Sources of phosphorus in two subcatchments of the Manawatu River, and discussion of mitigation measures to reduce the phosphorus load

• Published in: New Zealand journal of agricultural research 2013-09, Vol.56 (3), p.187-202
• Main Authors: Parfitt, RL, Frelat, M, Dymond, JR, Clark, M, Roygard, J
• Format: Article
• Language: English
• Subjects: beef; cows; Dairy farming; dissolved organic-P; DRP; Environmental aspects; farms; fertilizer; Fertilizers; forests; monitoring; nitrogen; pastures; periphyton; phosphate; Phosphates; Phosphorus; Phosphorus in agriculture; planting; rivers; sediments; sheep; soil; soil erosion; stream channels; subwatersheds; suspended particles; Suspended sediments; trees; water management; water quality
• ISSN: 1175-8775; 0028-8233; 1175-8775
• DOI: 10.1080/00288233.2013.799497

Dissolved phosphorus (P) from various sources contributes to periphyton growth in rivers. The Manawatu catchment is in the southern North Island, and since it flows through pastoral farmland with soft rock geology, it carries nitrogen, P and sediment that lead to growth of periphyton. We estimated long-term average sediment P in rivers in the catchment above Palmerston North (PNth) (3910 km²) with the NZeem® model, and also measured the total P on the bed of the river. The annual loss of P in eroded sediment made up 95% of the P flux, (1500 t P yr⁻¹ at PNth); 96% of the erosion occurred under pastures and 4% under forest. Dissolved P was measured at Hopelands and at PNth, and its origins—point sources, forests, the bed of the river and farmland—were assessed. The annual average dissolved P was estimated to be 75 t P at PNth, with 34 t P coming from sheep and beef farms, and this could be reduced with targeted planting of trees; 19 t P came from dairy farms, and could be reduced with changes to management of effluent, limiting soil Olsen P to the optimum agronomic range, and excluding cows from streams, banks and channels. Dissolved P from point sources (15 t P) could be decreased with changes to management of effluent. During low flows, sediment on the bed of the river released approximately 1.3 t dissolved P. These particulate-bound P losses could be reduced by targeted planting of trees on highly erodible land. The Upper Manawatu Water Management Zones (a priority catchment for mitigation) above Hopelands Bridge were also modelled. Council programmes to reduce soil erosion and total P, and to reduce dissolved P during low flow have the potential to reduce P loads in the river. Monitoring of P in the river should be carried out to define a more precise base line, and to monitor improvements to water quality as the programmes progress.