Nitrogen and ozone pollution: a threat to natural ecosystems

Abstract

During the last century, inputs of reactive nitrogen (N) to ecosystems have more than doubled as a result of anthropogenic activities. There is strong evidence that N has reduced the diversity of plant species in a range of habitats of high conservation value. The effects of atmospheric N deposition on the structure and functioning of natural and semi-natural ecosystems have been well studied. There has, however, been little investigation of the contrasting effects of reduced versus oxidised N, or of the importance of frequency of inputs. This thesis involves a study which investigates the response of a heathland plant community to different forms of N applied at low & frequent vs infrequent and high levels of N additions. Solutions were applied at one of five intervals: weekly, two weekly, monthly, two monthly and four monthly. In large parts of the UK, ozone (O3) critical levels for agricultural crops, forests and semi-natural vegetation are regularly exceeded. In grassland ecosystems, similar effects on major plant functional groups have been reported for both O3 and N, however, their combined effects have rarely been investigated. This thesis investigates how differing levels of N deposition modify the effects of O3 on calcifugous grassland species using an open top chamber fumigation facility at Silwood Park, Ascot. Mesocosms representing a mixed mesic plant community were exposed to different concentrations of O3 (FA, 40, 70 & 100 ppb) under two N regimes (0 & 50 kg N ha-1 yr-1) in 2008. In addition, the performance of a representative grass (Festuca ovina) and legume (Trifolium repens) species were tested under differing O3 and N concentrations in 2009. In a final part of the study, cultivars of French bean (Phaseolus vulgaris) bred for contrasting O3 sensitivity were exposed to a range of O3 concentrations under differing levels of N availability. The results revealed that N addition significantly affected the growth and productivity of heathland mesocosms with greater proportional increases in oxidised N rather than reduced treatment. Reduced N significantly lowered soil pH while the opposite effect (increased pH) was seen with the oxidised form. Low dose, more frequent N addition resulted in bigger responses than infrequent applications of larger amounts. The research presented provides an important step to increase knowledge of the influence of both pollutants (N and O3) on grassland within plant community’s functional groups. The overall community biomass of mesic grassland and grass/legume mixture declined due to O3 exposure which is a serious concern for habitat biodiversity. High N intensified the legumes sensitivity to elevated O3 and caused a greater biomass reduction than low N. Phaseolus genotypes (S-156 and R-123) used in the study responded significantly to O3 exposure which resulted in yield decline and poor performance at elevated O3 levels Nitrogen addition did not result in significant increase or decrease in the yield for either Phaseolus genotype but a significant interaction effect between O3 and N was observed for O3 injury in the sensitive genotype and less injury was recorded with the higher N treatment.