Cattle urine and dung additions differently affect nitrification pathways and greenhouse gas emission in a grassland soil

How cattle urine and dung addition, and their interaction with the mineral soil regulate cumulative autotrophic and heterotrophic nitrifications and greenhouse gas (GHG) emission is poorly understood. We investigated the effect of urine (applied at 425 mg N kg⁻¹ soil) and dung (applied at 200 mg total N kg⁻¹ soil) addition on cumulative autotrophic and heterotrophic nitrifications, and GHG emission in a grassland soil in a 35-day laboratory incubation experiment under six treatments: CK, unamended control; U, urine addition; D_s, dung added on the soil surface; U-D_s, urine addition + dung added on the soil surface; D_m, dung mixed into the soil; and U-D_m, urine addition + dung mixed into the soil. Compared with the CK, the U, U-D_s, and U-D_m treatments increased cumulative net nitrification (autotrophic + heterotrophic) by 548, 587, and 505% and cumulative autotrophic nitrification by 593, 618, and 508%, but the D_m treatment decreased cumulative net and autotrophic nitrifications by 77 and 83%, respectively. However, cumulative heterotrophic nitrification was less than 1% of the cumulative autotrophic nitrification in the CK and it was decreased by D_s and D_m as compared with the CK at the end of the incubation. Cumulative emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O) were in the order of CK < U < D_s < D_m < U-D_s < U-D_m. In addition, all urine and dung treatments reduced cumulative methane (CH₄) uptake (negative values of CH₄ emission) as compared with the CK. Overall, soil cumulative net and autotrophic nitrifications were higher, but GHG emission was lower, with dung added on the soil surface than with it mixed into the soil. We conclude that urine addition increases but dung addition decreases cumulative autotrophic nitrification; dung addition also decreases heterotrophic nitrification, and how dung is added to the soil is important in regulating the nitrification processes and GHG emission. Therefore, urine and dung management can alter soil N transformations and has implications for mitigating climate change.