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Title: FATE OF NITRATE AND BROMIDE IN UNSATURATED SANDY SOIL UNDER CITRUS PRODUCTION

Author
item PARAMASIVAM, S - SAVANNAH STATE UNIV, GA
item Alva, Ashok
item FARES, A - UNIV FLORIDA, LAKE ALFRED

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2001
Publication Date: 6/1/2002
Citation: PARAMASIVAM, S., ALVA, A.K., FARES, A. FATE OF NITRATE AND BROMIDE IN UNSATURATED SANDY SOIL UNDER CITRUS PRODUCTION. JOURNAL OF ENVIRONMENTAL QUALITY, 31:671-681. 2002.

Interpretive Summary: Negative ions such as nitrate and bromide are poorly retained in soils, therefore, can readily be transported with water. This process is quite rapid in sandy soils because of its high permeable nature. Rapid transport of nitrate below the rooting depth contributes to nitrate contamination of groundwater. This study was conducted in a sandy soil with sand content up pto 95%. Different rates of nitrogen and bromide were applied under a typical scheduled irrigation condition in a commercial 25+ year old orange trees. The transport of nitrate and bromide were monitored by repeated soil sampling to a depth of 2.5 meters. Within the depth of monitoring, the applied nitrogen and bromide were completely depleted by 42 and 214 days after application, respectively. This study showed rapid leaching of nitrate in this soil below 2.5 meters. A chemical transport model was used to predict the changes in concentrations of nitrogen and bromide at various stimes after application comparable to the time of sampling. The concentrations predicted by the chemical transport model were similar to those measured by soil sampling. Due to the rapid transport of nitrate in this soil, multiple applications of nitrogen at small doses and improved water management are highly recommended for minimizing leaching losses below the rooting depth.

Technical Abstract: Understanding the water and nutrients transport through the soil profile is important for efficient management of nutrient and irrigation to minimize nutrient leaching below the rootzone. We applied various N rates (28, 56, 84, and 112 kg N/hectare with one rate of bromide 80 kg Br/hectare to an Entisol in a citrus grove, to (i) determine the temporal changes in NO3-N distribution following broadcast application of water soluble dry granular fertilizer mixture; (ii) evaluate relationship between transport and distribution of NO3-N as function of water input (irrigation and rainfall); (iii) compare the transport and distribtuion of NO3-N and a non-reactive tracer (Br-); and (iv) evaluate the LEACHM predictions of NO3-N and Br- transport in the soil profile with the measured concentrations under standard citrus management practices. Soil samples were taken to the depth of 2.5 m within one or two days after rain or irrigation events until the profile NO3-N and Br- concentrations approached the background levels by 4 and 214 d, respectively. About 50 % reduction in concentration of NO3-N and Br- in the top 60 cm soil profile was observed after each soil sampling than the preceding sampling event. Volumetric soil water content and the concentrations of NO3-N and Br- at various depths within the entire soil profile predicted by LEACHM (Leaching Estimation and Chemistry Model) compared favorably with measured concentrations of the respective elements. LEACHM predicted a leaching loss of 12 to 25% of applied fertilizer N with drainage water at the bottom of the soil profile. This variation in losses of N was associated with increasing rates of fertilizer N application and occurred within the period of 28 and 42 days after fertilizer application.