Assessing resilience of heathland ecosystems to air-borne nitrogen deposition using plant and soil nutrient stoichiometry

Boateng, Emmanuel (2021) Assessing resilience of heathland ecosystems to air-borne nitrogen deposition using plant and soil nutrient stoichiometry. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Vegetation in heathland ecosystems are adapted to a low level of nitrogen (N) availability. Increased N deposition thus reduces the competitiveness of characteristic heathland plants, such as Calluna vulgaris and induces soil acidification resulting in the limitation of other nutrients, including phosphorus (P), making heathlands vulnerable ecosystems to increased N deposition. In this study, the main question addressed was: how would nutrient availability, particularly P and soil-plant nutrient stoichiometry (N: P ratio), influence the resilience of heathlands to the adverse effects of prolonged N deposition? This led to the assessment and selection of a suitable P extraction method to determine plant-available P across heathland communities. Effects of P availability to protect lower plants against the adverse effects of N deposition was examined in nutrient addition experiments and across heathland communities. Findings from method assessment suggest that Mehlich-I extracted-P most represented plant tissue P making Mehlich-I extraction method the 'best' determinant of plant-available P across the studied heathlands. However, water-extracted P also proved to be a good determinant of plant-available P showing a more sensitive measure of the readily available source of P. Observations from nutrient addition experiments indicated direct relationships between tissue and soil nutrient concentrations and stoichiometric ratios, but this was absent across heathland communities. Survey results support earlier works with negative relationships between nitrogen deposition and decreasing overall species richness. The response was stronger in lowland heaths (r2 = 0.26, p<0.05) than upland heaths (r2 = 0.15, p = 0.08). Sampling sites were characterised by high spatial variability in soil available P, but there were indications of increased lower plant richness on high P containing bedrocks relative to low P containing ones, although species resilience to N deposition effects was lacking. Further studies need to clarify the protective role of P for species (particularly lower plants) against the adverse effects of N deposition across heathland communities.