Short-term carbon and nitrogen cycling in urine patches assessed by combined carbon-13 and nitrogen-15 labelling

Urine deposition by grazing animals is known to induce large N2O emissions as a result of increased nitrification and denitrification in the soil. This is brought about by the increased N availability from the urine, in combination very likely also with increased organic C availability. Possible sources for C include the urine itself, increased solubility of soil C, lysis of microbial cells and leakage of C from scorched roots. The objective of this experiment was to test the hypothesis that: (i) urine deposition causes an increase in root-derived degradable C compounds in the soil, which (ii) fuel denitrification activity and N2O production. The study took advantage of carbon-13 pulse labelling the plant tissue combined with application of nitrogen-15 labelled synthetic urine as an attempt to identify the sources of N2O. Over a 6 weeks course, the CO2 evolved in response to urine application was equal to the quantity of organic C added. Immediately after the application, 87% of the respired CO2 appeared to be from the urine, and respiration of plant-derived C was temporarily decreased. The cumulated amount of respired 13C plant carbon, however, was unaltered by the urine treatment indicating that root death was not a significant source to available C. Nitrous oxide emissions accumulated to 7, 59, 142 and 77 mg N2O-N m−2, respectively, for control (0N), low urine N (LUN), high urine N (HUN) and high mineral N (HMN) treatments. Pair-wise comparisons indicated that HUN > LUN (P < 0.03), whereas HUN = HMN (P < 0.18). The N2O emission factors were 0.3% for the urine treatment, independent of urinary urea concentration and 0.15% for mineral N (NH4+). The 15N isotopic data indicated that denitrification of soil NO3− was the sole source for N2O production in the urine-affected soil after 12 days of incubation. The initial source of N2O could not be identified because of lack of ability to detect the soil 15NO3−. The source of N2O from added NH4+ was ambiguous since the isotopic signals of N2O, NH4+ and NO3− could not be discerned. Approximately, 50% of the urinary-N, independent of urea concentration, and 72% of the NH4+-was recovered after 6 weeks of incubation. This finding, in combination with the difference in the N2O losses, emphasizes the potential to control N-emissions from urine patches through dietary control of the urine N-content.