On soil boron turnover in soil–plant system with emphasis on organic matter

Fyodor Kot1, Ronit Farran1, Kunio Fujiwara2, Malik Kochva1, Avi Shaviv1, Takanobu Sugo3
1 Technion-Israel Institute of Technology
2 Department of Applied Chemistry and Biotechnology, Chiba University
3 KJK Co., Takasaki, Japan

O 11.2 in Weathering and chemical processes as keys to ecosystem functioning

15.07.2014, 11:20-11:40, H20

Unlike most other micronutrients, plant roots require a continuous external supply of trace amounts of boron (B), otherwise loss of membrane function occurs within minutes (Blaser-Grill et al., 1989). The concentration must be low enough not to have an adverse effect on plants. Apparently, the content of available B in the soil water-mobile phase (“soil solution”) must be controlled by an effective buffer system (Asad et al., 1997). Surprisingly, only a few works have considered B-organic/humus complexes as a source of bioavailable B (e.g. Kot, 2009; Kot et al., 2012). Still, the main challenging questions are: (1) the main sources of B to soils, and (2) the role of plant litter/debris, humification and humus substances in the soil-plant B turnover.

In order to evaluate soil available B potential and sources, a series of assays with soil water extract and plant litter was carried out. The objects were: (1) a collection of eastern Mediterranean soils: grumusol, terra rossa, loess, sandy, peaty-clay, and red sandy-loam; (2) leaves and litter of tree plants representing contrast ecosystem niches: oak Quercus robur, carob tree Ceratonia siliqua, and willow Salix babylonica. Soil water extracts were separated onto colloidal dimensions of 0.20, 0.45, and 1.20 µm. The leaves and litter was put into mini lysimeters and watered imitating rain events. Boron in the leachates and extracts was concentrated on B-specific ion-exchange stuff and analyzed with Azomethine-H method.

Concentration of B extracted from the soil water phase <0.45 µm was significantly larger in comparison with <0.20 µm phase, indicating water-mobile colloids as a major carrier of the element.

Sequential water extractions showed the soils’ high capability to compensate B to the soil water (“B regeneration effect”). For some soils, sequential extractions from <0.45 µm filtrate also gave some additional B; moreover, the first extraction showed a positive correlation with pH value only (R2 = 0.52, p<0.05), while for the subsequent extractions correlation with “dissolved” organic matter became more significant (R2 = 0.93, p<0.001). It may be concluded that the release of available B occurred from B-organic/humus complexes. We should expect that a major part of these complexes are inherited from humified plant components like lignin (Gaspar et al., 2011; Kot et al., 2012).

Treatment of the soils with a strong (5% v/w) formaldehyde solution resulted in some decrease in B release, indicating an involvement of microbiota.

The measured B leaching from the plant leaves and litter exceeded the evaluated B income with precipitation by orders of magnitude (29.5–191 and 0.06–5.40 mg B m-2 yr-1, correspondingly). Thus, one can suppose that B turnover in regular soil-plant systems is dominated by a semi-closed cycle “plants–litter–soil humus/humin–soil water mobile organic colloids”.

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last modified 2014-02-10