Biological nitrification inhibition (BNI) is a plant-mediated rhizosphere process where natural nitrification inhibitors (NIs) can be produced and released by roots to suppress nitrifier activity in soil. Several agricultural crops, such as rice, wheat, sorghum, and grasses, Brachiaria humidicola,have been found to have the ability to produce and release biological NIs from their roots. A few studies explored the effects of root exudates from grasses and crops (containing BNI activity) and specific BNI compounds on the transformation soil NH4+-N to NO3--N. However, less is known about the effects of biological NIs on soil emission of carbon dioxide (CO2), N gaseous emissions other than nitrous oxide (N2O), e.g. nitric oxide (NO)and dinitrogen(N2). Less is known about what soil, environmental and inhibitor properties such as temperature, pH, moisture, organic matter, NH4+-N content in soil, biological NI concentration and stability, affect their efficacy. Moreover, there is only a limited understanding of the effects of biological NIs on microbial populations and enzymes responsible for promoting nitrification, especially the mechanism through which biological NIs inhibit N2O emission. Hence, the studywas to determine the potential of biological NIsto reduce soil nitrogen (N)losses and improve nitrogen use efficiency (NUE)through improved understanding of the factors that control their efficacy in soil, and clarify the mechanisms of action of BNI. Effects of 1,9-decanediol (identified biological NI from rice), linoleic acid (LA, identified from tropical pasture grass, Brachiaria humidicola) and proven NI DCD, applied at two different rates (12.7 and 127 mg NI kg-1dry soil) on soil nitrification rates, greenhouse gas (GHG)(N2O and CO2) emissions, and also the ammonia oxidiser archaea (AOA) and bacteria (AOB) following NH4+-N application, were compared in Chapter 3.Results showedthatLA and 1,9-decanediol are ineffective to inhibit soil nitrification at relatively lower concentrations. However, DCD was effective in inhibitingsoil NH4+transformation to NO3-and N2O emissions under the same concentration.Thus, two higher concentration of LA and linolenic acid (LN) was added (635 and 1270 mg kg-1dry soil) to determine their effects on soil nitrification in Chapter 4. In addition,the stability, and direct or indirect nitrification inhibition of LA, LN andDCD are explored using 14C-labellingmethod, in a parallel incubation experiment. Results suggest that the apparent effect of LA and LN on soil NO3-concentration (≥635 mg kg-1dry soil) could be indirect under low-N conditions VII(no addition of fertiliser NH4+) due to the addition of sufficient labile C in the biological NIs stimulating either i) microbial immobilisation of soil NH4+or NO3-(under high C/N ratios), and/or ii) denitrification losses, such as N2O. We also demonstrated that LA and LN were much more rapidly mineralised than DCD in soil. The residual inhibitory effects of Brachiaria humidicola(Bh, containing BNI capacity) and Brachiaria ruziziensis(Br, not be able to release biological NIs)after sheep urine applicationare explored in Chapter5. Brachiaria humidicolainhibited N2O emissions during the first peak compared with Br, which indicates the potential strategy for using Bh grass in sheep-grazed pastures to reduce nitrification ratesand mitigate N2O emissions. Based on the possible indirect inhibition by easily mineralised biological NIs to stimulate soil denitrification,Chapter6evaluatedthe effect of different C compounds (identified from cattle slurry; glucose, vanillin, cellulose, glucosamine and butyric acid), fresh and aged cattle slurry on soil NO3-consumption, N2O and N2emissions during denitrification. Results showed that the liable C compounds (glucose, glucosamine and butyric acid) significantly stimulated soil N2O emissions via denitrification than complex C compound (e.g. cellulose) and fresh or aged cattle slurry. We conclude that the required doses of LA, LN and 1,9-decanediol to inhibit soil nitrification were significantly higher than the application rates of the proven synthetic NI, DCD. The efficacy of biological NIs were largely related to the initial biological NI concentration and stability in soil, which increased as the increasing of BNI concentration and decreasing mineralisation rates. The apparent reduction of soil NO3-concentration after the application of biological NIs may result from biological NIs 1) directly inhibiting the nitrification process; 2) providing a C source to stimulate soil NH4+and/or NO3-immobilisation; 3) providing a C source to promote soil denitrification. The synthetic NI, DCD, was confirmed to suppress the transformation of soil NH4+to NO3-, and reducesoil N2O emissions by impeding AOB but not AOA directly in a highly nitrifying soil. Further studies are necessaryto measure the effects of biological NIs on direct soil microbial immobilisation and denitrification to provide more evidence for the mechanism of biological NIs on soil nitrification.