In situ nitrous oxide and dinitrogen fluxes from a grazed pasture soil following cow urine application at two nitrogen rates

Abstract

Cattle grazing of pastures deposits urine onto the pasture soil at high nitrogen (N) rates that exceed the pasture's immediate N demands, increasing the risk of N loss. Nitrous oxide (N₂O), a greenhouse gas, and dinitrogen (N₂) are lost from the cattle urine patches. There is limited information on the in situ loss of N₂ from grazed-pasture systems which is needed for understanding pasture soil N dynamics and balances. The ¹⁵N flux method was used to determine N₂ and N₂O fluxes over time following synthetic urine- ¹⁵N application at either 400 or 800 kg N ha¯¹ to a grazed perennial pasture soil. Results showed that daily N₂O fluxes were higher under 800 kg N ha¯¹ than under 400 kg N ha¯¹, but there was no significant difference in N₂ fluxes. Cumulative N₂O emissions from soil with 400 kg N ha¯¹ and 800 kg N ha¯¹applied represented 0.16 ± 0.08% and 0.43 ± 0.08% of deposited N, respectively, while emitted N₂ accounted for 32.1 ± 4.1% and 14.4 ± 1.7%, respectively, over 95 days after urine application. Codenitrification and denitrification co-occurred, with denitrification accounting for 97.9 to 98.5% of total N₂ production. Recovery of urine-¹⁵N in pasture decreased with increasing N rate with 14.7 ± 0.5% and 9.9 ± 0.8% recovered at 400 and 800 kg N ha¯¹, respectively after 95 days. The N₂O/(N₂ + N₂O) product ratio was generally higher during periods of nitrification of urine-N (the first month after urine application) but with no clear relationship to other measured variables. Contrary to our hypothesis, an elevated urine-N rate did not enhance N₂ loss. This is speculated to be due to enhanced ammonia volatilisation and transfer of N as nitrate, to deeper soil layers. Soil relative gas diffusivity indicated that high N₂ fluxes resulted from entrapped N₂ diffusing from the draining soil.