Climate controls over phenology and amplitude of net ecosystem productivity in a wheat-maize rotation system in the North China plain

Investigating the response of net ecosystem productivity (NEP) to phenological variation for crop ecosystems is important for deeply understanding the impact of climate change on agriculture. However, its main controlling mechanisms have not been well understood for rotation cropland ecosystems. Using a 16 year (2003-2018) eddy covariance flux observation in a typical wheat-maize rotation system in the North China Plain (NCP), we explore the potential of carbon flux phenology (CFP) and abiotic factors in interpreting the interannual variability (IAV) of NEP. The results showed that the NEP of the wheat season (NEPwheat) was significantly controlled by the carbon uptake period (CUP), the end date of CUP (ECUP) and the peak value of NEP (NEPmax). The increase in spring temperature reduced soil water content (SWC) in the wheat season, leading to shorter CUP, earlier ECUP and lower NEPmax. However, NEP of the maize season (NEPmaize) was mainly affected by CUP, the start date of CUP (SCUP) and NEPmax. ECUP and CUP were negatively correlated with air temperature (Ta) and soil tem-perature (Ts) of the maize season, and NEPmax had a significant negative correlation with mean summer tem-perature (MTsu) and Ta. In addition, we found that NEPmaxcontributed the most to the IAV of NEPwheat (63%). However, CUP showed the most contributions in maize ecosystems, interpreting 30% IAV of NEPmaize. When considering the export of harvested biomass, the wheat season carbon budgets were close to neutral of 21 +/- 26 g C m- 2, the maize season was a carbon source of 102 +/- 18 g C m- 2, and the whole year behaved as a carbon source of 129 +/- 41 g C m- 2 during our study period. The study provides important insights into the response of carbon budget of cropland ecosystems to vegetation phenology shifts in the North China Plain under future climate change.