Impact of root water content on root biomass estimation using ground penetrating radar: evidence from forward simulations and field controlled experiments

Background and aims

The GPR indices used for predicting root biomass are measures of root radar reflectance. However, root radar reflectance is highly correlated with root water content. The objectives of this study are to assess the impact of root water content on GPR-based root biomass estimation and to develop more reliable approaches to quantify root biomass using GPR.

Methods

Four hundred nine roots of five plant species in a sandy area of northern China were examined to determine the general water content range of roots in sandy soils. Two sets of GPR simulation scenarios (including 492 synthesized radargrams in total) were then conducted to compare the changes of root radar signal and the accuracies of root biomass estimation by GPR at different root gravimetric water content levels. In the field, GPR transects were scanned for Ulmus pumila roots buried in sandy soils with three antenna center frequencies (0.5, 0.9, and 2.0 GHz). The performance of two new GPR-based root biomass quantification approaches (one using time interval GPR index and the other using a non-linear regression model) was then tested.

Results

All studied roots exhibited a broad range of gravimetric water content (>125 %), with the water contents of most roots ranging from 90 % to 150 %. Both field experiments and forward simulations indicated that 1) waveforms of root radar reflection, radar-reflectance related GPR indices, and root biomass estimation accuracy were all affected by root water content; and 2) using time interval index and establishing a nonlinear regression model of root biomass on GPR indices improved the accuracy of root biomass estimation, decreasing the prediction error (RMSE) by 4 to 30 % under field conditions.

Conclusions

The magnitude of GPR indices depends on both root biomass and root water content, and root water content affects root biomass estimation using GPR indices. Using a linear regression model of root biomass on radar-reflectance related GPR index for root biomass estimation would only be feasible for roots with a relative narrow range of water content (e.g., when gravimetric water contents of studied roots vary within 20 %). Appropriate GPR index and regression models should be selected based on the water content range of roots. The new protocol of root biomass quantification by GPR presented in this study improves the accuracy of root biomass estimation.     

Forward simulation of root’s ground penetrating radar signal: simulator development and validation

Background and aims

It remains unclear how the limiting factors (e.g., root size, root water content, spacing between roots, and soil water content) affect root investigation using ground penetrating radar (GPR). The objective of this study is to develop a theoretical forward simulation protocol of synthesizing root’s GPR signal and test the feasibility of our proposed simulation protocol in evaluating the impacts of limiting factors on GPR-based root detection and quantification.

Methods

The proposed forward simulation protocol was developed by integrating several existing numerical models, such as the Root Composition Model, the Root Dielectric Constant Model, the Root Electrical Conductivity Model, the Soil Dielectric Constant Model, the Soil Electrical Conductivity Model, and a newly-established model (Root Length-Biomass Model). Resolution and GPR index obtained from both field collected radargrams and corresponding simulations were compared to validate the accuracy of simulation.

Results

Simulated radargrams exhibit similar resolution with that of the in situ collected. The same trends of root radar signals against different levels of root size, root water content, interval between roots, root depth, and antenna frequency were observed on both in situ radargrams and simulated radargrams. Strong correlations (correlation coefficients ranging from 0.87 to 0.96) were found between GPR indices extracted from the simulated data and those from the field collected data.

Conclusions

Our proposed forward simulation is effective for assessing the impacts of limiting factors on root detection and quantification using GPR. This forward simulation protocol can be used to provide guidance for in situ GPR root investigation and can predict the accuracy of GPR-based root quantification under site-specific conditions.     

Ground-penetrating radar-based automatic reconstruction of three-dimensional coarse root system architecture

Background and aims

The diverse functions of roots set requirements on specific root system architecture (RSA). Investigation on RSA holds potentials for studying the adaptation of plants to environmental stresses and to interspecific competitions. Ground-penetrating radar (GPR) has provided a non-invasive method for studying in situ RSA. However, previous GPR method relied on manually connecting root points detected between radargrams to restore each root branch, resulting in limited accuracy and efficiency of reconstructing RSA. The objective of this study is to improve the effectiveness of 3D RSA reconstruction using GPR root detection data.

Methods

A total of 213 coarse root sections (with diameter >0.5 cm) were extracted from a distribution map of a reference shrub (Arctostaphylos pungens) root system to simulate the coarse roots identified by GPR. An automatic method was established to trace each root point to its optimum growing source point. Connections between discrete root points recovered the topology of the reference RSA. A spline curve smoothing method was applied to restore the 3D morphology of the reference RSA. The proposed protocol was then tested to rebuild the 3D RSA of a shrub (Caragana microphylla) growing in the sandy soils after in situ GPR survey. The accuracy of RSA reconstruction was quantitatively evaluated by a relationship matrix method and qualitatively assessed by direct comparisons between the reconstructed and the actual RSAs after in situ excavation.

Results

For both simulated and field collected GPR detection datasets, the reconstructed RSAs strongly corresponded to the real topology of the actual root systems. When adapting the best strategy, 186 of the 213 (87.32 %) root points on the reference root system of A. pungens were interlinked with correct topology, and the relationship matrix method detected an overall similarity of 82.75 % between the reconstructed and the actual RSAs.

Conclusion

The proposed automatic RSA reconstruction method greatly enhances the interpretation of GPR detection data regarding coarse roots, making in situ non-invasive and long-term mapping and monitoring of RSA possible.     

Iron transport, deposition and bioavailability in the wheat and barley grain

It has been reported that ground-penetrating radar (GPR) is a nondestructive tool that can be used to detect coarse roots in forest soils. However, successful GPR application for root detection has been site-specific and numerous factors can interfere with the resolution of the roots. We evaluated the effects of root diameter, root volumetric water content, and vertical and horizontal intervals between roots on the root detection of Cryptomeria japonica in sand using 900-MHz GPR. We found that roots greater than 19 mm in diameter were clearly detected. Roots having high volumetric water content were easily detected, but roots with less than 20% water content were not detected. Two roots that were located closely together were not individually distinguished. These results confirm that root diameter, root water content, and intervals between roots are important factors when using GPR for root detection and that these factors lead to an underestimation of root biomass.     

Comment on: “root orientation can affect detection accuracy of ground-penetrating radar”

Introduction

In a recent paper, Tanikawa et al. Plant Soil 373:317–327, (2013) reported a considerable impact of root orientation on the accuracy of root detection and root diameter estimation by ground-penetrating radar (GPR).

Methods

In Tanikawa et al. Plant Soil 373:317–327, (2013), buried root samples in a sand box were scanned from multiple cross angles between root orientation and GPR transecting line under controlled conditions. Changes in radar waveform parameter of roots to different cross angles were investigated.

Results

Tanikawa et al. Plant Soil 373:317–327, (2013) clarified that 1) the variation in amplitude area (a signal strength related waveform parameter) to different cross angles fitted a sinusoidal waveform; and 2) the impact of root orientation on root diameter estimation by GPR could be mathematically corrected by applying a grid transect survey. However, we found that the quantitative relationship established in Tanikawa et al. Plant Soil 373:317–327, (2013) between amplitude area and cross angle was incorrect, and the application of a grid transect survey still underestimated root diameter.

Conclusion

The change in amplitude area to cross angle between transecting line and root orientation fits a sinusoidal waveform but different to that reported in Tanikawa et al. Plant Soil 373:317–327, (2013). The polarization of GPR wave may explain such sinusoidal variation in amplitude area to cross angle. The effect of root orientation on GPR-based root diameter estimation remains to be calibrated.     

Detection of Cryptomeria japonica roots with ground penetrating radar

Abstract

Coarse tree roots, which are responsible for most root carbon storage, are usually measured by destructive methods such as excavation and coring. Ground penetrating radar (GPR) is a non-destructive tool that could be used to detect coarse roots in forest soils. In this study, we examined whether the roots of Cryptomeria japonica, a major plantation species in Japan, can be detected with GPR. We also looked for factors that impact the analysis and detection of roots. Roots and wooden dowels of C. japonica were buried 30 cm deep in sandy granite soil. From GPR measurements with a 900 MHz antenna, the distribution and diameter of samples in several transects were recorded. The buried roots were detected clearly and could be distinguished at diameters of 1.1–5.2 cm. There were significant positive relationships between root diameter and parameters extracted from the resultant GPR waveform. The difference in water content between roots and soil is a crucial factor impacting the ability to detect roots with GPR. We conclude that GPR can be used as a non-destructive tool, but further investigation is needed to determine optimal conditions (e.g. water content) and analytical methods for using GPR to examine roots in forest sites.     

Effect of species, root branching order and season on the root traits of 13 perennial grass species

Aims

Inter-specific comparisons of plant traits may vary depending on intra-specific variation. Here we examine the impact of root branching order and season on key functional root traits for grass species. We also compare root traits among co-existing grass species as a step towards defining root trait syndromes.

Methods

Monocultures of 13 grass species, grown under field conditions and subjected to intensive management, were used to record root trait values for coarse roots (1st order, >0.3?mm), fine roots (2nd and 3rd orders, <0.2?mm) and mixed root samples over three growing seasons.

Results

Branching order and species had a significant effect on root trait values, whereas season showed a marginal effect. The diameter of coarse roots was more variable than that of fine roots and, as expected, coarse roots had higher tissue density and lower specific root length values than fine roots. Principal component analysis run on eight root traits provided evidence for two trait syndromes related to resource acquisition and conservation strategies across grass species.

Conclusions

Our data show that root branching order is the main determinant of root trait variation among species. This highlights the necessity to include the proportion of fine vs coarse roots when measuring traits of mixed root samples.     

Root orientation can affect detection accuracy of ground-penetrating radar

Aim

Ground-penetrating radar (GPR) has been applied to detect coarse tree roots. The horizontal angle of a root crossing a scanning line is a factor that affects both root detection and waveform parameter values. The purpose of this study was to quantitatively evaluate the influence of root orientation (x, degree) on two major waveform parameters, amplitude area (A, dB × ns) and time interval between zero crossings (T, ns).

Methods

We scanned four diameter classes of dowels in a sandy bed as simulated roots using a 900 MHz antenna from multiple angles to clarify the relationships between the parameters and x.

Results

Angle x strongly affected reflection images and A values. The variation in A(x) fitted a sinusoidal waveform, whereas T was independent of x. The value of A scanning at 90° was estimated by A values of arbitrary x in two orthogonal transects. The sum of T in all reflected waveforms showed a significant linear correlation with dowel diameter.

Conclusions

We clarified that root orientation dramatically affected root detection and A values. The sum of T of all reflected waveforms was a suitable parameter for estimating root diameter. Applying grid transects can overcome the effects of root orientation.     

Grassland root demography responses to multiple climate change drivers depend on root morphology

Aims

We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers.

Methods

Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO₂ (TDCO₂).

Results

Air warming (C vs T) and the combined climate change treatment (C vs TDCO₂) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (≤ 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO₂ (TD vs TDCO₂); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO₂ and combined climate change drivers.

Conclusions

Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate.     

Drought changes the structure and elemental composition of very fine roots in seedlings of ten woody tree species. Implications for a drier climate

Background and aims

Water availability is often one of the most limiting factors for plants. Climate change predictions for many areas suggest an intensification of water limitation. The ability of a plant to modify its root characteristics can be an important mechanism for preventing drought stress.

Methods

We studied the drought response of seedlings of 10 woody species and compared the biomass allocation, vertical root distribution across different root diameters, and the key traits of very fine roots (root diameter <0.5 mm) under two water regimes (no water limitation and severe drought).

Results

Under drought conditions, the very fine roots had a higher specific root length (SRL, root length: biomass ratio), smaller root diameter and higher root tissue mass density, as well as a lower nitrogen concentration. A higher value of the mean root plasticity index was related to higher drought resistance. A quantitative literature review showed that there was a wide variation in the effect of the drought on SRL, thus there was not a clear effect of drought on SRL.

Conclusions

Certain species have the necessary root traits and plasticity to survive drought. We have identified plasticity in root characteristics as a whole-plant trait which plays a significant role in separating out species into those which are vulnerable and those which are resistant to drought.     

Leaf litter thickness,but not plant species,can affect root detection by ground penetrating radar

Aim

Ground penetrating radar (GPR), a nondestructive tool that can detect coarse tree roots, has not yet become a mature technology for use in forests. In this study, we asked two questions concerning this technology: (i) Does the leaf litter layer influence root detection and major indices based on the time interval between zero crossings (T) and the amplitude area (A)? (ii) Can GPR images discriminate roots of different plant species?

Methods

Roots buried in a sandy bed, which was covered with different thicknesses of leaf litter, were scanned using a 900 MHz GPR antenna. Roots of four plant species in the bed were also scanned.

Results

Leaf litter decreased root reflections without distorting the shape of the hyperbolas in the radar profile. A values decreased with increasing litter thickness, whereas T was independent of litter thickness. For all species combined, GPR indices were significantly correlated with root diameter.

Conclusions

Leaf litter dramatically decreased root detection, but the influence of the litter could be ignored when the sum of T for all reflection waveforms (ΣT) is adopted to estimate root diameter. To use A values to detect roots, litter should be removed or equalized in thickness. Radar profiles could not reliably differentiate among roots belonging to plants of different species.

     

Soil penetration resistance, a suitable soil property to account for variations in root elongation and branching

Background and Aims

Root system development is affected by soil conditions. The effects of bulk density, water content and penetration resistance on root development processes were investigated in peach trees.

Methods

Peach tree rootstocks were grown in various soil conditions, combining two bulk densities (1.2 and 1.5?g soil.cm^-3) and three water contents (0.14, 0.17 and 0.20?g.g^-1soil). Root parameters (tip diameter, length of apical unbranched zone, branching density and diameters of main and lateral roots) and plant growth (leaves, branches, trunk, root dry mass) were measured. Root growth processes (elongation, branching) were studied using relationships between root parameters.

Results

The proportion of biomass allocated to each plant compartment was similar whatever the soil conditions. Variations in root development were best explained by the variation in penetration resistance, rather than other soil properties. Increased soil penetration resistance reduced the root elongation rate, especially for thick roots. In addition, the branching pattern was affected. In soil with a high penetration resistance, the root system shape differs from a typical herringbone pattern.

Conclusions

These results allow quantification of the root system plasticity, and improve our understanding of the interactions between root development and soil properties.     

Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California

This research adds to the limited data on coarse and fine root biomass for blue oak (Quercus douglasii Hook and Arn.), a California deciduous oak species found extensively throughout the interior foothills surrounding the Central Valley. Root systems of six blue oak trees were analyzed using three methods — backhoe excavation, quantitative pits, and soil cores. Coarse root biomass ranged from 7 to 177 kg per tree. Rooting depth for the main root system ranged from 0.5 to 1.5 m, with an average of 70% of excavated root biomass located above 0.5 m. Of the total biomass in excavated central root systems, primary roots (including burls) accounted for 56% and large lateral roots (> 20 mm diameter) accounted for 36%. Data from cores indicated that most biomass outside of the root crown was located in fine roots and that fine root biomass decreased with depth. At surface depths (0–20 cm), small-fine (< 0.5 mm diameter) roots accounted for 71%, large-fine (0.5–2.0 mm) for 25%, and coarse (> 2 mm) for 4% of total root biomass collected with cores. Mean fine root biomass density in the top 50 cm was 0.43 kg m⁻³. Fine root biomass did not change with increasing distance from the trees (up to approximately 5 m). Thus, fine roots were not concentrated under the tree canopies. Our results emphasize the importance of the smallest size class of roots (<0.5 mm), which had both higher N concentration and, in the area outside the central root system, greater biomass than large fine (0.5–2.0 mm) or coarse (> 2.0 mm) roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Coarse root spatial distribution determined using a ground-penetrating radar technique in a subtropical evergreen broad-leaved forest, China

Coarse roots play a critical role in forest ecosystems and both abiotic and biotic factors affect their spatial distribution. To some extent, coarse root density may reflect the quantity of root biomass and biotic competition in forests. However, using traditional methods (e.g., excavation) to study coarse roots is challenging, because those methods are time-consuming and laborious. Furthermore, these destructive methods cannot be repeated in the same forests. Therefore, the discovery of non-destructive methods for root studies will be very significant. In this study, we used a ground-penetrating radar technique to detect the coarse root density of three habitats (ridge, slope and valley) and the dominant tree species (Castanopsis eyrei and Schima superba) in a subtropical forest. We found that (i) the mean of coarse root density for these three habitats was 88.04 roots m^?2, with roots being mainly distributed at depths of 0–40 cm. Coarse root densities were lower in deeper soils and in areas far from the trunk. (ii) Coarse root densities differed significantly among the three habitats studied here with slope habitat having the lowest coarse root density. Compared with S. superba, C. eyrei had more roots distributed in deeper soils. Furthermore, coarse roots with a diameter >3 cm occurred more frequently in the valleys, compared with root densities in ridge and slope habitats, and most coarse roots occurred at soil depths of 20–40 cm. (iii) The coarse root density correlated negatively with tree species richness at soil depths of 40–60 cm. The abundances of the dominant species, such as C. eyrei, Cyclobalanopsis glauca, Pinus massoniana, had significant impacts on coarse root density. (iv) The soil depth of 0–40 cm was the “basic distribution layer” for coarse roots since the majority of coarse roots were found in this soil layer with an average root density of 84.18 roots m^?2, which had no significant linear relationships with topography, tree species richness, rarefied tree species richness and tree density. Significant relationships between coarse root density and these factors were found at the soil depth of 40–60 cm, which was the “potential distribution layer” for coarse root distribution.     

Biomass of extramatrical ectomycorrhizal mycelium and fine roots in a young Norway spruce stand — a study using ingrowth bags with different substrates

Background and aims

The partitioning of below ground carbon inputs into roots and extramatrical ectomycorrhizal mycelium (ECM) is crucial for the C cycle in forest soils. Here we studied simultaneously the newly grown biomass of ECM and fine roots in a young Norway spruce stand.

Methods

Ingrowth mesh bags of 16 cm diameter and 12 cm height were placed in the upper soil and left for 12 to 16 months. The 2 mm mesh size allowed the ingrowth of fungal hyphae and roots whereas a 45 μm mesh size allowed only the ingrowth of hyphae. The mesh bags were filled with either EA horizon soil, pure quartz sand or crushed granite. Controls without any ingrowth were established for each substrate by solid tubes (2010) and by 1 μm mesh bags (2011). The fungal biomass in the substrates was estimated by the PLFA 18:2ω6,9 and ECM biomass was calculated as difference between fungal biomass in mesh bags and controls.

Results

The maximum ECM biomass was 438 kg ha^?1 in October 2010 in 2 mm mesh bags with EA substrate, and the minimum was close to zero in 2011 in 45 μm mesh bags with quartz sand. The high P content of the crushed granite did not influence the ECM biomass. Fine root biomass reached a maximum of 2,343 kg ha^?1 in October 2010 in mesh bags with quartz sand after 16 months exposure. In quartz sand and crushed granite, ECM biomass correlated positively with fine root biomass and the number of root tips, and negatively with specific root length.

Conclusion

The ratio of ECM biomass/fine root biomass in October ranged from 0.1 to 0.3 in quartz sand and crushed granite, but from 0.7 to 1.8 in the EA substrate. The results for the EA substrate suggest a large C flux to ECM under field conditions.     

Interactive effects of soil nutrient heterogeneity and belowground herbivory on the growth of plants with different root foraging traits

Aims

Plants with precise root foraging patterns can proliferate roots preferentially in nutrient-rich soil patches. When nutrients are distributed heterogeneously, this trait is often competitively advantageous in pot experiments but not field experiments. We hypothesized that this difference is due to belowground herbivory under field conditions.

Methods

We performed pot experiments using seedlings of Lolium perenne (a more precise root foraging species) and Plantago lanceolata (a less precise root foraging species). The experiment had a two-way factorial randomized block design, with nutrient distribution pattern (homogeneous or heterogeneous) and belowground herbivore (present or absent) as the two factors. Each pot contained one seedling of each species.

Results

With no herbivore present, plant biomass was smaller in the heterogeneous nutrient treatment than in the homogeneous treatment in P. lanceolata, but not in L. perenne. Under homogeneous nutrient distribution, plant biomass was lower in both species with a herbivore present than with no herbivore. Under heterogeneous nutrient distribution, biomass reduction due to herbivory occurred only in L. perenne.

Conclusions

Roots of the precise root foraging species were grazed more under the heterogeneous nutrient distribution, suggesting that the herbivore more efficiently foraged for roots in nutrient-rich soil patches.     

Spatial variation in root system activity of tomato (Solanum lycopersicum L.) in response to short and long-term waterlogging as determined by 15N uptake

Background and aim

Root system activity is affected by abiotic stresses, which often creates spatial differences in root conditions. This is expected to influence plants ability to cope with suboptimal conditions.

Methods

Changes in root system activity were determined as ¹⁵N root uptake in top and bottom layers of potted tomato plants (Solanum lycopersicum L.), after waterlogging the bottom layer for 24 h or 5 d. The plants were grown in peat-based media; non-compacted or highly-compacted, resulting in differences in gas diffusion, air permeability and oxygen availability.

Results

The roots were affected by short-term waterlogging (24 h) by decreasing uptake in the bottom layer and increasing uptake in the pot top layer. Long-term waterlogging (5 d) decreased the ¹⁵N root uptake more in both layers. Root uptake recovered fast (within 6 h) after short-term waterlogging, whereas recovery of long-term waterlogged roots took more than 24 h, suggesting production of new root biomass. Despite affecting physical properties, medium compaction did not affect root uptake. Aboveground biomass was affected by waterlogging by increasing the dry matter percentage, decreasing nitrogen (N) percentage and increasing starch content.

Conclusions

The results confirmed that root uptake in different layers of small pots could be distinguished by the ¹⁵N technique, which was applicable under potentially denitrifying conditions. The results demonstrated that during short-term stress in part of the root system plants increased uptake from the non-affected parts of the root system, probably as compensation for suboptimal conditions.     

Fine root biomass and turnover of two fast-growing poplar genotypes in a short-rotation coppice culture

Background and aims

The quantification of root dynamics remains a major challenge in ecological research because root sampling is laborious and prone to error due to unavoidable disturbance of the delicate soil-root interface. The objective of the present study was to quantify the distribution of the biomass and turnover of roots of poplars (Populus) and associated understory vegetation during the second growing season of a high-density short rotation coppice culture.

Methods

Roots were manually picked from soil samples collected with a soil core from narrow (75 cm apart) and wide rows (150 cm apart) of the double-row planting system from two genetically contrasting poplar genotypes. Several methods of estimating root production and turnover were compared.

Results

Poplar fine root biomass was higher in the narrow rows than in the wide rows. In spite of genetic differences in above-ground biomass, annual fine root productivity was similar for both genotypes (ca. 44 g DM m^?2 year^?1). Weed root biomass was equally distributed over the ground surface, and root productivity was more than two times higher compared to poplar fine roots (ca. 109 g DM m^?2 year^?1).

Conclusions

Early in SRC plantation development, weeds result in significant root competition to the crop tree poplars, but may confer certain ecosystem services such as carbon input to soil and retention of available soil N until the trees fully occupy the site.     

Root traits and soil properties in harvested perennial grassland,annual wheat,and never-tilled annual wheat

Background and aims

Root functional traits are determinants of soil carbon storage; plant productivity; and ecosystem properties. However, few studies look at both annual and perennial roots, soil properties, and productivity in the context of field scale agricultural systems.

Methods

In Long Term and Conversion studies in North Central Kansas, USA; root biomass and length, soil carbon and nitrogen, microbial biomass, nematode and micro-arthropod communities were measured to a depth of one meter in paired perennial grassland and cropland wheat sites as well as a grassland site that had been converted to cropland using no tillage five years prior.

Results

In the Long Term Study root biomass was three to seven times greater (9.4 Mg ha^?1 and 2.5 Mg ha^?1 in May), and root length two times greater (52.5 km m^?2 and 24.0 km m^?2 in May) in perennial grassland than in cropland. Soil organic carbon and microbial biomass carbon were larger, numbers of Orbatid mites greater (2084 vs 730 mites m^?2), and nematode communities more structured (Structure Index 67 vs 59) in perennial grassland versus annual cropland. Improved soil physical and biological properties in perennial grasslands were significantly correlated with larger, deeper root systems. In the Conversion Study root length and biomass, microbial biomass carbon, mite abundance and nematode community structure differed at some but not all dates and depths. Isotope analysis showed that five years after no-till conversion old perennial roots remained in soils of annual wheat fields and that all soil fractions except coarse particulate organic matter were derived from C₄ plants.

Conclusions

Significant correlation between larger, longer roots in grasslands compared to annual croplands and improved soil biological, physical and chemical properties suggest that perennial roots are an important factor allowing perennial grasslands to maintain productivity and soil quality with few inputs. Perennial roots may persist and continue to influence soil properties long after conversion to annual systems.