**12. Irrigation management to limit Rice arsenic accumulation**

Irrigation management of rice has been extensively studied to determine if restricted water application may result in reduced arsenic uptake [70–75]. In Missouri, Aide et al. [71], in a two-year rice study, investigated two irrigation regimes involving delayed flood and furrow irrigation on silt loam and clayey soils to assess arsenic uptake. Across both years and soil-types, rice total arsenic uptake was substantially reduced in rough rice seed for the furrow irrigated regime. Aide and Goldschmidt [72] in a two-year project involving 20 rice varieties similarly demonstrated that furrow irrigated rice had dramatically reduced arsenic concentrations in paddy (rough) rice compared to delayed flood irrigated rice. All furrow irrigated rice had rough rice total arsenic concentration below 0.1 mg kg−1, with 17 of the 20 rice varieties having less than 0.05 mg As kg−1. The mean arsenic concentrations for the delayed flood rice regime were approximately 0.28 mg As kg−1.

Aide demonstrated that furrow irrigation, involving three rice varieties in 2018 [75] and six varieties in 2019 [73], resulted in substantially smaller arsenic concentrations in rice straw and rough rice seed than delayed flood. Aide [74] in a review of water availability and research involving water-restricting irrigation regimes in Egypt, India and Eastern Asia demonstrated that alternate wetting and drying irrigation frequently was water conserving and limited arsenic uptake. Many of the research citations documented rice yields that were comparable to traditional irrigation regimes; however, additional research remains to be performed to provide consistency in yield attainment. An additional benefit of reduced irrigation of rice was a reduction in methane emission, a potent greenhouse gas [70].

Carrijo et al. [76] performed a compelling rice meta-study involving 56 studies comparing continuous flood with alternate wetting and drying (introduction of unsaturated soil water conditions), with most of the studies derived from Asia. They defined and partitioned alternate wetting and drying irrigation regimes into "safe" or "mild" (where the soil water matric potential was equal to or smaller than −20 kPa) and "severe" (where the soil water matric potential was below −20 kPa). The meta study documented the following: (i) the presence of unsaturated soil water conditions imposed during the entire growing season depressed rice yields, whereas unsaturated soil water conditions prior to either heading only (vegetative) or post heading (reproductive) only demonstrated little to zero yield loss, (ii) in most cases mild or safe alternate wetting and drying do not depress rice yields, whereas severe alternate wetting and drying showed yield reductions, (iii) yield losses were more significant in low organic matter soils or soils having alkaline pH levels, (iv) compared to the continuous flood system the alternate wetting and drying systems exhibited smaller water use rates and where mild alternate wetting and drying was practiced the water use efficiency was greater.

In China, He et al. [77] compared rice growth characteristics and yields in flood and non-flood systems and documented that rice root length density, leaf dry weight, shoot dry weight, and root activity were greater in the non-flood irrigation system at mid-tillering. Yields were typically greater in the flood system across all treatments. In California, Li et al. [78] investigated several alternate wetting-drying *An Emerging Global Understanding of Arsenic in Rice (*Oryza sativa*) and Agronomic Practices… DOI: http://dx.doi.org/10.5772/intechopen.105500*

irrigation systems with respect to continuous flood. The alternate wetting and drying were imposed at panicle initiation or 50% heading, with various degrees of drying established for each crop growth stage. At crop maturity total arsenic concentrations were greatest for the root system (14.8 mg As kg−1), whereas straw arsenic concentrations were 0.64 mg As kg−1. The arsenic concentrations in the root systems were primarily associated with Fe-plaque. Grain arsenic concentrations, when compared to continuous flood, were 57% redcued for brown rice and 63% reduced for polished rice. As the driest alternate wetting-drying episode, rice grain exhibited 78% less DMA and 40% less inorganic arsenic when compared to continuous flood. In California, Carrijo et al. [79] observed rice irrigation involving continuous flood and three alternate wetting and drying irrigation regimes with differences in drying severity (low, medium (−71 kPa) and high (−154 kPa) and three timings of the drying episodes (panicle initiation, booting and heading. Imposition of the medium and high drying episodes decreased arsenic uptake by 41 to 61%. The booting and heading drying episodes showed better arsenic mitigation responses.
