Nitrogen Recovery and Transformation from a Surface or Sub-Surface Application of Controlled-Release Fertilizer on a Sandy Soil

ABSTRACT

Controlled-release fertilizers (CRF) are used to reduce leaching of nutrients, especially nitrate-nitrogen (NO₃ ⁻-N) to groundwater, caused mainly by application of soluble N fertilizers to sandy soils in Florida. A leaching column study was conducted to evaluate N release and transformation from a CRF (CitriBlen) over a 16-week period when it was applied on the soil surface or incorporated into the soil. When one pore volume of water was applied to column weekly or biweekly, the CRF released urea-N slowly over time with three peaks of release on 3–4, 8, and 12 week after application. Both ammonium-nitrogen (NH₄ ⁺-N) and NO₃ ⁻-N were leached in large amounts on week 2, likely from soluble forms of N. Cumulatively, the most leached N at the end of study was in the NH₄ ⁺ form, followed by the NO₃ ⁻ form. The sum of all N forms leached and volatilized accounted for 53–69% of total N applied. Total N recovery was 70% and 93% of total N applied for surface and sub-surface application of the fertilizer, respectively. It was indicated that the better recovery rate found with sub-surface application may have been due to minimized N loss by volatilization. Sub-surface application of fertilizer resulted in more than three times NH₄ ⁺-N remained in soil, compared with surface application. On average for both application treatments throughout 16-week period, 5.8 h was required for ammonification and 4.7 d for nitrification to occur after N release from the fertilizer. Characterization of CRFs for specific soil type, leaching volume and cycle, and application manner as well as knowledge of N requirement of the crop will allow for the Best Management Practices of these fertilizers, thus obtaining optimum yields and minimizing nutrient losses from CRFs.

Keywords:

• nitrogen transport
• nitrogen transformation
• denitrification
• Florida

ACKNOWLEDGMENTS

This research was funded by Florida Department of Agriculture and Consumer Services and institutionally made possible by Southwest Florida Research and Education Center (SWFREC) of the University of Florida. Insightful comments from Dr. J.B. Sartain and Dr. E.H. Simonne of the University of Florida for the manuscript were greatly appreciated. Special thanks go to Mr. Benno Eidus and Dr. Kamal Mahmoud at SWFREC and Ms. Anna Zhao at Everglades Research and Education Center of the University of Florida for analytical assistance.

Notes

^†pH (soil:water = 1:2).

^‡Electric conductivity (soil:water = 1:2 v:v).

^§Total C was determined by a C-analyzer (NA 2500, ThermoQuest CE Instruments, Milano, Italy).

^¶Total N was determined by a Kjeldahl method (Bremner and Mulvaney, 1982).

^#Cation exchange capacity is sum of exchangeable cations by 1M NH₄OAc (pH 7) and exchangeable acidity (Thomas, 1982).

^††Determined by hydrometer method (Sheldrick and Wang, 1993).

^†Sum of all N forms in leachate and volatilization.

^‡Sum of urea-N, NH₄ ⁺-N, and NO₃ ⁻-N left in pellets.

^§Sum of each N form in all of 5 soil layers.

^¶Sum of all N forms in pellets and soil.

^#Total N recovery summing all N forms in leachate/volatilization, pellets, and soil.

^††Means followed by the same letter within a column are not significantly different at p = 0.05 between treatments.

^‡‡Number in parentheses denotes the percentage of each N form to total N applied (89 mg).

^†Water-filled pore space = volumetric water content/soil porosity (%). Average of all columns.

^†Weeks after application of fertilizer (wk).

^‡Standard deviation.

^§Time required for ammonification (d) = Urea-N (mg) / N release rate (mg d⁻¹).

^¶Time required for nitrification (d) = NH₄ ⁺-N + NH₃-N (mg) / N release rate (mg d⁻¹).

^#Time remained as nitrate (d) = Time internal between leachate collection (d)—Time required for ammonification (d)—Time required for nitrification (d), where N release rate = Sum of all N forms in leachate and volatilization (mg)/time interval between leachate collection (d).

^††Averages of all leaching events.