ALLELOPATHIC INHIBITION OF NITRIFICATION AND NITRIFYING BACTERIA IN A PONDEROSA PINE (PINUS PONDEROSA DOUGL.) COMMUNITY

Abstract

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.