Inhibition of Nitrification by Climax Ecosystems

Abstract

Three plots representing two stages of old‐field succession and the climax were selected in each of three vegetation types in Oklahoma: oak‐pine forest, post oak‐blackjack oak forest, and tall grass prairie. Soil samples from the 0–15 and 45–60 cm levels were analyzed every other month for 1 yr for exchangeable ammonium nitrogen and for nitrate. On alternate months numbers of Nitrosomonas and Nitrobacter were determined in the 0–15 cm level. The amount of ammonium nitrogen was lowest in the first successional stage, intermediate in the second successional stage, and highest in the climax stand. This trend was remarkably consistent throughout all sampling periods, all vegetation types, and both sampling levels in the soil. The amount of nitrate was highest in the first successional stage, intermediate in the second successional stage, and lowest in the climax stand in both sampling levels, all vegetation types, and virtually all sampling periods. The numbers of nitrifiers were high in the first successional stage, generally, and decreased to a very low level in the climax. In fact, there was often no Nitrobacter in the climax stands. These results indicate that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily in the climax as in the successional stages. Evidence from other geographic areas and vegetation types strongly supports this conclusion. This would certainly appear to be a logical trend in the evolution of ecosystems because of the increased conservation of nitrogen and energy. The ammonium ion is positively charged and is adsorbed on the negatively charged colloidal micelles, thus preventing leaching below the depth of rooting. On the other hand, nitrate ions are negatively charged, are repelled by the colloidal micelles in the soil, and thus readily leach below the depth of rooting or are washed away in surface drainage. There is growing evidence also that many plant species can use ammonium nitrogen as effectively or more so than nitrate nitrogen. If ammonium nitrogen is used directly, this eliminates four chemical steps because nitrogen which is oxidized to nitrite and then to nitrate must be reduced back to nitrite and then to ammonium nitrogen before it can react with keto‐acids in the formation of amino acids. The two reduction reactions require considerable expenditure of energy.

Allelopathy

“…Sterile aqueous extracts of roots and shoots of broomsedge also inhibited growth of two species of Rhizobium (Rice, 1972). Sterile aqueous extracts of roots and shoots of broomsedge also inhibited growth of two species of Rhizobium (Rice, 1972).…”

“…Concentrations of ammonium and nitrate nitrogen and numbers of nitrifiers were determined throughout the year in soils from two old-field successional stages and the climax in three vegetation types in Oklahoma, the tall grass prairie (TGP), the post oak-blackjack oak forest and the oak-pine forest (Rice and Pancholy, 1972). The inverse correlation between the concentration of nitrate nitrogen and the concentration of ammonium nitrogen in all plots was striking.…”

“…The concentration of ammonium increased from a low in the first successional stage to a high in the climax, whereas the concentration of nitrate decreased from a high in the first successional stage to a low value in the climax (Table 4.13 -only data for the TGP are shown). It was obvious from the general soil data that the April 1971 111 42 c 140 25 25 25 June 1971 3012 525 c 147 de 347 417 280 August 1971 334 158 50 e 62 72 3 dc October 1971 817 51 37 24 24 24 December 1971 177 26 c 32 e 23 23 18 February 1972 127 51 51 110 270 c 43 e a Data from Rice and Pancholy (1972). Thus, some factor or factors reduced the populations of nitrifiers during succession, resulting in an apparent reduction in the rate of oxidation of ammonium to nitrate.…”

Benchmarking Successional Progress in a Quantitative Food Web

“…Vitousek and Reiner [35] extended Odum's prediction by the “nutrient retention hypothesis” which predicts a balance of nutrient in- and outputs when a steady state in biomass accumulation is reached in late stages of succession. Empirical studies on nitrogen retention in old-field succession [36], streams [37] and temperate forests [38] confirmed this hypothesis, but data from tropical forests did not [39]. Here, we test if these predictions hold for natural temperate plankton systems which never reach an equilibrium state [11], [40].…”

ALLELOPATHIC EFFECTS OFCHENOPODIUM ALBUML. EXTRACTS ON NITRIFICATION

“…Rice and Pancholy (10)(11)(12) hypothesized that climax vegetation inhibits nitrification in soils and that is due to the production in such vegetation of phenolic compounds. This hypothesis has been supported by other workers (14)(15)(16)(20)(21)(22)(23)(24).…”