Nitrogen (N) is a key regulator of ecosystem development, and the cycling and competition for N resources in ecosystems remains poorly understood. Arid ecosystems are primarily found in North Africa, Middle East and Australia, and cover about one-third of the total global area. Soil organic N (SON) and C cycles are linked; both immobilization and mineralization of N pathways are linked by heterotrophic microorganisms that require C from organic material for production of energy and growth. Therefore, studies into N and C cycles are key to understanding biogeochemical cycles in these areas. Typical agricultural practices in Saudi Arabia (KSA) focus on the production of dates. This study focussed on Al-Hassa oasis - the largest oasis and date supplier in Eastern KSA. The aim of this thesis was to investigate the effect of changing soil temperature, water irrigation sources and fertiliser types on soil C and N cycles within the oasis. To increase our understanding of the effect of aridity on N and C cycle we used 14C techniques to investigate the rate of DON or DOC mineralization by soil microorganisms in response to different temperatures, moisture content and different water types. Initial experiments showed that soil properties decreased dissolved organic nitrogen (DON) and carbon (DOC) mineralization rate whereas, temperature increased mineralization rate in the soil by altering carbon dioxide (CO2) emissions and C partitioning between catabolic and anabolic processes within the microbial biomass. Therefore, we would recommend the farmers and government managers to reduce fertiliser on hot summer that can reduce environmental pollution and cost. Higher C mineralization rates were observed in the soil with the lowest contents of silt, clay, and salinity. The rate of 14CO2 evolution from DON is greater than from DOC compounds with shorter half-life for DON substrates because they are likely to be processed by different metabolic pathways inside the cell. Following on from this, further experiments showed that changing the irrigation water source significantly increased the mineralization of C contained in insoluble plant residues in comparison to that present in the soluble component. The rate of insoluble plant material mineralization was slower than for the soluble component leading to lower rates of 14CO2 loss and a ca. 11-fold longer half-life compared to soluble fractions. Results from the final experiments indicated that applying organic fertilisers would reduce nitrate (NO3-) leaching more than inorganic fertilisers. The information contained in this thesis has improved our fundamental understanding of C and N cycling in arid soil systems. We would recommend the farmers and government managers to reduce inorganic fertiliser and use more organic N fertiliser that can reduce environmental pollution, cost, save more water, and increased yield. Further studies are, however, still needed to investigate the long-term effects of changing soil temperature, water irrigation source and fertiliser types on soil microbial processes in arid soil for developing better water and fertiliser management and reducing N or C gaseous emissions.