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Ammonium and nitrate transport during saturated and unsaturated water flow through sandy soils

Kadyampakeni, Davie M., Nkedi-Kizza, Peter, Leiva, Jorge A., Muwamba, Augustine, Fletcher, Evelyn, Morgan, Kelly T.
Zeitschrift für Pflanzenernährung und Bodenkunde 2018 v.181 no.2 pp. 198-210
Citrus, E horizons, ammonium, aquifers, cation exchange, chlorides, crop production, environmental sustainability, fertigation, field experimentation, groundwater, hurricanes, ionic strength, leaching, nitrates, nitrogen, nitrogen fertilizers, nutrient management, pH, rain, rhizosphere, sand, sandy soils, simulation models, soil properties, soil quality, sorption, split application, summer, unsaturated flow, water uptake, Florida
Citrus production in Florida accounts for ≈ 60% of national production in USA. The sandy soil characteristic (> 95% sand) makes water and nutrient management extremely difficult, raising concerns about environmental sustainability as a result of nutrient inputs in citrus producing regions where sandy soils dominate. Thus, laboratory column and field experiments were conducted to better understand the leaching patterns of NH4 + and NO3 - ions in Florida's sandy soils. The soil columns were first saturated from the bottom with two pore volumes of simulated Florida rain followed by pumping a pulse of fertilizer mixture at a steady Darcy flux of 14 cm h⁻¹. Nitrate and Cl⁻ appeared earlier in the effluent than NH4 + in the A and Bₕ horizons, due to cation exchange of NH4 + . Essentially identical breakthrough curves (BTCs) for NH4 + and NO3 - were observed in the E‐horizon, due to very low sorption of NH4 + . The convective and dispersive equilibrium (CDE) model simulations were in good agreement with measured breakthrough curves (BTCs) for NH4 + , NO3 - , and Cl⁻. However, the sorption coefficient (KD) values used in the CDE model to simulate the BTCs for NH4 + were about 10 times less than the batch isotherm KD values. This was attributed to differences in pH, cation composition, and ionic strength between batch (static) and dynamic (leaching) systems. The field experiment showed that under unsaturated flow, improved short‐pulse fertigation systems (drip and microsprinkler) limited NH4 + and NO3 - transport beyond the root zone (top 30 cm), which might have promoted nutrient and water uptake in citrus. The column study revealed that under extreme weather events such as hurricanes or storm surge in Florida, saturated soil conditions can trigger N mobility below the root zone to surficial or groundwater aquifers. In the field experiment, the use of judicious, minimal and split applications and accurate placement of N‐fertilizers reduced leaching of N especially during heavy storms in the summer rainy months of Florida. The field experiment demonstrated that it is possible to manage inorganic N forms for optimal residence time for uptake and minimal leaching concerns.