The influence of New Zealand native vegetation on rhizosphere chemical concentration and speciation in riparian zones.

The quality of New Zealand’s surface waters has been identified as the most important environmental issue in agricultural landscapes. Waterways are degraded through nitrogen (N), phosphorus (P), sediments and pathogens that enter via stock access to drains, streams and rivers as well as runoff and leaching from adjacent fields. Often, NZ-native vegetation, which is depauperate in agricultural landscapes, is established in riparian zones to improve water quality by intercepting these contaminants before they enter waterways. Such plantings have expensive initial costs; it requires retirement of productive agricultural land, vegetation planting, labour costs and maintenance as well as fencing to prevent stock intrusion into the waterways. However, it is estimated that national-level planting initiatives could yield net benefits of $1.7 billion to $5.2 billion per year. Greenhouse studies have shown that the NZ-native species Mānuka (Leptospermum scoparium) and Kānuka (Kunzea robusta) affect the N-cycle, and may therefore reduce the flux of N into waterways. This finding indicates that the efficacy of riparian plantings, to reduce contaminant spill over, may be greatly influenced by the species used. I aimed to test the null hypothesis that there is no difference between NZ-native plant species when considering their effect on the concentration and speciation of N, P and other essential nutrients in soils adjacent to waterways. I measured chemical differences in the rhizosphere of Lolium perenne, Phormium tenax, Kunzea robusta, Leptospermum scoparium, Coprosma robusta and Pittosporum eugenioides at three different locations in New Zealand field conditions. Specifically, in each rhizosphere I measured (1) Soil pH and total C, (2) total N and N- speciation, (3) total P and Olsen-P, (4) Other essential nutrients that may affect the use of vegetation for freshwater protection. I found there were no significant interspecific differences in the rhizosphere concentrations of TN, NO₃⁻, NH₄⁺, TP and Olsen P under all five NZ-native species. However, TP was affected by cut-and-carry of Lolium perenne at SFFP. TS and SO₄²⁻ generally had significantly higher interspecific differences in the rhizosphere concentration of P. tenax and K. robusta at SFFP and Lake Ellesmere. All other NZ-species showed no significant differences. Soil pH and TC were not significantly influenced by different native species, which could be due to different cycling processes. There was higher Na concentration under L. perenne than P. tenax at SFFP. This was thought to be from wastewater application onto pasture. The age of plants may be a significant factor in determining interspecific differences in chemical concentrations. Lake Waikare showed no significant interspecific differences in rhizosphere concentrations of chemicals, but the NZ-species planted were less than 2 years of age. The lack of differences does not indicate that NZ-native plant species is not an important consideration for riparian plantings. There are many other potential benefits that specific NZ-species may exhibit, which were not been tested in this study. The selection of NZ-species for riparian plantings should not solely rely on the rhizosphere chemical differences, as found in this research, but should consider ecological and farm management requirements