The influence of temperature on emissions of nitrous oxide and dinitrogen from soils

Nitrification and denitrification are two major soil biological processes that release nitrous oxide (N₂O) from soils. N₂O production and reduction have been well-documented at temperatures below 35ºC, but are poorly understood at higher temperatures. N₂O production from nitrification was compared at a range of temperatures (10ºC to 45ºC) to mimic the typical temperatures encountered in soils from dairy pasture systems in Australia. Temperature was more important than soil type in controlling N₂O from nitrification, which was slow at 10 – 25ºC and peaked at 35 – 40ºC, suggesting a higher optimum temperature for N₂O production from nitrification than previous studies reported. Autotrophic nitrification produced N₂O predominantly below 35ºC, while heterotrophic nitrification, which used NH₄⁺ for nitrifying, released N₂O principally between 35ºC and 40ºC. Total N₂O emissions measured at different temperatures were influenced by the climatic region from which the soils were sourced. The magnitudes of N₂O emissions in the tropical soil exceeded those in the temperate soil under experimental conditions, although N₂O/NO₃⁻ from nitrification at different temperatures was independent of the climatic region from which soils were sourced. The N₂O/NO₃⁻ ratio was positively correlated with increased temperature and was above 1.0% at 35ºC, regardless of climate. Temperature interacted with soil moisture and NO₃⁻ availability to regulate N₂O from denitrification, while the conversion of N₂O to N₂ was affected principally by temperature. The highest denitrification (N₂O + N₂) was found at 35ºC in the soils treated at 75% FC and N contents between 100 – 150 kg N ha⁻¹. Low N₂O/N₂ ratios at 40 – 45ºC was due to the enhancement of N₂ production at these temperatures, suggesting greater soil NO₃⁻ loss as N₂ during summer, particularly in soils that are wet at that time. Interestingly, high NH₄⁺ availability was observed at 45ºC, which was hypothesised to relate to low nitrification rate and high rates of N mineralisation or dissimilatory nitrate reduction to ammonium at this temperature. This work has improved the knowledge of N cycling processes at high temperatures. Soil moisture or NO₃⁻ content alone are poor predictors of N₂O and N₂ production, since these elements interacted with temperature to control denitrification. High soil NH₄⁺ availability at 45ºC is a particularly interesting finding with potential to contribute to N losses. The findings confirm that management of soil moisture and NO₃⁻ availability, and a consideration of crop N demand are likely to reduce N losses as N₂O and N₂.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Agriculture, Food and Wine, 2016