Elsevier

Ecological Engineering

Volume 36, Issue 10, October 2010, Pages 1307-1313
Ecological Engineering

Effects of plant diversity on biomass production and substrate nitrogen in a subsurface vertical flow constructed wetland

https://doi.org/10.1016/j.ecoleng.2010.06.007Get rights and content

Abstract

Most biodiversity experiments have been conducted in grassland ecosystems with nitrogen limitation, while little research has been conducted on relationships between plant biomass production, substrate nitrogen retention and plant diversity in wetlands with continuous nitrogen supply. We conducted a plant diversity experiment in a subsurface vertical flow constructed wetland for treating domestic wastewater in southeastern China. Plant aboveground biomass production ranged from 20 to 3121 g m−2 yr−1 across all plant communities. In general, plant biomass production was positively correlated with species richness (P = 0.001) and functional group richness (P = 0.001). Substrate nitrate concentration increased significantly with increasing plant species richness (P = 0.046), but not with functional group richness (P = 0.550). Furthermore, legumes did not affect biomass production (P = 0.255), retention of substrate nitrate (P = 0.280) and ammonium (P = 0.269). Compared to the most productive of the corresponding monocultures, transgressive overyielding of mixed plant communities did not occur in most polycultures. Because greater diversity of plant community led to higher biomass production and substrate nitrogen retention, thus we recommend that plant biodiversity should be incorporated in constructed wetlands to improve wastewater treatment efficiency.

Introduction

Constructed wetlands (CWs) have recently been widely used around the world to purify wastewater because of their ability to remove pollutants at low cost (Knight et al., 1993, Tanaka et al., 2006, Liu et al., 2009). The nutrient retention function of CWs is closely related with the uptake and immobilization of nutrients by plants, microorganisms and substrate matrixes (Stottmeister et al., 2003, Coleman, 2001, Faulwetter et al., 2009). In addition, some have studied the relationship between biodiversity and CWs’ ability to clean wastewater (Engelhardt and Ritchie, 2001, Engelhardt and Ritchie, 2002). However, research on plant diversity-ecosystem function relationship in CWs is scarce.

Most studies on the relationship between biodiversity–ecosystem function were conducted in grassland ecosystems (Hector et al., 1999, Tilman et al., 2001, Roscher et al., 2004), which often have limited soil nitrogen (N) availability (Vitousek and Howarth, 1991, Zanetti et al., 1997). Many of these studies showed that greater plant species richness resulted in higher biomass production, i.e., productivity (Balvanera et al., 2006, Cardinale et al., 2006). When studies on biodiversity–ecosystem function were carried out with fertilization, productivity–species richness relationships were found to be stronger in fertilized than in unfertilized treatments (He et al., 2002, Wacker et al., 2009). Relationships between plant diversity and inorganic soil N availability are variable (Hooper and Vitousek, 1998, Niklaus et al., 2001, Palmborg et al., 2005), and several studies found decreases of substrate nitrate pool size with increased plant species diversity or functional diversity (Niklaus et al., 2001, Scherer-Lorenzen et al., 2003, Palmborg et al., 2005). Others reported that plant functional identity had no effect on N retention in substrates (Symstad et al., 1998). Many studies found that legumes played an important role in forming positive biodiversity–productivity relationships (Lambers et al., 2004, Fargione et al., 2007, Van Ruijven and Berendse, 2009), and complementary effects between legume and non-legume species existed in many grasslands with limited N availability (Tilman et al., 1997, Mulder et al., 2002, Palmborg et al., 2005).

Work on grasslands has suggested that greater plant species richness leads to more efficient uptake of nutrients and greater productivity (Tilman et al., 1997, Symstad et al., 1998, Hector et al., 1999). Meanwhile, CWs provide important ecosystem services, such as wastewater purification, which may depend on how plant diversity influences productivity and nutrient retention (Engelhardt and Ritchie, 2001). Philip and Alexander (2000) has reported that a mixture of plant species was more efficient than monocultures in wastewater treatment in CWs, since the former provided better conditions for denitrification. The subsurface vertical flow CW (SVFCW) with unsaturated flow is known to be particularly efficient in treating many types of wastewater (Molle et al., 2006, Tietz et al., 2008, Panuvatvanich et al., 2009, Cui et al., 2009). A better understanding of plant species effects on nutrient retention will greatly help us design CWs with greater removal efficiency of pollutants from treating wastewater. Therefore, a plant diversity experiment was conducted in a full-scale SVFCW system built near Zhoushan City, Zhejiang Province in southeastern China. The main objectives of this paper were to (1) evaluate relationships between plant diversity and productivity under high N input and (2) test effects of plant diversity on substrate N retention.

Section snippets

Site description and experimental design

A 1000 m2 of SVFCW system was built in 2005 at Zhujiajian (29°53N, 122°23E), Zhoushan City, Zhejiang Province, in southeast China. The goal was to remove the high level of inorganic nutrients contained in the post-treatment domestic wastewater which had been banned by the local government for release into the sea.

The SVFCW was constructed with a 3-layer filter, and its structure (length = 50 m, width = 20 m, depth = 1.2 m) was described in detail in Zhang et al. (2010). The PVC pipes with spraying

Plant aboveground biomass production

Plant aboveground biomass production ranged from 20 to 3121 g m−2 yr−1 in all plots (Fig. 1a), and was affected by plant species richness (P = 0.001) and functional group richness (P = 0.001, Table 3 and Fig. 1a and b). The presence of C3 grasses and C4 grasses affected on biomass production (P < 0.001 and P < 0.05, respectively), but the presence of legumes (P = 0.255) or forbs (P = 0.323) did not affect biomass production (Table 3).

The three species with the highest biomass when grown in monoculture were A.

Effects of plant diversity on plant biomass production

Plant diversity had a positive relationship with plant biomass production in this study. This likely resulted from both the complementary effect, i.e., different plant species may being able to better take advantage of the available resources (Tilman et al., 2001, Loreau, 1998, Fornara and Tilman, 2008), and the selection effect, i.e., more diverse communities had a greater chance of containing a very productive species (Aarssen, 1997, Huston, 1997), such as A. donax and S. arundinaceum in this

Conclusions

This biodiversity study in the SVFCW showed that increasing plant species richness increased plant biomass production and nitrate retention in the substrate and increased functional group richness increased plant biomass production but did not affect nitrate concentration in the substrate. Only a small number of the mixed species treatments produced a transgressive overyielding in our studied CW. Our system produced much higher plant biomass, especially the upper limit of plant biomass

Acknowledgements

We are grateful for the funding provided by the National Science Foundation of China through grant number 30870235. SXC was supported by a Discovery Grant from the Natural Science and Engineering Research Council of Canada (NSERC). We also thank Z.Y. Chu, H. Wang and J.M. Zhang for their help in the field work.

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