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Plant roots affect the soil shear strength, but the temporal dynamics of roots such as growth and decay due to different mortality means have rarely been considered. This study investigates the effects of root growth and decay on the changes in root biomechanical properties and the shearing behaviour of vegetated soils. Columns of compacted soils were grown with a grass species, Cynodon dactylon. The grass was grown for 6 months, followed by burning or herbicide applications to introduce root decay. The top part of each column was used for direct shear tests, whilst the roots collected from the bottom part were used for the measurements of root tensile and chemical properties. The vegetated soils displayed greater shear strength and larger dilatancy, which were attributable to the growth-induced increase in the root cellulose content, and thus the root tensile strength and modulus. Root decay, upon burning or herbicide use, caused significant declines in the root tensile properties, which translated to the reductions in soil dilatancy and soil shear strength lower than those of the fallow soil. Herbicide use introduced a much faster and significant loss of root reinforcement than burning because of the more severe decay and loss of root biomechanical properties. Key words: soil bioengineering, root reinforcement, root biomechanics, root decay, shear strength, dilatancy Les racines des plantes affectent la resistance au cisaillement du sol, mais la dynamique temporelle des racines, telle que la croissance et la decomposition dues a differents moyens de mortalite, a rarement ete prise en compte. Cette etude examine les effets de la croissance et de la decomposition des racines sur les changements des proprietes biomecaniques des racines et le comportement de cisaillement des sols vegetalises. Des colonnes de sols compactes ont ete cultivees avec une espece de graminee, Cynodon dactylon. L'herbe a ete cultivee pendant 6 mois, suivis d'un brulage ou d'applications d'herbicides pour introduire la decomposition des racines. La partie superieure de chaque colonne a ete utilisee pour les tests de cisaillement direct, tandis que les racines collectees dans la partie inferieure ont ete utilisees pour les mesures des proprietes de traction et chimiques des racines. Les sols vegetalises presentaient une plus grande resistance au cisaillement et une plus grande dilatation, qui etaient attribuables a l'augmentation, induite par la croissance, de la teneur en cellulose des racines et donc de la resistance a la traction et du module des racines. La decomposition des racines, lors du brulage ou de l'utilisation d'herbicides, a entraine une baisse significative des proprietes de traction des racines, qui s'est traduite par des reductions de la dilatance et de la resistance au cisaillement du sol inferieures a celles du sol en jachere. L'utilisation d'herbicides a entraine une perte beaucoup plus rapide et significative du renforcement des racines que le brulage, en raison de la decomposition plus severe et de la perte des proprietes biomecaniques des racines. [Traduit par la Redaction] Mots-cles : bio-ingenierie des sols, renforcement des racines, biomecanique des racines, decomposition des racines, resistance au cisaillement, dilatation, Introduction Plants in soil bioengineering have been used to reinforce shallow soils against landslides (Ng et al. 2019). Fibrous roots, as they grow, form a composite material, which displays greater [...]

Most forest soils contain substantial amounts of gravel. However, unlike the more widely known root resource uptake behaviors which respond to resource patches in substrate without gravels, how roots respond to substrate containing different gravel levels is poorly understood. We grew roots in substrates with five gravel levels (0%, 10%, 20%, 30%, and 40% of volume) in a subtropical Schima superba Gardn. et Champ plantation, determined fine root dynamics and turnover rate with minirhizotrons, measured fine root morphological, architectural, mycorrhizal colonization, chemistry, and mass allocation. The presence of gravel in the substrate delayed the timing of peak root growth. In the substrate with higher gravel content, plants produced more in roots in autumn, but there were fewer roots in summer and the roots tended to exhibit lower fine root turnover rate and mycorrhizal colonization, but higher root biomass allocation. The higher root biomass in the substrate with higher gravel content was associated with higher root carbon/nitrogen ratio. Our findings emphasize the complexity of root resource uptake behavior in response to gravel content and suggest that incorporating substrate gravel content into root studies may help to improve the prediction of patch exploitation and nutrient acquisition in stony soils. Key words: heterogeneous patches, root phenology, root morphology, root architecture, mycorrhizal colonization. La plupart des sols forestiers contiennent des quantites substantielles de gravier. Cependant, contrairement aux comportements plus largement connus concernant le prelevement des ressources par les racines qui reagissent a des ilots riches en ressources dans un substrat exempt de gravier, la facon dont les racines reagissent a un substrat qui contient differentes quantites de gravier est mal connue. Nous avons fait croitre les racines dans des substrats contenant differentes proportions (cinq) de gravier (0%, 10%, 20%, 30% et 40% du volume) dans une plantation subtropicale de Schima superba Gardn. et Champ; determine la dynamique et le taux de renouvellement des racines fines a l'aide de minirhizotrons; mesure la morphologie, l'architecture, la colonisation mycorhizienne et les caracteristiques chimiques des racines fines ainsi que la repartition de la biomasse. La presence de gravier dans le substrat a retarde le moment ou la croissance des racines a atteint son niveau maximum. Dans le substrat contenant le plus de gravier, la plus forte croissance est survenue dans les racines a l'automne. Par contre les racines etaient moins abondantes durant l'ete et le taux de renouvellement ainsi que la colonisation mycorhizienne des racines fines avaient tendance a etre plus faibles, mais l'allocation de biomasse racinaire etait plus prononcee. La biomasse racinaire plus elevee dans le substrat contenant le plus de gravier etait associee a un rapport carbone:azote plus eleve dans les racines. Nos resultats font ressortir la complexite du comportement relie au prelevement des ressources par les racines en reaction a la teneur en gravier et indiquent que l'incorporation de la teneur en gravier du substrat dans les etudes portant sur les racines pourrait contribuer a ameliorer la prediction de l'exploitation des ilots riches en ressources et l'acquisition des elements nutritifs dans les sols pierreux. [Traduit par la Redaction] Mots-cles : ilots heterogenes, phenologie racinaire, morphologie racinaire, architecture racinaire, colonisation mycorhizienne., 1. Introduction Resources in soil are heterogeneously distributed. Understanding fine root resource uptake strategies in heterogeneous patches is of primary importance for predicting plant growth and adaptation to changing environments [...]

Plant roots are known to provide mechanical reinforcement to soils upon shearing and seismic loading. However, the effects of different stress paths on root reinforcement remain unclear. Moreover, whether and how roots provide resistance to soil liquefaction upon cyclic loading have rarely been studied. The objective of this study is to conduct a series of undrained triaxial tests to investigate the monotonic and cyclic behaviour of rooted sand. Roots of vetiver grass (Chrysopogon zizanioides L.), which has been advocated for use in shallow slope stabilisation purposes, were used for testing. The root diameters ranged between 0.3 and 1.5 mm, while the root volume ratios (RVRs) were 0.23%, 0.45%, and 0.67%. It was discovered that the root reinforcement effect was anisotropic and path-dependent. Along the extension path, when the major principal stress was perpendicular to the predominant root orientation, the root-induced increase in soil friction angle was approximately 10[degrees]. This increase was much greater than that along the compression path, where the change was minimal. The presence of roots prevented the limited flow failure at some RVRs and cyclic stress ratios (CSRs) (which occurred in the unreinforced sand) and the failure mode of the root-reinforced soil switched to cyclic mobility. The liquefaction resistance was improved with an increase in root volume, and this improvement was more remarkable at higher CSRs. Key words: vegetation, root reinforcement, soil-root interaction, stress paths, liquefaction. Les racines des plantes sont connues pour apporter un renforcement mecanique aux sols en cas de cisaillement et de charge sismique. Cependant, les effets des differentes voies de stress sur le renforcement des racines ne sont pas clairs. En outre, on a rarement etudie si, et comment, les racines offrent une resistance a la liquefaction du sol en cas de charge cyclique. L'objectif de cette etude est de mener une serie de tests triaxiaux non draines pour etudier le comportement monotone et cyclique du sable enracine. Des racines de vetiver (Chrysopogon zizanioides L.), dont l'utilisation a ete preconisee pour la stabilisation des pentes peu profondes, ont ete utilisees pour des essais. Le diametre des racines variait entre 0,3 et 1,5 mm, tandis que les rapports de volume des racines (RVR) etaient de 0,23 %, 0,45 % et 0,67 %. On a decouvert que l'effet de renforcement des racines etait anisotrope et dependant de la voie. Le long du trajet d'extension, lorsque la contrainte principale majeure etait perpendiculaire a l'orientation predominante des racines, l'augmentation de l'angle de frottement du sol induite par les racines etait d'environ 10[degrees]. Cette augmentation a ete beaucoup plus importante que dans le cas du chemin de compression oU le changement a ete minime. La presence de racines a empeche la defaillance de l'ecoulement limite a certaines RVR et taux de stress cyclique (CSR) (qui s'est produite dans le sable non renforce), et le mode de defaillance du sol renforce par les racines est passe a la mobilite cyclique. La resistance a la liquefaction a ete amelioree par une augmentation du volume des racines, et cette amelioration a ete plus remarquable a des CSR plus eleves. [Traduit par la Redaction] Mots-cles : vegetation, renforcement des racines, interaction sol-racine, parcours de stress, liquefaction., Introduction Soil bioengineering using vegetation is considered an environmentally and ecologically friendly bioremediation technique for resisting shallow soil slope failure resulting from excessive rainfall (Stokes et al. 2014; Ni et [...]

Root turnover is an important contributor to ecosystem nutrient and carbon cycling, but seasonal aspects of root mortality are not well known. This study tests the hypothesis that in strongly seasonal climates, such as in Northern Ontario, Canada, perennial wetland monocots fall into two distinct categories with respect to their root overwintering strategy: complete senescence or survival over the winter. Root survival in late winter and early spring was tested for a total of 26 species using vitality staining with tetrazolium chloride. Root survival in spring was either over 85% (18 species) or 0% (8 species). Lateral root survival was marginally lower than that of basal roots. In some species, low nutrient supply slightly increased root winter mortality, but did not change the seasonal pattern. We conclude that in a northern temperate climate, the overwintering strategies of roots of herbaceous monocots are binary: either avoidance or tolerance of the long unfavourable season, similar to deciduous and evergreen leaves among woody plants. Roots do not gradually die during the unfavourable season, but either completely senesce in the autumn or survive the winter. The distinct root overwintering strategies presumably affect the species' resource economics and ecosystem processes. Keywords: phenology, root mortality, root turnover, seasonal climate, wetland. Resume : Le renouvellement racinaire contribue de maniere importante aux nutriments et au cycle du carbone d'un ecosysteme, mais les aspects saisonniers de la mortalite racinaire ne sont pas bien connus. Cette etude teste l'hypothese que dans des climats fortement saisonniers comme dans le nord de l'Ontario, Canada, les monocotyledones vivaces des milieux humides se distribuent en deux categories selon leur strategie de survie hiemale des racines : une senescence complete ou une survie au cours de l'hiver. La survie des racines tard dans l'hiver ou tot au printemps a ete mesuree chez 26 especes a l'aide de la coloration vitale au chlorure de tetrazolium. La survie des racines au printemps etait soit superieure a 85 % (18 especes), soit a 0%(8 especes). La survie des racines laterales etait un peu plus faible que celle des racines basales. Chez certaines especes, un faible apport nutritionnel augmentait legerement la mortalite des racines en hiver, mais ne changeait pas le patron saisonnier. Les auteurs concluent que dans un climat tempere nordique, les strategies de survie hiemale des racines des monocotyledones herbacees sont binaires : soit un evitement ou une tolerance a la longue saison froide, semblable a la strategie des feuilles decidues et sempervirentes des plantes ligneuses. Les racines ne meurent pas graduellement durant la saison froide, mais soit elles entrent completement en senescence a l'automne, soit elles survivent a l'hiver. Les strategies de survie hiemale des racines affectent probablement l'economie des ressources des especes et les processus des ecosystemes. [Traduit par la Redaction] Mots-cles : phenologie, mortalite racinaire, renouvellement racinaire, climat saisonnier, milieu humide., Introduction Understanding the relationships between plant species' functional traits and their ecological behaviour is necessary to predict species' responses to different environmental conditions, and to be able to develop appropriate [...]

The vertical distribution of belowground herbivores plays an important role in determining the performance of an individual plant, but we still do not know the effects of this distribution on plant populations. A grass (Lolium perenne L.) was, therefore, grown at two densities with three vertical distributions of the belowground herbivore Anomala cuprea Hope (Coleoptera: Scarabaeidae). The population yield decreased significantly in the treatments with a herbivore, and decreased most when the herbivore was in the top feeding zone (i.e., the shallowest soil). Plants only died when the herbivore was in the top zone or was free to move within the pot. At low plant density, the biomass of the three largest shoots decreased significantly in the presence of a herbivore, but that of shoots in the fourth and smaller ranks did not. At high plant density, shoot biomass was not significantly affected by herbivory, irrespective of plant size. The standard deviation of shoot size was larger at low densities than at high densities. At low, but not high plant densities, the standard deviation decreased when herbivory occurred in the shallowest soil layer. To our knowledge, this study is the first to demonstrate that the vertical distribution of a belowground herbivore can markedly affect the size dynamics of a plant population. Key words: belowground herbivory, plant-herbivore interaction, plant-soil interaction, population dynamics, population size structure. La distribution verticale des herbivores du sous-sol joue un role important dans la determination de la performance d'un plant individuel, mais on ne connait pas encore les effets de cette distribution sur les populations de vegetaux. Une herbe (Loliumperenne L.) a ete ainsi cultivee a deux densites avec trois distributions verticales de l'herbivore du sous-sol Anomala cuprea Hope (Coleoptera: Scarabaeidae). Le rendement de la population diminuait significativement en fonction des traitements en presence de l'herbivore, et diminuait davantage lorsque l'herbivore se trouvait dans la zone d'alimentation superieure (c'est-a-dire le sol le moins profond). Les plants mouraient seulement lorsque l'herbivore se trouvait dans la zone superieure ou qu'il etait libre de circuler a l'interieur du pot. A faible densite de vegetaux, la biomasse des trois plus grandes pousses diminuait significativement en presence d'un herbivore, contrairement a celle des pousses du 4ieme rang et moins. A haute densite de vegetaux, la biomasse des pousses n'etait pas significativement affectee par l'herbivorisme, peu importe la taille du plant. L'ecart-type de la taille des pousses etait plus grand a faible densite qu'a haute densite. A faible densite mais pas a haute densite de vegetaux, l'ecart-type diminuait en fonction de l'herbivorisme dans la couche de sol la moins profonde. A la connaissance des auteurs, leur etude est la premiere a demontrer que la distribution verticale d'un herbivore du sous-sol peut affecter de maniere importante la dynamique de la taille d'une population vegetale. [Traduit par la Redaction] Mots-cles : herbivorisme du sous-sol, interaction plante-herbivore, interaction plante-sol, dynamique de population, structure de la taille de la population., Introduction Soil organisms can markedly affect plant population and community dynamics (De Deyn et al. 2003; Bardgett and Wardle 2010; Dighton and Krumins 2014), because most of them feed on [...]

Root allocation and distribution patterns can influence forage yield during periods of moisture or nitrogen stress, as deep-rooted species access water and nutrients found deeper in the soil profile. In a greenhouse study, we examined rooting characteristics to a depth of 50 cm for 21 cool-season forage species (9 grasses, 6 legumes, and 6 forbs) exposed to drought and low N conditions. The goal of this research was to compare root distribution under uniform growing conditions for common grass, legume and non-leguminous forb species found in humid-temperate pastures of the northeast United States. Nitrogen or water stressed grasses generally had greater root biomass and a greater proportion of roots in the 30 to 40 cm soil layer than did stressed legumes or forbs. Low N significantly reduced root weight, but to a lesser extent than the decrease in shoot weight, resulting in an increase in root/shoot ratio. Drought stress reduced shoot weight but had no effect on root weight, also resulting in a greater root/shoot ratio. Drought stress also increased the proportion of deep roots compared with controls, whereas, N stress did not. Comparisons with previously published field studies suggested that information from container-grown plants could provide insights into field results, and also suggested that inherent species differences in root distribution could explain some, but not all, rooting patterns observed in multi-species forage mixtures. doi: 10.2135/cropsci2009.08.0461

Cold stress is a major factor that limits the success of environmentally sound cultivation of maize in central and northern Europe. This study evaluated the early growth of seven groups of Swiss Flint maize landraces (Zea mays L. ssp. mays) of different geographic origin under controlled cool conditions by assessing shoot and root traits. Measurements of plants, subjected to a permanent cold stress of 15/13[degrees]C (day/ night), were made at the three-leaf stage of the check Magister, a hybrid of approved good early vigor. The ranking (univariate analysis of variance) and the multivariate clustering of the groups of landraces differed when root and shoot traits were included in the evaluation. Furthermore, the association of phenotype with geographic parameters was successful only when root traits were considered. Groups of Swiss landrace accessions expressed three types of phenotypes under cold conditions: (i) large leaf area and an extensive axile root system for landraces from the Tessin and the Poschiavo valley (i.e., south of the Alps), (ii) large leaf area and a short axile root system in landraces from the Linth valley, the lower Rhine valley, and the posterior Rhine valley, all of which are north of the Alps, (iii) small leaf area and a short axile root system in landraces from the anterior Rhine valley and the Swiss Rh6ne valley. The method of root phenotyping (i.e., a simple and rapid determination of traits of axile roots) proved suitable for the characterization of a large number of samples, as is typically required in quantitative genetic analysis.

The mineralogy of bulk and rhizosphere soils was compared to assess the effect of roots on mineral weathering in a Typic Dystrochrept supporting Norway spruce (Picea abies [L.] Karst) and oak (Quercus sessiliflora Smith). In an experimental forest site (Breuil-Chenue, France), systematic soil sampling was performed in forty pits. The soil adhering to the roots was considered as rhizosphere soil. The remaining material was regarded as bulk soil. The mineralogy of the clay-sized particles of both fractions was determined by x-ray diffraction (XRD). Amorphous solid phases were estimated by extracting Fe and Al with hot Na-citrate and dithionite-citrate-bicarbonate. Total chemical analyzes were performed on the clay-sized particles via Ba[Cl.sub.2]-saturation and an iso-zirconium weathering balance was calculated. The XRD demonstrated an increase of illite-like minerals in the rhizosphere of both species and the selective extractions revealed a decrease in the amorphous phases. The total chemical analyzes showed that the rhizosphere clay-sized fraction contained significantly more Si and K (for oak only) and less Fe and Al than the bulk soil. By way of the iso-zirconium weathering balance, these losses of Fe and AI in the rhizosphere were estimated at several tens of kilograms by hectare for the surface horizon (0-3 cm). This study demonstrates that, despite the short duration of the contact between the active part of a root and the solid mineral phase, the intensity of the processes occurring in the rhizosphere significantly increases mineral weathering. Abbreviations: B, bulk soil; BS, base saturation; subscript c, hot Na-citate treatment; CEC, cation exchange capacity; subscript d, dithionite-citrate-bicarbonate treatment; EA, exchangeable acidity; EG, ethylene glycol treatment; HIS, hydroxy-interlayered smectite; HIV, hydroxy-interlayered vermiculite; ICP-AES, inductively coupled plasma spectrometry-atomic emission spectrometry; ICP-MS, inductively coupled plasma spectrometry- mass spectrometry; R, rhizosphere; subscript [tot], total; XRD, x-ray diffraction.

Chicory (Cichorium intybus L.), a deep-rooted forb, has potential for inclusion in pasture mixtures because of its reported drought tolerance and high productivity during summer months. This study examined how adding chicory to pasture mixtures affected forage yield, root growth, and soil moisture extraction under drought. The experiment was planted in August 2002. Movable rainout shelters were used to control water application in the field. Adding chicory to orchardgrass (Dactylis glomerata L.)-white clover (Trifolium repens L.) or perennial ryegrass (Lolium perenne L.)--white clover mixtures increased drought tolerance in 2003 when chicory constituted 24 to 39% of harvested forage biomass. Chicory mortality was high, decreasing from 39% of harvested forage yield under drought stress in early summer 2003 to 9% in late summer 2004. Improved yield under drought stress was not observed in 2004 when chicory constituted only 9 to 16% of the mixture. The three-species mixtures in 2003 had greater root counts than the two-species mixtures at soil depths below about 70 cm under well-watered conditions, but a greater proliferation of roots at depths below 70 cm was observed for the two-species mixture under drought stress. Both mixtures appeared capable of nighttime transfer of soil water from deep to shallow soil layers, thereby improving water availability to shallow roots. However, improved drought tolerance of the three-species mixture was probably related to improved water use efficiency rather than to greater access to and extraction of deep soil moisture.

Author(s): Gabriel Castrillo [1, 2]; Paulo Jos Pereira Lima Teixeira [1, 2]; Sur Herrera Paredes [1, 2, 3]; Theresa F. Law [1, 2]; Laura de Lorenzo [4]; Meghan E. Feltcher [...], Plants live in biogeochemically diverse soils with diverse microbiota. Plant organs associate intimately with a subset of these microbes, and the structure of the microbial community can be altered by soil nutrient content. Plant-associated microbes can compete with the plant and with each other for nutrients, but may also carry traits that increase the productivity of the plant. It is unknown how the plant immune system coordinates microbial recognition with nutritional cues during microbiome assembly. Here we establish that a genetic network controlling the phosphate stress response influences the structure of the root microbiome community, even under non-stress phosphate conditions. We define a molecular mechanism regulating coordination between nutrition and defence in the presence of a synthetic bacterial community. We further demonstrate that the master transcriptional regulators of phosphate stress response in Arabidopsis thaliana also directly repress defence, consistent with plant prioritization of nutritional stress over defence. Our work will further efforts to define and deploy useful microbes to enhance plant performance.

Rhizosphere conditions are of great importance fur nutrient uptake and thus plant growth. Nutrient availability in the rhizosphere may differ substantially from that of the bulk soil and specific sampling of the rhizosphere soil is crucial to the understanding of plant growth, particularly where roots are restricted in growth due to high bulk density of the soil. The purpose of this study was to develop suitable methods for investigating the chemical composition of soil around pores with and without roots in soils with high bulk density. Two different methods were undertaken: one approach was to perform sequential extractions with [H.sub.2]O, 0.01, 0.1, and 1 M N[H.sub.4]N[O.sub.3] on soil samples taken at different distances from the pore wall to determine differences in element availability. Another approach was to analyze resin-impregnated, undisturbed soil samples from the same site for elemental composition of pure wall material using a scanning electron microscope (SEM) equipped with an x-ray micro analyzer. In general, the sequential extraction detected few statistically significant differences in nutrient availability between pores containing routs compared with pores without roots. The SEM analysis showed lower amounts of easily weatherable minerals around pores containing roots. Both methods are suitable for investigating differences between rhizosphere anti bulk soil characteristics.

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil C[O.sub.2] fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 [+ or -] 0.001 mm * [cm.sup.-2] * [d.sup.-1] over winter, to 0.051 [+ or -] 0.015 mm * [cm.sup.-2] * [d.sup.-1] during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. Monthly rates of production and mortality were somewhat asynchronous because mortality tended to peak during fall and be minimal during periods of peak production. Production and mortality were, however, positively correlated across all tubes and time periods. Annual fine root production averaged 2.45 [+ or -] 0.31, 8.01 [+ or -] 1.39, and 2.53 [+ or -] 0.27 mm * [cm.sup.-2] * [yr.sup.-1] (means [+ or -] 1 SE) among the three sites, when averaged across years. Fine root survival and decomposition were measured by tracking and analyzing the fate of individual fine roots using mark-recapture techniques. Fine root survival was greatest during periods of peak root growth, and least over winter ([[phi].sub.time]). Roots first appearing in the middle of the growing season had higher survival rates than those first appearing early or late in the growing season, or over winter ([[phi].sub.cohort]), and risk of mortality decreased with root age ([[phi].sub.age]). Survival estimates translate to mean life spans of 108 [+ or -] 4 d during the growing season. While these values are in striking contrast to needle longevity and rates of aboveground litter decomposition, they are similar to many values found for temperate systems, supporting the notion that there are basic morphological and physiological traits of first-order roots that are common to most woody plant root systems. During the growing season, monthly fine root decomposition rates averaged 0.46 [+ or -] 0.01 per month, while decomposition rates over winter averaged 0.73 [+ or -] 0.01 per winter. These growing season estimates translate to 49 [+ or -] 2 d from the time a root was first observed as dead, to the time it disappeared. For roots that decomposed during the growing season, those with longer life spans decomposed more slowly after death. Comparing these results with other minirhizotron studies suggests that life-history traits of black spruce first-order roots are similar to those from temperate (and perhaps most) forest ecosystems. Annual production of fine roots averaged 228 [+ or -] 75 g biomass * [m.sup.-2] * [yr.sup.-1], constituting ~56% of total stand production. Aboveground production of trees (50 [+ or -] 14 g biomass * [m.sup.-2] * [yr.sup.-1], 13%) and shrubs (40 [+ or -] 2 g biomass * [m.sup.-2] * [yr.sup-1], 11%) contributed similarly to total production, while mosses (73 [+ or -] 14 g biomass * [m.sup.-2] * [yr.sup.-1], 20%) accounted for the largest component of aboveground production. Soil temperature had a strong control over both soil respiration ([Q.sub.10] = 2.21 [+ or -] 0.31) and root respiration ([Q.sub.10] = 2.30 [+ or -] 0.37). During the growing season (15 May to 15 September), ~56% of soil C[O.sub.2] efflux (580 [+ or -] 40 g C/[m.sup.2]) was derived from fine root respiration (329 [+ or -] 54 g C/[m.sup.2]). Although apparent rates of heterotrophic respiration (May through September) and total production did not differ, definitive estimates of net ecosystem production are impossible given the potentially large, unmeasured components of NPP (net primary production), such as root exudation and mycorrhizal production. Nevertheless, rates of fine root production, mortality, and decomposition indicate that in these black spruce ecosystems, fine roots are much more dynamic than would be predicted from patterns of aboveground processes, and that carbon, and presumably nutrients, are cycling through fine roots at rates several orders of magnitude faster than through aboveground tissues. Key words: belowground allocation; boreal forest; decomposition; fine roots; longevity; NPP.

Traditional methods of measuring tree root biomass are labor intensive and destructive in nature. We studied the utility of ground-penetrating radar (GPR) to measure tree root biomass in situ within a replicated, intensive culture forestry experiment planted with loblolly pine (Pinus taeda L.). The study site was located in Decatur County, Georgia, in an area of the Troup and Lucy (loamy, kaolinitic, thermic Grossarenic Kandiudults and Arenic Kandiudults, respectively) soils. With the aid of a digital signal processing GPR, estimates of root biomass to a depth of 30 cm were correlated to harvested root samples using soil cores. Significant effects of fertilizer application on signal attenuation were observed and corrected. The correlation coefficient between actual root biomass in soil cores and GPR estimates with corrections for fertilizer application were highly significant (r = 0.86, n = 60, p < 0.0001). Where site conditions are favorable to radar investigation, GPR can be a powerful cost-effective tool to measure root biomass. Verification with some destructive harvesting is required since universal calibrations for root biomass are unlikely, even across similar soil types. Use of GPR can drastically reduce the number of soil cores needed to assess tree root biomass and biomass distribution. The quality and quantity of information resulting from a detailed GPR survey, combined with soil cores on a subset of plots, can be used to rapidly estimate root biomass and provide a valuable assessment of lateral root biomass distribution and quantity.

1 Plant community theory often invokes competition to explain why species diversity declines as productivity increases. Competition for all resources might become more intense and lead to greater competitive exclusion or, alternatively, competition for light only could become more intense and exclude poor light competitors. 2 To test these hypotheses, we constructed communities of seven old-field species using combined monocultures. Constructs experienced no interspecific competition, only shoot competition or only root competition, with and without fertilizer. Diversity in these limited interaction communities was compared to diversity in unfertilized and fertilized mixtures of the seven species. 3 Diversity decreased with fertilization in mixtures and in communities with only root competition. Shoot competition had small effects on the community and did not contribute to changes in diversity with fertilization. 4 Root competition may strongly impact plant community structure in unproductive communities where light never becomes limiting, or under non-equilibrium conditions following human disturbances. Key-words: community structure, fertilization, old field, null community, combined monocultures design

What determines the way in which roots provide carbon to and interact with other components of the soil? Roots lose metabolites and signal molecules to the soil at rates of significance to soil organisms, and we need to know if the mechanisms of passive diffusion identified in hydroponics apply in soil, and whether other, active mechanisms complement them. New insights from biosensors into the heterogeneity and localization of exudation are transforming our understanding of root-microorganism relations. We need to know more about compounds that are exuded at subnutritional rates in soil and may act as signal molecules modifying the biology of soil organisms. Insights into one suite of such compounds is coming from studies of border cells. These cells are lost from the root cap at a rate regulated by the root and secrete compounds that alter the environment of and gene expression in soil microorganisms and fauna. The amount of root places an upper limit on the effect roots can have; carbon flow to the rhizosphere is a function of root growth. Top-down metabolic control analysis shows that the control over the rate at which roots grow is shared between root and shoot, with most control being in the shoot. Key words: biosensors; carbon flux; control analysis; exudation; rhizosphere; root border cells; root cap.

Plant roots play diverse roles in the rhizosphere. They function as organs responsible for structural support, for acquisition of mineral and water resources, and for fostering of symbiotic bacteria and fungi. They also sustain a complex food web of prokaryotes and eukaryotes in, on, and near the root. In addition to these well-known functions, roots have a recently discovered role with potentially profound coevolutionary implications for the rhizosphere food web, as well as for terrestrial ecological communities in general. Roots are both a source and a receptor of molecular signals important for mutualistic bacteria and perhaps other soil organisms. Fluid-borne rhizosphere compounds flow in the opposite direction from airborne signals that emanate from plant shoots. The powerful transpiration stream around roots naturally concentrates molecular data in a cafeteria of information about the surrounding soil. These molecular signals, such as N-acylated homoserine lactones and phenazines produced by bacteria, can be interpreted simultaneously by multiple species as well as by the root. We propose a model of multitrophic molecular signals in the rhizosphere that implies multiple roles of roots, as hosts, regulators, and sustainers of terrestrial productivity. We suggest a framework of regulation in the rhizosphere that functions through molecular 'control points.' We define control points as regulatory elements that are operated on by selection processes to confer fitness on individual organisms and thereby have effects that propagate through other trophic levels. Identification of a control point creates a hypothesis that can be tested to assess the quantitative significance of that regulatory element. Some control points may transmit or perceive signals between organisms, but others probably integrate changing environmental conditions or external resources into individual life histories and community functions. The promise of understanding the new molecular signals is that genes must closely underlie these control points. This could offer ecology access to the power of molecular biology and allow a deep understanding of the evolutionary significance of these phenomena. One major strength of rhizospheres for addressing these issues is that realistic ecological interactions can be examined in a restricted microcosm under environmentally controlled conditions with organisms whose genomes have been completely defined and/or partially modified. Key words: mutualism; regulation; roots; signals.

Rhizobial bacteria, endomycorrhizal fungi (also known as arbuscular mycorrhizas), and pseudomonad bacteria associated with plant roots can provide substantial benefits to the plants by fixing nitrogen, supplying phosphorus, or controlling root pathogens, respectively. A significant fraction of plant photosynthetic carbon may be used by these associated microorganisms, both to support their beneficial activities and for microbial growth and reproduction. Because many microbial individuals are associated with each individual plant, the individual benefit to a microbe that allocates more resources to its own reproduction (thereby allocating less to fixing [N.sub.2], supplying P, or producing antifungal metabolites) would exceed its individual loss from any resulting reduction in collective benefits (mainly plant carbon substrates). An initially rare 'free rider' mutant strain might therefore be expected to displace its more cooperative parental strain. Yet, the mycorrhizal and legume-rhizobium mutualisms have persisted (often coexisting with 'cheating') for millions of years. This paper discusses the importance of microbial cooperation (with plants and with other microbes) and possible reasons for its evolutionary persistence in the rhizosphere. In undisturbed soils, spatial structure can favor kin selection, but this may be counterbalanced by the increased likelihood that future competitors will be among the beneficiaries of current cooperation. In loose associations, direct fitness benefits to microorganisms may explain the evolutionary persistence of activities (e.g., production of antifungal compounds) that can benefit plants as a side effect. In closer, more symbiotic, relationships, host sanctions against individuals or clones that fail to perform their symbiotic function may be more important. New molecular methods and other research tools are facilitating research on this topic, and some of these conclusions soon may be revised. Key words: cheating; kin selection; mutualism; mycorrhizae; pseudomonads; rhizobia; roots; Tragedy of the Commons.

This study examined the temporal and landscape-scale patterns of root nitrogen concentration [N], root decomposition and N release from decomposing roots in oak-hickory forest ecosystems in southern Ohio. Sampling was conducted in three watershed-scale treatment units with different prescribed burning regimes, and each of these treatment units was divided into xeric, intermediate and mesic landscape positions. Root [N] decreased through the growing season in live roots but increased in dead roots. Root [N] was significantly lower in the xeric landscape positions, but only during the driest parts of the growing season. There was no consistent effect of prescribed burning on live or dead root [N]. Where differences among landscape position were detected, live root [N] was greatest in the most mesic landscape positions whereas dead root [N] was greatest in relatively xeric landscape positions. Overall, an average of 70% of original mass was lost from root litterbags and 80% of total N was released over one year of decomposition, with no significant differences among burning treatments in the instantaneous decay rate (k) or rate of N release. There were no differences among landscape positions in root decay rate or N release, even though prior studies have demonstrated strong landscape-position effects on leaf litter decomposition in similar sites. Overall, mass loss and N release from root litter is rapid in these ecosystems and relatively unaffected by either landscape position or fire frequency.

A two-dimensional model that combines root-system development and water extraction by roots is proposed to simulate the dynamic interaction between root growth and soil-water flow. Both of hydrotropismand gravitropism were considered as the controlling factors of root growth in the proposed root-system development model. The finite-element method was employed to compute the soil-water flow caused by water extraction, evaporation, and irrigation. We succeeded in simulating the plagiogravitropic elongations of lateral roots under a plane condition, and the asymmetric architecture of root system under a slope condition by the proposed model in which the root hydrotropism is considered. On the other hand, we cannot simulate such morphological characteristics of a root system by the use of the conventional model in which a random elongation factor is employed, and root hydrotropism is not considered. The results support the importance of hydrotropism in root-system development and the availability of the proposed model in which the hydrotropism is considered.

A model that couples the diffusion of [O.sub.2] to plant roots at the microscale to diffusion of [O.sub.2] through the soil at the macroscale is derived. The solution is complex, but if the length scales ([Z.sub.r] and [Z.sub.m]; see APPENDIX for list of symbols) are equated a simple analytical expression can be obtained. This model is used to investigate relationship between a critical air-filled porosity ([[theta].sub.m]) and the other parameters; viz temperature (T), [O.sub.2] concentration in the bulk soil (C*), [O.sub.2] concentration at the root surface ([C.sub.r]), root length density (L), the ratio of root radius (a) to the water film radius (R), microbial respiration ([M.sub.o]), and length scales ([Z.sub.m] and [Z.sub.r]) related to the depth to which microbial and plant respiration are active in the model using sensitivity analysis. The model shows that [[theta].sub.m] is not very sensitive to the [O.sub.2] concentration at the root surface ([C.sub.r]), or the ratio of root radius (a)/ water film radius (R), but is sensitive to all the other parameters in some part of their range. The results indicate that indices used to define soil aeration; [O.sub.2] diffusion rate (ODR) or [O.sub.2] flux, [O.sub.2] concentration, or air-filled porosity, which have been previously used, are related and a single critical value for these is unlikely. If a constant critical value exists for one of these indexes it cannot exist for the other two. It is also shown that it is highly unlikely that a universal critical parameter related to soil aeration exists for any of these parameters. It is concluded that more parameters than ODR, [O.sub.2] concentration, or air-filled porosity need to be measured if progress in soil aeration research is to be made.

Our understanding of fine-root decay processes is derived almost exclusively from litterbag studies. However, preparation of roots for litterbag studies and their subsequent decay within litterbags represent major departures from in situ conditions. We hypothesized that litterbag studies misrepresent fine-root decay and nutrient release rates during decomposition. To test these hypotheses we developed a new intact-core technique that requires no a priori root processing, retains natural rhizosphere associations, and maintains in situ decay conditions. Using both litterbags and intact cores, we measured annual decay rates and nitrogen release from newly senesced fine roots of silver maple, maize, and winter wheat. After one year, mass loss was 10-23% greater, and nitrogen release was 21-29% higher within intact cores. Differences appeared to result from litterbag-induced alterations to decomposer dynamics and from unavoidable changes to fine-root size-class composition within litterbags. Our results suggest that fine-root decay and nutrient turnover occur significantly faster than estimated from litterbag studies. By minimizing disturbances to roots, soil, and rhizosphere associates prior to root decay, the intact-core technique provides an improved alternative for measuring fine-root decomposition. Key words: decomposition; fine-root decay; fine roots; intact-core technique; litterbags alter decomposition dynamics; maize; maple; nitrogen release; nutrient release rates; wheat.

Quantitative description of root-water uptake under combined salinity and water stress is needed to optimize crop yields and water management in arid and semiarid regions. This study was conducted to develop a simple macroscopic root-water uptake model for nonuniform transient soil water content and salinity conditions in the root zone. This new model and previous models were tested against detailed experimental data obtained with Alfalfa (Medicago sativa L.) grown in the greenhouse in packed sandy loam (Typic Haplaquent) columns. Soil water content, pressure head, and osmotic head distributions in the root zone were varied by means of the amounts, application intervals, and salinities of the irrigation water. Experimental data under separate and combined stresses were used to test the various models using mean values of soil solution osmotic and pressure heads. The simple additive reduction function provided the worst agreement with the experimental data, while for most cases the multiplicative reduction functions could not adequately account for both water and salinity stress conditions. The newly proposed linear reduction function is neither additive nor multiplicative, but was assumed that both the intersect and slope of the reduction function increased with salinity. This model provided excellent agreement with the experimental data, particularly at higher soil solution salinities. The new reduction function could be used with any other nonlinear salinity reduction function.

Research indicates that root and shoot competition are interactive processes, rather than independent processes. Therefore, measuring one form of competition, such as resource availability or population size structure, is insufficient in providing information about overall plant growth.

Studies in global plant biogeography have almost exclusively analyzed relationships of abiotic and biotic factors with the distribution and structure of vegetation aboveground. The goal of this study was to extend such analyses to the belowground structure of vegetation by determining the biotic and abiotic factors that influence vertical root distributions in the soil, including soil, climate, and plant properties. The analysis used a database of vertical root profiles from the literature with 475 profiles from 209 geographic locations. Since most profiles were not sampled to the maximum rooting depth, several techniques were used to estimate the amount of roots at greater depths, to a maximum of 3 m in some systems. The accuracy of extrapolations was tested using a subset of deeply (>2 m) sampled or completely sampled profiles. Vertical root distributions for each profile were characterized by the interpolated 50% and 95% rooting depths (the depths above which 50% or 95% of all roots were located). General linear models incorporating plant life-form dominance, climate, and soil variables explained as much as 50% of the variance in rooting depths for various biomes and life-forms. Annual potential evapotranspiration (PET) and precipitation together accounted for the largest proportion of the variance (12-16% globally and 38% in some systems). Mean 95% rooting depths increased with decreasing latitude from 80[degrees] to 30[degrees] but showed no clear trend in the tropics. Annual PET, annual precipitation, and length of the warm season were all positively correlated with rooting depths. Rooting depths in tropical vegetation were only weakly correlated with climatic variables but were strongly correlated with sampling depths, suggesting that even after extrapolation, sampling depths there were often insufficient to characterize root profiles. Globally, >90% of all profiles had at least 50% of all roots in the upper 0.3 m of the soil profile (including organic horizons) and 95% of all roots in the upper 2 m. Deeper rooting depths were mainly found in waterlimited ecosystems. Deeper 95% rooting depths were also found for shrublands compared to grasslands, in sandy soils vs. clay or loam soils, and in systems with relatively shallow organic horizons compared with deeper organic horizons. Key words: biomes; climate; global ecology; global vegetation types; latitude; plant life-forms; potential evapotranspiration; precipitation; rooting depth; soil texture; vertical root distribution.

We studied the root distribution and the effects of leachates from the dominant shrub in rosemary scrub, Florida rosemary (Ceratiola ericoides), on germination of seven subordinate rosemary scrub species. For rosemary scrub specialists, (Eryngium cuneifolium and Hypericum cumulicola), germination was suppressed by the leaf and litter leachates. For species that are not found exclusively in rosemary scrub (Liatris ohlingerae, Polygonella basiramia, Paronychia chartacea, and Palofoxia feayi) litter and leaf leachate did not suppress germination significantly. Species limited to gaps in rosemary scrub (E. cuneifolium, H. cumulicola, and Lechea deckertii) showed reduced germination from rosemary leachates while species not limited to rosemary-free gaps (L. ohlingerae and P. feayi) were not affected by rosemary leachates. Rosemary root abundance was greatest near shrubs, at a shallow depth, and at sites not recently burned. As rosemary scrub patches age, rosemary roots are more likely to interact with herbaceous species in gaps. Key words: allelopathy; Ceratiola; Empetraceae; fire; Florida, USA; rosemary scrub.