Effects of Soil Osmotic Potential on Nitrification, Ammonification, N-Assimilation, and Nitrous Oxide Production

Authors

A. P. Low
John M. Stark, Utah State UniversityFollow
L. M. Dudley

Document Type

Article

Journal/Book Title/Conference

Soil Science

Volume

162

Publication Date

1997

First Page

16

Last Page

27

Abstract

Previous studies have examined the effects of soil osmotic potential (Ψ_s) on net rates of mineralization and nitrification. Because net rates represent the difference between gross production and consumption processes, it is unclear which process is being affected. We used an ¹⁵N isotopic dilution method to evaluate the effects of Ψ_s on gross rates of nitrification, ammonification, NH⁺₄ assimilation, and NO^-₃ assimilation, and net rates of nitrous oxide production in a Penoyer sandy loam at field capacity. To avoid creating specific ion toxicities that normally do not occur in this soil, we used a chemical equilibrium model to predict how solute concentrations in the soil solution change during evapo-concentration; then we used solutions containing these mixtures of solutes to create individual Ψs treatments. A nitrification potential assay was also performed to determine the effect of Ψ_s on nitrification rates at high substrate concentrations. In soil slurries with elevated NH⁺₄ concentration (1110 μM), nitrification rates declined exponentially with reduced Ψ_s (increased salt concentration); however, in soil samples incubated at field capacity without added NH⁺₄ (9.7 μM, or 2 mg N kg ^-1), the gross nitrification rate was independent of Ψ_s. The differential response between slurries and soil at field capacity was attributed to differences in NH⁺₄ concentrations, and indicated that the effects of Ψ_s were secondary to NH⁺₄ concentrations in controlling nitrification rates. Nitrification rates in slurries declined more when a single salt (K₂SO₄) was used than when the mixture of salts that more closely approximated the solute composition predicted to occur in the field was used to lower Ψ_s. This suggests that nitrifying bacteria are capable of adapting to specific ion toxicities. Gross rates of ammonification declined exponentially with decreased Ψ_s between 0 and -500 kPa but were independent of Ψ_s at potentials of -500 to -1750 kPa. Rates of microbial assimilation of NO^-₃ exceeded NH⁺₄ assimilation by a factor of 4, indicating that under NH⁺₄ limited conditions substantial NO^-₃ assimilation can occur. Microbial assimilation of both NH⁺₄ and NO^-₃ declined exponentially with decreased Ψ_s, and were insignificant at <-1500 kPa Ψ_s. Because NO^-₃ assimilation declined more rapidly than gross nitrification, net nitrification rates actually increased with declining Ψ_s. Rates of nitrous oxide (N₂O) production were also inversely correlated with Ψ_s. Our results indicate that in previous studies, measurement of net rates, use of inappropriate salts, and addition of substrate may have resulted in over-estimation of the adverse effects of low Ψ_s on rates of N-transformations.

Recommended Citation

Low, A.P., J.M. Stark, and L.M. Dudley. 1997. Effects of soil osmotic potential on nitrification, ammonification, N-assimilation, and nitrous oxide production. Soil Science 162:16-27.