Boron in humus and inorganic components of Hamra and Grumosol soils irrigated with reclaimed wastewater

Abstract

The role of organic matter in soil boron (B) turnover and availability is not well understood. The forms and mobility of B are of special concern in soils irrigated with reclaimed wastewaters (RWW). We studied B distribution and binding in major components of two irrigated Mediterranean soils, with special emphasis on humus and water-mobile phases. The results showed that most B in the sandy loam Hamra soil and a large part in the clayey calcareous Grumosol was bound to extractable humus fractions and, in the Grumosol, to organic/mineral refractory residue, along with fractions of free (non-silicate) iron/aluminium (Fe/Al) minerals and aluminosilicates. Among humus fractions, the major B carriers were humin, Fe/Al-humates (complexed firmly, presumably through polyvalent Fe/Al cations), and calcium/magnesium (Ca/Mg)-humates (bridged to soil particles through divalent cations), and to a much lesser extent fulvic-Fe/Al (Hamra) and fulvic-Ca/Mg (Grumosol) complexes. The mode of B preferential binding indicates an origin of the soil humus from lignin of plant cell walls and membranes. In water extract, B was bound firmly (non-exchangeable) to coarse colloids >0.20 μm, presumably of organic/bacterial origin. Boron was not detected in the exchangeable fraction. This raises the question of the forms of bioavailable B in the soils. It can be assumed that the bulk of B in the soil–plant system circulates among plants (lignin) and the inherited soil organic matter/humified material. It is noteworthy that irrigation with RWW resulted in a slight increase of mannitol-extractable B and a redistribution of humus-B in favour of firmly bound Fe/Al-humate complexes.

Boron in the Environment

“…122 In arid and semiarid areas, B toxicity is a result of high levels of B in soils due to high evaporation rate together with additions of B via irrigation water. 43,45,119,125 In the mean time, Hingston 126 and Yermiyahu et al 127 reported higher sorption of B on soil organic matter than on clay; at similar pH levels and B concentration, the sorption was one to two orders of magnitude higher than on clays. In addition, human pollution may increase soil B to levels that are toxic for plants.…”

“…Temporary deficiency of available B can be triggered by liming of acid soils due to increased B adsorption at higher soil pH. 43 Furthermore, in soil, there are plenty of B-binding agents besides hydroxyl anions from dissociated water molecules. 123 The highest naturally occurring concentrations of soil B are in soils derived from marine evaporates and marine argillaceous sediments.…”

“…The most important source is irrigation water, but others include waste from surface mining, fly Boron in the Environment ash, and industrial chemicals. A bulk of organically bound soil B seems to be inherited from the plant lignin: Kot et al 43 have found a major portion of the soil B to be bound firmly in refractory fractions of the soil organic matter, such as humin and claye/ironealuminumehumus complexes. 121a Some authors suppose that the chemistry of soil B, compared to other nutrient elements, is very simple for the following reasons 124 : (1) B does not undergo oxidationereduction or volatilization reactions in soils, (2) boric acid is a very weak, monobasic acid that acts as a Lewis acid by accepting a hydroxyl ion to form the borate anion, and (3) B-containing minerals are either very insoluble (tourmalines) or very soluble (hydrated B minerals) and these minerals generally do not control solubility of B in soil solution.…”