EFFECT OF PHOSPHORUS LEVEL ON NITRIFICATION RATES IN THREE LOW-PHOSPHORUS ULTISOLS

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Abstract
Using perfusion and batch incubation, we determined minimum levels of inorganic phosphorus (P) required for maximum activity of nitrifying bacteria in soils. Three Pdeficient subsoil materials (Arenic Paleudult, Typic Paleudult, and Aquic Hapludult) were limed to approximately pH 7 and treated with various rates of P as KH2PO4 and Ca(H2PO4)2 to establish comparable P concentrations in perfusates and soil solutions. Ammonium sulfate and KNO2 were used as ammonium and nitrite substrates; an enriched inoculant was added to ensure an equal number of nitrifiers. We monitored nitrite, NO3-, pH, and P 2 d after the initiation of experiments and weekly thereafter. Nitrification at each P level was graphed as a function of time, to determine the effect of P on delay times and maximum nitrification rates. Inorganic P concentrations above which NH4+ oxidizers would not respond to additional P were 0.59 ± 0.24 μM for the minimum delay time and 0.60 ± 0.28 μM for the maximum nitrification rate. Nitrite oxidizers required no more than 0.13 and 0.24 ± 0.07 μM P for the minimum delay time and the maximum oxidation rate, respectively. Using perfusion and batch incubation, we determined minimum levels of inorganic phosphorus (P) required for maximum activity of nitrifying bacteria in soils. Three Pdeficient subsoil materials (Arenic Paleudult, Typic Paleudult, and Aquic Hapludult) were limed to approximately pH 7 and treated with various rates of P as KH2PO4 and Ca(H2PO4)2 to establish comparable P concentrations in perfusates and soil solutions. Ammonium sulfate and KNO2 were used as ammonium and nitrite substrates; an enriched inoculant was added to ensure an equal number of nitrifiers. We monitored nitrite, NO3-, pH, and P 2 d after the initiation of experiments and weekly thereafter. Nitrification at each P level was graphed as a function of time, to determine the effect of P on delay times and maximum nitrification rates. Inorganic P concentrations above which NH4+ oxidizers would not respond to additional P were 0.59 ± 0.24 μM for the minimum delay time and 0.60 ± 0.28 μM for the maximum nitrification rate. Nitrite oxidizers required no more than 0.13 and 0.24 ± 0.07 μM P for the minimum delay time and the maximum oxidation rate, respectively. © Williams & Wilkins 1984. All Rights Reserved.