Boron in humus and inorganic components of Hamra and Grumosol soils irrigated with reclaimed wastewater F. S. Kot A,B , R. Farran A , M. Kochva A , and A. Shaviv A A Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel. B Corresponding author. Email: fskot@tx.technion.ac.il 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 mm, 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. Received 7 September 2011, accepted 16 January 2012, published online 20 February 2012 Introduction Boron (B) is involved in important soil processes (Kovda 1973): the intra-soil biological cycle; humification; isomorphic substitution in clay minerals and the formation of colloids; and illuviation. Goldschmidt (1937) found that B is concentrated in the uppermost humus layer of forest soils. Viets (1962) supposed that much of the B in soils is complexed with humus, probably as adsorbed, chelated, or complexed ions, where its release to plants is presumably dependent on soil moisture. Berger and Pratt (1963) stated that a large part of the total B in soils is held in the organic matter in tightly bound compounds that have been formed in the growing plants themselves; B in organic matter is primarily released as available form through the action of microbes. Agulhon (1910) showed that B stimulates the growth of several vascular plants in water culture and that B is abundant in lignified tissues. Matoh et al.(1992) assigned 95–98% of the total plant B to cell wall under B-limiting conditions, leaving only a small fraction for possible involvement in other plant functions. Increasing evidence suggests one or more functions of B beyond cell-wall structure for a large variety of organisms, including plants, animals, and bacteria. Several critical reviews of B chemistry and its role in higher plants have been published since 1997 (Hu and Brown 1997; Nable et al. 1997; Power and Woods 1997; Dembitsky et al. 2002; Bolaños et al. 2004; Goldbach and Wimmer 2007; Tanaka and Fujiwara 2008; Kot 2009; Lehto et al. 2010). Knowledge on biogeochemistry of B in soil–plant systems may be crucial for most cultivated soils of different climatic areas, including those deficient in water resources and forced to utilise down-graded and reclaimed wastewater (RWW). Wastewaters tend to be relatively high in B concentration, mostly due to its presence in detergents. Studies on B fertilisation have shown that the limits between B deficiency and toxicity are very narrow, and that applications of B can be extremely toxic to some plants at concentrations only slightly above optimum for others (Cartwright et al. 1984; Gupta et al. 1985). Irrigation water does not usually contain enough B to injure plants directly; rather, it is the continued use and concentration in the soil due to evapotranspiration that leads to the eventual toxicity problems (Eaton and Wilcox 1939; Gupta et al. 1985). In general, both total and plant-available B can be very high in arid or semi-arid areas where leaching is limited. Boron is a constituent of cell walls and membranes of vascular plants and is also essential for algae, diatoms, and cyanobacteria, whereby it enters soil humification processes. Though studies on B in soil and soil–plant systems focus mostly on inorganic B interactions, a few works have considered B-organic/humus forms in soil (e.g. Kot 2009). Unlike most Journal compilation Ó CSIRO 2012 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2012, 50, 30–43 http://dx.doi.org/10.1071/SR11232