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Split phosphorus fertiliser applications as a strategy to reduce incidental phosphorus losses in surface runoff

González Jiménez, J.L., Daly, K., Roberts, W.M., Healy, M.G.
Journal of environmental management 2019 v.242 pp. 114-120
climate, commercial farms, fertilizer application, organic soils, overland flow, phosphorus fertilizers, rain, rainfall simulation, reactive phosphorus, risk, risk reduction, runoff, sorption, split application, superphosphate
Organic soils have low sorption capacities for phosphorus (P), and may pose a risk of P loss to water if P applications to these soils coincide with runoff events. Little is known about the magnitude of exports of P in overland flow following application of P fertiliser onto these soils, or on the influence of the frequency on P losses and persistence. The number of P fertiliser applications was surveyed across 39 commercial farms to assess current practice and inform the design of a rainfall runoff experiment to evaluate the effect of frequency of P applications on losses and persistence across time. Superphosphate (16% P) was applied in single (equivalent to 30 and 55 kg P ha−1 applied at day 0) and split (equivalent to 15 and 27.5 kg P ha−1 applied in two doses at days 0 and 55) applications to an organic soil inclined at a slope of 6% in a rainfall simulator experiment. The surface runoff of dissolved reactive phosphorus (DRP) was measured in controlled 30-min rainfall simulations conducted intermittently over an 85-day period. The DRP losses in surface runoff after the first rainfall event were 44.6 and 97.8 mg L−1 for single applications of 30 and 55 kg ha−1, respectively, and 13.3 and 21.8 mg L−1 for the same rates split in two doses, indicating that single P applications had disproportionately bigger impacts on losses than split applications. This supports the idea that frequent, but smaller, P applications can minimise the impact of fertilisation on waters. Dissolved reactive P concentrations remained significantly higher than those from the control samples until the end the experiment for almost all the P treatments, highlighting the long-lasting effects of added P and the elevated risk of P losses on organic soils. For climates with frequent rainfall events, which are likely to coincide with fertiliser applications, smaller but more frequent P applications can reduce the risk of P transfer as opposed to one single application.