The effects of increased atmospheric reactive nitrogen deposition upon rates of biological nitrogen fixation in peatlands and temperate forests

• Main Author: Saiz Val, Ernesto
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: Q Science (General); Northern Sweden
• Online Access: https://eprints.keele.ac.uk/id/eprint/8320/; https://eprints.keele.ac.uk/id/eprint/8320/1/SaizValPhD2020.pdf

Biological nitrogen fixation (BNF) is a microbial process that fixes atmospheric nitrogen (N2) gas into reactive, bioavailable forms (Nr). Naturally, lightning and BNF accounted for the entire Nr available for plant and microbial metabolic demands. However, following industrialization, Nr has also been produced anthropogenically (e.g. burning fossil fuel, fertilizers). The UK experiences high rates of anthropogenic Nr deposition (>26 kg ha-1yr-1) not being clear if increased Nr deposition slows or shuts down BNF in peatlands and temperate forests, as Nr is freely available. Therefore, it was critical to investigate it. The research objectives were to establish a robust method to measure BNF, to evaluate the impact of chronic Nr deposition on BNF and to examine the main factors controlling BNF. Following comparative evaluation of the two common methods for quantifying BNF in peatlands, the acetylene reduction assay underestimated BNF by 53% compared to the direct 15N2 assimilation method. Across a gradient of Nr deposition, higher rates of in situ BNF were found in areas with lower Nr deposition rates and BNF decreased as Nr increased; however, BNF did not shut down completely. Under experimental long-term addition of Nr and sulphur into a peatland in northern Sweden, BNF decreased by 94%. Sphagnum mosses exposed to higher Nr deposition exhibited an increase in BNF by 83% following the addition of P and K. Addition of microbial respiratory metabolites (CH4, CO2 and N2O) to mosses enhanced BNF 80%. Asymbiotic BNF in deciduous temperate forest soils exposed to ~22 kg Nr annual deposition increased by 368% under elevated CO2 fumigation. This research demonstrates that BNF does not shut down under chronic atmospheric Nr and that increased availability of nutrients and energy for microbes can further boost BNF. Therefore, BNF must be considered even in polluted areas when modelling the N economy of their ecosystems.