16 17 As the world's population increases, demands on staple crops like rice (Oryza sativa L.) will also 18 increase, requiring additional fresh water supplies for irrigation of rice fields. Safe alternate wetting 19 and drying (AWD) is a water management technique that is being adopted across a number of 20 countries to reduce the water input for rice cultivation. The impact of AWD on plant growth, yield 21 and grain quality is not well understood. A field trial of AWD was conducted at Mymensingh, 22 Bangladesh over two boro (dry) seasons using eight field plots, four under AWD and four 23 continuously flooded (CF). This manuscript describes the results of check cultivar BRRI dhan28 which 24 was replicated in 35-40 rows per plot giving a total of 140-160 replicates per treatment. A study on 25 the soil solution concentration of many elements indicated that manganese, iron, zinc, and arsenic 26 were different under AWD conditions compared to CF on a number of sampling time points, but did 27 not show a pattern related to the AWD treatment. A survey of soil strength using a penetrometer 28 detected a small, but significant, hardening of the surface soil of the AWD plots. At harvest the shoot 29 and grain mass was significantly greater for the plants grown under AWD (9.0-9.4% and 12.0-15.4%, 30 respectively) with the plants grown under AWD having a greater number of productive tillers. 31Physiological examination in the first year showed that although AWD decreased (~21%) leaf 32 elongation rate (LER) of recently transplanted seedlings during the first drying cycle, subsequent 33 drying cycles did not affect LER, while tillering was slightly increased by AWD and there was evidence 34 of higher leaf abscisic acid (ABA) in AWD plants. In the second year analysis of six phytohormones 35 revealed that AWD increased plant foliar iso-pentenyladenine (iP) concentrations by 37% while leaf 36 trans-zeatin concentrations decreased (36%) compared to CF plants. The elemental composition of 37 the shoots and grains was also examined. In both years AWD decreased grain concentration of 38 sulphur (by 4% and 15%), calcium (by 6% and 9%), iron (by 11% and 16%), and arsenic (by 14% and 39 26%), while it increased the grain concentration of manganese (by 19% and 28%), copper (by 81% 40 and 37%), and cadmium (by 28% and 67%). These results indicate that plants grown under safe AWD 41 conditions at this site have an increased grain mass compared to plants grown under CF, and this 42 may be partly due to a high number of productive tillers. AWD decreases the concentration of 43 arsenic in the grains in this site, but it elevates the concentration of cadmium. 44 45 Key words: Rice, alternate wetting and drying, arsenic, cadmium, pore water 46 48 between 35-60% of their dietary calorie intake (Fageria, 2007). Irrigated lowland rice systems 49 produce ~75% of global rice (Fageria, 2007). Producing high yield under irrigated systems requires 50 large quantities of water (Bouman, 2009). It is estimated that to produce 1 kg of rice gra...

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Water saving techniques, such as alternate wetting and drying (AWD), are becoming a necessity in modern rice farming because of climate change mitigation and growing water use scarcity. Reducing water can vastly reduce methane (CH 4 ) emissions; however, this net climate benefit may be offset by enhanced carbon dioxide (CO 2 ) emissions from soil. The main aims of this study were: to determine the effects of AWD on yield and ecosystem C dynamics, and to establish the underlying mechanistic basis for observed trends in net ecosystem C gain or loss in an Italian rice paddy. We investigated the effects of conventional water management (i.e. conventionally flooded paddy; CF) and AWD on biomass accumulation (aboveground, belowground, grain), key ecosystem C fluxes (net ecosystem exchange (NEE), net primary productivity (NPP), gross primary productivity (GPP), ecosystem respiration (ER), autotrophic respiration (RA), heterotrophic respiration (RH)), and soil organic matter (SOM) decay for four common commercial European rice cultivars. The most significant finding was that neither treatment nor cultivar affected NEE, GPP, ER or SOM decomposition. RA was the dominant contributor to ER for both CF and AWD treatments. Cultivar and treatment affected the total biomass of the rice plants; specifically, with greater root production in CF compared to AWD. Importantly, there was no effect of treatment on the overall yield for any cultivar. Possibly, the wetting-drying cycles may have been insufficient to allow substantial soil C metabolism or there was a lack of labile substrate in the soil. These results imply that AWD systems may not be at risk of enhancing soil C loss, making it a viable solution for climate change mitigation and water conservation. Although more studies are needed, the initial outlook for AWD in Europe is positive; with no net loss of soil C from SOM decomposition, whilst also maintaining yield.

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