Thesis: The balance between microbial nitrification and nitrogen immobilisation rates and variation in nitrifying communities in Western Australian semi-arid soils
Nitrogen (N) is an essential plant nutrient, however conversion of N to nitrate by nitrification in agricultural soil exposes N to gaseous or leaching losses. Losses as nitrous oxide or nitrate leaching are an environmental concern, and in low input arable systems like those found in Western Australia, N losses may also mean a loss of production. Preventing these losses may be achieved by managing soil microbial N processes to increase microbial immobilisation and storage of N (the retention pathway) and/or to decrease nitrification (the loss pathway). An index to measure the relative importance of these two processes is the nitrification to immobilisation ratio (N/I ratio). The N/I ratio is correlated with leaching losses in temperate agricultural soils, but it is not known if this index is suitable for predicting N losses from semi-arid soils. Semi-arid soils are found in the Western Australian wheatbelt and are important for grain production. This region has cool, wet winters (the growing season) and hot, dry summers. Microorganisms in soil N cycling are affected by soil temperature, organic carbon (C) availability and water content, as well as other factors. In addition, nitrification may be carried out by ammonia-oxidising bacteria (AOB) and/or archaea (AOA), though little is known about how and why the relative population abundance of AOA and AOB varies in semi-arid soils. This project aims to understand soil N cycling, especially immobilisation and nitrification, in arable soils of semi-arid regions, the microbial communities that drive the N cycle, and to evaluate land management strategies that may change immobilisation and nitrification rates and communities. Specifically, the project will: i) investigate how soil organic C availability and soil temperature affect microbial N cycling rates and the risk of N loss; ii) evaluate the ability of a nitrification inhibitor to manage the risk of N loss, and the effect of the inhibitor on the communities and activity of AOA and AOB; and iii) examine seasonal changes in the communities of AOB and AOA in a semi-arid arable soil.
Why my research is important
Nitrogen (N) is an essential plant nutrient and one of the primary regulators of plant growth. In order to increase crop production, N is often appliedto agricultural soils. In the arable soils of semi-arid regions, such as the central grainbelt of Western Australia, crops are only grown during the winter, and the soil is left fallow in summer. The absence of plant uptake during the fallow period means that residual N in the soil is at risk of loss, especially after summer rainfall events when microbial activity is stimulated and drainage is occuring. Preventing these losses is important because besides being a loss of a valuable nutrient, N lost to water supplies is a health hazard and contributes to eutrophication, while gaseous N losses as nitrous oxide and nitric oxide contribute to global warming.