Soil nitrogen availability in olive orchards after mulching legume cover crop residues

- Mulching pure legume cover crops may not be a sustainable option for soil management in olive orchards.
- An in situ incubation detected only a slight increase in N available in the soil after the legumes had been mulched.
- The N nutritional status of the trees suggests that the transfer of N from legumes to olive trees was low.
- Most of N present in the legume species biomass was probably lost without having entered the soil.

A natural source of nitrogen (N) is needed to increase soil fertility in traditional olive orchards and to maintain the N nutritional status of olive trees. With this aim, we studied the introduction of legume cover crops in two olive orchards, which were converted into mulches and left on the ground as phytomass after they had been cut. The experiments were carried out in Suçães and Qta do Carrascal, NE Portugal, from October 2009 to January 2012. In Suçães, the ground-cover treatments were lupine (Lupinus albus L.), a mixture of 11 self-reseeding annual legume species, natural vegetation fertilized with 60 kg N hm−2 and natural vegetation not fertilized. The treatments imposed in Qta do Carrascal were lupine, hairy vetch (Vicia villosa Roth.), a mixture of 11 self-reseeding annual legumes and natural vegetation. Soil N availability in the year following the establishment of the mulches was monitored by an in situ incubation technique. Olive yields and tree nutritional status were also measured. A peak of net N mineralization was recorded early in the autumn in the plots where the legume cover crops had been grown in the last season, in comparison with the natural vegetation plot. In the next spring, soil N availability was negligible even in the plots previously cropped with legume species. The effect of the legume cover crops on soil inorganic-N availability seems to have been slight and short-lived taking into account the high amounts of N contained in the mulched phytomass. The effect of legume cover crops on olive yield and leaf N concentration was statistically significant only in few occasions. The results of these experiments recommended some caution in the management of pure legume cover crops as a mulch in olive orchards due to the reduced transfer of N from legumes to olive trees.