Availability of urea to autotrophic ammonia-oxidizing bacteria as related to the fate of 14C- and 15N-labeled urea added to soil

Abstract

Nitrate has been found to accumulate more rapidly in soils fertilized with urea than with inorganic sources of NH4 +, despite the fact that nitrification must be preceded by hydrolytic decomposition. For acidic conditions, this finding has been attributed to limited uptake of NH4 + by ammonium-oxidizing bacteria (also reported herein), suggesting an advantage for direct utilization of a nonionizable N substrate such as urea. If the same advantage applies to urea-C, nitrification of urea-N would also be promoted in neutral or alkaline soils, as reported in numerous studies. To ascertain whether urea-C can be utilized directly by nitrifying organisms, NO2 − production was measured for Nitrosomonas europaea and Nitrosospira sp. NPAV in minimal media with urea as the sole source of either C or C and N. Nitrite accumulated only with the latter organism, in which case nearly quantitative recovery was observed for N added as NH4 + and/or urea. In a subsequent study, recovery of 14C and 15N in gaseous, extractable, and hydrolyzable forms was determined after incubation with labeled urea for up to 29 days, by using two soils that differed markedly in physiochemical properties affecting nutrient availability. Results obtained in correlating 14C incorporation in the amino acid fraction with 15N accumulation as NO3 − were consistent with the stoichiometry that would be expected if C fixation were driven by autotrophic nitrification. Our findings demonstrate unequivocally that urea is utilized as a source of C and N by nitrifying microorganisms, which may account for rapid nitrification of urea-N in soils.