Seasonal variations in net ecosystem exchange of CO 2 and conservation of water use efficiency of cotton in the Mississippi Delta

Abstract Accurately quantifying the net ecosystem exchange of CO₂ (NEE) is a critical prerequisite for developing alternative farm management strategies to enhance carbon sequestration in biological systems and reduce greenhouse gas emissions. A two-year study quantified the NEE of CO₂ from cotton (Gossypium hirsutum L.) on silty clay in farm-scale fields using an eddy covariance approach (EC). On a seasonal scale, NEE and evapotranspiration (ET) were 8,836 kg CO₂ ha⁻ ¹ and 367 mm, respectively, in 2017, and 10,759 kg CO₂ ha⁻ ¹ and 430 mm, respectively, in 2018. Harvested cotton lint yields were 1,269 and 1,569 kg ha⁻ ¹ in 2017 and 2018, respectively. The water use efficiency (WUE) for lint production (WUElint) and ecosystem level WUE for NEE (WUENEE) across the two seasons was nearly constant. WUElint were 3.5 kg lint ha⁻ ¹ mm⁻ ¹ and 3.6 kg lint ha⁻ ¹ mm⁻ ¹, respectively, in 2017 and 2018, and WUENEE were 24 kg CO₂ ha⁻ ¹ mm⁻ ¹ and 25 kg CO₂ ha⁻ ¹ mm⁻ ¹. The NEE and WUE measured in this study can be used as a benchmark for comparing carbon sequestration potential in cotton production. However, further research is needed to understand NEE responses to long-term climate variability and to develop climate-smart crop-soil management strategies.