NITROGEN MINERALIZATION FROM CITRUS TREE RESIDUES UNDER DIFFERENT PRODUCTION CONDITIONS

Under most citrus production conditions, dry leaves and other vegetati ve parts are returned to the soil under the trees. These materials and fibrous roots decompose on a regular basis. The objective here was to measure annual N contribution to citrus trees from organic residue de composition. Mineralization of N was measured during 2 yr under citrus trees of two age groups each on Tavares fine sand (hyperthermic, unco ated Typic Quartzipsamment) and Wabasso sand (sandy, siliceous, hypert hermic Alfic Haplaquod). Mineralization of N from tree residues under the canopy accounted for 58 to 84 and 126 to 153 kg N ha(-1) yr(-1) fo r the 4- and 20-yr-old trees, respectively, on the Tavares fine sand a nd 39 to 64 and 121 to 126 kg N ha(-1) yr(-1) for the 7- and 40-yr-old trees, respectively, on the Wabasso sand. The transformation of poten tially mineralizable N (i.e., total N in dry crop residue and fibrous roots under the canopy) into inorganic Forms (i.e., percentage mineral ization) was greater in Wabasso sand (41-86 %) compared with that in T avares fine sand (17-71 %). Significant correlations (r = 0.82 [signif icant at P = 0.05] and 0.84 [significant at P = 0.01]) were found betw een measured quantities of mineralized N and potentially mineralizable N. There were also significant correlations between mineralized N and either rainfall or average temperature during each incubation interva l. This study demonstrated that the quantity of net N mineralized from organic residues under the tree accounts for a significant portion of annual N requirement.