Allelopathic inhibition of nitrification and nitrifying bacteria in a ponderosa pine (Pinus ponderosa Dougl.) community.

The dynamics of ponderosa pine (Pinus ponderosa Dougl.) stands in western North Dakota were studied to determine the influence of plant-produced chemicals on nitrification rates and competitive interactions within the stands. Ponderosa pine accounted for more than 98% of all tree and shrub stratum stems in this climax community. Low levels of nitrate-nitrogen relative to ammonium-nitrogen and low numbers of Nitrosomonas and Nitrobacter in the soils indicated that nitrification rates were low. Inhibition of nitrification is often attributed to low soil pH in coniferous forests, but the slightly alkaline soils in this study (pH 7.25-7.75) suggested that another factor caused the low nitrification. Evidence obtained suggested that the reduction in nitrate synthesis was due to the production and subsequent transfer to the soil of secondary plant chemicals that were toxic to Nitrosomonas. Chemical inhibitors of nitrification, including caffeic acid, chlorogenic acid, quercitin, and condensed tannins, were found in extracts from ponderosa pine needles, bark, and A horizon soils. These extracts proved to be toxic to soil suspensions of Nitrosomonas causing reductions of from 68-93% of the control. These findings indicate that climax ponderosa pine communities minimize the conversion of ammonia-nitrogen to nitrate-nitrogen by chemically inhibiting nitrification. NITRIFICATION is the microorganism-mediated oxidation of ammonia to nitrate. Since nitrate-nitrogen (NO3-N) and ammonium-nitrogen (NH4-N) are the only inorganic forms of nitrogen known to be readily assimilated and utilized by plants, and since all plant species are not equal in their abilities to absorb and/or utilize nitrate-nitrogen and ammonium-nitrogen (Bigg and Daniel, 1978; Gigon and Rorison, 1972; Krajina, Madoc-Jones, and Mellor, 1973; Van Den Driessche, 1978), the absolute quantity of each form of nitrogen available to plants is of utmost importance to the mineral nutrition of those plants. The differential fitness among certain forest species may in part be directly related to their nitrogen uptake abilities in soils with a predominance of one or the other form of available inorganic nitrogen. Therefore, the presence or absence of appreciable nitrification is an important community 1 Received for publication 28 November 1979; revision accepted 21 January 1980. We are indebted to Elmer Finck, Lloyd Hulbert, Elroy Rice, Chris Smith, Peter Wong, and two anonymous reviewers for their invaluable critiques of various drafts of this manuscript. We would also like to convey our thanks to Dick Bares and Jim Crompton for assistance with the field work. Partial funding was provided by a grant from the North Dakota Regional Environmental Assessment Program and laboratory facilities were provided by Project Reclamation at the University of North Dakota. 2 Present address: Sec. of Ecology and Systematics, Langmuir Lab., Cornell University, Ithaca, New York 14850. characteristic which can influence the predominant mode of nitrogen uptake by plants. Recent research indicates that nitrification sometimes is inhibited by plant-produced chemicals which are toxic to one or both of the microorganisms (Nitrobacter and Nitrosomonas) responsible for nitrate production in soils (Basaraba, 1964; Lodhi, 1977, 1978b; Munro, 1966a, b; Rice, 1964, 1974; Rice and Pancholy, 1972, 1973, 1974). The concomitant decreases in NO3-N that result from the curtailment of nitrification can have a significant positive impact on the mineral budget of an ecosystem because, as Likens, Bormann, and Johnson (1969) have shown, high nitrification rates can initiate large scale losses of anions and cations from forest communities. Rice and Pancholy (1972, 1973, 1974) have suggested that nitrification rates decrease along successional clines until they reach a minimum in climax communities. This minimum was shown to be related to plant-produced chemicals which inhibited nitrification in tallgrass prairie and deciduous forest climax communities. Additional information from other climax communities, which is needed to bolster or reject this hypothesis, has been lacking. Much confusion exists as to the relationship between soil pH and nitrification rate. Alexander (1961) and Brady (1974) indicate that nitrification is severely suppressed in acid soils (below pH 6.0), yet others (Likens, Bormann, and Johnson, 1969; Smith, Bormann, and Li-