Magdalena Ramírez-Sandoval

Agricultural soils account for about 60% of the global atmospheric emissions of the potent greenhouse gas nitrous oxide (N₂O). One of the main processes producing N₂O is denitrification, which occurs under oxygen-limiting conditions when carbon is readily available. On grazed pastures, urine patches create ideal conditions for denitrification, especially in soils with high organic matter content, like Andisols. This lab study looks at the effects of Urine-urea-N load on the Andisol potential to emit N₂O. For this, we investigated the effects of three levels of urea-N concentrations in cow urine on emissions of N₂O, N₂, and CO₂ under controlled conditions optimised for denitrification to occur. Results show total N₂O emissions increased with increasing urine-N concentration and indicate that denitrification was the main N₂O-producing process during the first 2–3 days after urine application, though it was most likely soil native N rather than urine-N being utilised at this stage. An increase in soil nitrate indicates that a second peak of N₂O emissions was most likely due to the nitrification of ammonium hydrolysed from the added urine, showing that nitrification and denitrification have the potential to play a big part in N losses and greenhouse gas production from these soils.

Urea present in cattle urine contributes large amounts of nitrogen (N) to grazed pastures, which can be the equivalent to approximately 1000 kg N ha⁻¹. However, there are no studies in volcanic soils of southern Chile on the effect of different concentrations of urinary N deposited in the soil, nor of the effect different wetting and drying conditions mimicking the variation in weather conditions on the nitrification process from urea to NH₄⁺ and total oxidized nitrogen (TON) over time. In addition, the inhibition of nitrification driven by the accumulation of NH₃ at high rates of N applied to Andisol has not been evaluated. Fresh cattle urine was applied at three different rates of N equivalent to 247 kg N ha⁻¹ (Low N), 461 kg N ha⁻¹ (Medium N), and 921 kg N ha⁻¹ (High N), as well as deionized water as a control. Further, three moisture conditions were imposed: constant moisture (CM), drying–rewetting (DRW) cycles at 7-day intervals, and soil drying (SD). Destructive soil core samples were evaluated for top and bottom halves individually every 7 days over a 36-day period to measure changes on inorganic N and pH. There were no interaction effects for N rates and soil moisture. The main effect of the different rates of N on mineralization was significant throughout the incubation period, while the effect of the different moisture conditions was variable over time. High N was associated with elevated NH₃ concentrations and could explain why total N mineralization was partially inhibited. These results suggest that the presence of different nitrifying microorganisms in soil under different chemical and physical conditions determines nitrification, and thus, the oxidation of ammonia should be studied in more detail as the first step of nitrification, specifically in volcanic soils.

The objective of this study was to assess N dynamics in the soil, and the response in regard to dry matter yield (DM yield; kg·DM·ha⁻¹), N yield (g·N·m⁻²) and N concentration (g·N·kg⁻¹ DM) under urine patches and dung pats and in the affected zone of the grassland soil over 304 days. The amount of N under the urine and dung patches increased until day 10 and then started to decrease until the end of the experiment, reaching control treatment levels. The DM yield in the urine patch zone and the 0–10 cm zone around the dung pat was higher than in the control treatment throughout the evaluated period, while in the 0–30 cm zone around the urine patch and the 10–20 cm zone around the dung pat the DM yield was negatively affected by the application of excreta. The maximum accumulated N yield was up to two times higher than the control in the 0–10 cm zone of the dung pat, while the mean concentration of N throughout the period was 42% higher than the control for the “urine patch” zone and 47% higher in the 0–10 cm area around the dung pat. The total absorbed N was 19% and 15% for urine and dung excreta, respectively. Further research is needed to explain the variables that might affect the results obtained, and it is necessary to evaluate the botanical composition as a factor that contributes to this effect.