Diffuse Fertilization or Lack Thereof: A Multisite Synthesis of Water and Carbon Fluxes

Abstract

Aerosols or clouds in the atmosphere scatter incoming shortwave radiation and increase diffuse radiation. At the Earth's surface, diffuse radiation can penetrate deeper into vertically complex vegetation. Leaf photosynthesis and transpiration can be coupled through stomatal conductance and are thought to increase together at low diffuse fractions (K_d) until an optimal point, after which diffuse enhancement decreases. Because ground-based measurements of diffuse radiation are sparse, prior studies have used atmospheric proxies to determine K_d. With eddy covariance observations from the National Ecological Observatory Network (NEON) and evapotranspiration (ET) retrievals from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS), we quantify the relationship between K_d, ET, and net ecosystem exchange (NEE) across 5 years, 32 sites, and 6 plant functional types (PFTs). At most NEON sites (26 out of 32), ET decreases with increasing K_d without an optimal point or diffuse enhancement. Moisture-dominant NEON sites experience less enhancement of ET under diffuse conditions compared to radiation-dominant sites. Cloud contamination limits the availability of ECOSTRESS ET data and prevents quantification of the K_d-ET relationship on broader spatial scales. Although the optimal points in the K_d-NEE relationship are variable across NEON sites, a majority show diffuse enhancement of NEE (19 out of 32), predominantly in forest and shrub/scrub PFTs. Most NEON sites also show an enhancement in the net carbon-water ratio (NEE/ET; 25 out of 32). We partially attribute differences in NEON results from prior studies to differences between ground-based observations and modeled products of diffuse radiation.