Exploring multi-functional soil resistance and resilience across an agricultural mosaic landscape : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Soils provide key ecosystem services that support terrestrial life on Earth. This natural substrate is vulnerable to disturbances, and due to the slow formation rate of soils, it is considered a non renewable resource on human timescales. Degradation of soils has been identified as a serious threat for Aotearoa|New Zealand, as over half of the country’s economy is reliant upon services and goods related to soils. The capacity of soils to resist and recover from disturbances is associated with edaphic properties, parent material, plant cover, climate, and soil microbiota. Understanding how these factors can be manipulated to design resilient landscapes that support soil multifunctionality is a growing area of research. The primary aim of my PhD research was to evaluate how soil microbial communities in a productive landscape in Aotearoa|New Zealand respond to disturbances relevant both locally and globally, and whether soil resistance and resilience can be stimulated through microbial inoculations. This research was conducted in Te Kaha, rohe|territory of Te Whānau-ā-Apanui, which provided contrasting levels of agricultural land management and aligned with the core values of Bioprotection Aotearoa. To facilitate the overarching aims of this research, a baseline characterisation was first performed on soils from paired kiwifruit (Actinidia deliciosa) and maize (Zea mays) sites. This study found that microbial communities were distinct across these landscapes, and that kiwifruit soils were associated with higher soil organic carbon, microbial activity, microbial biomass, as well as species richness and evenness, all of which are traditional indicators for soil quality. To assess whether these soil quality indicators could be transferred — and if so, whether this could improve soil response to disturbances — subsequent laboratory and field experiments used 1% and 5% (v/v) soil transfers of kiwifruit soil into maize soil. Drought intensity and frequency is projected to increase globally and has been identified as a risk in Aotearoa|New Zealand, particularly in Te Ika-a-Māui|the North Island. A laboratory-based successive drought study was conducted to assess how kiwifruit and maize soil respond to this type of disturbance and if resistance and resilience could be improved through disturbance conditioning or soil transfers. Soil microbial community analysis found that the response of microbes to drought varied with land use — kiwifruit soils were associated with higher bacterial community stability whereas maize soils supported a more stable fungal community. This highlights the importance of characterising both fungal and bacterial populations during disturbance studies. Repeated disturbances generally did not induce an improved soil response and while soil transfers did have a small, transient influence on microbial community structure, it did not meaningfully influence functionality. Land-use change, particularly with relation to agriculture, is another global driver of soil degradation. In Te Kaha, maize monocultures are being converted to kiwifruit monocultures, driven by economic benefits and perceived soil health concerns associated with continuous maize cultivation. This study established an opportunistic field experiment in a recently converted orchard to monitor shifts in soil attributes during land-use conversion. By the end of the study, the chemical profile of the converted site more closely resembled kiwifruit soils than maize. While microbial communities were still more similar to their historic land use, both fungal and bacterial communities displayed consistent directional changes, becoming less similar to maize and more similar to kiwifruit. Unlike the incubation study, soil transfers did not significantly alter microbial community structure, although some potentially transferred microorganisms persisted throughout the study duration. This finding underscores the legacy of historic land use on soils and provides rare time-series insight into microbial shifts occurring under active land-use conversion. This PhD thesis utilised molecular microbial ecology techniques to characterise soils in an understudied region of Aotearoa|New Zealand and is the first to compare microbial communities between kiwifruit and maize systems. This research, which was guided by kōrero|discourse with Te Whānau-ā-Apanui, provided a seasonal soil baseline of key crop systems in Te Moana-a-Toitehuatahi|the Bay of Plenty, evaluated soil transfers as an inoculum source in both laboratory- and field-based experiments, and assessed microbial community structure and potential functionality in response to two global drivers of soil degradation.