Quantification of root water uptake in soil using X‐ray computed tomography and image‐based modelling

Spatially averaged models of root–soil interactions are often used to calculate plant water uptake. Using a combination of X‐ray computed tomography (CT) and image‐based modelling, we tested the accuracy of this spatial averaging by directly calculating plant water uptake for young wheat plants in two soil types. The root system was imaged using X‐ray CT at 2, 4, 6, 8 and 12 d after transplanting. The roots were segmented using semi‐automated root tracking for speed and reproducibility. The segmented geometries were converted to a mesh suitable for the numerical solution of Richards' equation. Richards' equation was parameterized using existing pore scale studies of soil hydraulic properties in the rhizosphere of wheat plants. Image‐based modelling allows the spatial distribution of water around the root to be visualized and the fluxes into the root to be calculated. By comparing the results obtained through image‐based modelling to spatially averaged models, the impact of root architecture and geometry in water uptake was quantified. We observed that the spatially averaged models performed well in comparison to the image‐based models with <2% difference in uptake. However, the spatial averaging loses important information regarding the spatial distribution of water near the root system.     

A method to separate plant roots from soil and analyze root surface area

Analysis of the effects of soil management practices on crop production requires knowledge of these effects on plant roots. Much time is required to wash plant roots from soil and separate the living plant roots from organic debris and previous years’ roots. We developed a root washer that can accommodate relatively large soil samples for washing. The root washer has a rotary design and will accommodate up to 24 samples (100 mm diam. by 240 mm long) at one time. We used a flat-bed scanner to digitize an image of the roots from each sample and used a grid system with commercially-available image analysis software to analyze each sample for root surface area. Sensitivity analysis and subsequent comparisons of ‘dirty’ samples containing the roots and all the organic debris contained in the sample and ‘clean’ samples where the organic debris was manually removed from each sample showed that up to 15% of the projected image could be coveredwith debris without affecting accuracy and precision of root surface area measurements. Samples containing a large amount of debris may need to be partitioned into more than one scanning tray to allow accurate measurements of the root surface area. Sample processing time was reduced from 20 h, when hand separation of roots from debris was used, to about 0.5 h, when analyzing the image from an uncleaned sample. The method minimizes the need for preprocessing steps such as dying the roots to get better image contrast for image analysis. Some information, such as root length, root diameter classes and root weights, is not obtained when using this technique. Root length measurements, if needed, could be made by hand on the digital images. Root weight measurement would require sample cleaning and the advantage of less processing time per sample with this method would be lost. The significance of the tradeoff between information not obtained using this technique and the ability to process a greater number of samples with the time and personnel resources available must be determined by the individual researcher and research objectives.     

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.     

The behaviour of roots encountering cracks in soil

Summary It is shown that probabilities of root penetration across cracks in soil can be calculated effectively using a mathematical model involving root stress and soil distributions and penetrometer/root stress ratios. Penetration criteria are developed, and it is found that the effective penetrometer/root stress ratios take values of about 4 for crack widths smaller than about 2 mm and about 8 for wider cracks. Root swelling does not appear to contribute significantly to the probability of root penetration through any effect on root buckling stress. Suggestions are made for further work on the effects of soil structure and strength on root behaviour.     

Nitrogen‐mediated effects of elevated CO2 on intra‐aggregate soil pore structure

Soil pore structure has a strong influence on water retention, and is itself influenced by plant and microbial dynamics such as root proliferation and microbial exudation. Although increased nitrogen (N) availability and elevated atmospheric CO₂ concentrations (eCO₂) often have interacting effects on root and microbial dynamics, it is unclear whether these biotic effects can translate into altered soil pore structure and water retention. This study was based on a long‐term experiment (7 yr at the time of sampling) in which a C₄ pasture grass (Paspalum notatum) was grown on a sandy loam soil while provided factorial additions of N and CO₂. Through an analysis of soil aggregate fractal properties supported by 3D microtomographic imagery, we found that N fertilization induced an increase in intra‐aggregate porosity and a simultaneous shift toward greater accumulation of pore space in larger aggregates. These effects were enhanced by eCO₂ and yielded an increase in water retention at pressure potentials near the wilting point of plants. However, eCO₂ alone induced changes in the opposite direction, with larger aggregates containing less pore space than under control conditions, and water retention decreasing accordingly. Results on biotic factors further suggested that organic matter gains or losses induced the observed structural changes. Based on our results, we postulate that the pore structure of many mineral soils could undergo N‐dependent changes as atmospheric CO₂ concentrations rise, having global‐scale implications for water balance, carbon storage, and related rhizosphere functions.     

植物群落向土壤有机碳输入及其对气候变暖的响应研究进展

植物群落作为陆地生态系统土壤有机碳的主要来源,可通过地表凋落物分解、细根周转和根系分泌物等方式将光合作用同化的碳输入到土壤中。全球气候变暖正深刻地影响植物群落的分布、结构与功能,改变森林地上和地下凋落物产量与分解速率和根系分泌过程,从而改变植物群落向土壤输入有机碳数量。本文综述了植物群落向土壤有机碳输入过程及其对气候变暖的响应研究进展。研究表明,气候变暖可通过影响植物群落生产直接影响凋落物产量和根系分泌过程,还可通过改变凋落物分解环境条件、凋落物基质质量和分解者群落结构与活性等非生物与生物因子而间接作用于凋落物向土壤有机碳输入过程。气候变暖还可通过影响植物根系性状、根系分泌物化学组成等间接影响植物根系向土壤输入的碳量,但其具体机制还需深入探讨。未来的研究应该关注气候变暖导致植物群落结构改变进而影响土壤有机碳输入的具体机制以及粗木质残体对土壤有机碳输入的贡献,同时还应注重植物凋落物与根系分泌过程的整合研究,以期更全面地认识气候变暖背景下植物群落对土壤碳库及碳循环过程的贡献。     

Soil strength influences wheat root interactions with soil macropores

Deep rooting is critical for access to water and nutrients found in subsoil. However, damage to soil structure and the natural increase in soil strength with depth, often impedes root penetration. Evidence suggests that roots use macropores (soil cavities greater than 75 μm) to bypass strong soil layers. If roots have to exploit structures, a key trait conferring deep rooting will be the ability to locate existing pore networks; a trait called trematotropism. In this study, artificial macropores were created in repacked soil columns at bulk densities of 1.6 g cm⁻³ and 1.2 g cm⁻³, representing compact and loose soil. Near isogenic lines of wheat, Rht-B1a and Rht-B1c, were planted and root–macropore interactions were visualized and quantified using X-ray computed tomography. In compact soil, 68.8% of root–macropore interactions resulted in pore colonization, compared with 12.5% in loose soil. Changes in root growth trajectory following pore interaction were also quantified, with 21.0% of roots changing direction (±3°) in loose soil compared with 76.0% in compact soil. These results indicate that colonization of macropores is an important strategy of wheat roots in compacted subsoil. Management practices to reduce subsoil compaction and encourage macropore formation could offer significant advantage in helping wheat roots penetrate deeper into subsoil.     

The effect of soil warming on bulk soil vs. rhizosphere respiration

There has been considerable debate on whether root/rhizosphere respiration or bulk soil respiration is more sensitive to long-term temperature changes. We investigated the response of belowground respiration to soil warming by 3 °C above ambient in bare soil plots and plots planted with wheat and maize. Initially, belowground respiration responded more to the soil warming in bare soil plots than in planted plots. However, as the growing season progressed, a greater soil-warming response developed in the planted plots as the contribution of root/rhizosphere respiration to belowground respiration declined. A negative correlation was observed between the contribution of root/rhizosphere respiration to total belowground respiration and the magnitude of the soil-warming response indicating that bulk soil respiration is more temperature sensitive than root/rhizosphere respiration. The dependence of root/rhizosphere respiration on substrate provision from photosynthesis is the most probable explanation for the observed lower temperature sensitivity of root/rhizosphere respiration. At harvest in late September, final crop biomass did not differ between the two soil temperature treatments in either the maize or wheat plots. Postharvest, flux measurements during the winter months indicated that the response of belowground respiration to the soil-warming treatment increased in magnitude (response equated to a Q ₁₀ value of 5.7 compared with ∼2.3 during the growing season). However, it appeared that this response was partly caused by a strong indirect effect of soil warming. When measurements were made at a common temperature, belowground respiration remained higher in the warmed subplots suggesting soil warming had maintained a more active microbial community through the winter months. It is proposed that any changes in winter temperatures, resulting from global warming, could alter the sink strength of terrestrial ecosystems considerably.     

Root-soil contact of maize,as measured by a thin-section technique

In models of oxygen, water and nutrient uptake by plant roots, the degree of root-soil contact is an important parameter. An observation technique is required to evaluate to what extent root-soil contact depends on plant species, soil texture and structure. Thin sections for studying soil structure may be used for this purpose, provided that roots do not shrink during section preparation, and that all root cross sections are recognized.Maize was grown in pots with soil aggregates obtained by sieving and compacting to three bulk densities. Thin sections were made by freeze-drying samples before impregnating the soil with resin. Two checks were made on the validity of the method. Firstly, visual appearance of roots with intact epidermis, cortex and other tissues did not show signs of shrinkage. Secondly, the agreement was checked between root lengths obtained by washing duplicate soil samples and the number of root cross sections counted on horizonal and vertical thin sections. For the latter, the angle at which roots intersected the thin-section plane was determined from the shape of the cross sections. The frequency distribution of calculated angles was in agreement with the frequency distribution expected for a randomly oriented set of cylinders when an error term was included in the simulated measurements.Some results are presented for a field test of the thin-section method with barley on a calcareous marine sandy loam. Root hairs, apparently undamaged by sample preparation, are important for bridging the gap between roots and soil in this situation. According to the experience presented, the thin-section technique is suitable to derive the degree of root-soil contact, as influenced by species, soil texture and structure, in samples obtained from pot or field experiments.Communication No. 43 of the Dutch Programme on Soil Ecology of Arable Farming Systems.Communication No. 43 of the Dutch Programme on Soil Ecology of Arable Farming Systems.     

Challenges and opportunities for quantifying roots and rhizosphere interactions through imaging and image analysis

The morphology of roots and root systems influences the efficiency by which plants acquire nutrients and water, anchor themselves and provide stability to the surrounding soil. Plant genotype and the biotic and abiotic environment significantly influence root morphology, growth and ultimately crop yield. The challenge for researchers interested in phenotyping root systems is, therefore, not just to measure roots and link their phenotype to the plant genotype, but also to understand how the growth of roots is influenced by their environment. This review discusses progress in quantifying root system parameters (e.g. in terms of size, shape and dynamics) using imaging and image analysis technologies and also discusses their potential for providing a better understanding of root:soil interactions. Significant progress has been made in image acquisition techniques, however trade‐offs exist between sample throughput, sample size, image resolution and information gained. All of these factors impact on downstream image analysis processes. While there have been significant advances in computation power, limitations still exist in statistical processes involved in image analysis. Utilizing and combining different imaging systems, integrating measurements and image analysis where possible, and amalgamating data will allow researchers to gain a better understanding of root:soil interactions.     

The soil as an environment for plant parasitic nematodes

British arable soils are uniform in texture to plough depth but vary in structure and the content of new organic matter according to recent cultivations. Subsoils are less uniform and little influenced by cultivations. Although remarkably uniform in structure and outward appearance nematodes vary greatly in length and girth. Root ectoparasites live wholly in the soil but root endoparasites may spend only brief periods there. Nematodes that spend much time in the soil surface are subject to a harsher microclimate than those inhabiting deeper layers. The movement and activity of nematodes in soil is influenced by the thickness of water films, the amount of pore space with dimensions the nematodes can traverse, the stability and packing of aggregates, the oxygen consumption of competing organisms and the rate of supply. Other factors of which little is known are the degree of continuity of the usable spaces and their tortuosity. The moisture characteristic curve which relates water withdrawn to the suction pressure (matric potential) applied is a useful tool which enables the space available to a given nematode to be related to its cross-sectional diameter. Although total pore space and usable pore space are correlated in a general way with each other and with texture (e.g.% sand), and bulk density is correlated with pore space within textural classes, none of these measures can be substituted for an estimate of the usable pore space appropriate to the size of the nematode being studied. For most purposes the solid matrix of arable soils can be regarded as an inert skeleton supporting the pore space. The stability of the matrix is therefore an important parameter, determining changes of space and time. Little is known about the distribution of macro-voids in soils and rapid methods of assessing them are needed. These may be more important for and more easily used by long nematodes than short ones. For nematodes living near the soil surface diurnal temperature fluctuations may be large. Their movement and activity should be related to temperature fluctuations via the Q₁₀-curve. For nematodes living deeper in the soil diurnal fluctuations are unimportant. So development can be mirrored by curves of accumulated temperature in day degrees above basal development temperature. Unlike the pattern of rainfall accumulation, which varies from place to place and year to year, and must greatly affect nematode multiplication, crop damage and activity, the pattern of temperature accumulation is remarkably stable. Differences from place to place and year to year are moderated by the changes in sowing date they impose. Consequently a nematode species and the host plant it infests usually develop together under similar soil climatic conditions from planting date onwards. In real soils the spaces in which nematodes live are partly filled by water in winter but are progressively drained as the season advances and become air filled. Then water films are too thin to permit movement except while the soil is draining after rainfall. Therefore the duration of activity is proportional to rainfall, and total activity throughout development is proportional to accumulated rainfall after discounting amounts too small to penetrate the soil. The activity of root ectoparasitic nematodes could be modelled by multiplying the duration of activity by temperature, using the Q₁₀ curve if necessary (i.e. by the rate of activity). The soil environment imposes constraints on the animals that live in it. Populations are relatively static and inbred. The pore space also limits the properties of substances that link parasite with parasite (pheromones) and parasite with host (phytomones). The many additional complications that arise when plants are grown are depicted diagrammatically.     

X-ray absorption and phase contrast imaging to study the interplay between plant roots and soil structure

Plant performance is, at least partly, linked to the location of roots with respect to soil structure features and the micro-environment surrounding roots. Measurements of root distributions from intact samples, using optical microscopy and field tracings have been partially successful but are imprecise and labour-intensive. Theoretically, X-ray computed micro-tomography represents an ideal solution for non-invasive imaging of plant roots and soil structure. However, before it becomes fast enough and affordable or easily accessible, there is still a need for a diagnostic tool to investigate root/soil interplay. Here, a method for detection of undisturbed plant roots and their immediate physical environment is presented. X-ray absorption and phase contrast imaging are combined to produce projection images of soil sections from which root distributions and soil structure can be analyzed. The clarity of roots on the X-ray film is sufficient to allow manual tracing on an acetate sheet fixed over the film. In its current version, the method suffers limitations mainly related to (i) the degree of subjectivity associated with manual tracing and (ii) the difficulty of separating live and dead roots. The method represents a simple and relatively inexpensive way to detect and quantify roots from intact samples and has scope for further improvements. In this paper, the main steps of the method, sampling, image acquisition and image processing are documented. The potential use of the method in an agronomic perspective is illustrated using surface and sub-surface soil samples from a controlled wheat trial. Quantitative characterization of root attributes, e.g. radius, length density, branching intensity and the complex interplay between roots and soil structure, is presented and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessment of soil contamination--a functional perspective

In many industrialized countries the use of land is impeded bysoil pollution from a variety of sources. Decisions on clean-up, management or set-aside ofcontaminated land are based on various considerations, including human health risks, butecological arguments do not have a strong position in such assessments. This paper analyses whythis should be so, and what ecotoxicology and theoretical ecology can improve on thesituation. It seems that soil assessment suffers from a fundamental weakness, which relatesto the absence of a commonly accepted framework that may act as a reference. Soilcontamination can be assessed both from a functional perspective and a structuralperspective. The relationship between structure and function in ecosystems is a fundamentalquestion of ecology which receives a lot of attention in recent literature, however, ageneral concept that may guide ecotoxicological assessments has not yet arisen. On the experimentalside, a good deal of progress has been made in the development and standardized useof terrestrial model ecosystems (TME). In such systems, usually consisting of intactsoil columns incubated in the laboratory under conditions allowing plant growth and drainageof water, a compromise is sought between field relevance and experimental manageability.A great variety of measurements can be made on such systems, including microbiologicalprocesses and activities, but also activities of the decomposer soil fauna.I propose that these TMEs can be useful instruments in ecological soil quality assessments. Inaddition a ``bioinformatics approach' to the analysis of data obtained in TME experimentsis proposed. Soil function should be considered as a multidimensional concept and thevarious measurements can be considered as indicators, whose combined values define the``normal operating range' of the system. Deviations from the normal operating range indicatethat the system is in a condition of stress. It is hoped that more work along this line willimprove the prospects for ecological arguments in soil quality assessment.     

生物炭对土壤有机碳矿化的激发效应及其机理研究进展

近年来由于生物炭具有碳素稳定性强和孔隙结构发达等特性,其在土壤固碳减排方面的作用研究受到广泛关注.然而当生物炭进入土壤环境后最终是增加土壤碳的储存还是促进土壤碳的排放?目前学术界对该问题仍存在争议.生物炭对土壤有机碳的激发效应及其机理研究有待进一步深入开展.本文在分析生物炭自身碳素组分和稳定性、孔隙结构及表面形态特征的基础上,综述了添加生物炭对土壤本底有机碳矿化产生激发效应的研究进展,分别阐述了产生正激发和负激发效应(即促进和抑制矿化)的机制机理,认为正激发效应主要是基于生物炭促进土壤微生物活性增强、生物炭中易分解组分的优先矿化以及由此引发的土壤微生物的共代谢作用,而负激发效应主要是基于生物炭内部孔隙结构和外表面对土壤有机质的包封作用和吸附保护作用、生物炭促进土壤有机-无机复合体形成的稳定化作用、生物炭对土壤微生物及其酶活性的抑制作用.最后对今后相关研究方向进行了展望,以期为生物炭在土壤固碳减排方面的应用提供理论依据.     

Bringing function to structure: Root–soil interactions shaping phosphatase activity throughout a soil profile in Puerto Rico

1. Large areas of highly productive tropical forests occur on weathered soils with low concentrations of available phosphorus (P). In such forests, root and microbial production of acid phosphatase enzymes capable of mineralizing organic phosphorus is considered vital to increasing available P for plant uptake.
2. We measured both root and soil phosphatase throughout depth and alongside a variety of root and soil factors to better understand the potential of roots and soil biota to increase P availability and to constrain estimates of the biochemical mineralization within ecosystem models.
3. We measured soil phosphatase down to 1 m, root phosphatase to 30 cm, and collected data on fine‐root mass density, specific root length, soil P, bulk density, and soil texture using soil cores in four tropical forests within the Luquillo Experimental Forest in Puerto Rico.
4. We found that soil phosphatase decreased with soil depth, but not root phosphatase. Furthermore, when both soil and root phosphatase were expressed per soil volume, soil phosphatase was 100‐fold higher that root phosphatase.
5. Both root and soil factors influenced soil and root phosphatase. Soil phosphatase increased with fine‐root mass density and organic P, which together explained over 50% of the variation in soil phosphatase. Over 80% of the variation in root phosphatase per unit root mass was attributed to specific root length (positive correlation) and available (resin) P (negative correlation).
6. Synthesis: Fine‐root traits and soil P data are necessary to understand and represent soil and root phosphatase activity throughout the soil column and across sites with different soil conditions and tree species. These findings can be used to parameterize or benchmark estimates of biochemical mineralization in ecosystem models that contain fine‐root biomass and soil P distributions throughout depth.

     

The deformation of soil by penetrometers and root tips ofPisum sativum

Channels were formed by seminal roots ofPisum sativum and a steel penetrometer of similar dimeter in blocks of remoulded and weathered soil. For both types of channels, the soil was equilibrated and maintained at –12kPa matric water potential during formation. Small samples of soil containing channels were then excavated and examined using a scanning electron microscope. Sections of root channels were found to contain a clearly differentiated zone of newly remoulded soil containing oriented clay. In contrast to channels created by the rigid steel probe, the newly remoulded zone surrounding root channels did not exhibit either a region of maximum soil compression at the channel surface or a radial pattern of shear failure and compression. This micromorphological evidence suggests that exudates may have an additional role to play in reducing the mechanical strength of soil in the proximity of the root tip. The mechanism is thought to operate through an accumulation of soil water related to solute potential and a resultant increase in matric potential.     

The dynamics of root meristem distribution in the soil

Plants must develop efficient root architectures to secure access to nutrients and water in soil. This is achieved during plant development through a series of expansion and branching processes, mostly in the proximity of root apical meristems, where the plant senses the environment and explores immediate regions of the soil. We have developed a new approach to study the dynamics of root meristem distribution in soil, using the relationship between the increase in root length density and the root meristem density. Initiated at the seed, the location of root meristems in barley seedlings was shown to propagate, wave‐like, through the soil, leaving behind a permanent network of roots for the plant to acquire water and nutrients. Data from observations on barley roots were used to construct mathematical models to describe the density of root meristems in space. These models suggested that the morphology of the waves of meristems was a function of specific root developmental processes. The waves of meristems observed in root systems of barley seedlings exploring the soil might represent a more general and fundamental aspect of plant rooting strategies for securing soil resources.     

Mycorrhizas and soil structure

In addition to their well-recognized roles in plant nutrition and communities, mycorrhizas can influence the key ecosystem process of soil aggregation. Here we review the contribution of mycorrhizas, mostly focused on arbuscular mycorrhizal fungi (AMF), to soil structure at various hierarchical levels: plant community; individual root; and the soil mycelium. There are a suite of mechanisms by which mycorrhizal fungi can influence soil aggregation at each of these various scales. By extension of these mechanisms to the question of fungal diversity, it is recognized that different species or communities of fungi can promote soil aggregation to different degrees. We argue that soil aggregation should be included in a more complete 'multifunctional' perspective of mycorrhizal ecology, and that in-depth understanding of mycorrhizas/soil process relationships will require analyses emphasizing feedbacks between soil structure and mycorrhizas, rather than a uni-directional approach simply addressing mycorrhizal effects on soils. We finish the discussion by highlighting new tools, developments and foci that will probably be crucial in further understanding mycorrhizal contributions to soil structure.     

In situ-determination of the phosphate-gradient around a root by radioautography of frozen soil sections

Summary A radioautographic method is described which allows the determination of phosphate concentration profiles around a root in situ,i.e. under conditions of radial diffusional flow. The device consisted of a soil column containing a distinct layer labelled with³³P which was kept separated from the rest of the content of a (modified) Kich-Brauckmann vessel. The primary root of a maize plant was directed into the special soil core whereas the other roots were allowed to develop into the unlabelled portion of the pot. Two or five days after the roots had penetrated the labelled soil sections the soil blocks were immediately frozen in liquid nitrogen and, ommitting any further embedding procedures, sliced perpendicular to the growth direction of the root by means of a stone cutting saw. From the frozen soil slices radioautograms were prepared and densitometrically analysed for phosphate content within and outside the root. The P-depletion zones around the root as well as areas of P-accumulation within the root coincided well with anatomical and morphological root parameters as determined with maize plants grown under similar conditions thus mutually corroborating the findings. Interestingly, the P-depletion zone around the primary root did not exceed the area of the root hair cylinder. Although soil composition and the extent of water supply to the pot somewhat limit the applicability of the presented technique, it should be appropriate for the investigation of a variety of agricultural soils. Since laterals did not interfere with the analysis this method should also allow long-term studies to be performed.     

Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration

Radiocarbon (¹⁴C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of ¹⁴C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ¹⁴C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil. Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using ¹⁴C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the ¹⁴C values in soil pore space CO₂, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.