Rainfed cotton crop in central India is a strong net CO2 sink: An eddy covariance-based analysis of ecosystem fluxes

The present study reports ecosystem CO₂, H₂O exchanges (at diurnal, daily, and seasonal-scale) from a rainfed (dryland) cotton crop (Gossypium sp) grown in deep black soil of tropical central India during 2019–20 and 2020–21, using an Eddy Covariance (EC) system. The half-hourly CO₂ fluxes were found to vary significantly across the growth stages of cotton crop with peak values during first boll opening to first picking stage [mean Net Ecosystem CO₂ Exchange (NEE), −16 to −18 µmol m⁻² s⁻¹; mean Gross Primary production (GPP), 20–22 µmol m⁻² s⁻¹; and mean Ecosystem Respiration (Rₑcₒ), 5–6 µmol m⁻² s⁻¹]. The response of daytime GPP to Photosynthetic Photon Flux Density (PPFD) was found to be poor during 1–30 days after sowing (DAS) with R²∼0.4. It improved during 30–150 DAS (R²∼0.6), while deteriorating further as the crop started senescing. The rainfed cotton crop was found to be a strong net CO₂ sink, with seasonal NEE, GPP, Rₑcₒ, and evapotranspiration (ET) of − 333.66 gC m⁻², 990.16 gC m⁻², 656.50 gC m⁻² and 468.32 mm during 2019–20, respectively. The cotton season of 2020–21 had seasonal NEE, GPP, Rₑcₒ, and ET of − 391.85 gC m⁻², 1063.89 gC m⁻², 672.04 gC m⁻², and 545.25 mm respectively. The seasonal Ecosystem Water Use Efficiency (EWUE) was estimated to be 1.9 and 2.1 gC kg⁻¹ H₂O during 2019–20 and 2020–21, respectively. Maximum values of crop coefficient (Kc) were observed to be 1–1.2 during 30–75 DAS, signifying favourable conditions for crop growth. The inter-seasonal differences in the CO₂ and H₂O exchanges were mainly driven by the differences in incoming PPFD due to cloudiness. Results of the present study will help in understanding the ecosystem processes and exchanges from a rainfed cotton-based cropping system, which in turn can significantly contribute to the global carbon and moisture budget.