Long-term monitoring of buffer zone efficiency under different cultivation techniques in boreal conditions

Vegetative buffer zones (BZs) between fields and watercourses are one way to minimize phosphorus (P) losses from fields to surface runoff and thus decrease eutrophication and blue-green algal blooms in lakes and coastal waters. A 6-plot experimental field was established in clay soil (Vertic Cambisol) in SW Finland in 1991 to study long-term changes in functioning of BZs and their retention capacity for total solids (TS), total P (TP), dissolved reactive P (DRP) and particulate P (PP) in different seasons. The steep slope in lower sections of four plots (18m wide×70m long) was planted with 10-m-wide mowed grass buffer zones (GBZ) or unmowed vegetated buffer zones (VBZ) growing natural herbage and shrubs. Surface runoff water samples from the GBZ and VBZ plots were compared to samples from plots cultivated without a buffer (NBZ). The source field area in all plots and the steep slope (12-18%) on the NBZ were ploughed in autumn, and sown with barley (Hordeum vulgare) or oats (Avena sativa) in spring (conventional tillage, 1991-2001), sown with grass and grazed (72-234 cow grazing days ha⁻¹ yr⁻¹; 2003-2005) and direct drilled without tillage (2006-2008). Surface soil samples (0-2cm) were taken from the BZs in autumn and spring to evaluate the level of plant-available P (P Ac) extracted with 0.5M NH₄-acetate-0.5M acetic acid at pH 4.65. The BZs were most effective at decreasing TS, TP and PP with conventional tillage, less so with direct drilling and least effective with grazing. In a conventionally tilled field, the TS and TP removal efficiencies were over 50% and 27-36%, respectively, for the BZs as compared to the NBZ. In the VBZ plots, the DRP load was, however, 60% greater than in the NBZ or GBZ plots. In direct drilling, the surface runoff losses were smaller than in conventionally tilled NBZ plots. The lowest losses of TS, TP and PP were found during grazing for all plots, but with grazing the DRP load, 0.3-0.4kgha⁻¹ yr⁻¹, was higher than during grain growing in all treatments. The GBZ and VBZ were effective in retaining P in summer and autumn, whereas in spring their retention capacity was decreased. The reason for high DRP losses in spring was the high P Ac in surface soil and frozen broken plant tissues in the VBZ and the grazed source field. Mowing and removing of swathe from the GBZ decreased the DRP losses.