Ecosystem carbon dioxide exchange and water use efficiency in a triple-cropping rice paddy in Southern India: A two-year field observation

The lowland tropical triple-cropping rice system has unique characteristics that affect the hydrological, nutrient, and atmospheric environments. To better understand the ecosystem carbon and water dynamics of a triple-cropping rice paddy from the perspective of sustainability, ecosystem-level CO₂ flux and ecosystem water use efficiency (eWUE) were observed using eddy covariance over 2 years (2016–2018) at an experimental field site in southern India, and gross primary production (GPP) and ecosystem respiration (RE) were derived using the flux partitioning technique. Results showed that among the three crop seasons per year, GPP and RE were higher (887.2 and 570.2 g C m⁻², respectively) in Thaladi (October–January: wet season) than in Kuruvai (June–September: dry season; 773.4 and 568.9 g C m⁻², respectively) and summer rice (February–May; 694.0 and 499.7 g C m⁻², respectively) owing to the longer growing season. Triple-cropping meant that the quasi-annual GPP of 2598 g C m⁻² (i.e., the total value for the three consecutive seasons, including the corresponding fallow periods) was much greater than the quasi-annual RE of 1974 g C m⁻². Consequently, the net ecosystem production value was positive (624 g C m⁻²). Evapotranspiration was also high on the annual scale (1681 mm); that is, 48 % greater than mean annual precipitation (1139 mm). Analysis revealed that Thaladi had higher eWUE (2.21 g C (kg H₂O)⁻¹) than that of Kuruvai (1.46 g C (kg H₂O)⁻¹) and summer rice (1.57 g C (kg H₂O)⁻¹) owing to decreased water loss in cloudy weather. Intense solar radiation is generally recognized as advantageous for crop growth in most regions, but not for Kuruvai and summer rice, when too strong solar radiation increases loss of water unused for photosynthesis. The findings indicate that water-saving techniques should be targeted on the Kuruvai and summer rice seasons.