Retention of phosphorus in soil and vegetation of a buffer zone area during snowmelt peak flow in southern Finland

In regions with a distinct winter-period, nearly half of the annual runoff occurs during a short snow-melt period early in spring. During this time functioning buffer zones are important means of preventing the leaching of phosphorus from forest land into downstream watercourses. Sparse vegetation restricts biological P accumulation and high flows reduce the capacity of deeper soil layers to adsorb P, indicating that the effectiveness of buffer zones may be low. Our aim was to increase the understanding of phosphorus retention in a buffer zone area under such unfavourable conditions for P removal, and to estimate the amount of P sorbed by soil and taken up by vascular plants and mosses. Over a five-day period in spring we added 10kg (60kg ha^-1) of PO₄-P and 185MBq (1100MBq ha^-1) ³²P to a 25-50m wide buffer zone area (0.17ha) in southern Finland. We measured the total P retention and recovery during ten days after the beginning of the experiment. Recovery of ³²P was 16% of added P, of which 90% was in soil, 3% in vascular plants and 5% in mosses. Thus, our results showed that the total P retention was low, most likely because the water flow did not slow down sufficiently and penetrate deeply enough to enable a close contact between P in the runoff and the soil matrix. Most of the recovered P was in the soil suggesting that adsorption by soil was the most important sink for P under early spring high flow conditions.