Dynamics of a pasture soil microbial community after deposition of cattle urine amended with [¹³C]urea

Within grazed pastures, urine patches are hot spots of nitrogen turnover, since dietary N surpluses are excreted mainly as urea in the urine. This short-term experiment investigated ¹³C uptake in microbial lipids after simulated deposition of cattle urine at 10.0 and 17.1 g of urea C m ^-2. Confined field plots without or with cattle urine amendment were sampled after 4 and 14 days, and soil from 0- to 5-cm and 10- to 20-cm depths was analyzed for content and composition of phospliolipid fatty acids (PLFAs) and for the distribution of area-derived ¹³C among individual PLFAs. Carbon dioxide emissions were quantified, and the contributions derived from urea were assessed. Initial changes in PLFA composition were greater at the lower level of urea, as revealed by a principal-component analysis. At the higher urea level, osmotic stress was indicated by the dynamics of cyclopropane fatty acids and branched-chain fatty acids. Incorporation of ¹³C from [¹³C]urea was low but significant, and the largest amounts of urea-derived C were found in common fatty acids (i.e., 16:0, 16:1ω7c, and 18:1ω7) that would be consistent with growth of typical NH ₄⁺-oxidizing (Nitrosomonas) and NO₂ ^--oxidizing (Nitrobacter) bacteria. Surprisingly, a 20‰ depletion of ¹³C in the cyclopropane fatty acid cy17:0 was observed after 4 days, which was replaced by a 10 to 20‰ depletion of that in cy19:0 after 14 days. Possible reasons for this pattern are discussed. Autotrophic nitrifiers could not be implicated in urea hydrolysis to any large extent, but PLFA dynamics and the incorporation of urea-derived ¹³C in PLFAs indicated a response of nitrifiers which differed between the two urea concentrations.