Seasonal variation of net ecosystem CO2 exchange and its influencing factors in an apple orchard in the Loess Plateau

The Loess Plateau is the largest apple cultivation region in the world. However, the role of rain-fed apple orchards as carbon sinks or sources, including the dynamic variation and influencing factors, are still unclear. In this study, the net ecosystem CO2 exchange (NEE) was monitored by an eddy covariance (EC) system in Loess Plateau apple orchards during 2016-2017. The results demonstrated that the annual NEE was higher in 2016 (- 698.0 g C m(-2) year(-1)) than in 2017 (- 554.0 g C m(-2) year(-1)). Particularly, the amount of orchard CO2 uptake was significantly greater in 2016 (- 772.0 g C m(-2)) than in 2017 (- 642.1 g C m(-2)) during the carbon sink period. This difference may be attributed to the higher NEE in 2016 compared to 2017 during the fast and slow growth periods. In addition, a higher daily NEE occurred to the higher air temperature (T-a), which promoted early sprouting in 2016 (- 3.91 g C m(-2) day(-1)) compared to 2017 (- 2.86 g C m(-2) day(-1)) during the fast growth period. The daily NEE in 2016 (- 2.59 g C m(-2) day(-1)) was remarkably higher than that in 2017 (- 1.41 g C m(-2) day(-1)) during the slow growth period, owing to the greater number of cloudy and rainy days and lower temperatures in 2017. Overall, the present study demonstrated the key role played by the amount of precipitation and temperature in regulating the NEE during the growth season and provided accurate quantitative information on the carbon budget in apple orchards.