Lower Root Density Research Articles

Spatial Patterns of Roots in a Semiarid Grassland: Abundance of Canopy Openings and Regeneration Gaps

1 Spatial patterns of root biomass and plant cover were quantified in 10 late-successional, shortgrass steppe communities in which a large proportion of the soil is bare and regeneration is frequently limited by soil water. Our main objectives were to evaluate how patterns of root density were associated with previously documented variation in recruitment in canopy openings of different sizes and to estimate the abundance of openings with low root density. 2 Root biomass in the top 30 cm of soil was much lower in openings of all sizes than under plants and declined steeply as opening size increased. Biomass of light-coloured roots (i.e. those presumed to be functional) in centres of 10-, 20and 60-cm openings was 62, 33 and 4%, respectively, of that under plants. 3 Openings more than 5 cm across made up 34% of the surface. Most were small: 86% of openings were 50 cm across), which are known to enhance regeneration, had low root density. Such openings (2% of total) occupied 2% of the area and nearly all were caused by disturbance. However, many openings caused by disturbance were 30-50 cm across, a size range where interference from neighbours changes from strong to weak, and some were even smaller. Most of the area (>99.5%) was within the range of the root system of the dominant grass, Bouteloua gracilis. 5 We infer that most openings large enough to support enhanced recruitment are explored by roots of dominant bunchgrasses and that gap dynamics in shortgrass steppe involves constraints on water use in B. gracilis root systems. Because large openings are rare, variation in below-ground competition in the abundant, smaller openings may be important for regeneration.

Landscape Distribution of Organisms and the Scaling of Soil Resources

We demonstrate that the competitive ability and landscape distribution of organisms can be affected by the interaction between their size and scaling properties and the scaling properties of the resources they consume. First, we prove that the landscape distribution of plants can be significantly affected by the interaction between their root lateral spread (RLS) and the scaling of soil N. Furthermore, we show that the nature of this interaction is highly dependent on the levels of soil N and plant age (adults or seedlings). We also demonstrate how the optimal RLS depends on the combination of soil N scaling properties at different levels of resolution. We made the ALLOCATE model spatially explicit for RLS and soil N scaling to analyze plant competition in a fractal environment. We show that, on the average, plants with low RLS and high root density (RD) dominate under low N stress, while plants with high RLS and low RD dominate at high N stress and that the scaling of soil N significantly controls the degree of dominance and the performance of seedlings. These results may cast some light regarding present disagreements as to which plant trait may be optimal in the competition for soil nutrients.

Microbial biomass, N mineralization, and the activities of various enzymes in relation to nitrate leaching and root distribution in a slurry-amended grassland

High rates of cattle slurry application induce NO inf3 sup- leaching from grassland soils. Therefore, field and lysimeter trials were conducted at Gumpenstein (Austria) to determine the residual effect of various rates of cattle slurry on microbial biomass, N mineralization, activities of soil enzymes, root densities, and N leaching in a grassland soil profile (Orthic Luvisol, sandy silt, pH 6.6). The cattle slurry applications corresponded to rates of 0, 96, 240, and 480 kg N ha-1. N leaching was estimated in the lysimeter trial from 1981 to 1991. At a depth of 0.50 m, N leaching was elevated in the plot with the highest slurry application. In October 1991, deeper soil layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) from control and slurry-amended plots (480 kg N ha-1) were investigated. Soil biological properties decreased with soil depth. N mineralization, nitrification, and enzymes involved in N cycling (protease, deaminase, and urease) were enhanced significantly (P<0.05) at all soil depths of the slurry-amended grassland. High rates of cattle slurry application reduced the weight of root dry matter and changed the root distribution in the different soil layers. In the slurry-amended plots the roots were mainly located in the topsoil (0–10 cm). As a result of this study, low root densities and high N mineralization rates are held to be the main reasons for NO inf3 sup- leaching after heavy slurry applications on grassland.

Soil and crop responses to different tillage systems

An experiment was carried out over eight consecutive years at three sites, on clay or clay loam soils. In a split-plot design, two main treatments (mouldboard ploughing to 25 cm depth and disc or springtine cultivation to 13 cm depth) w3re combined with two seeddbed preparation treatments (three passes with a conventional harrow vs. one pass with a power take off (PTO) driven harrow). Seedbed characteristics and bulk soil properties investigated at one of the sites in 1991 were similar in the different treatments in the 0–13 cm layer. In the 13–25 cm laayer shallow cultivation resulted in significantly higher bulk density, degree of compactness and penetration resistance, and lower root density than in mouldboard ploughing. A reduced number of tractor passes achieved by using the PTO driven harrow resulted in significantly lower bulk density and penetration resistance in the unploughed soil, while still providing an adequate seedbed. At 25–30 cm depth, the volume of pores with equivalent diameter > 100 μm, saturated hydraulic cconductivity and are permeability were higher with ploughless tillage than with conventional tillage. Pore continuity was greater in unploughed soil at all depths investigated. In unploughed plots there was a concentration of organic carbon and potassium in the upper 13 cm. Phosphorus distribution and pH were not altered by the tillage systems. The yield was improved by the PTO driven harrow both in ploughed and unploughed plots.

Elevated CO2 alters deployment of roots in "small" growth containers.

Previously we examined how limited rooting space and nutrient supply influenced plant growth under elevated atmospheric CO2 concentrations (McConnaughay et al. 1993). We demonstrated that plant growth enhancement under elevated CO2 was influenced more by the concentration of nutrients added to growth containers than to either the total nutrient content per pot or amount or the dimensions of available rooting space. To gain insight into how elevated CO2 atmospheres affect how plants utilize available belowground space when rooting space and nutrient supply are limited we measured the deployment of roots within pots through time. Contrary to aboveground responses, patterns of belowground deployment were most strongly influenced by elevated CO2 in pots of different volume and shape. Further, elevated CO2 conditions interacted differently with limited belowground space for the two species we studied,Abutilon theophrasti, a C3 dicot with a deep taproot, andSetaria faberii, a C4 monocot with a shallow fibrous root system. ForSetaria, elevated CO2 increased the size of the largest region of low root density at the pot surface in larger rooting volumes independent of nutrient content, thereby decreasing their efficiency of deployment. ForAbutilon, plants responded to elevated CO2 concentrations by equalizing the pattern of deployment in all the pots. Nutrient concentration, and not pot size or shape, greatly influenced the density of root growth. Root densities forAbutilon andSetaria were similar to those observed in field conditions, for annual dicots and monocots respectively, suggesting that studies using pots may successfully mimic natural conditions.

Contributory factors to soil spatial variability in an ultisol. II. Retention of living treesin situfollowing land clearing

Abstract The effect of retaining living trees in situ following manual land clearing on soil physical properties was studied in a Typic Kandiudult in southern Cameroon. Soil compaction in the surface 100 mm was greatest with complete clearing and least under forest, with retention of living trees resulting in soil compaction levels which were intermediate to both the former. Soil compaction in the surface 100 mm also increased with increasing distance from the tree trunk (or tree stump with complete clearing). Both the above observations were attributed to a combination of high root density, high macrofaunal activity, high ground cover, high organic matter content and low traffic under forest and at the base of trees or tree stumps. Increasing soil compaction also occurred with increasing depth, and was attributed to the existence of few biopores in the subsoil horizons. Absence of biopores was thought to be due to low root densities in the subsoil caused by a combination of low macroporosity, low air por...

Root system analysis of seedlings of seven tree species from a tropical dry forest in Mexico

Root attributes of tree seedlings of seven species from the tropical deciduous forest along the Pacific Coast of Mexico are described using morphometirc root system analysis. Mean relative growth rate, root/shoot ratios, specific root length, root density, mean number of roots tips and root length/leaf area ratio were determined in seedlings grown for 35 days inside growth chambers. All the species had low relative growth rates, low root/shoot ratios and low root densities (<0.5 cm/cm3). The species associated with disturbed habitats, in contrast to the species characteristic of undisturbed areas, presented small seeds, a dichotomous root branching pattern and large specific root length. No relationship was found between seed size and mean relative growth rate among the species studied.

Analysis of the spatial variability in maize root density

In a maize field, one inter-row out of two was compacted two years down to 30-cm depth. This compacted inter-row (CIR) had a low root density down to 85-cm depth, while the soil below the row and the non compacted inter-row (NCIR) was densely rooted. Soil water status was monitored in each of these three compartments using tensiometers, neutron probe and gravimetric measurements. Both years, the rate of water extraction was about one half in the CIR compared with the row and the NCIR. As a consequence, appreciable differences in soil water potential were observed between colonized and sparsely colonized zones of each layer. These horizontal gradients were steeper than the vertical gradient between layers. This calls into question the suitability of one-dimensional models of water extraction for non-regular root systems, which are common in the field.

ROOT DISTRIBUTION OF AND WATER UPTAKE BY FIELD-GROWN BARLEY IN A BLACK SOLOD

A field study was conducted in 1984 and 1985 to determine the spatial distribution with time of root length density of spring barley (Hordeum vulgare L.) growing in a Black Solod in northwestern Alberta. The weakly solonetzic Bnt horizon present in the solodic soil appeared not to inhibit root growth, and roots were present to 90 cm depth of soil. Drought in 1985 reduced root growth in general, and in particular in the surface soil (0 – 15 cm depth) between crop rows. Root growth in both years continued well after ear emergence and attained a maximum total length (14.5 and 9.5 km m−2 in 1984 and 1985, respectively) some time into grain-filling. Water uptake rates of up to 1.3 cm3 m−1 d−1 were observed; this maximum rate was associated with younger roots in the 60- to 90-cm depth in 1984. Low availability of subsoil water in 1985, however, resulted in low root density and water uptake rates in the 60- to 90-cm depth. The weighted mean uptake rate for the entire root system was slightly more than 0.4 cm3 m−1 d−1 in 1984 and about half that in 1985. Key words: Barley, Hordeum vulgare L., solonetzic soil, water inflow, root growth, root length density

Spatial Heterogeneity of the Root System of Grasses in the Patagonian Arid Steppe

SORIANO, A., R. A. GOLLUSCIO AND E. SATORRE (Depto. Ecol., Facultad Agron., Univ. Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires, Argentina). Spatial heterogeneity of the root system of grasses in the Patagonian arid steppe. Bull. Torrey Bot. Club 114:103-108. 1987.Root biomass distribution and root growth into traps buried in the soil have been studied in the three dominant grasses (Stipa speciosa Trin. et Rupr., S. humilis Vahl and Poa ligularis Nees ap. Steud.) of extensive plant communities of the Patagonian arid steppe. Root biomass decreases sharply from center to the periphery of young and old plants of the three species. As a result of this attenuation, low root density pockets, corresponding to bare soil patches between tussocks, are found in the upper soil layer. At these pockets, water from small rains would be underutilized. Comparison of root biomass and spring root growth into traps supports the conclusion that, in a growing season, the three grasses were able to rebuild the roots only to the size previous to ablation provoked by trap installation. It is, therefore, postulated that lack of full colonization of low density root pockets by established root systems is determined by morphological constraints that limit their size and geometry. Potentiality seems to exist for the establishment of new individuals that would contribute to the formation of a continuous layer of uniform root density in the upper soil. Better use would thus be made of small rains, amounting to 57% of total rainfall events.

THE EFFECT OF ROOT DENSITY, INOCULUM PLACEMENT AND INFECTIVITY OF INOCULUM ON THE DEVELOPMENT OF VESICULAR–ARBUSCULAR MYCORRHIZAS

SummaryWe examined, in three glasshouse experiments, the development of vesicular‐arbuscular (VA) mycorrhizas formed by Glomus fasciculatum (Thaxter sensu. Gerd.) Gerd. &amp; Trappe on Trifolium subtẽrraneum L. in relation to plant denser and inoculum quantity. The effect of inoculum placement on the formation of mycorrhizas on Medicago truncatula Gaertn. by another Glomus sp. was studied in a fourth experiment. More mycorrhizal root was formed by Glomus sp. when inoculum was dispersed throughout the soil than when it was localized either near the top or bottom of the pot. The absolute rate of mycorrhiza formation was initially more rapid with localized inoculum. However, a greater weight of mycorrhizal root was subsequently obtained with dispersed inoculum as roots continually intercepted propagules. Infection by G. fasciculatum spread deeper in one soil than in two others, perhaps reflecting differences in root growth and density associated with differences in phosphorus status of the soils for plant growth. The weight of roots which became infected increased with increasing plant density, but not when shoot growth was greatly decreased due to competition between plants. The relative rate of mycorrhizal infection remained unaffected by treatments in each experiment. When a low root density and a high inoculum level were combined, the amount of roots which became infected was greater than expected. These conditions favoured a longer exponential phase in the development of infection.

Root Density and Water Potential Gradients near the Plant Root

The models of Gardner (1960) and Cowan (1965) for water transfer to the plant root are used to estimate the differences in water potential between the root and the bulk soil for a wide range of root densities and water extraction rates at a series of matric potentials for a Yolo light clay. For root densities and extraction rates reported both in the literature and in this paper there is good evidence to suggest that the large potential gradients originally predicted by Gardner and Cowan are restricted to situations involving very low root densities and high extraction rates in relatively dry soil.

Rooting Density and Water Extraction Patterns for Corn (Zea mays L.)1

AbstractBecause they occur at greater distances from the plant stem, roots deep within the profile often are considered less effective than those near the soil surface.An experiment was conducted to compare water‐absorbing efficiency, per centimeter of root, of corn (Zea mays L.) roots deep in the profile with that of roots near the soil surface. Plants were grown in a rhizotron compartment with rainfall excluded by a metal cover over the soil. Soil water content was determined with a neutron probe; rooting density, from measurements of roots on the glass viewing surface of the compartment. Leaf area was calculated by a length‐width method and plant height was measured daily. Stomatal aperture was estimated with a pressure drop promoter twice daily.With few exceptions, soil water content decreased and rooting density increased during the experiment. For the first weeks, transpiration exceeded pan evaporation, but toward the end of the experiment it was about half as much as pan evaporation. Water uptake per centimeter of root was affected most by soil hydraulic, conductivity, and at a given conductivity, it was greater at lower root densities. This effect of root density probably occurred because the roots were younger and more permeable at low root densities than at high root densities. Thus, for the conditions of these experiments, roots deep in the profile were probably more effective per centimeter of root for water uptake than shallow roots because they were younger and were in wetter soil.