Rice cultivation under continuous flooding vs alternate wetting and drying: implications for biomass, nitrogen cycling and greenhouse gas flux

Rice uses 34-43% of the global irrigation water and is responsible for the usage of 24-30% of the world's total freshwater. More than 75% of rice produced in India is cultivated using the traditional continuous flooding (CF) irrigation method, which is a labour-intensive, time, water and energy-consuming process and a key source of global methane emissions. Alternate Wetting and Drying (AWD) is a popular water-saving approach trailed in Asia including India to reduce water use and methane emissions, whilst sustaining rice production. AWD is a method of periodic soil saturation followed by drying compared to CF. The objective of this research was to evaluate greenhouse gas (GHG) fluxes and internal and external nitrogen cycling processes as influenced by AWD and CF management regimes. A mesocosm experiment was set up in the laboratory using imported Indian paddy soil where Jasmine rice (var KDML 105) was grown. Our results depicted that plant biomass (52.57%), root biomass (28.57%), height (24.77%), effective tiller number (45.15%), stem sheath diameter (53.38%) and stomatal conductance (66.49%) were significantly (p<0.05) higher in CF compared to AWD treatment. A similar trend was observed in rice leaf chlorophyll (Chl a, b and total chl) contents. Interestingly, the chlorophyll a and b ratio observed was higher (1.63) in AWD compared to CF (1.03) conditions. This was likely during the process of chlorophyll b degradation and conversion to Chl a, thus resulting in the increase of a to b ratio to cope with the stress by maintaining the leaf photosynthetic efficacy. Soil enzyme activity revealed that β-glucosidase (BG), β-N-acetyl-glucosaminidase (NAG), and acid phosphatase (AP) were higher in AWD, whereas leucine aminopeptidase (LAP) activity was significantly higher in CF. Higher LAP activity might be a response to limited nutrient availability, as LAP helps to release amino acids that serves as a source for N mineralization and N supply. The ¹⁵N isotope tracing study revealed that denitrified N₂O flux was significantly (p<0.05) higher in CF compared to AWD where source partitioning (% N₂O denitrified) was 99.32% in CF and 27.01% in AWD. Higher gross mineralization was observed under AWD (3.92 ± 0.31µg^-1 g^-1 d^-1) due to the promotion of aerobic microbial activity compared to CF (1.31 ± 0.31µg^-1 g^-1 d^-1). A similar trend was observed for the consumption and immobilization of NH₄⁺ and gross nitrification rates. GHG emissions rate viz., CH₄-C, CO₂-C, and N₂O-N emissions were significantly higher under CF by 61, 3 and 72.%, respectively. Moreover, the global warming potential projected was higher under CF averaging at 10.92 mg kg^-1 soil compared to 2.19 mg CO₂ kg^-1 soil under AWD. Reduced GHG emissions under AWD provides for a significant negative feedback to global warming potential and future initiatives should keep emphasizing the optimization of this practice for its significant contribution to both climate change mitigation and sustainable agriculture.