Prescriptions for optimal management of stony soils at Te Whenua Hou in Canterbury, New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Te Whenua Hou (Eyrewell forest) is a Ngāi Tahu plantation forestry-to-dairy conversion which also has ecological values related to a threatened and residual native flora and fauna. Eyrewell soil is typified by its high stoniness with high drought vulnerability and potential high susceptibility to rapid leaching of water and nutrients. An abundance of rock fragments provides a restrictive water storage capacity. These factors raise a number of environmental concerns, both for transfer of nitrates to the wider environment and for the estabilishment of vegetation. Mediation of these concerns clearly would be of value both to agriculture and to restoration. The present study aims to provide an optimal prescription for stony soil management in Eyrewell to achieve successful land use outcomes. The present research project includes an investigation of current soil status at Te Whenua Hou, laboratory-, greenhouse-, and lysimeter-scale trials, and field research. Eyrewell soil has not been properly investigated since the land conversion, so the physical condition of current Eyrewell soil was identified by excavating soil profiles. Then, water flow patterns were observed by visualizing flow paths using a dye tracer. In the laboratory, the effects of rock fragment content and sizes on water flow rates were studied within repacked soil columns. The combined effect of rock fragments and living plant roots on nutrient leaching was investigated in a greenhouse pot trial. In addition, the effect of rock fragments on solute transport was explored using established lysimeters which had formerly supported crops and were likely to have contained decayed root channels. Lastly, a field investigation was conducted on the relationship between rock fragment content, properties of adjacent soils, and tension infiltration rates. The results revealed that rock fragments have a complicated effect on water flow and solute transport depending on rock fragment content and their contribution to continuous pore space. Intermediate rock fragment content reduced water flow rates, but high rock fragment content induced rapid and intensive leaching by reducing the water-holding capacity of soils and creating large voids. Living plant roots could reduce the nutrient leaching in stony soil, but dead or decayed root channels became the main pathways of preferential flow, which enhanced nutrient leaching. Intermediate rock fragment content contributed to decreasing nutrient leaching through those root channels by increasing the tortuosity of the channels. However, high rock fragment content created additional preferential flow pathways and increased solute transport. The findings of this research project show that the effects of changing rock fragment content are often couter-intuitive. Furthermore, there are substantial differences in hydrology between repacked soil columns and in-situ field studies. Laboratory studies must be extrapolated with care. Integration of the findings of this research project led to a commendation to remove 6-10 % of rock fragments from 0-40 cm depth, and 26-30 % of rock fragments from 40-80 cm depth. This removal would be likely to reduce up to 25 % of water leaching and 21 % of nutrient leaching.