Riparian buffer zones of the Canterbury agricultural landscape and their effectiveness for nitrogen and phosphorus retention.

Waterways in New Zealand have been severely degraded by agricultural activities during the past two centuries and land-use intensification is set to continue. Diffuse-source inputs are widely perceived to be a significant problem in low-lying areas, and riparian buffer zones are identified as a strategy to reduce land-use impacts. I surveyed the width and vegetative composition of riparian areas adjacent to 88 narrow farm waterways on the Canterbury Plains during the summer of 2009-2010. I investigated soil phosphorus and nitrogen availability at agriculture/riparian boundaries adjacent to high-intensity (dairy) and low-intensity (dry-stock grazing) pastures, and compared nutrient retention by fenced-off grass filter strips with that of native-planted buffers. Finally, I used growth metrics of three species commonly occurring in native buffers (Cordyline australis, Plagianthus regius and Pittosporum tenuifolium) and seasonal grass biomass accumulation, to assess growth response in relation to nutrient and moisture status of shallow (10cm) riparian soils. Riparian margins of Canterbury’s agricultural waterways were commonly between 2 – 5 metres wide. Most were dominated by exotic pasture grasses and annual/perennial forbs or, exotic shrubs and trees, as opposed to native vegetation. Nutrient availability at the agriculture/riparian boundary appeared to be influenced more by seasonal effects and site-specific characteristics, than land-use. While nutrient retention was variable, it was generally higher in native-vegetated buffers than in grass buffers and also strongly influenced by season and site context. Accumulated grass biomass, and to a lesser extent native riparian species, appeared to be influenced primarily by soil moisture within shallow riparian soils, rather than nutrient status. Of the three trees assessed, the unique surface-root structures of Cordyline australis may confer an advantage for uptake of near-surface water and nutrients. However, on lowland plains, established native riparian trees that acquire nutrient resources from throughout the soil profile and shallow ground-waters also constitute an essential component of buffer systems to ensure optimal nutrient removal. This thesis recommends continuing use of narrow grass buffers in combination with down-slope planting with a range of native species to optimise nutrient uptake depth and storage longevity. It also suggests further study of plant/soil/nutrient interactions and, nutrient flow within riparian buffers in the context of topography to ensure that land-manager revegetation efforts live up to expectations