Effect of manure and digestate amendments on apple seedlings in the presence of homospecific residues

Biotic factors connected to pathogens activity are among the causes recognized for soil sickness disease. Allelopathic causes have been also demonstrated to play a role in the complex etiology, due to the presence of homospecific residues and their evolution over time. A trial was set on apple scion cultivar ‘Gala’ grafted on M9 grown in 1.5-L pots filled with a clay soil not subjected to replant, focusing on the effect of organic amendment application in the presence of crop residues. Two variability factors were considered. First: the presence/absence of residues derived from grinded shoot mixed with the soil, at a concentration of 2% in volume. Second: the application below the scion root level of two organic amendments: goat manure and digestate from biogas production. The two amendments were compared with an inorganic fertilizer (Nitrophoska Gold) applied superficially, at a dose calibrated to provide in all case 1 g of N pot-1. A 50% release of N was estimated for both amendments during the whole trial. The effect of amendment application either in presence and absence of residues was evaluated in terms of root growth and topography over a period of 84 days on 13 replicates per treatment. Canopy biomass and metabolism were also monitored. Residues application did not compromise plant activity for any of the parameters. Total root biomass was not modified by amendment application although differences were recorded on biomass partitioning over the profile. At canopy level, manure application increased stomatal conductance and photosynthesis. The trial demonstrated how, in a short term, the exclusive presence of a low concentration of crop residues is not able to reproduce soil sickness symptoms, thus confirming a complex etiology for the disease. The trial also highlighted root plastic capability especially expressed in terms of spatial allocation.