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

Abstract

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.