Characteristics of carbon emissions in cotton fields under mulched drip irrigation

As the second carbon storage pool, soil is easily influenced by human activities. Mulched drip irrigation is a water-saving irrigation technique used widely in arid and semi-arid regions. However, information about the response of CO₂ exchange to mulched drip irrigation, such as the wetter and warmer soil, is limited. To identify the carbon emissions effects of mulched drip irrigation, we carried out a field experiment using drip irrigation with and without clear plastic mulching during the cotton growing seasons of 2015 and 2016. We monitored the temporal and spatial variation of soil moisture, soil temperature, cotton growth stage, biomass, lint yield, CO₂ emissions, and the relationship between soil respiration rate and soil climate. The results showed that plastic mulching drip irrigation increased soil moisture and soil temperature, especially during the early and middle growth stages of cotton. The soil respiration rate was related positively to the higher soil temperature and moisture conditions promoted by plastic film mulching, although the coefficients of determination were low (R² were 0.480 and 0.205, corresponding p-value was both 0.000, respectively). The highest value of soil respiration was obtained within the narrow rows under the drip tape, regardless of the practice of mulching or not. The soil respiration rate under plastic mulch in the narrow and wide rows were on average 28.35 % and 22.48 % higher than non-mulched control. Meanwhile, the amount of total CO₂ emissions was significantly increased by 25.34 % and 28.90 % in these same rows, respectively (p-values were 0.006 at narrow rows and 0.010 at wide rows in the first year, and 0.000 at same rows in the second year). The differences of CO₂ emission in the bare soil was not significant between mulched plots and non-mulched control (p-values were 0.757 and 0.918 in the first and second growing seasons, respectively). In addition, plastic mulching significantly improved the biomass and yield of cotton, by 61.49 % and 12.83 % on average (p-values were 0.034 and 0.039 in 2015, 0.024 and 0.032 in 2016), respectively. The results indicate that the application of drip irrigation under plastic mulch could increase soil water content and temperature, promote cotton growth, and improve lint yield. However, it may also lead to increased CO₂ emissions, which can intensify the warming of the climate.