Aerosol pollution alters the diurnal dynamics of sun and shade leaf photosynthesis through different mechanisms

Benefit of aerosol reduction to winter wheat during China's clean air action: A case study of Henan Province

A strongly declining aerosol radiative effect has been observed in China since 2013 after implementing the clean air action, yet its impact on wheat (Triticum aestivum L.) production remains unclear. We use satellite measures and a biophysical crop model to assess the impact of aerosol-induced radiative perturbations on winter wheat production in the agricultural belt of Henan province from 2013 to 2018. After calibrating parameters with the extended Fourier Amplitude Sensitivity Test (EFAST) and the generalized likelihood uncertainty estimation (GLUE) method, the DSSAT CERES-Wheat model was able to simulate crop biomass and yield more accurately. We found that the aerosol negatively impacted wheat biomass by 21.87% and yield by 22.48% from 2006 to 2018, and the biomass effects from planting to anthesis were more significant compared to anthesis to maturity. Due to the strict clean air action, under all-sky conditions, the surface solar shortwave radiation (SSR) in 2018 increased by about 7.08% over 2006-2013 during the wheat growing seasons. As a result of the improvement of crop photosynthesis, winter wheat biomass and yield increased by an average of 5.46% and 2.9%, respectively. Our findings show that crop carbon uptake and yield will benefit from the clean air action in China, helping to ensure national food and health security.

Elevated aerosol enhances plant water-use efficiency by increasing carbon uptake while reducing water loss

Aerosols could significantly influence ecosystem carbon and water fluxes, potentially altering their interconnected dynamics, typically characterized by water-use efficiency (WUE). However, our understanding of the underlying ecophysiological mechanisms remains limited due to insufficient field observations. We conducted 4-yr measurements of leaf photosynthesis and transpiration, as well as 3-yr measurements of stem growth (SG) and sap flow of poplar trees exposed to natural aerosol fluctuation, to elucidate aerosol's impact on plant WUE. We found that aerosol improved sun leaf WUE mainly because a sharp decline in photosynthetically active radiation (PAR) inhibited its transpiration, while photosynthesis was less affected, as the negative effect induced by declined PAR was offset by the positive effect induced by low leaf vapor pressure deficit (VPDleaf). Conversely, diffuse radiation fertilization (DRF) effect stimulated shade leaf photosynthesis with minimal impact on transpiration, leading to an improved WUE. The responses were further verified by a strong DRF on SG and a decrease in sap flow due to the suppresses in total radiation and VPD. Our field observations indicate that, contrary to the commonly assumed coupling response, carbon uptake and water use exhibited dissimilar reactions to aerosol pollution, ultimately enhancing WUE at the leaf and canopy level.

Elevation-dependent pattern of net CO2 uptake across China

The elevation gradient has long been known to be vital in shaping the structure and function of terrestrial ecosystems, but little is known about the elevation-dependent pattern of net CO2 uptake, denoted by net ecosystem productivity (NEP). Here, by analyzing data from 203 eddy covariance sites across China, we report a negative linear elevation-dependent pattern of NEP, collectively shaped by varying hydrothermal factors, nutrient supply, and ecosystem types. Furthermore, the NEP shows a higher temperature sensitivity in high-elevation environments (3000-5000 m) compared with the lower-elevation environments (<3000 m). Model ensemble and satellite-based observations consistently reveal more rapid relative changes in NEP in high-elevation environments during the last four decades. Machine learning also predicts a stronger relative increase in high-elevation environments, whereas less change is expected at lower elevations. We therefore conclude a varying elevation-dependent pattern of the NEP of terrestrial ecosystems in China, although there is significant uncertainty involved.

Soil moisture shapes the environmental control mechanism on canopy conductance in a natural oak forest

Canopy conductance (g_c) is an important biophysical parameter closely related to ecosystem energy partitioning and carbon sequestration, which can be used to judge drought effect on forest ecosystems. It is very important to explore how soil moisture change affects the environmental control mechanism of g_c, especially in natural oak forests in Central China where frequent extreme precipitation (P) and drought will occur in a context of climate change. In this study, variations of g_c and its environmental control mechanisms in a warm-temperate forest over three consecutive years under different hydroclimatic conditions were examined by using eddy-covariance technique. Results showed that the averaged g_c in the three growing seasons were 11.2, 11.3 and 7.8 mms^-1, respectively, with a CV of 19.7 %. The lowest g_c occurred in the year with the lowest P. Using three years of data, we found that vapor pressure deficit (VPD) exhibited the dominate effect on g_c, both diffuse photosynthetically active radiation (PAR_dif) and air temperature (T_a) were positively correlated with g_c. When relative extractable water content (REW) was larger than 0.4, however, inhibiting effect of high VPD on g_c disappeared and the effect of direct photosynthetically active radiation (PAR_dir) on g_c was larger compared to PAR_dif. When REW was <0.1, the positive relationship between T_a and g_c became negative. Our results indicated that soil moisture ultimately shapes the environmental control mechanism of g_c in a natural oak forest.