Pathways and processes of phosphorus loss from pastures grazed by sheep Alice Rowena Melland B.Agr.Sc. (Hons1), The University of Melbourne A thesis submitted in total fulfilment of the requirement of the degree of Doctor of Philosophy in the School of Agriculture and Food Systems Institute of Land and Food Resources The University of Melbourne, Victoria, Australia December 2003 ABSTRACT Sheep producers in Victoria are applying more phosphorus (P) fertiliser and increasing stocking rates to increase production. At the same time, there is increasing awareness amongst research, community and producer groups that P-rich runoff water from agricultural land can contribute to the growth of undesirable algal blooms in surface waters. The loads, concentrations and forms of P in surface and subsurface hydrological flow pathways were estimated or measured directly on high and low P fertility hillslope plots in south-west Victoria to assess how this practice change could affect the P status of runoff and drainage water. Small- plot rainfall simulator studies were also conducted to investigate pasture management treatment effects. The spatial and temporal distribution of P loss from hillslope pastures, and the processes of P mobilisation in runoff identified in this study were used to identify appropriate management practises to help minimise P losses in runoff. Runoff P concentrations were then related to properties of pasture soils and runoff at a range of locations across Victoria to identify whether simple models and/or soil P tests could be used to predict P concentrations in runoff. Results from the study suggested that P concentrations in runoff from both low (8 kg P applied/ha) and high (25-40 kg P applied/ha) P fertility sheep pastures were unacceptably high relative to targets set for stream water quality in Victoria (annual mean concentrations >0.04 mg P/L), and that the dissolved reactive P fraction is the P fraction in runoff that increases the most as pasture P fertility increases. The volume of runoff was the primary factor influencing total P loads. Runoff was generated mainly by saturation excess flow during winter and spring from waterlogged soils that occupied <25% of the hillslope area. Subsurface hydrological pathways were only more important than surface pathways for P movement in dry years. Key management practises that are therefore recommended to minimise P losses from pastures in Victoria, are to avoid applying high rates of P fertiliser to landscape areas that become seasonally waterlogged, to consider retiring these areas from production and to maintain groundcover year-round to minimise erosion. An empirical model that uses a linear dissociation constant to describe the partitioning of P between runoff sediment and solution phases was reasonably well suited to predicting annual average DP concentrations in runoff from sheep pastures. The results also suggested, however, that factors such as runoff volume and fertiliser management practices (eg timing of application) should be considered in predictive models. The model was analogous to simple relationships developed between runoff P concentrations and soil test P. For the soils examined, which had a limited range of P sorption capacity (Pmax 862-1374 mg/kg), the POlsen soil test was a better predictor of runoff P concentrations than water extractable P or the degree of soil P I saturation. Relationships between soil P status and hillslope runoff water quality need to be investigated for a wider range of soils, agricultural practices and catchments in Australia if environmentally critical soil P thresholds are to be set. II DECLARATION This is to certify that, (i) the thesis comprises only of my original work (ii) due acknowledgment has been made in the text to all other material used, (iii) the thesis is less than 100 000 words in length, exclusive of tables, figures and references …………………………………… Alice Melland Date III Blank Page IV ACKNOWLEDGMENTS Firstly I would like to thank my supervisors Prof. David Chapman, Prof. Bob White and Dr. Malcolm McCaskill for the opportunity to undertake this degree, for your technical and scientific guidance and for the support and encouragement you offered throughout the project. From my interactions with you I have learnt the power of positive feedback and will endeavour to emulate this with colleagues in the future. I would also like to thank Dr. Jim Cox, Cyril Ciesiolka, Gavin Kearney and Dr. Tony Weatherley for specialised advice in hydrology, runoff measuring equipment, biometrics and soil chemistry respectively. Thanks also to Brendan Christy for guidance in using the daily soil water balance simulation spreadsheet which he, Bob White and others developed. For providing career development opportunities, encouragement and inspiration, I would like to thank Dr. George Riffkin, Prof. Nancy Millis, Geoff Saul and Dr. Cameron Gourley. I am also most grateful for the financial support of the DPI (then DNRE) Nancy Millis Postgraduate Award, the DPI Wool Program and in-kind support from SGS national experiment funding. Thanks also to Robert and Marilyn Lyons for allowing me to establish a peg and instrument farm on their sheep property and for their ongoing interest, support and enthusiasm for the research. To all the technical staff at DPI (then DNRE) Hamilton, you were an amazing team to work with and I have learned so much from you all. Thanks for your lateral thought, practical solutions, local wisdom, great company and above all your positive attitudes to work and life. In particular I would like to extend a big thank you to Jean Lamb – for providing the stamina and drive to complete 400 botanal readings in a day, for your competence, versatility, positive attitude and personal support, to Dion Borg – for managing the sheep and pastures at the Vasey field site and for teaching me how to pour concrete, use a shovel without breaking my back and for always finding a solution, and to Keiran Williams – for driving the drill rig on so many occasions, sharing philosophies on life and for the bentonite fights! Thanks also to Tim Plozza, Darren Gordon, Reto Zollinger, Paul Quinn, Belinda Gardener and the Farm staff for helping with soil sampling, fencing, rainfall simulating and lab support throughout the project. Thanks to all the staff at the Pastoral and Veterinary Institute in Hamilton for being so friendly and making me feel welcome, and for helping me out on matters of library searches, administration, finances, laboratory and sample processing techniques and for imparting aspects of your local knowledge with me. I would like to extend special thanks to Cassie Schefe for teaching me how to use the rainfall simulator, for giving me the opportunity to perform rainfall simulations at Maindample in conjunction with your Honours project, and for sharing your suspended sediment and runoff V data with me. Thanks also to Andrew Smith from the Institute of Land and Food Resources (ILFR) at The University of Melbourne for performing the laboratory procedures for the soil water retention characteristics described in Chapter 3. Thanks also to Ron Teo at ILFR for laboratory and IT advice and assistance, and to Margaret O’Dowd and Catriona Ridland for trying to keep me on the administrative straight and narrow throughout the project. I am very grateful to George Croatto and the inorganic chemistry unit of the State Chemistry Laboratory in Werribee for instructing and assisting me in the use of soil grinding and analytical equipment (eg Milestone microwave digestion units, Carey UV-VIS spectrophotometer and the Skalar flow injection analyser). Thanks also to Ron Walsh (SCL) for performing the ICP-AES elemental analyses of my ammonium oxalate soil extracts. From the Department of Primary Industries Ellinbank I would like to thank David Nash for assisting with setting up the Vasey field site set up, providing access to the Ellinbank laboratory and a runoff field site in Gippsland, and historical data from that site as well as technical advice throughout my candidature, and David Halliwell for providing laboratory access at Ellinbank, and for instructing and assisting me in chemical methods and the use of analytical equipment (LaChat FI autoanalyser). Thanks also to Joanne Coventry and Megan Burns for laboratory assistance. Maindample event volume and P concentrations were kindly supplied for use in this thesis by Brendan Christy and Terry McClean at DPI Rutherglen. For camaraderie and peer support I am most grateful to my friends Raquel, Sam, Brendan, Leanne, buddies from the Western Outstationed Postgrad Student group and the Melbourne University Volleyball Club, and to Lucy, previously of La Trobe University, for being my travel companion to my first overseas conference, for hours of stimulating conservation about P (I mean it!) and for kindly sharing P sorption data for 90 agricultural soils across Australia with me. A big thanks also to fellow postgrads in ILFR Room 201, particularly to Li Yong for managing to teach me the basics of Linux computer language!, to Karen for your counsel and enthusiasm, and to Marlos, Arshad, Rodrigo, Najib and Lilanga for sharing some of your data, humour and cultures with me. I’ve already referred to over 50 people who have helped smooth out some of the bumps during the ride of my PhD journey. There are a few people, however, without whom I simply could not have reached my final destination. To my dear friends Megan, Bettina and Cheryl and to Mum, Rachel and Dad, I cannot thank you and your partners