Publication n°121 of the International Association of Hydrological Sciences Proceedings of the Anaheim Symposium, December 197G NITRIFICATION IN EVERGLADES HISTOSOLS: A POTENTIAL ROLE IN SOIL SUBSIDENCE

Subsidence of the Everglades Histosols occurs at a rate of approximately 3 cm/year. The major cause of this loss of soil is the oxidation of the organic matter by the microbial community. One of the products of this microbial activity is inorganic nitrogen. Assuming an annual subsidence rate of 3 cm, approximately 1400 kg/ha of nitrogen is released per year. Much of this nitrogen accumulates in the soil as nitrate. The microbial reaction sequence involved in the production of this nitrate is initiated by mineralization of the soil organic matter by the heterotrophic bacteria and fungi. The major nitrogenous product of this process is ammonium. This is the primary step involved in soil subsidence. The ammonium formed through the mineralization is then oxidized by the nitrifying bacteria to nitrate. Two groups of microorganisms are known to participate in this process, heterotrophic and autotrophic nitrifiers. Nitrate can be removed from the soil by denitrification, assimilation by the plant community or microbial community, or run off in the drainage waters. Examination of the microbial community involved in nitrogen metabolism in Histosols suggests that the nitrate concentration found in the soil can be controlled through prudent control of the water table. Raising the water table would decrease the rate of soil subsidence, hence decreasing the amount of inorganic nitrogen formed, as well as inhibit nitrification and stimulate denitrification. Preservation of one natural resource, the organic soils, will help in the preservation of the surrounding lakes and streams. The Everglades Histosols are subsiding at a rate of approximately 3 cm/ year (Stephens and Speir, 1969). Among the reasons proposed for this loss of soil elevation are compaction, shrinkage due to drying, burning, wind erosion and microbial oxidation (Stephens and Speir, 1969). The primary cause must be considered to be microbial oxidation. This aerobic process which commenced upon drainage of the soil accounts for 58 to 73 percent of the observed subsidence (Volk, 1972). These values were obtained by comparing the amount of carbon dioxide produced by microbial oxidation of the soil with the known rate of subsidence of the soils in the field. Among the products of this microbial oxidation of soil organic matter are the inorganic nitrogenous compounds nitrate and ammonium. With the soil organic nitrogen content of 3.5 percent (Davis and Lucas, 1959) in Everglades Histosols, approximately 1400 kg/ha inorganic nitrogen can be expected to be produced by the subsidence of 3 cm of soil. The accumulation of much of this nitrogen as nitrate was demonstrated by Neller (1944) when he observed nitrate accumulation in fallow soil and soil planted to corn. The nitrate concentration was measured in surface (0-15 cm) and subsurface (15-30 cm). Concentrations of 239 and 127 ppm nitrate-nitrogen (dry soil) were found in the surface and subsurface fallow soils, respectively. The growth of the corn resulted in a decrease in the nitrate in the surface soil to 145 ppm. No change was observed in the subsurface sample. Recent studies have demonstrated nitrateFlorida Agricultural Experiment Stations Journal Series No. Ov'l