An inflatable minirhizotron system for root observations with improved soil/tube contact

Commonly used minirhizotrons consisting of a transparent tube inserted into the soil seldom attain good contact between the tube and the soil, which leads to root growth occurring in a gap rather than in the soil. A new system is described involving an inflatable flexible rubber wall, made from a modified motorcycle tube. Pressure ensures a proper tube/soil contact so that the environmental circumstances for root growth along the tube more closely correspond to those in the undisturbed soil. Before the endoscope slide is introduced into the minirhizotron for taking pictures, the inflatable tube is removed, so that there is no-often opaque-wall between the endoscope and the roots. This improves the picture quality and facilitates the analysis of root images.     

An inflatable minirhizotron system for stony soils

Commonly used minirhizotron methodologies imply the introduction of rigid frames into the soil. This can be a problem if the hole presents an irregular surface, because the presence of gaps can modify the normal behaviour of roots growing along the soil-tube interface. Some authors have developed inflatable adaptations of the technique, but some of these still need the introduction of a rigid frame and others have the often added problem of the wall opaqueness. An inflatable adaptation of the minirhizotron technique has been developed to be used in stony soils. These kind of soils, common in many forests, as in mediterranean forested areas, would present too irregular surface holes to place rigid frames inside. The adaptation proposed allows the use of minirhizotrons in stony soils, with the added advantage of not having anything between the endoscope and the roots when recordings are made, which highly improves the image quality.     

Automatic discrimination of fine roots in minirhizotron images

Minirhizotrons provide detailed information on the production, life history and mortality of fine roots. However, manual processing of minirhizotron images is time-consuming, limiting the number and size of experiments that can reasonably be analysed. Previously, an algorithm was developed to automatically detect and measure individual roots in minirhizotron images. Here, species-specific root classifiers were developed to discriminate detected roots from bright background artifacts. Classifiers were developed from training images of peach (Prunus persica), freeman maple (Acer x freemanii) and sweetbay magnolia (Magnolia virginiana) using the Adaboost algorithm. True- and false-positive rates for classifiers were estimated using receiver operating characteristic curves. Classifiers gave true positive rates of 89-94% and false positive rates of 3-7% when applied to nontraining images of the species for which they were developed. The application of a classifier trained on one species to images from another species resulted in little or no reduction in accuracy. These results suggest that a single root classifier can be used to distinguish roots from background objects across multiple minirhizotron experiments. By incorporating root detection and discrimination algorithms into an open-source minirhizotron image analysis application, many analysis tasks that are currently performed by hand can be automated.     

An approach to minirhizotron root image analysis

Minirhizotrons speed up research on root demography, but image quality often hampers standardization of the image processing method. A simple procedure working on the blue band of colour images was tested on fibrous roots of sugarbeet (Beta vulgaris var. saccharifera). With respect to green and red, the blue band allows better detection of roots when their luminance is very similar to that of the background. The method makes use of an exponential algorithm of contrast stretching, which takes luminance frequency distribution into account. Based on a single threshold level, the procedure includes skeletonization. A minimum segment length was adopted to discriminate roots from extraneous objects. Although a specific minimum root length (MRL) value was calculated for each soil type, results show that a single value can be applied, indicating that this method can be profitably used for processing large samples of images. This revised version was published online in June 2006 with corrections to the Cover Date.     

A comparison of minirhizotron techniques for estimating root length density in soils of different bulk densities

Flexible- and rigid-walled minirhizotron techniques were compared for estimating root length density of 14- to 28-day-old Pinto bean (Phaseolus vulgaris L.) and spring whet (Triticum aestivum L.) plants in soil boxes under controlled environment conditions at three soil bulk densities (1.3, 1.5 and 1.7 g cm^–3). The flexible-tube system consisted of bicycle inner tubes inflated inside augered access holes and removed only when measurements were taken. Rigid tubes were constructed of extruded polybutyrate plastic. In both cases tubes were oriented horizontally. Despite similar root densities for wheat and beans based on measurements obtained from soil cores, root densities estimated from both types of minirhizotron were higher in bean than in wheat in uncompacted soil. Estimates of root density by the flexible tube minirhizotron were more closely correlated with soil core image analysis estimates than were those by the rigid minirhizotron system. At high soil bulk density, rigid tube measurements consistently overestimated actual rooting density of both wheat and bean. The relationship between estimated and actual rooting densities in the case of flexible tube measurements was not significantly influenced by soil bulk density. These findings were consistent with the theory that preferential root growth is induced by gaps at the soil-observation tube interface, inherent in the rigid tube technique, and was accentuated under conditions of high soil strength.     

Evaluating minirhizotron estimates of fine root longevity and production in the forest floor of a temperate broadleaf forest

The minirhizotron technique (MR) for in situ measurement of fine root dynamics offers the opportunity to obtain accurate and unbiased estimates of root production in perennial vegetation only if MR tubes do not affect the longevity of fine roots. Assuming fine root biomass is near steady-state, fine root production (g m^–2 yr^–1) can be estimated as the ratio of fine root biomass (g m^–2) to median fine root longevity (yr). This study evaluates the critical question of whether MR access tubes affect the longevity of fine roots, by comparing fine root survivorship obtained using MR with those from a non-intrusive in situ screen method in the forest floor horizons of a northern hardwood forest in New Hampshire, USA. Fine root survivorship was measured in 380 root screens during 1993–1997 and in six horizontal minirhizotron tubes during 1996–1997. No statistically significant difference was found between estimates of survivorship of fine roots (<1 mm dia.) at this site from MR versus from in situ screens, suggesting that MR tubes do not substantially affect fine root longevity in the forest floor of this northern hardwood forest and providing greater confidence in measurements of fine root production using the MR technique. Furthermore, the methodology for estimating fine root production from MR longevity data was evaluated by comparison of fine root longevity and production estimates made using single vs. multiple root cohorts, and using root-number, root-length, and root-mass weighted methods. Our results indicate that fine root-length longevity estimates based on multiple root cohorts throughout the year can be used to approximate fine root biomass production. Using this method, we estimated fine root longevity and production in the forest floor at this site to be 314 days (or 0.86 yr) and 303 g m^–2 yr^–1, respectively. Fine root production in this northern hardwood forest is approximately equivalent to standing biomass and was previously underestimated by root in-growth cores. We conclude that the use of MR to estimate fine root longevity and production as outlined here may result in improved estimates of fine root production in perennial vegetation.     

Fine root heterogeneity by branch order: exploring the discrepancy in root turnover estimates between minirhizotron and carbon isotopic methods

Fine roots constitute a large and dynamic component of the carbon cycles of terrestrial ecosystems. The reported fivefold discrepancy in turnover estimates between median longevity (ML) from minirhizotrons and mean residence time (MRT) using carbon isotopes may have global consequences. Here, a root branch order-based model and a simulated factorial experiment were used to examine four sources of error. Inherent differences between ML, a number-based measure, and MRT, a mass-based measure, and the inability of the MRT method to account for multiple replacements of rapidly cycling roots were the two sources of error that contributed more to the disparity than did the improper choice of root age distribution models and sampling bias. Sensitivity analysis showed that the rate at which root longevity increases as order increases was the most important factor influencing the disparity between ML and MRT. Assessing root populations for each branch order may substantially reduce the errors in longevity estimates of the fine root guild. Our results point to the need to acquire longevity estimates of different orders, particularly those of higher orders.     

Soil temperature effects from minirhizotron lighting systems

Observing root dynamics or soil fauna with minirhizotrons requires the use of incandescent or ultraviolet (UV) lighting systems. These light sources can generate heat which would be transferred to the surrounding soil adjacent to the minirhizotron observation tubes and thus may influence root growth and development or fauna activity. The objective of this study was to determine the effect of incandescent and UV light from a minirhizotron camera system on soil temperatures next to minirhizotron tubes. Temperature probes were attached next to and at 0.5 cm from the tube surface and the tubes were then placed in boxes with either a fine sand or a loamy clay soil. Incandescent light was operated stationary for 5 min or moved at 1 cm increments every 10 s down the tube for both dry and wet soils. The UV light was used in a stationary position for 10 minutes in both dry soils. Maximum temperature increases were 3.41–3.52 °C and 1.69–2.14 °C next to the tube for the dry and wet soils, respectively with 5 min of stationary incandescent light. Ultraviolet lights increased soil temperatures to a maximum of approximately 2.5 °C in the dry soil. Probes placed 0.5 cm from the tube surface also showed temperature increases up to 2.15 °C. Moving the light source every 10 s, however, resulted in lower temperature increases (<0.8 °C). Therefore short durations of light resulted in small temperature increases suggesting minimal impact on root development. Increased soil temperatures from longer durations of light, however, may alter root growth and development as well as soil fauna activity and warrants further study.     

Temperature-induced alteration to root longevity in Lolium perenne

The production and mortality of roots is an important factor determining nutrient and carbon fluxes in terrestrial ecosystems. However, the effect of temperature on the longevity of roots is not understood. In this study the impact of changes in temperature on the longevity of Lolium perenne roots was determined.Plants were grown at 15 °C, 21 °C and 27 °C and their roots observed using a minirhizotron system. Major effects of temperature were measured with large reductions in root longevity occurring at higher temperatures. The implications of these data are discussed.     

A color composite technique for detecting root dynamics of barley (Hordeum vulgare L.) from minirhizotron images

Quantification of root dynamics by destructive methods is confounded by high coefficients of variation and loss of fine roots. The minirhizotron technique is non-destructive and allows for sequential root observations to be made at the same depth in situ. Observations can be stored on video tape which facilitates data handling and computer-aided image processing. A color composite technique using digital image analyses was adapted in this study to detect barley root dynamics from sequential minirhizotron images. Plants were grown in the greenhouse in boxes (80 × 80 × 75 cm) containing soil from a surface horizon of a Typic Cryoboroll. A minirhizotron was installed at a 45°C angle in each box. Roots intersecting the minirhizotron were observed and video-recorded at tillering, stem extension, heading, dough and ripening growth stages. The images from a particular depth were digitized from the analog video then registered to each other. Discrimination of roots from the soil matrix gave quantitative estimates of root appearance and disappearance. Changes in root appearance and disappearance were detected by assigning a separate primary color (red, green, blue) to selected growth stages, then overlaying the images to create red-green and red-green-blue color composites. The resulting composites allowed for a visual interpretation and quantification of barley root dynamics in situ.     

Soil core and minirhizotron comparison for the determination of root length density

Detailed knowledge of the distribution of roots in the soil is important in understanding the extraction of water and nutrients from soil. Various techniques have been developed to monitor root-length density under field conditions. Excavation techniques, including soil cores, have long been considered to give reliable estimates of root-length density, but these techniques are laborious in sample collection and tedious in determination of root lengths. An attractive alternative for monitoring root-length density has been the minirhizotron whereby a periscope is inserted into a clear tube permanently installed in the soil for repeated and rapid measures of root development. The objective of this study was to compare the ability of the minirhizotron technique to measure root-length density as compared to the root-core technique.As in previous studies, substantial disagreement existed between the two techniques in the top 30-cm of the soil. The results from the minirhizotron consistently indicated a much lower root population than the root-core technique in the surface layer of soil. This is especially worrisome because more than 45% of the root-length density was found in this layer with the root-core technique. At deeper soil layers, the minirhizotron data proved to be no less variable than the root-core technique making the determination of statistically significant results difficult. Finally, the relationship between the minirhizotron and soil-core results varied with time even when the observations from the soil surface layer were ignored. Attempts to directly translate minirhizotron observations into a root-length density using a correlation approach would be suspect based on the results of this experiment.Mention of company names or commercial products does not imply recommendation or endorsement by the United States Department of Agriculture over others not mentioned.     

Arbuscular mycorrhizal fungi induced alteration to root longevity of poplar

Root longevity is an important element determining the fluxes of nutrients and carbon within terrestrial ecosystems. The effect of colonisation by an arbuscular mycorrhizal (AM) fungus on root longevity of poplar was determined using a minirhizotron camera to make direct observation of roots in situ in conjunction with image analysis. significant influences of colonisation on the longevity of roots were observed with most colonised roots surviving for a shorter period than non-colonised roots: only 16% of colonised roots survived for longer than 49 days compared to 49% of non-colonised roots. After 21 days approximately 5 times as many colonised as non-colonised roots had died. Possible mechanisms are discussed.     

Comparison of tomato root distributions by minirhizotron and destructive sampling

Calibration of minirhizotron data against root length density (RLD) was carried out in a field trial where three drip irrigation depths: surface (R0) and subsurface, 0.20 m (RI) and 0.40 m depth (RII) and two processing tomato cultivars: `Brigade' (CI) and `H3044' (CII) were imposed. For each treatment three minirhizotron tubes were located at 10, 37.5 and 75 cm of the way from one plant row to the next. Roots intersecting the minirizotrons walls were expressed as root length intensity (L _a) and number of roots per unit of minirhizotron wall area (N _ra). Root length density (RLD) was calculated from core samples taken for each minirhizotron tube at two locations: near the top of the minirhizotron (BI) and 15 cm apart from it, facing the minirhizotron wall opposite the plant row (BII). Minirhizotron data were regressed against RLD obtained at BI and BII and with their respective means. The results show that for all the situations studied, better correlations were obtained when RLD was regressed with L _a than with N _ra. Also was evident that the relationship between L _a and RLD was strongly influenced by the location of soil coring. RLD was correlated with L _a trough linear and cubic equations, having the last ones higher determination coefficients. For instance at 10 cm from the plant row when values from the top layer (0–40 cm) were analysed separately, L _a was significantly regressed with RLD measured at BII and described by the equations: RLD = 0.5448 + 0.0071 L _a (R ² = 0.51) and RLD = 0.4823 + 0.0074L _a + 8×10^–5 L _a ² – 5×10^–7 L _a ³ (R ² = 0.61). Under the 40 cm depth the highest coefficients of determination for the linear and cubic equations were respectively 0.47 and 0.88, found when L _a was regressed with RLD measured at BI. For minirhizotrons located at 75 cm from the plant row and for location BI it was possible to analyse jointly data from all depths with coefficients of determination of 0.45 and 0.59 for the linear and cubic equations respectively.     

Impacts of Water Input Manipulations on Fine Root Production and Mortality in a Mature Hardwood Forest

In order to examine the below ground response of a mature upland hardwood forest in the southeastern U.S., to increases and decreases in water inputs, the gross production, mortality, and net production of fine roots were examined over the first and third years of a long-term water manipulation experiment (Throughfall Displacement Experiment). Treatments involved a 33% decrease (DRY), 33% increase (WET), and ambient (AMB) levels of throughfall to the forest floor, begun in July, 1993. Video images of roots appearing on minirhizotron faces installed on both upper and lower slopes were recorded biweekly to a depth of 90 cm from April through October of 1994 and of 1996. Comparisons were made between treatments in amounts of new root elongation, root mortality, and calculated net root production. Minirhizotron observations during 1994 growing season, immediately following winter 1994 installation, revealed a strong effect of installation disturbance and were therefore not considered valid reflections of the response of the stand to the treatments. The 1996 data, on the other hand, exhibited absence of installation biases inherent in 1994 data because of a longer period since treatment initiation (2 2/3 yr vs. 8 mths), and favorable root growth conditions in all treatments during a greater portion of the year. The 1996 data were, therefore, considered realistic measures of below ground treatment responses. During 1996, net root production at 0-30 cm depth, at the upper slope positions, was significantly greater in DRY than in WET and AMB. Net root production was also greater at the lower slope position, but not significantly so. Treatment differences were the result of gross root production, as patterns of mortality did not differ across treatments. Nor were there significant treatment differences at depths below 30 cm. Whether trees in DRY produced more roots to replace root biomass lost during a previous drought year, or whether a new root:shoot ratio was beginning to develop in response to treatments, will require observations from the response of the stand in future years to be determined.     

Root dynamics in plant and ratoon crops of sugar cane

The root system of a sugar cane crop on an Ultisol in northeastern Brazil was examined throughout the plant and first ratoon crop cycles, using both coring and minirhizotron methods. Total root masses (living plus dead, 0.9–1.1 kg m^-2) and live root lengths (14.0–17.5 km m^-2) were greater during the ratoon cycle than at the end of the plant cane cycle (0.75 kg m^-2 and 13.8 km m^-2, respectively). Root die-back during the two weeks following ratoon harvest was estimated to be 0.15 kg m^-2, about 17% of the total root mass. Root die-back after the plant cane harvest was lower because fire was not used at this harvest and soil humidity was higher under the accumulated litter. A small amount of fine roots proliferated in the litter layer, amounting to 1% of the total mass and 3% of the total length. Root turnover could not be accurately assessed from minirhizotron observations due to variation in the relationship between coring data and the minirhizotron data with both time and soil depth.     

高寒草甸植物群落不同根序根系特征对降雨量变化的响应

根系是草原生态系统中最重要的碳库之一,分析高寒草甸植物群落生物量和地下不同径级根系碳分配特征及根系的生长特征对降雨变化的响应,有利于了解全球变化背景下高寒草甸植物根系、土壤碳氮循环及其过程。采用微根管技术原位监测5种降雨处理下(增雨50%:1.5P、自然降雨:1.0P、减雨30%:0.7P、减雨50%:0.5P、减雨90%:0.1P)高寒草甸植物群落和根系属性(现存量、生产量、死亡量、根系寿命和周转速率)的变化特征,结果表明:(1)降雨变化对地上植物群落生物量无显著影响,但0.5P和0.1P显著增加禾本科生物量(P<0.05)。(2)总根系现存量在处理间无显著差异,但随着降雨量减少呈先增加后降低的趋势。土层间不同径级根系现存量差异显著,0-10 cm土层1.5P和0.7P1级根现存量显著增加,2级和3级根现存量显著降低;在10-20 cm土层,1.0P2级根系现存量显著高于其余处理(P<0.05)。(3)总根生产量与死亡量随降雨减少而降低,在0-10 cm土层,1.0P总根生产量和死亡量最高,0.1P显著降低了1级根生产量(P<0.05)。(4)0.1P显著增加10-20 cm土层1级根和总根寿命(P<0.05)。(5)根系周转随降雨量减少呈降低趋势,但无显著差异(P>0.05)。(6)结构方程模型进一步表明:根系现存量和生产量受土层和水分的直接影响,土层和养分对根系周转有负效应。综上所述,降雨量的变化并未显著改变地下总根系生物量,但少量降雨变化(0.7P、1.5P)会降低植物对2、3级根生物量的分配,投入更多资源以促进1级根的生长;而水分下降至轻度水分胁迫(0.1P),植物会减少地下各径级根系生物量的分配,保持低根系生物量消耗和低根系生长来维持其正常的生长状态,完成其正常的生态功能。     

Modelling the contribution of arbuscular mycorrhizal fungi to plant phosphate uptake

In this paper, we investigate the role of arbuscular mycorrhizal fungi in plant phosphorus nutrition. We develop a mathematical model which quantitatively assesses the contribution of external fungal hyphae to plant phosphate uptake.We derive an equation for solute uptake by a growing fungal mycelium which we couple with a model for root uptake. We analyse the model using nondimensionalization and numerical simulations.Simulations predict that removal of phosphate from soil is dominated by hyphal uptake as opposed to root uptake. Model analysis shows that the depletion zones around hyphae overlap within 8 h and that the transfer between fungus and root is a critical step for the behaviour of phosphorus within the mycelial phase. We also show that the volume fraction of mycelium is negligibly small in comparison to other soil phases.This is the first model to quantify the contribution of mycorrhizal fungi to plant phosphate uptake. A full data set for model parametrization and validation is not currently available. Therefore, more complete sets of experimental measurements are necessary to make this model more applicable.     

Root dynamics in a salt marsh over three consecutive years

The minirhizotron technique has been used to study root development in a salt marsh in the western part of the Nationalpark Niedersächsisches Wattenmeer during a three-year period. The objective of our study was to evaluate root depth distribution and seasonal changes in growth activities of natural plant root systems. Root number was counted at monthly intervals in the top soil layer (0–0.2 m) for every 2 cm soil depth. The number of roots was regarded as an easily detectable parameter reflecting root growth and decay.In general, highest rooting intensity was found in the soil's subsurface layer (0–0.08 m). The number of roots significantly decreased in deeper horizons of the soil. There was also a significant increase and decrease in the number of roots in the course of a year. The highest rooting intensity was found in late winter to early spring, which substantially decreased towards mid summer when the plants were in their reproductive phase. The data indicate that there is a clear seasonal pattern of root growth of salt marsh species.