115
Plant diversity effect on horizontal subsurface-flow constructed wetlands (HSSF-
CWs): Are two species better than one?
M. Rodríguez, J. Brisson
Institut de Recherche en Biologie Végétale, Université de Montréal. 4101 East, Sherbrooke St,
Montreal (Quebec), H1X 2B2 CANADA.
Introduction
Under the context of constructed wetlands, it has been hypothesized that plant polycultures
could be more efficient than monocultures, by means of functional complementarity (Zhang
et al. 2010). Assuming that a monoculture of species A is better than species B for pollutant
removal (A>B), how will the combination of the two species AB perform? Will AB
perform as well as the best monoculture (AB=A>B)? Or will it outcompete the
monocultures (AB>A>B), supporting the hypothesis that plant diversity improves pollutant
removal?
Phragmites australis
and
Phalaris arundinacea
are commonly used species for wastewater
treatment with complementary traits (root system arrangement and seasonality) (Vymazal
et al. 2005). Based on the differences between the two species, we evaluated if the
combination of plant species influences the efficiency of pollutant removal in HSSF-CWs.
The aim of our study is to answer the question: Are two species better than one?
Methods
Experimental set-up:
Sixteen mesocosm scale experimental constructed wetland units
were set-up in a controlled greenhouse environment and evaluated for 12 months (Jul.
2012-Jul. 2013). Each experimental unit consisted of two coupled mesocosms (L 70cm W
51cm H 36cm), respectively planted according to each of the following four treatments:
monocultures of
P. australis
(AA) and
P. arundinacea
(BB) and the combination of the
two plant species,
P. australis
followed by
P. arundinacea
(AB), as well as
P. arundinacea
followed by
P. australis
(BA) (Fig.1). Experimental units were divided into two separate
mesocosms to avoid one species dominating the polycultures. Each treatment was
replicated four times following a randomized block design. The mesocosms were filled
with granitic river gravel (10-15 mm diameter). Following a period of plant establishment,
the experimental units were fed from Apr. 2012 with 15 L d
1
of reconstituted wastewater
from diluted fish farm sludge, at a hydraulic loading rate of 42 L·m
−2
·d
−1
. Average influent
concentration (in g·m
−2
·d
−1
) was TSS 8.2; COD 14.3; PO
4
-P 0.6; NH
4
-N 1.1. The
temperature of the greenhouse ranged from 35ºC in summer to 5ºC in winter.
