Search

Infection by ectomycorrhizal (ECM) fungi may benefit hybrid poplar growing in contaminated soils by providing greater access to water and nutrients and possibly protecting the trees from direct contact with toxic contaminants. The objective of this research was to determine the effect of colonization of the ECM fungus Pisolithus tinctorius(Pers.) Coker & Couch on hybrid poplar fine root production, biomass and N and P uptake when grown in diesel‐contaminated soil (5000 mg diesel fuel kg soil−1). Commercially available Mycogrow Tree Tabs were the source of inoculum. A minirhizotron camera was used to provide the data necessary for estimating fine root production. Colonization of hybrid poplar roots (P. deltoides× [P. laurifolia× P. nigra] cv. Walker) by P. tinctoriusincreased total fine root production in diesel‐contaminated soil to 56.58 g m−2compared to 22.59 g m−2in the uncolonized, diesel‐contaminated treatment. Hybrid poplar leaf N and P concentrations were significantly greater in the diesel‐contaminated/ECM‐colonized treatment compared to the diesel‐contaminated/uncolonized treatment after 12 wk, while significantly less diesel fuel was recovered from the soil of the uncolonized treatment compared to the colonized treatment. Both planted treatments removed more contaminants from the soil than an unplanted control. Significantly greater concentrations of total petroleum hydrocarbons (TPH) were found sequestered in hybrid poplar root/fungal‐sheath complexes from the colonized treatment compared to the roots of the uncolonized treatment. The results of this study indicate that over a 12‐wk growth period, ECM colonization of hybrid poplar in diesel‐contaminated soils increased fine root production and whole‐plant biomass, but inhibited removal of TPH from the soil.

Several indicators have been identified for the conservation and maintenance of soil criterion in the Montreal Protocol. The objective of this study was to use soil compaction and disturbance measures to determine harvesting impacts at a landscape scale in the boreal forest of Saskatchewan. Forest harvesting impacts were studied pre and postharvest for five harvested sites by (i) sampling soil bulk density (Db) at prescribed grid‐points, and (ii) measuring soil disturbance regimes on two 30‐m transects at each grid‐point. Mean soil Dbin the harvested area increased significantly (8–11%) from pre to postharvest conditions for the two winter‐harvested sites at both the 10‐ and 20‐cm depths, while two of the three summer harvested sites also showed significant Dbincreases (7–15%) at the 10‐cm depth. Combining all five sites, showed that after harvest 32% of all the grid‐points had an increased Dbof >15%. Mean soil Dbat a 10‐cm depth for roadways and landings was significantly higher (8–14%) than postharvest Dbfor postharvest levels at four of the five harvested sites. Surface soil disturbance regimes were higher for the summer‐harvested sites than that for the winter‐harvested sites. Landscape position showed no significant differences in Dbbetween the shoulder, backslope, and footslope positions; however, within each landscape position, significant differences in Dbwere found between pre and postharvest conditions. Soil Dband soil disturbance regimes measured on a grid basis provided a simple, but reliable method for monitoring soil compaction and disturbance effects from harvesting at a landscape scale.

Land conversion and cultivation effects on belowground biomass and C were assessed at a fescue (Festuca altaicaTrin. ssp. hallii) prairie site and an adjacent cultivated field seeded to spring wheat (Triticum aestivumL.). Belowground biomass and soil organic C (SOC) were measured in three landform element groups (upper level and convex shoulder elements [UL], low‐catchment‐area footslope elements [FS], and low‐elevation level and high‐catchment‐area footslope elements [LL]) at both sites. Upper level, FS, and LL landscape positions in the prairie had, respectively, eight, 12, and 13 times more belowground biomass in the top 60 cm of soil than their cultivated field counterparts. In the upper 1.8 m of soil, belowground biomass in the prairie grassland increased downslope (UL = 1849 ± 306, FS = 2533 ± 899, and LL = 3663 ± 1248 g m−1). In the cultivated field, LL landscape positions had higher levels of belowground biomass than either UL or FS positions (UL = 228 ± 54.5, FS = 200 ± 47.0, and LL = 294 ± 76.8 g m−2). Losses of C from belowground biomass accounted for 17.6% of total SOC losses in UL landscape positions, and 71.7% of losses in FS positions after cultivation. Lower level positions had the greatest loss of belowground plant C (−15.6 Mg ha−1) but overall the SOC had increased by 9.0 Mg ha−1In these position the loss of belowground C was offset by significantly higher SOC additions to the surface through soil redistribution. On a whole landscape basis, the C loss because of changes in belowground biomass is a large part of the C change that occurs during the transition from native prairie to arable agriculture.

The application of diffusion/mass‐flow theory to K uptake by slash pine (Pinus elliottiiEngelm. var. elliottii) seedlings from low‐K‐supplying soils is presented. Simulations for K uptake were evaluated by the Barber‐Cushman model (BCM) and the Baldwin‐Nye‐Tinker model (BNTM) under greenhouse and field conditions. Potassium uptake was overpredicted by 31 to 50% in greenhouse studies; overestimates, however, are attributed to nonuniform distribution of roots along pot walls. Simulations underpredicted K uptake by 0.48 and 0.83 times that observed in seedlings grown in the field. Although ectomycorrhizal hyphae were abundant in the field, their contribution to K uptake was not incorporated. Predictions of K uptake were excellent for seedlings grown at a tree nursery when fertilizer amendments were included. The BNTM, based on simplifying assumptions, predicted K‐uptake values similar to the BCM. Sensitivity analysis showed that root growth and initial K concentration in soil solution were the two most important factors affecting K uptake by slash pine. Soil processes that control K concentration were the limiting factors to K uptake on these soils. Monte Carlo procedures indicated that, for two harvests in the field, simulated estimates of uptake had CVs ranging from 34 to 64%.

Knowledge of the composition and quantity of organic substances released from roots of different plant species is necessary for understanding the chemical and biological processes in the rhizosphere. The present study was undertaken to quantify low molecular weight organic acids (LMWOAs) released from roots of five cultivars/lines of durum wheat (Triticum turgidum var. durum L.): Kyle, Sceptre, DT618, DT627, and DT637 and four cultivars/ lines of flax (Linum usitatissiumum L.): Somme, Flanders, AC Emerson, and YSED 2. Plants were grown in sterile nutrient solution cultures and amounts of organic acids exuded by roots were analyzed by gas chromatography. The LMWOAs varied significantly among both durum wheat and flax cultivars and oxalic, malonic, fumaric, succinic, acetic, malic, citric and tartaric acids were detected in root exudates of both species. Generally, oxalic and acetic acids were predominant in durum wheat exudates and oxalic, acetic and malic acids were predominant in flax root exudates. High oxalic acid concentrations occurred in root exudates of durum wheat cultivars DT627 and DT637, and flax cultivar YSED 2. Compared with the other durum wheat cultivars, Kyle released the lowest total amount of LMWOAs, whereas among the flax cultivars, YSED 2 had the highest total amount of acids secreted from roots. The data showed that the release of LMWOAs from roots was cultivar dependent. The results provide valuable background information for studying the role of root exudates in soil-plant relationships.

The low‐molecular‐weight organic acids (LMWOAs) secreted by plant roots modify the mobility of Cd through formation of soluble complexes in the soil rhizosphere; the kinetics of Cd release as influenced by organic acids and its impact on Cd bioaccumulation remains to be established. The influence of LMWOAs (10−3and 10−2M), viz., acetic, citric, oxalic, fumaric, and succinic acids, which are present in the soil rhizosphere, on the kinetics of Cd release from selected soils (Luseland, Waitville, and Jedbergh) of Saskatchewan, Canada, was investigated at 25°C and at an ionic strength of 0.1 MNaNO3. It was observed that the Cd release from the soils increased initially up to a reaction period of about 2 h and then slowly decreased with time. With time, the LMWOAs were apparently microbially degraded and the soil solution Cd2+, which was transformed from the Cd‐LMWOA complexes by microbial degradation, was adsorbed onto the negatively charged soil particles. Increasing amounts of Cd were released from the soils to the soil solution with the renewal of LMWOAs after every 2 h. The short‐term (reaction period of 0.25 to 1 h) kinetics of Cd release from the soils was described by a parabolic diffusion kinetic model. The overall diffusion coefficient of the Cd release from the soil and the Cd released by the renewal of LMWOAs followed the same trend as the Cd availability index of the soils: Luseland >> Waitville > Jedbergh. Thus, the results presented in this investigation indicate the importance of the kinetics of the Cd release from the soils by the LMWOAs excreted by root exudates in understanding the availability of soil Cd.

The underlying mechanisms for root competition are not completely understood; yet, it is important to accurately define the zone of influence around individual roots for nutrient or water uptake, particularly when modeling nutrient uptake processes. The objectives of this study were to investigate the use of the Dirichlet tessellation for quantifying soil polygons around roots of spring wheat (Triticum aestivumL. cv. Katepwa) grown on three landscape positions and to quantify the root spatial patterns. Roots were exposed on a horizontal soil plane at 2‐, 5‐, and 10‐cm depths and mapped on acetate sheets at 21, 45, and 88 days after planting (DAP) for shoulder, backslope, and footslope positions. Tessellation of scanned root maps showed that soil polygons decreased in size during the sampling period; however, polygon areas increased with soil depth regardless of slope position and ranged from 0.08 to 52.77 cm2. Eccentricity averaged 0.74 ± 0.01, indicating that polygons were more circular than elliptical and root displacement averaged one‐third the distance from the polygon center to the polygon edge. Root spatial patterns using R‐index values, showed that roots on shoulder positions had clumped to random patterns while roots at the footslope had random to regular patterns. Root spatial patterns and displacement for wheat violated the assumptions used in nutrient uptake modeling and should be incorporated into future modeling efforts. Dirichlet tessellation is an effective procedure for analyzing root competition and has promise for multispecies competition if different root systems can be weighted by morphological or physiological characteristics.

The distribution of hydroxy‐interlayered minerals (HIM) was studied in Ultic Haplaquods with respect to horizon and location within the soil matrix. Mineralogy of the clay fraction and of tenacious grain coatings was determined for selected horizons by means of x‐ray diffraction and differential scanning calorimetry. Smectite and kaolinite were the dominant layer silicates in A and E horizons, where sand grains were stripped of coatings and clay occurred as floccules. Hydroxy‐interlayered minerals were the dominant layer silicates in Bh and E′ horizons where grains were coated. Tenacious coatings removed by ultrasound from sands occurring in the Bh and E′ horizons contained more HIM and less quartz than any other portion of the soil matrix. Podzolic stripping of coatings in A and E horizons and transformation of HIM to smectite via interlayer depolymerization could explain the observed HIM depth distribution. Higher HIM concentrations in tenacious coatings in Bh and E′ horizons suggest that the coatings predated podzolization. Hydroxy‐interlayered minerals in Btg1 horizons dispersed in deionized water more readily than did kaolinite, suggesting that differential dispersion in conjunction with a fluctuating water table may be a factor in pedogenic redistribution of clay minerals.

Root length distribution of slash pine (Pinus elliottiiEngelm. var. elliottii) and associated understory vegetation was investigated for an Ultic Haplaquod in north‐central Florida. One‐third of the length of pine roots (<1 mm in diameter) was present in the argillic horizon below the 1 m depth. Slash pine taproots extended between 200 and 330 cm below the surface, (i.e., 1‐2.3 m into the argillic horizon). Slash pine roots were often found growing in large diameter, dead palmetto roots. Two‐thirds of the soil samples taken from the E horizon had all the pine roots growing in dead palmetto roots. Stable strontium (Sr) was placed in the argillic horizon and in the litter layer. Strontium concentration in the slash pine foliage increased 30 to 50% (depending on the age of the foliage sampled) when the argillic horizon was treated. There was no apparent Sr increase in needle tissue from spiking the litter layer. This effect was most likely due to the competition from associated vegetation.

Various definitions of the nutrient buffer power of soils, based on ion‐exchange isotherms, have been proposed in the literature. However, incorrect definitions can result in erroneous estimations of solute diffusion coefficients and nutrient replenishment of soil solution, hence, rates of nutrient supply for plant nutrient uptake. This note presents conditions under which each definition of buffer power is valid, and situations where inherent assumptions may not be valid.