Abstract
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 (CH4) emissions; however, this net climate benefit may be offset by enhanced carbon dioxide (CO2) 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.
Highlights
Irrigated rice (Oryza sativa L.) is the largest consumer of water in the agricultural sector (Thakur et al, 2014) and can require up to 2500 L of water per kg yield, depending on the rice ecosystem and local climate (Bouman, 2009)
Belowground net primary productivity (NPP) (BNPP) will be greater in AWD compared to continuously flooded (CF), while aboveground NPP (ANPP) will show the opposite trend, in-line with plant resource allocation theory
One possible explanation is that the 1–2 week wetting-drying cycle for our AWD system was not sufficient to cause an observable shift in the carbon metabolism of the soil; this interpretation is supported by findings from our leaf litter decomposition experiment, which showed no significant difference in decay rates between AWD or CF treatments, implying that the underlying carbon metabolism of the soil was not altered by AWD
Summary
Irrigated rice (Oryza sativa L.) is the largest consumer of water in the agricultural sector (Thakur et al, 2014) and can require up to 2500 L of water per kg yield, depending on the rice ecosystem and local climate (Bouman, 2009). The total European rice contribution is only 0.4% of the total global figure (FAO, 2014; USDA, 2015), it has economic, sociocultural and ecological importance in several Mediterranean countries, including the Ebro Delta in Spain, Rhone Delta in France and Lombardy in Italy. In these regions, does rice production contribute to local economies, but rice fields play a key role in managing local ornithological fauna populations and macroinvertebrate communities (Faure and Mazaud, 1995; Ibáñez and Caiola, 2018; Longoni, 2010; Lupi et al, 2013), and the harvested area is continually expanding (Ferrero, 2007; Ferrero and Vidotto, 2010).
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