Item

Predicting the changes in environmentally and agronomically significant phosphorus forms following the cessation of phosphorus fertilizer applications to grassland

Dodd, Rosalind
McDowell, Richard
Condron, LM
Date
2012-06
Type
Journal Article
Fields of Research
ANZSRC::0503 Soil Sciences , ANZSRC::05 Environmental Sciences , ANZSRC::0703 Crop and Pasture Production , ANZSRC::3004 Crop and pasture production , ANZSRC::4106 Soil sciences
Abstract
Phosphorus (P) loss from soil can impair surface water quality. Losses from soil are related to soil P concentrations, but agronomic measures such as Olsen P do not in many cases predict the potential for P loss. One possible strategy to decrease P loss is to stop applying P fertilizers. We examined the changes in both agronomic (Olsen P) and environmental [water-extractable P (WEP) and calcium chloride-extractable P (CaCl₂-P)] P tests, and the potential implications following a halt to P fertilizer application to four long-term grassland field trials on different soil types. Exponential decreases in Olsen P and WEP concentration over time were observed in three of the four trials, but only in one trial for CaCl₂-P. The rates of decrease in Olsen P (OP) and WEP concentration were best correlated with initial WEP (WEPi) concentration and the quotient of Olsen Pi⁄P retention (PR, a measure of Al- and Fe-oxides), respectively. The equation t = 1⁄(–0.035 x ln OPi⁄PR – 0.0455) x (ln WEPt – ln WEPi) was used to predict the time (t) taken for WEP concentrations at the sites to decrease to 0.02 mg⁄L (WEPt), which is proposed as a limit for dissolved reactive phosphorus in overland flow, and the result was 23–44 yr. Results from a similar equation for Olsen P predicted a quicker rate of WEP. A significant decline in dry matter (DM) yield was observed at one trial site. For this site, the rate of decline in DM yield was of a similar magnitude to the rate of decline in WEP concentration. This suggests that halting P fertilizer application to decrease P loss as measured by WEP concentration may decrease farm productivity. An alternative, more financially acceptable, strategy is required, such as a negative P balance while maintaining yields with N fertilizer, but further work is required to assess both the agronomic and environmental implications of this strategy.
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© 2012 The Authors. Journal compilation © 2012 British Society of Soil Science.
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