T. Jot Smyth

Altered aluminum inhibition of soybean root elongation in the presence of magnesium

Variations in genotype rankings among screenings for Al tolerance in hydroponics may be related to differences in the composition of the solutions. In the present study, we investigated the involvement of Mg ions in modifying Al rhizotoxicity in soybeans. Root elongation was strongly inhibited by Al in a simple, 800 μM CaSO4 solution, but elongation increased noticeably when the solutions also contained Mg. Amelioration of Al rhizotoxicity was not associated with an increase in ionic strength of treatment solutions because Al3+ activities were kept constant. Concentration series experiments indicated that the Mg effect occurred in the μM range, while Ca amelioration of Al toxicity occurred at mM concentrations. The positive effect of Mg on root elongation was greatest for Al-sensitive genotypes and minimized genotypic differences for Al-tolerance. The Mg protection against Al rhizotoxicity apparently does not occur with all species, because it was not observed in Atlas and Scout 66 wheat varieties. The ability of Mg to ameliorate Al toxicity in soybean at μM levels suggests the involvement of distinct physiological factors.

Comparative Effects of Two Forage Species on Rhizosphere Acidification and Solubilization of Phosphate Rocks of Different Reactivity

ABSTRACT Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H+) and the removal of the dissolved products [calcium (Ca2 +) and dihydrogen phosphate (H2PO4 −)]. Plant roots may excrete H+ or OH− in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H+ or OH− may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N2 fixation, which induced a significant H+ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO3 −) comprising 92% of total anion uptake. Nitrate and sulfate (SO4 2 −) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH−), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH− extrusion from roots.

Soybean root growth in relation to ionic composition in magnesium-amended acid subsoils: Implications on root citrate ameliorating aluminum constraints

Abstract Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that µmol L−1 additions of Mg2+ were as effective in ameliorating Al rhizotoxicity as additions of Ca2+in the mmol L−1 concentration range. The objectives of this study were to assess the ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of native equilibrium soil solution concentrations in each soil (50 or 100 µmol L−1) and MgCl2 additions to achieve 150 and 300 µmol L−1 Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO3 treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of the soil solution Al species (Al3+ and Al-hydroxyl species) and Mg2+. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Ca2+), (Al3+)/(Ca2+) and (Al3+)/([Ca2+] + [Mg2+]), suggesting that Ca could be a primary factor ameliorating Al and H+ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by micromolar Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al, may be less important in acid subsoils with low native Ca levels.

Assessment of rhizotoxic aluminum in soil solutions by computer and chromogenic speciation

Abstract Numerous chemical species of aluminum (Al) occur in acid soils and simple but accurate methods are needed to characterize phytotoxicity. Kinetic reaction studies between Al and ferron were used to separate Al into instantaneously reactive (Ala), metastable (Alb), and non‐reactive (Alc) fractions in soil solutions of two North Carolina Ultisols. Relative root growth of field‐grown corn (Zea mays L.) was used to assess rhizotoxicity of kinetically measured Al forms. Reaction of Alb with ferron conformed to first order kinetics, with rate constant values ranging from 0.53 to 9.13 minute‐1. The Ala fraction correlated significantly with Al reacting in 30 seconds (r2=0.99), but the former averaged 10% less than the latter. Both Ala and Al reacting in 30 seconds correlated significantly with the Alb fraction (r2≥0.89), but not with the Alc fraction (r2<0.1). Correlation exercises between ferronreactive Al fractions and various Al species predicted by the GEOCHEM‐PC speciation program revealed a strong ...

Crop Utilization of Nitrogen in Swine Lagoon Sludge

Swine lagoon sludge is commonly applied to soil as a source of nitrogen (N) for crop production but the fate of applied N not recovered from the soil by the receiver crop has received little attention. The objectives of this study were to (1) assess the yield and N accumulation responses of corn (Zea mays L.) and wheat (Triticum aestivum) to different levels of N applied as swine lagoon sludge, (2) quantify recovery of residual N accumulation by the second and third crops after sludge application, and (3) evaluate the effect of different sludge N rates on nitrate (NO3-N) concentrations in the soil. Sludge N trials were conducted with wheat on two swine farms and with corn on one swine farm in the coastal plain of North Carolina. Agronomic optimum N rates for wheat grown at two locations was 360 kg total sludge N ha−1 and the optimum N rate for corn at one location was 327 kg total sludge N ha−1. Residual N recovered by subsequent wheat and corn crops following the corn crop that received lagoon sludge was 3 and 12 kg N ha−1, respectively, on a whole-plant basis and 2 and 10 kg N ha−1, respectively, on a grain basis at the agronomic optimum N rate for corn (327 kg sludge N ha−1). From the 327 kg ha−1 of sludge N applied to corn, 249 kg N ha−1 were not recovered after harvest of three crops for grain. Accumulation in recalcitrant soil organic N pools, ammonia (NH3) volatilization during sludge application, return of N in stover/straw to the soil, and leaching of NO3 from the root zone probably account for much of the nonutilized N. At the agronomic sludge N rate for corn (327 kg N ha−1), downward movement of NO3-N through the soil was similar to that for the 168 kg N ha−1 urea ammonium nitrate (UAN) treatment. Thus, potential N pollution of groundwater by land application of lagoon sludge would not exceed that caused by UAN application.

Lack of soybean root elongation responses to micromolar magnesium additions and fate of root-exuded citrate in acid subsoils.

Additions of micromolar concentrations of magnesium (Mg) to hydroponics enhance aluminum (Al) tolerance of soybean by increasing citrate secretion from roots and external complexation of toxic Al species. The objective of this study was to assess the ameliorative effect of Mg additions on soybean root elongation into mineralogically different acid soils. Roots of soybean seedlings grew for 28 days into acid soils treated with three Mg levels in their soil solution (Control, 150 and 300 μM) and lime. Root growth in the acid soils and aboveground dry matter responses to the Mg treatments were less than for the lime treatments. Citrate fate in the acid soils revealed that 66–99% of added citrate was either adsorbed or biodegraded, suggesting that root secreting citrate in the soil abundant with Al and iron (Fe) hydroxides potentially reduces the availability to complex rhizotoxic Al. A calcium (Ca) deficiency may have constrained root growth response to the Mg-treated soils.