Predicting the changes in environmentally and agronomically significant phosphorus forms following the cessation of phosphorus fertilizer applications to grassland |
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| Authors: | R J Dodd R W Mcdowell L M Condron |
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| Institution: | 1. AgResearch Ltd, Invermay Agricultural Centre, Private Bag 50034, Mosgiel 9053, New Zealand;2. Faculty of Agriculture and Life Sciences, Lincoln University, PO Box 84, Lincoln 7647, Christchurch, New Zealand |
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| 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 × ln OPi/PR ? 0.0455) × (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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| Keywords: | Productivity soil test phosphorus superphosphate water quality |
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