Short-term N2O, CO2, NH3 fluxes, and N/C transfers in a Danish grass-clover pasture after simulated urine deposition in autumn

Urine patches are significant hot-spots of C and N transformations. To investigate the effects of urine composition on C and N turnover and gaseous emissions from a Danish pasture soil, a field plot study was carried out in September 2001. Cattle urine was amended with two levels of C-13- and N-15-labeled urea, corresponding to 5.58 and 9.54 g urea-NI-1, to reflect two levels of protein intake. Urine was then added to a sandy-loam pasture soil equivalent to a rate of 23.3 or 39.8 g urea-N m(-2). Pools and isotopic labeling of nitrous oxide (N2O) and CO2 emissions, extractable urea, ammonium (NH4+), and nitrate (NO3-), and plant uptake were monitored during a 14 d period, while ammonia (NH3) losses were estimated in separate plots amended with unlabeled urine. Ammonia volatilization was estimated to account for 14% and 12% of the urea-N applied in the low (UL) and high (UM urea treatment, respectively. The recovery of urea-derived N as NH4+ increased during the first several days, but isotopic dilu4 tion was significant, possibly as a result of stress-induced microbial metabolism. After a 2 d lag phase, nitrification proceeded at similar rates in UL and UH despite a significant difference in NH4+ availability. Nitrous oxide fluxes were low, but generally increased during the 14 d period, as did the proportion derived from urea-N. On day 14, the contribution from urea was 23% (UL) and 13% (UH treatment), respectively. Cumulative total losses of N2O during the 14 d period corresponded to 0.021% (UL) and 0.015% (UM of applied urea-N. Nitrification was probably the source of N2O. Emission of urea-derived C as CO2 was only detectable within the first 24 h. Urea-derived C and N in above-ground plant material was only significant at the first sampling, indicating that uptake of urine-C and N via the leaves was small. Urine composition did not influence the potential for N2O emissions from urine patches under the experimental conditions, but the importance of site conditions and season should be investigated further.