Compost and rootstock effects on fibrous root physiology and bacterial community composition in young citrus trees under endemic Huanglongbing.

Citrus production in Florida has declined drastically due to Huanglongbing (HLB), a bacterial disease associated with Candidatus Liberibacter asiaticus. This disease significantly reduces fruit yield and quality and shortens the tree's lifespan, severely impacting Florida's economy. As no resistant citrus cultivars are currently available, intensive management is essential to maintain tree health and productivity. We conducted a commercial-scale field study in southwest Florida to test whether biannual compost applications improve soil health and belowground functioning of young 'Valencia' sweet orange trees grafted on four different rootstocks under endemic HLB. Compost increased soil organic matter, pH, nutrient content, and cation exchange capacity, while also increasing soil moisture and electrical conductivity. This translated into higher fibrous root respiration and increases in first-order specific root length. Metabolomics analysis revealed strong compost-associated reshaping of the fibrous root metabolome, including enrichment of several nitrogen-related amino acids and central carbon intermediates. Endorhizosphere profiling showed that compost altered the active bacterial community composition with limited effects on alpha diversity. Despite these belowground gains, compost did not affect tree size and productivity - rather, rootstock was the primary determinant. This suggests that the belowground improvements from compost are insufficient to overcome the physiological constraints imposed by HLB.