Ecosystem-atmosphere exchanges of carbon dioxide, water vapour and energy in India: a synthesis of insights from eddy covariance measurements

India is a large country characterised by diverse bioclimatic regions and semi-natural and managed ecosystems, with some of the largest areas of arable land and mangroves, globally. Eddy covariance represents the state-or-the-art for directly quantifying the exchange of mass and energy between land surface and atmosphere. Here, we collate eddy covariance flux observations from several sites across India, covering major land use and vegetation types and spanning twenty-seven site-years. The pattern of maximum and minimum CO2 exchange differ widely among the sites and ecosystems. Croplands exhibit maximum CO2 uptake during the monsoon in response to rainfall. Some forests, croplands, and mangroves behave as well-watered ecosystems, whereas others oscillate between well-watered and water-stressed states, due to temperature and moisture dynamics. Respiration changes commensurately with photosynthetic CO2 uptake, primarily comprising growth respiration. Grasslands have a higher carbon retention capacity, followed by croplands, forests, and mangroves. CO2, water, and sensible and latent heat fluxes peaked during different times of the day across ecosystems, imprinting phase-lags that vary by site and season. Water-limited ecosystems register the highest ecosystem water use efficiency (WUE), whereas the irrigated croplands have the lowest WUE. Forests have intermediate WUE of these two; however, Indian forests (predominantly tropical and subtropical) have lower WUE than their temperate and boreal counterparts. Canopy-atmosphere coupling is tightest during the dry periods, with their physiological controls regulating the properties of the surface atmosphere. This is reversed during the monsoon when environmental control dominates physiological control. This information is essential for the long-term monitoring of these ecosystems and climate studies and will be useful to different communities, including scientists, economists, resource managers, and policymakers.