Nitrogen and Biomass Content, and Nitrogen and Water Uptake Parameters of Citrus Grown on Sandy Soils in Central Florida Ph.D. Exit Seminar Soil and Water Science Department K. T. Morgan

Seminar Overview General background Biomass and N partitioning Crop water use by citrus N uptake dynamics of mature citrus trees

Florida Citrus Production Central, East Coast, and West Coast production areas 11,233,000 metric ton 2002 production million dollars in % of US and 18% of World citrus production Source: Florida Agricultural Statistics, Citrus Summary, 2003

Citrus Production Constraints Water availability – Increasing competition with commercial and residential users requiring reductions in permitted use Water quality – Nitrate contamination of drinking water sources in central Florida linked to agricultural fertilizer application

Citrus Water Issues Greater restrictions on water use Lack of effective water storage Cost of alternate sources such as desalination and reclamation Currently few tools available to improve water use efficiency

Nitrate Leaching Problem 63% wells in Florida contain detectable NO 3 -N 15% of drinking wells were above maximum contamination level (MCL) statewide 89% of wells above MCL located in Lake, Polk, and Highlands counties Preliminary Best Management Practice (BMP) for N fertilization was adopted in 1994 Final BMP was accepted by FDACS in 2002

Goals Quantify tissue mass and nitrogen content changes as affected by tree age and rootstock Develop improved citrus irrigation scheduling constants Acquire data needed for seasonal mature tree nitrogen balance

Biomass and N Partitioning

Objectives Determine biomass and N content changes with increase in tree size Explore the effect of rootstocks on mature tree biomass and N distribution Develop pattern of root system development with increase in tree size Determine the effect of rootstocks on root length density distribution

Hypotheses Biomass and N partitioning of specific tissue categories with tree size can be captured in generic relationships Rootstock has significant effect on citrus growth and biomass partitioning Vertical and horizontal root length density distribution changes with rootstock and tree size

Methods Experiment 1 - Mature ( 14 years old) Hamlin orange six trees on each of two rootstocks (Swingle and Carrizo) Experiment years to > 20 years ( m) Valencia orange trees Measurements Fresh weights, dry weights, and total N by tissue category Quantify root length density at different soil depths/ locations

Tissue Categories Above-ground Mature Leaves Current Year Leaves Twigs < 0.5 cm dia. Branches cm Branches cm Branches > 3 cm Trunk Below-ground Roots < 4 mm Roots > 4 mm Taproot Soil Cores at 15 cm increments to 90 cm

Relationship of Dry Weight to Canopy Volume

Relationship of Trunk Diameter to Canopy Volume

Rootstock Effects on N Distribution Total Leaves Total twigs Branches Sm. Med. Lg. Total (% Total N) Carrizo Swingle Stock NS * * Significant at p=0.1 level

Effect of Canopy Volume on Root Length Density

Effect of Rootstock on Root Length Density Distribution Carrizo Length

Effect of Rootstock on Root Length Density Distribution Swingle

Conclusions Biomass and N partitioning changes with tree size Rootstock has significant effect on biomass and N content Rootstock has significant effect on root length density distribution

Crop Water Use by Citrus

Objectives Determine seasonal water uptake patterns Estimate seasonal crop coefficient (K c ) using reference ET Develop relationship of reduced soil water uptake with decreased soil water content (K s )

Hypotheses Seasonal maximum daily water uptake, relative to reference evapotranspiration, follows predictable pattern (K c ) Water uptake decreases with soil water content (K s ) Crop water stress determined by water availability in soil layer with greatest root density

Estimated Crop Evapotranspiration ET c = ET o * K c * K s – ET c = Crop evapotranspiration – ET o = Potential evapotranspiration – K c = Crop coefficient – K s = Soil limited uptake coefficient

Methods Soil water content determined by capacitance probes placed adjacent to three mature Hamlin orange trees Data collected at 0.5 h intervals Sensors: Depths : 10, 20, 40, and 80 cm Spacing: 75 & 150 cm (in-row) 90, 180, & 270 cm (between rows)

Seasonal Crop Coefficient (K c ) Rogers et al ET c /ET o

Effect of Soil Water Content on Soil Water Uptake (K s ) Allen et al ET c /ET o *K c

Effect of Root Length on Soil Water Uptake (K s ) Root Length Density

Conclusions Seasonal water uptake relative to daily ET o follows a predictable pattern Soil water uptake depended on soil water depletion Water uptake was limited by layer with lowest soil water concentration

N Uptake Dynamics of Mature Citrus Trees

Objectives Determine seasonal changes in tissue N concentrations Evaluate seasonal tree biomass and N loss Determine potential N losses, and nitrification rates after N applications Quantify N uptake rates

Hypotheses Leaf N concentration decreases with leaf age due to N dilution N reserves in woody tissue change during periods of rapid tree growth Fertilizer-N is rapidly converted to NO 3 -N Seasonal N uptake rate is related to leaf N status

Methods Samples of all categories analyzed for total N Samples collected at 6-week intervals from mid-February to mid-October Bloom, fruit, and leaf material collected twice monthly.

Methods Months= March, May and September Application Rates = 15 and 30 kg N ha -1 Variety = Hamlin grafted on a) Swingle b) Carizzo Depths = 0-15cm, cm, and cm 4 replicates (trees): 10 composite samples Sampling at 0, 24, 48, 72, and 96 h

Seasonal Change in Citrus Tissue N Concentration

Seasonal Change in Citrus Tissue N Concentration

Seasonal Tree N Loss

Soil N Change with Time After Application

Seasonal N Uptake Rates

Conclusions Seasonal leaf N was lowest during fruit set and increased vegetative growth Tree N reserves recovered prior to harvest Nitrification was rapid under Florida conditions

Summary Focal points: Biomass and N content accumulation with tree size rootstock Seasonal ET parameters Seasonal tree biomass and N changes, and loss Mature tree N uptake rates

Acknowledgements Grant support by: Florida Department of Agriculture and Consumer Services Cargill Inc. Advisors: Drs. Tom Obreza, Johan Scholberg Committee members: Drs. Comerford, Jones, and Wheaton All those that helped in my project