ARS | Publication request: Availability of Urea to Autotrophic Ammonia-Oxidizing Bacteria As Related to the Fate of 14c and 15n-Labeled Urea Added to Soil

Submitted to: Biol Fertil Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 14, 2005
Publication Date: June 1, 2005
Citation: Marsh, K.L., Sims, G.K., Mulvaney, R.L. 2005. Availability of urea to autotriphic ammonia-oxidizing bacteria as related to the fate of 145c- and 15N-labeled urea added to soil. Biol Fertil Soils. 102(1):DOI:10.1007/s00374-005-0004-2.

Interpretive Summary: Like other fertilizers that produce ammonium in soils, urea can serve as a source of energy for growth of ammonium-oxidizing bacteria (AOB), resulting in the formation of nitrate (nitrification). In fact, the use of urea as N fertilizer results in unusually rapid nitrification, even faster than ammonium itself. It has been suggested that ammonium is taken up more slowly than urea in acidic soils, which may explain why urea undergoes rapid nitrification in such environments. However, urea is nitrified more rapidly than ammonium fertilizers under most conditions, which cannot be explained by uptake properties alone. Unlike other nitrogen fertilizers, urea also produces carbon dioxide, which can potentially be used for growth of AOB, since they are autotrophic (use carbon dioxide as a carbon source) like plants. Though it has been shown that some AOB can release carbon dioxide from urea, use of this carbon dioxide for growth has not yet been shown. If AOB can acquire carbon directly from urea, this factor, along with other properties of urea, may explain rapid nitrification of urea in non-acidic soils. We measured nitrate production by the AOB, Nitrosomonas europaea and Nitrosospira sp. NPAV in pure culture with urea as the sole source of either C or C and N. Nitrification occurred only with the latter organism, which was known to possess an enzyme for decomposition of urea. The organism was able to use urea as a source of nitrogen, carbon, and energy. Uptake studies supported the theory that ammonium is more slowly taken up by AOB under acidic conditions. Using labeled urea in soils, we were able to demonstrate that urea carbon was being utilized to produce cell protein in amounts expected based on the amount of nitrification that occurred. Our findings demonstrate unequivocally that urea is utilized as a source of C and N by AOB, which, when combined with our uptake results, account for rapid nitrification of urea-N in soils ranging widely in pH. The impact of this work is that it explains the rapid nitrification of urea under a range of conditions, and provides a basis for development of N management techniques that may reduce nitrate pollution by agriculture. Furthermore, methods used in this experiment will be applicable for examining the environmental behavior of other agricultural amendments containing nitrogen and carbon (such as herbicides).

Technical Abstract: Nitrate has been found to accumulate more rapidly in soils fertilized with urea than with inorganic sources of NH+4, despite the fact that nitrification must be preceded by hydrolytic decomposition. For acidic conditions, this finding has been attributed to limited uptake of NH+4 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 of alkaline soils, as reported in numerous studies. To ascertain whether urea-C can be utilized directly by nitrifying organisms, NO-2 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. Nitrate accumulated only with the latter organism, in which case nearly quantitative recovery was observed for N added as NH+4 and/or urea. In a subsequent study, recovery of 14C and 15N in gaseous, extractable, and hydrolysable 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 NO-3 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.