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

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.