title: Developing 15N natural abundance technologies for soil nitrogen transformations in native forest ecosystems of subtropical Australia
Creators: name: Bai, Yifan
Publisher: name: Griffith University
Year: 2024
Subjects: subject: forest soil N cycling
subject: soil moisture content
subject: denitrification
subject: south-east Queensland

description: The nitrogen (N) cycle is of pivotal importance in terrestrial ecosystems. Atmospheric inert N2 gas is biologically and chemically fixed as reactive N, and reactive N is applied as chemical fertilizer and biologically fixed into the soil, where it undergoes a series of soil N transformations to provide essential nutrients for microorganisms and plants. Finally, it returns to the atmosphere as either reactive N gases such as nitrous oxide (N2O) or inert N2. However, as the greenhouse gas effect has become more severe in recent decades and the N cycle has become less complete due to climate change-induced weather extremes such as prolonged droughts and flash floods, the proportion of N2O in the N returned to the atmosphere has been increasing over the years. N₂O is a greenhouse gas with a warming potential 298 times greater than that of carbon dioxide (CO₂). In addition to having an atmospheric lifetime of 120 years, N2O is also oxidized in the stratosphere to nitric oxide (NO), which is responsible for the ozone depletion. Furthermore, the inability to utilize N2O results in the permanent loss of N from terrestrial ecosystems, thereby reducing the N availability, which in turn undermines the productivity of terrestrial ecosystems. Soil represents a significant contributor to global N2O emissions. Natural forests, which previously served as a N reservoir, have also experienced N2O emissions in recent years as a consequence of atmospheric deposition and extreme weather events. In contrast to agricultural soils, which are typically fertilized year-round, natural forest soils tend to exhibit higher organic matter content and more active microbial communities. Given the relatively stable environment of these forests, research on N2O emissions in natural forest soil is a pressing necessity. As an intermediate product of denitrification, a reduction reaction performed by microorganisms consuming energy, N2O production is simultaneously regulated by soil temperature, pH value, oxygen flux, redox potential and organic matter content. Furthermore, the upstream N reactions of N mineralization, nitrification and N immobilization also influence the integrity of denitrification through the reaction substrate, nitrate (NO3-). Under natural settings, these factors are ultimately attributed to soil moisture content (SMC). Given the strong relationship between SMC and climate change, the impact of triggered weather extremes on soil N transformation can be quantified in terms of SMC and N content, thereby reflecting to some extent the relationship between climate change and soil N2O emissions. As a consequence of climate change, the frequency and intensity of floods are gradually increasing, accompanied by higher discharge in a shorter period of time. The majority of current studies concentrate on the long-term loss and emission of N from terrestrial ecosystems. Nevertheless, there is a paucity of research and understanding of soil N transformations in the short term. The loss of soil N in a relatively short period of time caused by flash floods represents a significant challenge brought about by climate change. This study therefore concentrated on the short-term soil N transformations in response to SMC. Furthermore, an innovative approach was employed to examine the dynamics of soil mineral N pools in the short term, thereby enabling a more detailed analysis of the dynamic process of soil N transformations. In view of the exceptional capacity of biochar to retain water and fertilizer in the soil, this study also investigated whether biochar reduce the risk of N2O emission through the analysis of soil N transformations. The objectives of this study were as follows: (1) To quantify the short-term effects of SMC on soil C and N pools, as well as soil N transformations. (2) To investigate soil mineral N contents and transformations in response to different biochar application rates. (3) To investigate the effects of air-drying, to simulate drought, on soil N pools and mineral N transformations of 0-5 cm surface soils. (4) To compare similarities and differences in C and N pools and N transformations in soils that were at upslope and low slope. [...]