The influence of cover cropping on carbon sequestration and water use efficiency in an irrigated Mediterranean olive agrosystem

Olive groves are one of the most extensive crops in the Mediterranean region, hence, their management practices can result in significant environmental, social, and economic impacts. Given the generalized water stress conditions across the Mediterranean region, irrigation is usually applied to increase olive crop yield. In Spain, the country with the largest olive crop extension, 29% of the olive cultivated area is irrigated. Cover cropping (i.e. the maintenance of annuals and perennials in between tree rows) can be considered as one of the most widespread conservation practices. It is being increasingly adopted as a sustainable strategy to increase soil organic carbon content and mitigate soil degradation problems caused by soil erosion, apart from other benefits such as the increase of microfauna biodiversity. On the other hand, cover crops may also increase evapotranspiration and, consequently, water demand in olive production systems. While the influence of cover crops on carbon sequestration capacity of olive groves has been previously demonstrated to be positive, their effects on evapotranspiration and water use efficiency, defined as the ratio between carbon uptake and evapotranspiration, remain uncertain.

In this study, we aim to assess the effect of cover crops on microclimate conditions, carbon sequestration, evapotranspiration and water use efficiency of an irrigated olive grove located in Jaén (SE Spain), where two adjacent areas were subjected to two different treatments: 1) weed-free treatment, in which a glyphosate-based herbicide is applied annually to avoid spontaneous weed growth (generally in early spring), and 2) weed-cover treatment (i.e. cover crop), where spontaneous weed cover was kept from autumn to spring. Both treatments are equipped with a wide range of environmental sensors to characterize short- and long-term variations in ambient conditions (e.g. air temperature, relative humidity, precipitation, incoming/reflected short- and long-wave radiation, soil moisture and temperature, soil heat flux). In addition, two eddy covariance towers allow the direct measurement of atmosphere-ecosystem exchanges of water, heat, carbon dioxide and momentum at a high temporal resolution (<1h). This study was carried out over three consecutive hydrological years from October 2015 to September 2018.

Preliminary results, based on data of the first hydrological year (2015-2016), show that cover crops increase ecosystem evapotranspiration. However, the net carbon uptake was higher in the weed-cover treatment compared to the weed-free treatment, leading to a net increase in ecosystem water use efficiency. Further, the Bowen ratio – conceived as the ratio between sensible and latent heat fluxes – was lower in the weed-cover than in the weed-free treatment. Therefore, despite the fact that cover crops increase water loss through evapotranspiration, they mitigate the carbon footprint of the agrosystem, likely having a positive effect on crop water use efficiency by maximizing the ratio of carbon assimilation to water loss.