116
Fig. 1. Experimental set-up. Polycultures (AB, BA) and monocultures (AA, BB). (Drawing V.
Gagnon)
Physical-chemical analyses.
Treatment performances were monitored in eight sampling
periods, three times in summer (Jul.-Aug. 2012, Jul. 2013), twice in autumn (Oct.-Nov.
2012) and spring (Apr.-May 2013) and once in winter (Mar.). Inlet and outflow samples of
both mesocosms of the experimental units were collected and the following parameters
were measured: TSS, TN, TP, PO
4
-P, S
2-
. Removal efficiencies were calculated based on
mass balance.
Plant features.
Aboveground biomass dry weight, stem length and density were measured
in Nov. 2012. Root development was monitored by 360º belowground images captured
with a root scanner (CID-600) in transparent acrylic tubes buried in the mesocosms.
Results and discussion
P. australis
and
P. arundinacea
showed different above and belowground biomass
distributions.
P. australis
had lower stem density (586m
-2
), but taller stems (2.4m),
compared to
P. arundinacea
with very high density (1600m
-2
) of thin and shorter stems
(1.7m).
However, average dry biomass was similar for both species (2.2 kg·m
-2
).
Belowground pictures showed differences between the root system of both species and their
evolution through the seasons.
P. arundinacea
root systems were characterized by a very
high density of fine roots, particularly in the top layer of the mesocosms. In contrast, deep
rhizomes and a low density of fine roots characterized
P. australis
. Moreover, in early
spring 2012, mesocosms planted with
P. arundinacea
developed a black precipitate at the
bottom layer, typical of sulfide production in anaerobic environments. In winter, the middle
layers turned gray, at the same as time sulfide concentrations measured at the outflow were
found to decrease.
P. australis
mesocosms, on the other hand, did not develop a black layer
during the experiment, and sulfide concentrations were lower. These results suggest that
sulfate reducing bacteria may develop differently and be influenced by differences in root
exudates and oxygen levels in the root system between the two plant species.
In terms of pollutant removal efficiency, both species showed very good performances, and
no notable differences were found for any of the parameters tested. In general,
monocultures (AA, BB) versus polycultures (AB, BA) were also comparable. For example,
TSS and COD removal were excellent (85-95%) for all plant species combinations tested,
during the eight sampling periods. Ammonium removal was more variable, both between
