From sunlight to seed: Assessing limits to solar radiation capture and conversion in agro-ecosystems

Abstract Yield depends upon the amount of photosynthetically active radiation (PAR) absorbed by the crop during the growing season and its conversion into harvestable biomass. Little is known about attainable and actual efficiencies involved in absorbing incident PAR (ea) and converting it into yield (ec) in producer fields. We developed a novel approach consisting of producer data, satellite imagery, and crop modelling to assess ea and ec in agro-ecosystems. Simulated phenology and satellite imagery were used to estimate incident (IPAR) and absorbed (APAR) PAR during the crop season for 3096 soybean fields sown across the US North Central region during 2014–2016. Quantile regression was used to derive upper limits for ea and ec and multiple-regression analysis was performed to identify biophysical drivers of the observed variation in these two efficiencies. Differences in weather, soil, and management resulted in a wide range in crop cycle length (58–126 d), total IPAR (580–1250 MJ m−2), and seed yield (0.8–6.3 Mg ha−1). The relationship between yield and total APAR was curvilinear, indicating marginal yield gains in the upper APAR range. Attainable ea represented 65% of total IPAR, while attainable ec ranged from 0.6 to 1.2 g seed MJ−1 depending upon total APAR. Average producer ea and ec were 14 and 29% below their attainable efficiencies. Average efficiency in converting IPAR into yield was 0.8%, with an upper limit of 1.1% derived from the 95th percentile of the field data distribution. Although weak, relationships between efficiencies and meteorological factors such as IPAR, temperature, water balance, and diffuse radiation were consistent with previous literature. There was a strong trade-off between ec and ea indicating that maximizing both efficiencies simultaneously was not possible in producer fields. Our approach can be used to determine attainable and actual efficiencies in capturing and converting radiation into yield, set realistic yield limits, and understand relationships between producer yield and management practices.