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

摘要

Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that omol Lp# additions of Mgpo were as effective in ameliorating Al rhizotoxicity as additions of Capoin the mmol Lp# 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 omol Lp#) and MgCl additions to achieve 150 and 300 omol Lp# Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO 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 (Alpd and Al-hydroxyl species) and Mgpo. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Capo), (Alpd)/(Capo) and (Alpd)/([Capo] + [Mgpo]), 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.