COMPARATIVE INHIBITION OF NITRIFIERS AND NITRIFICATION IN A FOREST COMMUNITY AS A RESULT OF THE ALLELOPATHIC NATURE OF VARIOUS TREE SPECIES

Inhibition of nitrification and nitrifiers was significantly variable under different tree species in the same forest ecosystem. Nitrate nitrogen was always lower than ammonium nitrogen when compared under each species. On the other hand, low numbers of Nitrosoinonas and Nitrobacter showed an inverse relationship with large amounts of ammonium in most samples. Aforementioned variation is due to the variable tree litter under different species, which in turn produces inhibitors of nitrification and nitrifiers. Inhibition of nitrification in natural ecosystems increases the amounts of ammonium nitrogen thus resulting in a conservation of energy, if plants utilize such nitrogen directly. THERE have been several reports of inhibition of nitrification in grasslands and a few reports in which such inhibition in forest soil was demonstrated. Nye and Greenland (1960) reported that in evergreen forest, the numbers of nitrifiers and the rate of nitrification were low. Viro (1963) found that in spruce dominant forest, most of the available nitrogen was ammonia nitrogen. Rice and Pancholy (1972, 1973, 1974) reported significant increases in concentrations of NH4+-N, significant decreases in N03--N and bacteria as succession progressed toward the climax in several types of vegetation. They inferred, therefore, that climax vegetation inhibits nitrification. Lodhi (1977) reported that the amounts of NH4+N and N03--N were variable in forest soil under different dominant tree stands. However, NH4+N was always considerably higher than N03--N. Such variations under different trees were attributed to the variable tree litter which plays an important role in controlling the soil pH, nitrification and mineralization. This project was initiated to determine: (1) the number of nitrifying bacteria in different tree stands, (2) the relationship of number of nitrifiers to variable amounts of NH4+-N and N03--N under different tree species, and (3) the physio-ecological influence of different species on nitrifiers and nitrifications. MATERIALS AND METHODS-The study area was the same forest described previously by Lodhi (1976, 1977). Soil samples were collected from underneath each tree stand of Platanus occidentalis (sycamore), Celtis occidentalis (hackberry), Quercus borealis (red oak), Q. alba (white oak), and Ulmus americana (American elm). Nomenclature follows Fernald (1950). Ten 1 Received for publication 21 April 1978; revision accepted 1 August 1978. soil samples from 0-25 cm were taken with the use of a soil-auger. Visible pieces of organic matter were removed, and soils were analyzed for pH, nitrate nitrogen by a specific ion electrode, and ammonium nitrogen according to Bremner (1965). The nitrifying bacteria populations under each stand of trees were quantified using ten soil samples from each stand which were combined in five samples. All five combined samples from each stand were analyzed for numbers of Nitrosomonas and Nitrobacter by the most probable numbers method following Alexander and Clark (1965). RESULTS-Appreciable differences in pH, NH4+-N, N03--N, Nitrosomonas and Nitrobacter numbers were found in soils associated with different tree species (Table 1). Mean values showed a large variation, considering that all dominant trees occurred in the same community. The amount of NH4+-N was always significantly higher than N03--N within each species. Moreover, the numbers of Nitrosomonas and Nitrobacter consistently showed a direct relationship with the amounts of N03--N and an inverse relationship with amounts of NH4+-N (Table 1). Such a correlation is even more pronounced when the pooled mean values are compared (Table 1). Since soil pH plays important roles in nitrification, further soil comparisons were made between elm, hackberry and white oak because of the range of pH occurence between them (Lodhi, 1977). Mean values for soil properties were significantly different among the three tree species (Table 2), except for NH4+-N and Nitrobacter between elm and hackberry. DISCUSSION-The significantly larger amounts of NH4+-N versus N03--N under all dominant trees is a clear sign of inhibition of nitrification. A larger amount of NH4+-N than NOK--N is even