Changes in nitrogen and phosphorus concentrations in soil, soil water and surface run-off following grading of irrigation bays used for intensive grazing

Soil tests are often used to identify areas at risk of excessive phosphorus (P) exports. We investigated the changes in soil P (0-20 mm) in four recently laser-graded (<1 year) and four established (>10 years) irrigated pastures in south-eastern Australia before and after 3 years of irrigated dairy production. At the second sampling, soil water P and nitrogen (N), and P and N in surface run-off (overland flow) were also measured enabling comparison of P in surface run-off with measures of soil P. In surface soil (0-20 mm), grading reduced measures of soil P, while P sorption increased. Over 3 years, in the graded bays, Olsen P, Colwell P and P sorption decreased and water extractable P and P sorption saturation increased, while Olsen P and Colwell P decreased in the established bays. After 3 years, total dissolved P (TDP) concentrations in soil water were greater in the established bays, but dissolved reactive P (DRP) concentrations were unaffected. Organic P in soil water comprised 70 and 32% of TDP in the established and graded bays, respectively. The soil water analyses were reflected in surface run-off. After 3 years, laser grading decreased TDP, TDN, TP and TN exports in wetting front run-off by 40, 29, 41 and 36%, respectively, compared with established bays. This is an important result for the management of dairy systems as it suggests that the regular cultivation used to renovate pasture on more intensive dairy farms decreases the exports of P and N