Nitrogen and phosphorus variation in shallow groundwater and assimilation in plants in complex riparian buffer zones

The study of purification efficiency and nutrient assimilation in plants was made in two riparian buffer zones with a complex of wet meadow and grey alder (Alnus incana) stand. In the less polluted Porijõgi test site, the 31 m wide buffer zone removed 40% of total nitrogen (total-N) and 78% of total phosphorus (total-P), while a heavily polluted 51 m wide buffer zone in Viiratsi retained 85% of total-N and 84% of total-P. The input of nutrients and purification efficiency displayed a significant relationship. The total-N removal in buffer zone was negative when the input value was less than 0.3 mg l^-1 and the purification efficiency was always positive when the input value exceeded 5 mg l^-1. The purification efficiency of total-P was positive when the input value exceeded 0.15 mg l^-1. Grass vegetation plays an important role in nutrient retention in riparian buffer strips. The maximum phytomass production was measured in Porijõgi site where production of the Filipendula ulmaria community was up to 2,358 g m^-2, assimilation of N 32.1 and of P 4.9 g m^-2, respectively. This is much higher than the biomass production and N and P uptake of the grey alders (Alnus incana) at the same site - 1,730, 20.5 and 1.5 g m^-2, respectively.