3.2 Interannual variability and trends of carbon fluxes
Annual NEP is positive and showed an increasing trend (5.65 gC m-2 yr-1 yr-1) from 2009 to 2021 (p < 0.05) (Figure 4). The mean NEP was 91.61±36.17 gC m-2 yr-1 (CV = 39.49%) over the study period (2009–2021). The maximum value was 168.30 gC m-2 yr-1 in 2020 and the minimum value was 34.70 gC m-2 yr-1 in 2010 (Figure 4).
The mean values of accumulative NEP in spring, summer, and autumn were 31.07, 60.91, and 25.31 gC m-2, which account for 33.9, 66.5, and 27.6% of the annual NEP, respectively. The accumulative NEP in spring displayed a statistically significant increasing trend, with an increase rate of 3.18 gC m-2 yr-1 yr-1 (p = 0.01) (Figure 5a), which accounted for 56% of the annual increase rate. No statistically significant trend was observed for the other seasons (Figure 5b, c).
NEP is determined by GEP and Re (NEP = GEP - Re); thus, we analysed the long-term trends of GEP and Re. Both annual and spring GEP showed a statistically significant increasing trend over the 13 years (p < 0.05), with an increasing rate of 8.63 and 4.65 gC m-2 yr-1 yr-1, respectively (Figure 4, 5a), which was consistent with the trend of NEP. The annual Re showed a non-significant increasing trend (p > 0.05) (Figure 4). Re in spring, summer, and autumn did not exhibit a significant increasing trend (Figure 5). It can be seen that the growth trend of NEP is mainly caused by the growth of GEP, especially in spring.