Soil biogeochemistry and the benefits of grasses as companion plants : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Intercropping and other forms of mixed cropping are both widely used and beneficial in many agricultural systems. One of the best-known examples is the grass-clover production model in pastures, in which nitrogen-fixation by symbiotic bacteria (e.g. Rhizobium spp.) benefits the grass as well as the host species of clover (Trifolium spp.) or other legume species. This is one example of transgressive overyielding, which refers to situations when the yield of two species growing together exceeds the yield of either of the species growing individually on the same area of land. This doctoral study aimed to investigate the benefits of companion plants in terms of soil biogeochemistry, focusing particularly on species that are companion plants to grasses. Literature is reviewed of plant species competition and co-existence, and rhizosphere biogeochemistry, primarily in the context of pastoral grasslands in the New Zealand High Country. The research project includes an investigation of whether there are nutritional benefits to nitrogen (N) -fixing plants when they grow in combination with grasses, rather than this being simply a relationship that involves spillover of nitrogen from legumes to grasses. This is extended to studies of phosphorus (P) -mobilizing plants and grasses, to investigate whether grasses provide reciprocal nutrients to cluster-rooted plants in the Proteaceae. The hypothesis underlying this study was that rhizosphere processes in grasses may be more adept than clovers at exploiting key trace elements in soils, and that these trace elements can be exchanged for N or P, thus also providing an explanation for species coexistence. Practical investigations consisted of pot experiments, a transplanted-soil core mesocosm experiment and field sampling, primarily concerned with nutrient deficient soils. This experimental work analysed the effects of different species combinations on (i) yield and (ii) plant and soil nutrient status (macronutrients and trace elements). Plant species combinations included both exotic and native species of grasses and legumes. The work was partly carried out on the Lincoln University Campus and at Mt. Grand Station, a university-owned high country pastoral farm in Hawea, Central Otago, from 2019 – 2022, with funding support from the Miss E.L. Hellaby Indigenous Grasslands Research Trust. The results showed that grasses played a critical role in acquisition of soil nutrients when they were growing with companion legumes or proteas in degraded soil. I found that legumes contained more nutrients (e.g. P, potassium sulphur, molybdenum and boron) when they were growing with grasses. Both native tussock grasses and exotic pasture grasses had significant positive impacts on native and exotic legumes in terms of nutrient uptake of a range of elements including Nitrogen and Sulfur. Both legumes and grasses benefited from growing together. Cluster-rooted species of Proteacea and grasses were found to have higher foliar concentrations of nutrients when they were growing together. The findings of this research provide evidence of facilitation between plant species with contrasting root systems when they are growing together in terms of procurement of key plant nutrients. Combinations of species can better exploit soil nutrients under conditions where fertility is constrained. This work is interpreted in the context of pasture production systems, protection of native biodiversity and land sharing by exotic and native species in the New Zealand High Country. The thesis is presented as an introduction to the study, a broad literature review, the inclusion of three published papers 1-3, one recently-submitted manuscript 4, a chapter describing additional work carried out during the PhD study, and a chapter containing the interpretive discussion and conclusions.