Nutrient Management Research Database

Agricultural soils are a major source of nitrous oxide (N₂O) emissions and an understanding of factors regulating such emissions across contrasting soil types is critical for improved estimation through modelling and mitigation of N₂O. In this study we investigated the role of soil texture and its interaction with plants in regulating the N₂O fluxes in agricultural systems. A measurement system that combined weighing lysimeters with automated chambers was used to directly compare continuously measured surface N₂O fluxes, leaching losses of water and nitrogen and evapotranspiration in three contrasting soils types of the Riverine Plain, NSW, Australia. The soils comprised a deep sand, a loam and a clay loam with and without the presence of wheat plants. All soils were under the same fertilizer management and irrigation was applied according to plant water requirements. In fallow soils, texture significantly affected N₂O emissions in the order clay loam > loam > sand. However, when planted, the difference in N₂O emissions among the three soils types became less pronounced. Nitrous oxide emissions were 6.2 and 2.4 times higher from fallow clay loam and loam cores, respectively, compared with cores planted with wheat. This is considered to be due to plant uptake of water and nitrogen which resulted in reduced amounts of soil water and available nitrogen, and therefore less favourable soil conditions for denitrification. The effect of plants on N₂O emissions was not apparent in the coarse textured sandy soil probably because of aerobic soil conditions, likely caused by low water holding capacity and rapid drainage irrespective of plant presence resulting in reduced denitrification activity. More than 90% of N₂O emissions were derived from denitrification in the fine-textured clay loam—determined for a two week period using K¹⁵NO₃ fertilizer. The proportion of N₂O that was not derived from K¹⁵NO₃ was higher in the coarse-textured sand and loam, which may have been derived from soil N through nitrification or denitrification of mineralized N. Water filled pore space was a poorer predictor of N₂O emissions compared with volumetric water content because of variable bulk density among soil types. The data may better inform the calibration of greenhouse gas prediction models as soil texture is one of the primary factors that explain spatial variation in N₂O emissions by regulating soil oxygen. Defining the significance of N₂O emissions between planted and fallow soils may enable improved yield scaled N₂O emission assessment, water and nitrogen scheduling in the pre-watering phase during early crop establishment and within rotations of irrigated arable cropping systems.

• This study investigated how N2O emissions from soils of varying textures were influenced by the presence of a wheat crop. Mineral N levels and soil moisture were also tracked in the course of the experiment to help explain the mechanisms responsible for N2O emission patterns.
• The three soil textures analyzed were a clay loam, loam, and sandy soil.
• Yield data for the three soil types was also measured.

• N2O emissions were higher in more finely textured soils. The pattern of N2O emission was clay loam > loam > sand. N2O emissions are more readily stimulated in finely textured soils as they take longer to drain.
• The presence of a wheat crop limited N2O emissions in the clay loam and loam soils, while no differences were observed for the sandy soil. N2O emissions were as much as 6x higher when a wheat crop was not present.
• Despite similar levels of N fertilization (a total of 89 lbs N/ac), yields were higher in the more finely textured soils, with a pattern similar to the N2O emissions.
• The results of this study show that the presence of a crop can help reduce N2O emissions, while also highlighting the importance of considering yield differences in addition to N2O emissions levels (so-called “yield scaled" N2O emissions).