Cover cropping in olive agrosystems: a Spanish case study to evaluate the trade-off between carbon mitigation, microclimate enhancement and water use efficiency

Abstract:

Olive groves are one of the most extensive crops in the Mediterranean region. 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 usually applied to increase soil organic carbon and mitigate soil degradation problems caused by erosion. However, cover crops may also increase evapotranspiration (ET) and, consequently, water demand in olive production systems. While the influence of cover crops on the carbon sequestration capacity of olive groves has been previously demonstrated to be positive, their effects on ET and water use efficiency (WUE), remain uncertain.

In this study, we aim to assess the effect of cover crops on microclimate conditions, carbon sequestration, ET and WUE of an irrigated olive grove located in SE Spain, where two adjacent areas were subjected to two different treatments: 1) weed-free treatment (WF), in which a glyphosate-based herbicide is applied to avoid spontaneous weed growth, and 2) weed-cover treatment (WC, i.e. cover crop), where spontaneous weed cover was kept from autumn to spring. Two fully equipped eddy covariance stations measured atmosphere-ecosystem exchange of water, heat and carbon dioxide and ambient conditions at each treatment over three hydrological years (2015-2018). Preliminary results show that spring net carbon sequestration is higher in the WC treatment for all hydrological years, however, the presence of cover crops increased ET during spring months. Nevertheless, spring WUE was 15, 6 and 61% higher in the WC treatment for the three study years, respectively. On an annual basis, ET was 12, 19 and 3% higher in the WC treatment over the three years of the study. Regarding microclimate, soil water content was higher during spring and winter in the WC treatment while the daily bowen ratio was higher in the WF treatment over autumn, spring and winter. Milder temperatures were registered in the WC treatment during summer, when daily soil and air thermal amplitude was 1°C and 3°C higher in the WF treatment. Overall, despite the fact that cover crops increase water loss through ET, cover cropping might enhance microclimatic conditions as well as WUE in olive agrosystems.