优缺点.其次,重点对XCT和MRI法进行了比较,有研究表明,MRI法不适用于根系三维构型的观测.指出自然土壤介质下,XCT是根系三维构型定量化分析未来的发展趋势.     

Evapotranspiration of Phragmites communis community in Panjin wetland and its control factors

Based on the data in the whole year of 2005 using microclimate gradient observation, eddy covariance observation, and plant ecophysiological observation at Panjin Wetland Ecosystem Research Station, Institute of Atmospheric Environment, China Meteorological Administration, the daily evapotranspiration variation and the control factors of the Phragmites communis community are analyzed. The results show that daily evapotranspiration variation among different months can be expressed as a single-peak curve that is lower in the morning and evening, and higher around noon. Meteorological and biological factors affect the evapotranspiration obviously. According to the correlation analysis, the evapotranspiration variation of Phragmites communis wetland has good correspondence with such environmental factors as net radiation, air temperature, surface temperature, relative humidity, wind speed and soil moisture. Regression analysis indicated that the major factors during the growing season include net radiation, soil moisture, relative humidity, air temperature and surface temperature, while the factors during the non-growing season include net radiation, soil surface temperature and wind speed. At the same time, growth situation and biological characteristics of vegetation have significant effects on the evapotranspiration of Phragmites communis, especially the leaf index and leaf conductance.     

箱式气体交换观测系统及其在植物生态系统气体交换研究中的应用

基于箱式法构建的箱式气体交换观测系统是植物生态系统碳水循环研究的常用方法,其在土壤和植物组织尺度的气体交换研究中的作用尤为重要.本文根据箱式气体交换观测系统的气路结构,对闭路、半闭路、开路和半开路系统的原理和计算方法进行了详细总结;汇总了目前常用的商业箱式气体交换观测系统,并回顾了自制系统的研制和应用历程;最后简要探讨了箱式气体交换观测系统存在的问题及其改进方式.     

植物源和土壤有机质源土壤呼吸组分的拆分原理、方法与应用进展

区分土壤呼吸组分并揭示其与环境因素的相关关系,对于准确评估土壤碳过程及其环境影响机制至关重要。根据底物来源和作用机制的差异,土壤呼吸主要包括根系呼吸、根际微生物呼吸、凋落物分解、自然条件下和激发效应下土壤有机质（SOM）分解。现有土壤呼吸组分拆分方法可以分为基于植物源CO₂测定或土壤有机质源CO₂测定的差分拆分方法,以及基于土壤呼吸组分同位素信号差异的拆分方法。土壤呼吸组分拆分研究可以解决不同土壤呼吸组分对环境变化的响应机制、植物光合碳输入与地下土壤呼吸组分的交互作用、土壤呼吸组分变化对土壤碳库周转的影响机制等科学问题,但其理论假设、观测技术方法、潜在的误差来源等仍需要继续关注并系统研究。     

Root, mycorrhiza and earthworm interactions: their effects on soil structuring processes, plant and soil nutrient concentration and plant biomass

Earthworms, arbuscular mycorrhiza fungi (AMF) and roots are important components of the belowground part of terrestrial ecosystem. However, their interacting effects on soil properties and plant growth are still poorly understood. A compartmental experimental design was used in a climate chamber in order to investigate, without phosphorus (P) addition, the single and combined effects of earthworms (Allolobophora chlorotica), AMF (Glomus intraradices) and roots (Allium porrum) on soil structure, nutrient concentration and plant growth. In our experimental conditions, plant roots improved soil structure stability (at the level of macroaggregates) whereas earthworms decreased it. AMF had no effect on soil structure stability but increased P transfer from the soil to the plant and significantly increased plant biomass. Earthworms had no direct influence on P uptake or plant biomass, and the N/P ratio measured in the shoots indicated that P was limiting. Interactions between AMF and earthworms were also observed on total C and N content in the soil and on total root biomass. Their effects varied temporally and between the different soil compartments (bulk soil, rhizosphere and drilosphere). After comparison with other similar studies, we suggest that effects of earthworms and AMF on plant production may depend on the limiting factors in the soil, mainly N or P. Our experiment highlights the importance of measuring physical and chemical soil parameters when studying soil organism interactions and their influence on plant performance.     

Nests of subterranean fungus-growing termites (Isoptera, Macrotermitinae) as nutrient patches for grasses in savannah ecosystems

Savannah are ecosystems in which mineral nitrogen is considered as a limiting factor for plant productivity. They are heterogeneous and spatially structured in patches, or islands, where mineral nitrogen content is concentrated. Among the soil macrofauna, termites of the Macrotermitinae subfamily are major determinants of soil heterogeneity through the biogenic underground nest structures (fungus‐comb chambers) they produce. To study the role of the heterogeneity created by termites on Pennisetum pedicellatum, an herbaceous grass species was grown in greenhouse. This was carried out using an homogeneous soil poor in mineral nitrogen, and an heterogeneous soil with patch, made of (i) Ancistrotermes cavithorax fungus‐comb chamber wall and (ii) soil with the same mineral nitrogen content as the termite handled soil. Plants exhibited a better growth on patch of termite‐modified soil whereas no significant differences were shown with the supply of mineral nitrogen. The presence of fungus‐comb chamber wall material resulted in an increase of fine root biomass and root/shoot ratio. We conclude that termites, through their building activities, may create nutrient patches available to grasses. Concurrently, our data illustrate that the higher mineral nitrogen content in termite‐built structures is not the only factor responsible for plant growth.     

Plant composition and diversity in a semi-natural Mediterranean island landscape: the importance of environmental factors

Abstract

Islands crucially contribute to the Mediterranean Basin’s high floristic diversity, which, however, is at risk facing climate and land-use changes. Besides the identification of highly diverse areas, the knowledge about factors favouring diversity is of great importance. We analysed plant species diversity and composition related to environmental factors over varied vegetation units on a former Italian prison island in the northwest of Sardinia. Due to a long history of land use with grazing and later abandonment the nowadays protected island features a semi-natural landscape and can serve as an example for strongly anthropogenic altered insular ecosystems. Floristic composition, soil properties, microclimate and ungulate abundance were assessed. Relationships of vegetation composition and diversity with abiotic variables were examined by Canonical Correspondence Analysis, which indicated the importance of air temperature, soil moisture, slope gradient and C/N ratio for floristic differentiation. Most important abiotic factors for plant species richness were relative air humidity and soil moisture, while floristic diversity was mainly determined by air temperature and pH. Furthermore, observation data pointed to an adverse influence of ungulate abundance for plant species diversity. Regarding nature conservation, grazing intensity thus must be critically taken into account, especially for sensitive vegetation units like the coastal garrigue.     

Plant growth response to direct and indirect temperature effects varies by vegetation type and elevation in a subarctic tundra

There has been growing recent use of elevational gradients as tools for assessing effects of temperature changes on vegetation properties, because these gradients enable temperature effects to be considered over larger spatial and temporal scales than is possible through conventional experiments. While many studies have explored the direct effects of temperature, the indirect effects of temperature through its long‐term influence on soil abiotic or biotic properties remain essentially unexplored. We performed two climate chamber experiments using soils from a subarctic elevational gradient in Abisko, Sweden to investigate the direct effects of temperature, and indirect effects of temperature via soil legacies, on growth of two grass species. The soils were collected from each of two vegetation types (heath, dominated by dwarf shrubs, and meadow, dominated by graminoids and herbs) at each of three elevations. We found that plants responded to both the direct effect of temperature and its indirect effect via soil legacies, and that direct and indirect effects were largely decoupled. Vegetation type was a major determinant of plant responses to both the direct and indirect effects of temperature; responses to soils from increasing elevation were stronger and showed a more linear decline for meadow than for heath soils. The influence of soil biota on plant growth was independent of elevation, with a positive influence across all elevations regardless of soil origin for meadow soils but not for heath soils. Taken together, this means that responses of plant growth to soil legacy effects of temperature across the elevational gradient were driven primarily by soil abiotic, and not biotic, factors. These findings emphasize that vegetation type is a strong determinant of how temperature variation across elevational gradients impacts on plant growth, and highlight the need for considering both direct and indirect effects of temperature on plant responses to future climate change.     

Interactions between biochar and mycorrhizal fungi in a water-stressed agricultural soil

Biochar may alleviate plant water stress in association with arbuscular mycorrhizal (AM) fungi but research has not been conclusive. Therefore, a glasshouse experiment was conducted to understand how interactions between AM fungi and plants respond to biochar application under water-stressed conditions. A twin chamber pot system was used to determine whether a woody biochar increased root colonisation by a natural AM fungal population in a pasture soil (‘field’ chamber) and whether this was associated with increased growth of extraradical AM fungal hyphae detected by plants growing in an adjacent (‘bait’) chamber containing irradiated soil. The two chambers were separated by a mesh that excluded roots. Subterranean clover was grown with and without water stress and harvested after 35, 49 and 63 days from each chamber. When biochar was applied to the field chamber under water-stressed conditions, shoot mass increased in parallel with mycorrhizal colonisation, extraradical hyphal length and shoot phosphorus concentration. AM fungal colonisation of roots in the bait chamber indicated an increase in extraradical mycorrhizal hyphae in the field chamber. Biochar had little effect on AM fungi or plant growth under well-watered conditions. The biochar-induced increase in mycorrhizal colonisation was associated with increased growth of extraradical AM fungal hyphae in the pasture soil under water-stressed conditions.     

贺兰山东坡典型植物群落多样性垂直分布特征与土壤因子的关系

深入探究山地生态系统植物群落多样性与关键土壤因子的关系及土壤因子在植物群落构建与多样性维持中的重要作用是区域生物多样性保护和生态系统服务功能综合提升的基础。本文采用冗余分析（Redundancy analysis,RDA）和相关分析揭示了贺兰山东坡典型植物群落多样性垂直分布格局与土壤因子的关系。结果表明：（1）贺兰山东坡植物群落多样性具有明显的垂直分布格局,针阔叶混交林植物物种组成最为丰富,中域效应明显;不同层次植物多样性随海拔升高变化趋势不一致且各自在不同海拔间差异显著（P<0.05）,植物多样性大体表现为草本层 > 灌木层 > 乔木层。（2）研究区土壤因子沿海拔梯度变化趋势不同,均存在显著差异（P<0.05）,其中土壤含水量（SM）、有机碳（SOC）和全氮（TN）含量空间变异性较大;高山灌丛草甸土壤养分含量最高,高海拔区土壤P限制高。（3）9个土壤因子解释了研究区总体植物多样性89.68%的生态信息,与植物多样性具有较强的相关性,影响不同层次植物多样性垂直分异的关键土壤因子略有不同。综上,土壤有机碳、pH、碳磷比、含水量是影响贺兰山东坡植物多样性空间分布的主要驱动因子。     

Seedling disease in an annual legume: consequences for seedling mortality, plant size, and population seed production

The effect of seed and seedling mortality on plant population dynamics depends on the degree to which the growth and reproduction of surviving individuals can compensate for the deaths that occur. To explore this issue, we sowed seeds of the annual Kummerowia stipulacea at three densities in sunken pots in the field, which contained either field soil, microwaved field soil, or microwaved field soil augmented with oospores of three Pythium species. High sowing density reduced seedling establishment and seedling size, but these effects were independent of the soil treatment. In the oospore-augmented soil, seed and seedling survival was low. The surviving plants were initially smaller but, at maturity, average plant size was greatest in the oospore-augmented soil, compared to the other treatments. Total population seed production was unaffected by soil treatment, suggesting that the effect of disease was limited to the seedling stage, with surviving plants released from intraspecific competition. To test the hypothesis that the surviving plants in the oospore-augmented soil were more disease-resistant, seeds from each of the sowing density-soil type treatments were sown in a growth chamber inoculation study. No evidence for selection for resistance was found. A second inoculation experiment revealed that oospore inoculum reduced plant numbers and mass regardless of whether field or microwaved soil was used, suggesting that results from the field experiment were not dependent on the use of microwaved soil. The findings of this study indicate that the ecological effects of disease on individual plants and on plant populations are not necessarily equivalent. Received: 13 January 1999 / Accepted: 21 September 1999     

The effects of chamber pressurization on soil-surface CO2 flux and the implications for NEE measurements under elevated CO2

Soil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas-exchange experiments and a soil CO₂ transport model to characterize the effects of chamber pressure on soil CO₂ efflux in an annual California grassland. For greater than ambient chamber pressures, experimental data show that soil-surface CO₂ flux decreases as a nonlinear function of increasing chamber pressure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO₂ efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO₂ transport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO₂ from the soil by generating a downward flow of air through the soil air-filled pore space. This advective flow of air reduces the CO₂ concentration gradient across the soil–atmosphere interface, resulting in a smaller diffusive flux into the chamber head space. Simulations also show that the reduction in diffusive flux is a function of chamber pressure, soil moisture, soil texture, the depth distribution of soil CO₂ generation, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portion of the frequently observed increase in net ecosystem carbon uptake under elevated CO₂ may be an artifact resulting from the impact of chamber pressurization on soil CO₂ efflux.     

植物氮形态利用策略及对外来植物入侵性的影响

氮是影响外来植物入侵性的重要因素之一, 但相关研究多关注土壤氮水平的效应, 较少考虑氮形态的作用。为从土壤氮形态利用的角度阐释外来植物的入侵机制, 本文在植物氮形态利用策略分析的基础上, 综述了外来植物氮形态利用的偏好性及其对入侵性的影响。植物的氮形态利用策略有偏好性和可塑性两种, 这可能与植物对土壤氮形态特性的长期适应有关; 植物不仅可以对土壤氮形态做出响应, 而且还能改造土壤氮形态, 并对改变后的土壤氮形态做出反馈响应。很多外来植物入侵硝态氮占优势的干扰生境, 偏好硝态氮的外来植物与本地植物竞争硝态氮; 而偏好铵态氮的外来植物通过抑制土壤硝化作用, 营造铵态氮环境, 促进自身生长, 同时抑制偏好硝态氮的本地植物生长。然而, 植物氮形态利用策略不是一成不变的, 而是受多种生物和非生物因素共同作用影响的复杂过程, 今后应加强多因素交互作用对外来入侵植物氮形态利用策略的影响及机制研究, 更好地揭示氮形态利用策略, 尤其是氮形态利用的可塑性与外来植物入侵性的关系。