Effect of land use and moisture on phosphorus forms in upland stream beds in South Otago, New Zealand
Richard W. McDowellAgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel 9053, New Zealand. Email: richard.mcdowell@agresearch.co.nz
Marine and Freshwater Research 60(7) 619-625 https://doi.org/10.1071/MF08047
Submitted: 23 February 2008 Accepted: 26 November 2008 Published: 28 July 2009
Abstract
Land use can influence stream sediment composition and water quality, whereas moisture status affects sediment phosphorus (P) bioavailability to algae. Declining upland surface-water quality in South Otago, New Zealand, may reflect land-use changes from sheep- to dairy-farming. I sampled sediment (0–20 cm) from streams draining 12 dairy- and 12 sheep-farmed catchments in spring (wet) and autumn (dry). 31P nuclear magnetic resonance (NMR) spectroscopy and the EDTA-fractionation scheme were used to determine different P forms and infer P bioavailability. Significantly more P was present in the sediment of streams draining dairy- than sheep-farmed catchments. Total P did not differ with the moisture regime; however, changes occurred in the following P fractions: acid-soluble organic P, NaOH-P, CaCO3≈P, Fe(OOH)≈P and residual organic P. Extraction for 31P NMR analysis removed 78–85% of sediment total P and isolated five P classes. More bioavailable P such as orthophosphate (23–40% of P extracted) and diesters (2–6% of P extracted) was present in dry than in wet sediments, and in sediments draining dairy streams than in those from sheep-farmed catchments. This indicates substantial reserves of bioavailable P in sediment from these catchments, especially from dairy-farmed catchments, sustaining in-stream P concentrations for many years even without additional P input from land.
Additional keywords: depth, diesters, fractionation, monoesters, 31P NMR.
Acknowledgements
I thank Ian Stewart of the Department of Chemistry, University of Otago, New Zealand, for generating the spectra. Funding for this work was provided by the New Zealand Foundation for Research Science and Technology under contract C10X0320. The paper was enhanced by the suggestions of both reviewers and the guest editor.
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