The purpose of this chapter is to provide plant scientists with a background on the nature of soil salinity with a particular emphasis on irrigated agriculture. Since the chemistry of soil solutions plays a major role in soil salinity, considerable details on this topic are offered. Chemical speciation in the soil solution should be of importance to plant scientists. The dynamic nature of soil salinity in the rootzone affects performance of plants. Profile distribution of salts is affected by leaching fraction and changes with changing water content from irrigation and rootwater extraction. Soluble salts in soils are highly mobile and transported by water through mass flow and dispersion. Irrigation water management is one of the keys in maintaining salt balance in the rootzone. Growing regulations on the disposal and management of poor quality drainage waters is now exacerbating the maintenance of salt balance in the rootzone in irrigated lands.
IntroductionThe world's surface area occupies about 13.2 billion ha, but no more than 7 billion ha are arable and 1.5 billion ha are cultivated (Massoud, 1981). Of the cultivated lands, about 0.34 billion ha (23%) are saline (salt-affected) and another 0.56 billion ha (37%) are sodic (sodium-affected). Thus, saline and sodic soils cover about 10% of the total arable lands and exist in over 100 countries. Another set of database (FAO, 1989) indicates that the world has about 227 million ha of irrigated lands of which 20% are salt-affected. Since irrigated agriculture provides about one third of the world food supply, secondary salinization of irrigated lands is of major concern. And of the remaining 1,247 million ha of non-irrigated lands, 31.2 million ha are salinized. Ghassemi et al. (1995)
A flow‐through wetland system was established in the Tulare Lake Drainage District (TLDD) in California to determine if selenium (Se) from saline irrigation drainage can be removed prior to impoundment in evaporation basins to reduce potential toxicity to waterbirds. The objective of this research was to evaluate Se speciation, accumulation, and fractionation in the waters and sediments of the newly developed wetland system. The inlet water was dominated by selenate [Se(VI), 92%], with smaller percentages of selenite [Se(IV), 5%] and organic Se [org‐Se(−II), 3%]. For the outflow water, the average percentage of Se(VI) was 72% in November 1997 and 59% in February 1999. This change may be due to an increase in either residence time and/or accumulation of organic detrital matter, which may enhance Se(VI) reduction processes. Selenium accumulation, transformation, and incorporation with the solid phase were all intensified in the surface sediment (<20 cm). The highest total Se concentrations in the sediments were found in the top 5 cm and concentrations dramatically decreased with depth. Elemental Se [Se(0)], as extracted by Na2SO3, was the largest fraction (average of 46%) of the total sediment Se, followed by organic matter‐associated Se (OM‐Se) extracted by NaOH (average of 34%). Soluble, adsorbed, and carbonate‐associated Se, as extracted by KCl, K2HPO4 (pH 8.0), and NaOAc (pH 5.0), were about 3, 10, and 3% of the total sediment Se, respectively. After establishing the wetland for 2 yr, significant Se removal from the flowing water was observed. The major sink mechanisms in the sediment are reduction to Se(0) and immobilization into the organic phase.
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