Spatial and Temporal Nutrient Dynamics and Water Management of Huanglongbing-Affected Mature Citrus Trees on Florida Sandy Soils

Abstract

Nutrients are vital for plant growth, development, and aid in disease control because nutrients affect host plant and pathogen interactions. Once a citrus tree is infected with the phloem-limited, Gram-negative bacteria, Candidatus Liberibacter asiaticus (CLas), huanglongbing (HLB; citrus greening), it would fall under threat of survival as the disease has no known control mechanism discovered thus far. The objective of this study was to determine if split soil applications of essential nutrients improve the availability and accumulation, reduce leaching of these nutrients beyond the root zone, and promote root growth and water dynamics of HLB-affected citrus trees in the soil–plant–atmosphere continuum. Split soil applications of three N rates (168, 224, and 280 kg ha−1 year−1) were the main blocks. Micronutrients were randomly applied to the sub-blocks assigned in a split-split plot design, applied in three splits annually. The micronutrients were applied to foliage and soil as follows: foliar only 1× (1×), foliar 1× and soil-applied 1× (2×), and foliar 1× and soil-applied 2× (3×)× (1× = 9 kg ha−1 year−1 of Mn and Zn to each foliar and soil along with 2.3 kg ha−1 year−1 of B). Significant soil NH4-N and NO3-N, Zn, and Mn were retained within the active soil root zone (0–30 cm). Higher soil acidity was detected when trees received the highest micronutrient rate in the upper soil layers (0–15 cm) as compared with the middle (15–30 cm) and the lowest (30–45 cm) soil layers. Fine root length density (FRLD) was significantly lower at the highest micronutrient rates, manifesting root growth negatively associated with high Mn and low soil pH. Invariably, the water dynamics: stem water potential (ψstem), stomata conductance (gs), and sap flow were also negatively affected when trees received foliar 1× and soil 2× (3×) treatment as compared with the other treatments. Split application of nutrients had a significant effect on FRLD growth, retaining soil-applied nutrients within the active root zone, and improved water use efficiency.