How fertilization regime should be adapted when almond trees are grown under deficit irrigation and with cover crops? Insights and open questions from a field trial

The nutrient requirements of almond trees grown under optimum conditions are well known. Nowadays, it is necessary to adapt fertilization to face new challenges, such as the increasing scarcity of water resources that force farmers to implement deficit irrigation strategies, or the cultivation of fruit trees intercropped with cover crops or native vegetation that promote additional ecosystem services. The objective of this work is to know the effects on the nutritional status of almond trees subjected to deficit irrigation and cover vegetation (CV), in order to be able to readapt fertilizer recommendations to this approach more in line with the new EU Farm-to-Fork strategy. In a field trial established in southeastern Spain under semi-arid conditions we tested 5 treatments; 1) control trees (irrigated and fertilized at 100%), 2) trees irrigated and fertilized at 66% and where the inter-row was tilled, 3) trees irrigated and fertilized at 66% and with CV, 4) trees irrigated and fertilized at 33%, where the inter-row was tilled, and 5) trees irrigated and fertilized at 33% and with CV. Our results indicate that when the application of water and nutrients decreases, trees maintain a nutrient homeostasis by reducing growth and yield, and that a 66% decrease in water and nutrient supply can be a good strategy where irrigation water is scarce. The cover vegetation reduced growth and yield, but the results do not show a detrimental effect of the CV on the water and macronutrient status of the tree. We observed a reduction of Fe, Mn and Zn in the leaves of trees with cover vegetation, which was more evident in the almond trees under more severe restriction (33%) that probably had to expand their root system and therefore compete more directly with the CV. These results highlight the need to readjust fertilizers when deficit irrigation techniques are applied, as well as to reincorporate the nutrients sequestered by the tree canopy and the CV into the soil.