Short-term C and N dynamics in urine patches: Response of nitrifiers to different urinary urea levels

Summary

Managed grasslands can accumulate soil organic carbon and thereby remove CO2 from the atmosphere. When grazed, however, the interaction between N inputs (fertilization, feeding, N fixation) and excretal returns can lead to sward deterioration and elevated emissions of N2O in urine patches. Both effects may reduce the positive effect of carbon storage on the greenhouse gas balance.
A field plot study was carried out between 19 September and 4 October 2001, in which cattle urine amended with 13C- and 15N-labelled urea was added to a sandy loam pasture. Two levels of urea, corresponding to 23.3 and 39.8 g urea-N m-2, were compared with an unamended control. Ammonia losses corresponded to 14 and 12% of the urea-N applied in the low (UL) and high (UH) urea treatment, respectively. Nitrification was initially delayed, but then proceeded almost linearly until the end of the two-week experiment. Nitrous oxide emission rates generally increased during the 14-d period, as did the proportion derived from urea-N. The results indicated qualitative differences in the patterns of urea-C and -N turnover, but similar relative losses of N2O at the two levels of urinary urea used.
After 4 and 14 days, soil from 0-5 and 10-20 cm depth was analyzed for content and composition of phospholipid fatty acids (PLFAs), and for distribution of urea-derived 13C in individual PLFAs. Initial changes in PLFA composition were greater at the lower level of urea, as revealed by a principal component analysis. Incorporation of 13C from [13C]urea was low, but significant, and fatty acids typical for Nitrosomonas spp. (i.e., 16:0, 16:1w7c) and Nitrobacter spp. (i.e., 16:0, 18:1w7) contained the highest amounts of 13C. Notably, a -20‰ depletion of the cyclopropane fatty acid (CFA) cy17:0 was observed after 4 days, which was replaced by a –10 to -20‰ depletion of cy19:0 after 14 days. Possible reasons for this pattern are discussed. Autotrophic nitrifiers could not be implicated in urea hydrolysis in urine patches to any large extent. Yet, the incorporation of urea-derived 13C in PLFAs indicated a selective response of nitrifiers which differed between the two urea concentrations.