Soil chemistry and ecology on a restoration trajectory of a coastal sandplain forest, Punakaiki, New Zealand

Abstract

This research was carried out in order to better understand the interactive role of vegetation and soil biogeochemistry on an ecological restoration trajectory on the West Coast of New Zealand. The Punakaiki Coastal Restoration Project (PCRP) was developed to restore degraded land to a more natural vegetation, resembling the original sandplain forest that has largely disappeared. Ecological restoration at the site, in terms of practice and research, has mainly focused on plant establishment and faunal colonization. The present study investigated whether restoration of soils is an integral part of this process. The project aimed to understand whether ecological restoration significantly modifies soils and, vice versa, whether physio-chemical variability of soils significantly influences the restoration trajectory. This research is based on a combination of laboratory, glasshouse and field-based studies. Incubation of native plant litters in soil was found to change soil chemical properties, including nitrogen (N) dynamics. It was found that two native species, Kunzea robusta and Olearia paniculata, may have the potential to ameliorate concerns associated with nitrate leaching and nitrous oxide production. Restored vegetation at the study site modified the dynamics of dissolved organic carbon (DOC) and mobile N in soil solution and increased rates of N mineralization. Interactions between vegetation and soil biota have significantly impacted these changes; changed soil conditions have also altered the composition of soil faunal communities. Study of soil pedogenesis revealed a formerly unknown spatial variability of the soil template. As soils have aged this has been reflected in a loss of soil total phosphorus (P), increase of occluded P and an increasing proportional importance of soil organic P. The dynamics of soil P fractionation on a short-term soil chronosequence across the site provided a better understanding of the response of soil biogeochemistry to the trajectory of ecological restoration on old and young soils. Key parameters were shown to be soil pH, organic matter, organic P and the variability of different P fractions. A detailed comparison of remnants of New Zealand Flax and Nikau Palm, and abandoned agricultural grassland, provided an opportunity to investigate the effects of these different types of vegetation on soil development. Multiple variables were found to be significant, including differences in plant physiology, soil organisms, hydrological gradient of an alluvial fan, and guano deposition, all of which modified soil P fractionation and secondary iron/aluminium (Fe/Al) minerals. In a glasshouse experiment, soil dehydrogenase activity and biologically based P (CaCl2-P, citrate-P and HCl-P) were significantly increased through interactions of earthworms and guano; the dynamic of soil P was modified by additional interactions with flax plants. The relationships between soil chemistry, biodiversity and plants on the restoration trajectory at PCRP were synthesized using multivariate analysis. A conceptual model was developed, elucidating changes of soil physio-chemistry on the restoration trajectory. The success of the PCRP restoration and establishment of flora and fauna are strongly influenced by soil variability, but the developing plant communities also substantially modify soil physio-chemistry. The study illustrates that a preliminary investigation of site-specific soils should be an essential part of restoration practice.