A new device was designed to study root biomass and distribution in the subsurface flow gravel-bed constructed wetland. The cylindrical device is made of a stainless steel mesh with a wire thickness of 1.38 mm and a mesh size of 1.80 cm. This device is 50 cm in height and 20 cm in diameter. The cylinder is installed vertically between plant rows with the top just above the gravel sediment of the wetland and filled with gravel medium as the substrate for plant root establishment. The gravel inside the device can be retrieved to observe root distribution and to measure root biomass and placed back when completed. During this study, 15 cylinders were installed in a small gravel-bed constructed wetland [8 m × 3 m × 0.7 m (L × W × D)] vegetated with 5 wetland plant species. Our results indicate that this method was effective and easy to use.
Total root biomass from two measurements during the one year study was 331.8 g m^-2, of which 174.4 g m^-2 in the medium depth of 0-5 cm, 142.1 g m^-2 in 5-15 cm, and 15.3 g m^-2 in below 15 cm. Measurements on five macrophytes (Canna indica Linn., Cyperus alternifolius subsp. flabelliformis (Rottb.) Kukenth, Hymenocallis littoralis (Jacq.) Salisb., Pennisetum purpureum Schum. and Phragmites communis Trin.) revealed significant differences in root biomass among these species, with the highest root biomass of 182.4 g m^-2 in C. indica and the lowest root biomass of 1.38 g m^-2 in H. littoralis. Root biomass of different species seemed to have different seasonal patterns. P. purpureum had larger root biomass in July, as it showed more vigorous shoot growth during the earlier part of the year, and the other species had higher root biomass in December.
Vertical distribution of root biomass differed markedly among species. H. littoralis and P. communis, which have rhizomes or thick roots, presented higher percentages of root biomass in sizes greater than 1 mm, deeper root distribution and less root biomass in the shallow layer. C. indica, C. flabelliformis and P. purpureum, which have a fibrous root system, showed greater than 80% to 100% of root biomass within 1 mm size, and shallow root distribution.