Mineralization of Nitrogen in Organic Soils

The effect of fluctuating seasonal temperatures on the release of soluble NO3-N, NHJ-N, soluble organic N (SON), and solube organic C (SOC) into drainage effluent during soil organic matter mineralization was measured on organic soil profiles obtained intact from various locations in Florida. Soil columns were leached once every 25 days, followed by applying a suction of 100 cm. Soil columns were flooded for 25 days each in the months of July and August to simulate normal agronomic practices. Leachate was analyzed for NO,, NHi, SON, and SOC. The amounts of N mineralized ranged from 410 to 938 kg N-ha~' •year" for cultivated soils and 874 to 1,250 kg-ha"'-year~' for virgin soils. Nitrate accounted for 48 to 81% of the total N in the effluent, while SON accounted for about 16 to 45% of the total N. Remaining 2 to 7% of the total N in the effluent was NHJ. Soluble organic C in the leachate accounted for about 1,795 to 3,516 kg C-ha~'year'. Seasonal fluctuations in temperatures had minimal effects on the release of SON, SOC, and NHJ, but the release of NO 7 was increased with an increase in average daily temperature (9.4 to 30°C) with Q10 values ranging from 1.3 to 1.9. Flooding the organic soils increased the SON and SOC release into the effluents by about twofold, compared to drained conditions. Total N loss in 1 year as a result of organic matter mineralization was about 1.3 to 4.2% of the total soil organic N. Additional Index Words: decomposition, ammonification, nitrification, drainage effluents, soluble organic N, soluble organic C. Reddy, K. R. 1982. Mineralization of nitrogen in organic soils. Soil Sci. Soc. Am. J. 46:561-566. O soils (Histosols) are naturally productive and contain large amounts of organic C, N, and P. These soils are poorly drained and have high waterholding capacities. Artificial drainage is often necessary for agricultural development which results in rapid breakdown of soil organic matter causing soil subsidence (Stephens, 1969). This process also results in the release of various forms of N, as NO I, NHJ, and soluble organic N (SON) into drainage water. Drainage water N discharge from organic soils used for agricultrual purposes ranged from 12 to 40 kg N-ha~'-year~' in the Florida Everglades area (Florida Sugar Cane League, 1978) and about 56 kg N-ha^-year" in the central Florida area (Reddy et al., 1982). Other studies reported a drainage water N discharge of about 37 to 245 kg N-ha"-year" for organic soils located in New York (Duxbury and Peverly, 1978). The biochemical processes controlling N release into drainage water include ammonification, nitrification, and dentrification. These processes along with soil and environmental factors (moisture and temperature) dictate the amount of N released into drainage water. A few studies have been reported on N mineralization in organic soils (Avnimelech, 1971; Isi1 Florida Agric. Exp. Stn. Journal Series no. 3573. Received 31 July 1981. Approved 8 Feb. 1982. 2 Assistant Professor, Agricultural Research and Education Center, University of Forida, P.O. Box 909, Sanford, FL 32771. rimah and Keeney, 1973; Guthrie and Duxbury, 1978; Terry, 1980). Mineralization rates (360 to 686 kg Nha"'-year~') in these studies were based on either batch or short-term incubation at constant temperature. None of the studies reported account for the effect of temperature fluctuations and management practices (summer flooding) on mineralization of soil organic N. The purpose of this investigation was to measure the effect of fluctuating seasonal temperatures and flooding on the release of NO J, NH}, SON, and SOC into drainage water during soil organic matter decomposition. Natural soil profiles collected from various locations in Florida were used in the study. MATERIALS AND METHODS