Modification of soil biogeochemistry by plants during a restoration trajectory (Punakaiki, New Zealand) : A thesis submitted in partial fulfilment of the requirements for the Master of International Nature Conservation at Lincoln University

Abstract

The Punakaiki Coastal Restoration Project (PCRP) was carried out to restore native vegetation in a farmed pasture, aiming to establish a functioning sandplain. More than 30 species and >150,000 individual plants were transplanted from containers-grown specimens across the 70 ha site on a pragmatic and adhoc basis over several years. A restoration trajectory is also an ecological succession with temporal changes in species composition of plant communities. The present study focuses soil biogeochemistry in areas of the site where different plant species have been planted. Plants and soils were sampled from different locations at PCRP, and chemical properties of these samples were analysed using ICP-OES to determine macronutrient and trace elements. Olsen-P, mineral nitrogen, soil pH, and organic matter were also analysed. Olsen- P was determined using colorimetrically, while mineral nitrogen was determined using a triple-channel CHN analyser. The results showed that soil chemistry varied beneath different plant species, indicating that plants have different potentials to influence soil nutrient concentrations. Nutrient concentrations also varied with soil depth. Soil concentrations of NO3-N, Fe, Mn, Mo, Na, P, and S differed significantly under different species. Patterns of similarity and differences within the data were investigated. The study also found that a major factor causing the element difference under different plants was site variation. Soil samples were taken from the transects with different soil ages. Younger soil was more likely to contain more abundant trace and major elements and more biological activity, and soil pH was also impacted by soil age. Nutrient concentrations in soil and foliage were often either positively or negatively correlated. Whist primary minerals usually determine major nutrients and trace elements in the soil, differences can be due to passive or active absorptive channels in the soil and by physiological differences between plants. Rhizosphere processes can also alter soil chemistry. Coprosma robusta showed a passive response to salinity, suggesting it might be unable to regulate Na uptake, with a positive correlation between soil Na and foliage Na. By contrast, Coprosma propinqua appeared to actively control uptake of Al, Mg, Na, and Ni; it was the only species that showed a potential mechanism for exclusion of some elements, with a negative correlation of these elements between foliage and soil. The effect of rhizosphere processes on soil organic matter depends on plant species and soil properties, and microorganism activity and community play fundamental roles in driving organic carbon accumulation and decomposition. This study found that Olsen-P and soil organic matter tended to be higher in rhizosphere soil than in bulk soil, although the difference was not statistically significant. This may have been due to microbial activity differing between rhizospheric and bulk soil, probably associated with root morphology and exudates. An incubation experiment of litter of different plant species mixed with soil showed that plant litter could significantly change soil chemistry, and this varied between plant species. Plant litter reduced soil acidity and increased soil Olsen-P, but this differed in magnitude between plant species. Soil concentrations of NO3-N and NH4-N decreased after incubation; it is argued this might result from microorganism consumption of soil organic carbon. This research also aimed to evaluate finding of an earlier study on the role of seabird guano on soil nutrient concentrations and the accumulation of nutrients in New Zealand flax (Phormium tenax). The present study indicated that there was no significant difference in soil P levels under flax compared to other plants, contradicting the earlier findings of Zhong (2016). It was suggested that the variation in P levels were more likely to be due to soil age and location differences. Furthermore, P. tenax was found to have a significantly higher concentration of sodium than other plants, suggesting its ability to absorb substantial amounts of sodium from the soil. The findings also raise the question of whether P. tenax captures sodium in cuticular wax from marine spray due to its morphological features, which could be explored in further research. Overall, this study has shown that soil and plant nutrients differ between plant species and that plants have significant effects on soil chemistry. Plant and soil nutrients interact with litter quality and other environmental factors. It is concluded that understanding soil biogeochemistry has a role to play in ecological restoration, although this is currently poorly understood.