Growth and nutrient dynamics of soft-stem bulrush in constructed wetlands treating nutrient-rich wastewaters

The growth characteristics and nutritional status of Schoenoplectus tabernaemontani (C.C. Gmelin) Palla (softstem bulrush or lake clubrush) were investigated during the second and third growth seasons in four equivalent subsurface-flow, gravel-bed constructed treatment wetlands. Each wetland was supplied with a different hydraulic loading rate of agricultural wastewater, covering the range commonly applied to such systems. Harvest and demographic techniques were combined to determine seasonal patterns and gradients of growth and nutrient allocation, and net annual primary productivity (NAPP). Marked seasonal patterns of early spring emergence, summer growth and autumn senescence were observed, with little over-wintering of live above-ground biomass. Starch, the dominant long-term storage substance, comprised ∼20% of rhizome dry weight (DW) in autumn. Mobilization during spring reduced concentrations by around half, with a trend of increasing depletion in the higher loaded wetlands. NAPP, including above-ground mortality, during the second growth season ranged between ∼2.5 and 3.5 kg DW m -2, with 10-23% allocated to below-ground growth. Mean above-ground live and dead biomass ranged between ∼1.75 and 2.65 kg DW m -2 by mid-summer, with below to above-ground biomass ratios similar in all wetlands at between 0.6 and 0.7. Rhizomes, which comprised around 80% of the below-ground biomass, were generally restricted to the upper 10 cm of the substratum and over half the root biomass also occurred in this zone, with very few roots penetrating below 30 cm depth. High culm concentrations of N, P, Mg and Zn in spring declined markedly over the growth season, while S and Ca showed general increases, and K, Fe and Cu remained relatively stable. Gradients of decreasing tissue concentration of most macronutrients were noted with increasing distance from wastewater inflows. Plant accumulation of N rose by 20-35 g m -2 and P by 4-9 g m -2 with seasonal regrowth of above-ground shoots. Net plant N and P uptake rates rose to maximum values of 0.3 g N m -2 d -1 and 0.1 g P m -2 d -1 in early summer, declining markedly during late summer and autumn. Mass balance assessments of N and P accumulation in plants at near maximum seasonal biomass, after three growth seasons, showed that only 6 to 11% of the N removal and 6 to 13% of the P removal recorded from wastewaters applied to the wetlands could be ascribed to plant uptake and accumulation.