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 CO 2 , H 2 O 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 CO 2 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 CO 2 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 (R eco ), 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 R 2 ∼0.4. It improved during 30–150 DAS (R 2 ∼0.6), while deteriorating further as the crop started senescing. The rainfed cotton crop was found to be a strong net CO 2 sink, with seasonal NEE, GPP, R eco , 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, R eco , 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 H 2 O during 2019–20 and 2020–21, respectively. Maximum values of crop coefficient (K c ) were observed to be 1–1.2 during 30–75 DAS, signifying favourable conditions for crop growth. The inter-seasonal differences in the CO 2 and H 2 O 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. • Rainfed cotton crop was a strong net CO 2 sink with peak net CO 2 influxes of − 16 to − 18 µmol m -2 s -1 . • NEE − 333 gC m -2 , GPP 990 gC m -2 , and ET 468 mm in the crop season of 2019–20. • NEE − 392 gC m -2 , GPP 1064 gC m -2 , and ET 545 mm in the crop season of 2020–21. • Seasonal EWUE was 1.9–2.1 gC kg -1 H 2 O with maximum K c values of 1–1.2 during the mid-season.