Phosphorus and nitrogen cycles in the vegetation of differently managed buffer zones

Vegetated buffer zones (BZs) between a cultivated field and a watercourse reduce erosion and load of particle-bound phosphorus (P), but decay of abundant vegetation increases the potential of BZs to act as a source of readily algal-available P. To quantify temporal variations in P and nitrogen (N) contents of the grassy vegetation of BZs on a clay soil (Vertic Cambisol) in south-western Finland, plant samples were collected six times between May 2005 and April 2006 from natural BZs, BZs grazed by cattle and BZs harvested by cutting and removal of the yield. The total dry weight biomass peaked in early August at 2,130-2,360 and 5,500-6,270 kg ha⁻¹ for the grazed and the other BZs, respectively. In August, 3,840-4,830 kg ha⁻¹ were removed from the harvested BZs while the entire biomass of the non-harvested BZs remained in the field. In October, total P and N contents varied from 2.4-10.2 to 19-72 kg ha⁻¹, respectively, the lowest amounts being for the young harvested BZ and the highest for the non-harvested BZs. A considerable decrease of P and N contents occurred in the biomass up to 6.1 and almost 30 kg ha⁻¹, respectively, after the first frosts. Harvesting of BZs is recommended to decrease the amount of P and N in the BZs and reduce the risk of P and N leaching outside the growing season.