A 10‐year study of phosphorus balances and the impact of grazed grassland on total P redistribution within the soil profile |
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Authors: | C J Watson D I Matthews |
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Institution: | aAgriculture, Food and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK, and bApplied Plant Science and Biometrics Division Branch, Agri‐Food and Biosciences Institute, Newforge Lane, Belfast BT9 5PX, UK |
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Abstract: | Phosphorus (P) inputs (wet deposition and fertilizer P) and outputs (animal product and drainflow) were studied on reseeded grazed grassland swards receiving different nitrogen (N) inputs (100–500 kg N ha?1 year?1) for 10 years (March 1989–February 1999), at an experimental site in Northern Ireland. All plots received the same maintenance application of P fertilizer (8.5 kg P ha?1 year?1) to meet grass requirements, to minimize the P surplus and to quantify the impact on P losses to land drainage water. The annual flow weighted mean total P concentrations in drainflow ranged from 187 to 273 μg P litre?1 and were well above the concentrations believed to trigger eutrophication. Annual total P lost to drainage water ranged from 0.28 to 1.73 kg P ha?1, but was unaffected by N input. As the average annual P balance was zero, there was no significant change in total P in the top 15 cm of soil. However, there was a highly significant redistribution of P to the soil surface from the 10–15 cm depth, possibly as a result of root acquisition and earthworm activity. Total P in the top 5 cm of soil increased from 0.85 g kg?1 to 1.04 g kg?1, over the 10 years of the study, despite there being no net P input. This P accumulation in the top few cm of soil is likely to exacerbate P losses in overland flow and make improvements in water quality difficult to achieve. |
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