Interannual variation in plant phenology can lead to major modifications in the interannual variation of net ecosystem production (NEP) and net biome production (NBP) as a result of recent climate change in croplands. Continuous measurements of carbon flux using the eddy covariance technique were conducted in two winter wheat and summer maize double-cropped croplands during 2003–2012 in Yucheng and during 2007–2012 in Luancheng on the North China Plain. Our results showed that the difference between the NEP and the NBP, i.e., the crop economic yield, was conservative even though the NEP and the NBP for both sites exhibited marked fluctuations during the years of observation. A significant and positive relationship was found between the annual carbon uptake period (CUP) and the NEP as well as the NBP. The NEP and the NBP would increase by 14.8±5.2 and 14.7±6.6 g C m−2 yr−1, respectively, if one CUP-day was extended. A positive relationship also existed between the CUP and the NEP as well as the NBP for winter wheat and summer maize, respectively. The annual air temperature, through its negative effect on the start date of the CUP, determined the length of the CUP. The spring temperature was the main indirect factor controlling the annual carbon sequestration when a one-season crop (winter wheat) was considered. Thus, global warming can be expected to extend the length of the CUP and thus increase carbon sequestration in croplands.

The ecosystem light response parameters, i.e. apparent quantum yield (α), maximum rate of ecosystem gross photosynthesis (Amax), and daytime ecosystem respiration (Rd), are very important when estimating regional carbon budgets. But they are not well understood in double cropping systems. Here, continuous flux data were collected from two rotation croplands in Yucheng (YC) and in Luancheng (LC) to describe the among-year variations in α, Amax, and Rd, and to investigate variation mechanism on an annual scale. The three parameters exhibited marked fluctuations during the observation years. The annual α, Amax, and Rd ranged from 0.0022–0.0059 mg CO2 μmol photon−1, from 2.33–4.43 mg CO2 m−2 s−1, and from 0.19–0.47 mg CO2 m−2 s−1 at YC, and from 0.0016–0.0021 mg CO2 μmol photon−1, from 3.00–6.30 mg CO2 m−2 s−1, and from 0.06–0.19 mg CO2 m−2 s−1 at LC, respectively. Annual α and Rd declined significantly when vapor pressure deficit (VPD) exceeded 1.05 kPa and increased significantly when canopy conductance (gc) exceed 6.33 mm/s at YC, but changed slightly when VPD and gc exceeded 1.16 kPa and 7.77 mm/s at LC, respectively. The fact that the negative effects of VPD and gc on α and Rd at LC were not as significant as they were at YC may be attributed to different climate conditions and planting species. A negative relationship (R2 = 0.90 for YC and 0.89 for LC) existed between VPD and gc. Therefore, the VPD, through its negative effect on gc, inhibited α and Rd indirectly. Among-year Amax variation was mainly influenced by the annual mean surface soil temperature (Ts) of non-growing season of wheat significantly (R2 = 0.59, P < 0.01). Therefore, in future climate change scenarios, these environmental effects need to be included in carbon cycle models so that the accuracy of the carbon budget estimation can be improved.

Backround Changes in photosynthetic traits (PTs) during the long‐term genetic improvement of soybean (Glycine max (L.) Merr.) yield have been studied, but detailed information on whether PT responses to environmental stress have improved, and their correlations with seed yield, are still unknown. Our objectives were to describe the changes in soybean PTs – leaf area index (LAI), leaf chlorophyll content (Chl), net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) – during decades of genetic improvement, and to detect whether the responses to increasing fertilizer application rates (FRs) of the PTs of 13 different soybean cultivars released in various decades differed. Results All of the soybean PTs increased significantly along with the year in which each cultivar was released, under different FR treatments, indicating that PTs have improved during decades of genetic breeding. Medium FR (nitrogen) treatment (150 kg ha −1) increased PT values, to different extents, at all the investigated growth stages. Leaf area index, Chl, and PN of the old and middle cultivar groups at the full bloom (R2), full seed (R6), and beginning maturity (R7) stages decreased significantly under high FR treatment (300 kg ha−1) compared with the medium FR treatment. The former had no effect on any of the PTs of new cultivar group, or had promotive effects. Thus, the photosynthetic capacities of the new cultivars are more tolerant to high FR‐related stress than older cultivars. Conclusions The photosynthetic capacities, and tolerance to high FR‐related stress, of soybean cultivars that were released in different years improved after long‐term genetic breeding. © 2021 Society of Chemical Industry