Nitrogen (N) is an essential nutrient for plant growth and therefore a detailed mechanistic understanding of soil N cycling and plant N uptake is necessary to ensure sustainable food production. In addition, N availability can have significant effects on vegetation community structure and therefore climate-change induced shifts in N availability could affect ecosystem functioning and climate change feedbacks. The potential importance of organic N for plant nutrition has only recently been realised, and knowledge of its cycling and the
competition for this resource in plant-soil systems is lacking.
The first aim of this PhD thesis was to investigate the potential for plants to compete with soil microbes for N at an early stage in protein breakdown, rather than having to rely on the breakdown of proteins to amino acids and subsequent mineralization to NH4+/NO3- . The second aim of this thesis was to study N cycling processes in subsoil environments and their potential importance in plant N acquisition. The final aim was to examine how increased N inputs affected C and N cycling in Arctic tundra ecosystems, in order to gain a better
understanding of potential climate change impacts on this vulnerable area.
Experimentation with isotopically labelled N forms revealed differential mobility of the different N forms in soil. Firstly, NO3-proved extremely mobile in comparison to NH4+, which was retained both on the exchange phase and rapidly captured by soil microorganisms. High molecular weight (HMW) dissolved organic nitrogen (DON) showed
greater potential for leaching than its low molecular weight (LMW) counterpart, which was also rapidly assimilated by soil microbes. Soil microbes showed a preference for peptide-N over other N forms, whilst wheat plants showed a greater affinity for NH4+ when grown under sterile conditions. When in competition with microbes, wheat mainly took up NO3-, with the capture of alanine and peptides being comparatively low. The presence of peptide-N decreased the ability of wheat to use NH4+, whilst the presence of other N forms depressed wheat’s use of peptide-N. Studies of arable topsoil and subsoil revealed that C limitation was limiting microbial processes at depth, which was likely caused in part by low root length density and low rates of C input. This caused slower C and N mineralization processes at depth. Deep soils therefore appear unlikely to be a major N source for plants, but this could be stimulated by a labile C input. Surface moisture limitation appeared unlikely to cause a reliance on subsoil N, as there was no significant difference in the change in soil water content with depth during drought conditions. Increased organic N inputs to the Arctic significantly affected plant composition and increased plant litter degradation rates. This has implications for the Arctic’s ability to store C and suggests a positive feedback to climate change.
In conclusion, this thesis has shown that DON input s are highly important for plant and microbial nutrition, and that DON can influence C degradation in the Arctic. Subsoil could potentially contribute to plant nutrition, but low inputs of organic material limits microbial activity, so wheat’s main N supply appears to come from surface horizons where N cycling is greatest. Plants can readily uptake peptides, although the results suggest that agricultural plants may struggle to compete with microbes for this resource. Furthermore, when added in combination with other reduced forms of N, the uptake of both peptide and the other N forms decreases.