Non-Stomatal Processes are Responsible for the Decrease in Gross Primary Production of a Potato Crop During Edaphic Drought

Abstract

Soil water stress is one of the main limiting factors for agrosystem functioning, causing a reduction in gross primary production (GPP). However, it is still unclear whether GPP limitations originate from a direct stomatal response (SOL) or from other non-stomatal causes (NSOL). In this study, the effects of edaphic drought on a potato crop were investigated by using eddy covariance data from the Lonzée ICOS station (BE-Lon) in Belgium during four cropping periods (2006, 2010, 2014 and 2018). For the years when GPP anomalies were detected, the apparent maximum carboxylation rate (corresponding to a NSOL) started to decrease when the relative extractable water in the soil (REW) passed below a threshold corresponding to 76% of the maximum available water. At the same time, the slope parameter in the stomatal conductance optimality model remained constant or increased (absence of SOL). Consequently, the non-consideration of NSOL in the modeling of GPP led to an overestimation of GPP. At an hourly timescale, the carbon and water fluxes tended to decouple when soil water availability decreased, exhibiting a strict control of NSOL in hourly dynamics of GPP in opposition to transpiration. Our results show the need for better representation of the drought influence on photosynthesis processes by reducing photosynthetic capacities to properly model GPP and transpiration dynamics of croplands during dry episodes.

Keywords: GPP, eddy-covariance, drought, model, potato, stomatal conductance