Effects of plant species on macrophyte decomposition under three nutrient conditions in a eutrophic shallow lake, North China
Highlights
► Macrophyte decomposition rates are species and site specific. ► Plant species is the main determinant of detritus mass loss. ► Influences of initial detritus quality indicators on detritus mass loss increase with time. ► The effect of site conditions on detritus mass loss vary with plant species.
Introduction
Macrophyte decomposition is an essential process in carbon (C) storage and nutrient cycling for eutrophic shallow lakes (Carpenter, 1980, Debusk and Reddy, 2005). Enhanced primary production due to enriched environments and increased organic material accumulation because of the incomplete decomposition can reduce lake water area and raise water level, accelerating lake terrestrialization (Partanen and Luoto, 2006, Papastergiadou et al., 2007, Liao et al., 2008). Therefore, improved knowledge of characteristics of decomposition for different plant species is needed to better understand ecological processes in eutrophic shallow lakes.
Detritus quality, defined as chemical characteristics of decomposition substrate, is considered as the main determinant of detritus decomposition (Nziguheba et al., 1998, Villar et al., 2001, Kim and Rejmánková, 2005, Chimney and Pietro, 2006, Shilla et al., 2006). Plant species vary widely in detritus nutrient contents and differ greatly in the decomposition rates (Hoorens et al., 2003, Alemanno et al., 2007). High decomposition rate species are generally characterized by high contents of initial nitrogen (N) and phosphorus (P), and low values of initial C:N, C:P, and N:P ratios. Concentrations of detritus nutrients significantly impact early leaching and subsequent chemical decomposition stages (Aerts and De Caluwe, 1997, Berg and McClaugherty, 2008). Recently, many studies have suggested that lignin:N ratio, an indicator of carbon quality, was significantly related to decomposition rate (De Neiff et al., 2006). However, the C:N ratio performs better than the lignin:N ratio in describing decomposition rate when plant detritus contains a broad range of lignin content (Taylor et al., 1989).
Aquatic environmental conditions also influence detritus decomposition. In aquatic ecosystems, temperature, pH, and nutrient availability in water columns are considered to be important indexes that control detritus decomposition (Royer and Minshall, 2001, Rejmánková and Houdková, 2006, Cunha-Santino and Bianchini, 2008). It is recognized that higher nutrient availability in the environment generally supports faster decomposition rates (Newman et al., 2001, Gulis and Suberkropp, 2003, Gulis et al., 2006, Breeuwer et al., 2008). Most studies assumed that the effects of environmental condition on decomposition are identical for all types of species (Hobbie, 2000, Hobbie and Gough, 2004, Breeuwer et al., 2008, Austin et al., 2009, Lang et al., 2009). However, the significant interaction between species and site in these studies suggest that site quality effects on decomposition may vary with the plant species.
In eutrophic lake ecosystems, the enriched nutrient content can create a favorable habitat for aquatic macrophyte growth. As a result, macrophytes usually spread rapidly in this kind of ecosystem. Macrophyte decompositions are important for the development of lakes. Although plant detritus from enriched environments is often characterized with high nutrient contents and high decomposition rates (Aerts and De Caluwe, 1997, Villar et al., 2001, Debusk and Reddy, 2005), a substantial part of dead plant tissues can accumulate in lake sediment due to incomplete decomposition and contribute to organic material accumulation (Chimney and Pietro, 2006). Lake sediment layers are raised and further stimulate the overgrowth of macrophytes. Because the increased macrophytes and reduced lake area can greatly damage animal habitats and accelerate lake succession, detritus decomposition is important for many ecological processes of nutrient-enriched lake ecosystems. Meanwhile, since nutrient conditions in eutrophic lakes can vary spatially due to various pollution sources, detritus decomposition often demonstrates spatial heterogeneity and further complicated the decomposition studies in this kind of lakes. To better understand how detritus decomposition is controlled by internal and external factors, the effects of detritus and site on decomposition were analyzed and studied through simulated modeling.
Recently, there are numerous methods to simulate and predict decomposition processes (Moorhead et al., 1996, Cunha-Santino and Bianchini, 2008). Ecological models have been used to describe decomposition process. Some of these models are processed based and can simulate the underlying processes in detritus decay (Palosuo et al., 2005, Zhang et al., 2008). Some other models, like statistical and mathematical models, are more concerned with system behavior at a specific level of interest, such as nutrient cycles at the system level, microbial growth at the population level (Moorhead et al., 1996). The most common model is a negative, exponential model proposed by Olson (1963) to describe mass loss from detritus. Because effects of site and detritus may be enhances in a nutrient-enriched lake due to spatial heterogeneity in nutrient distribution, a more detailed model considering both site and species effects on decomposition is expected. Here, we developed a two-stage regression model based on the field experiment to identify contributions of both detritus and site factors to detritus decomposition.
Our study area is Lake Baiyangdian which is a shallow eutrophic lake in North China. Due to elevated water nutrients, high primary production has been observed in past decades in the lake. The aims of this study are (1) to compare decomposition processes of different dominant species under three nutrient conditions; (2) to test the role of plant species on decomposition processes and determine main detritus quality factors contributing to decompositions; (3) to demonstrate the effects of environmental conditions on decomposition and its interaction with plant species; and (4) to develop a mathematic mode to analyze species and site effects on detritus mass loss.
Section snippets
Site description
Lake Baiyangdian is the largest freshwater lake in North China (38°21′N, 115°54′E, see Appendix A in online supplement). Climate in this area is a temperate continental monsoon climate with an annual mean precipitation of 510 mm. The lake has a surface area of 366 km2 with high nutrient contents, high macrophyte density, and low water depth. Until the middle of the 20th century, the lake had been seldom affected by human activities, and the average water depth was 2.5 m. Since the 1950s,
Initial detritus quality characteristics and site chemical conditions
Wide ranges in initial detritus nutrient contents were observed of the six macrophytes (Table 1). C. demersum exhibited the highest detritus quality with the highest contents of N and P as well as the lowest ratios of C:N, C:P, and N:P. The nutrient contents in P. australis and T. angustifolia were relatively low compared with other species. The N content in T. angustifolia was about one half of mean N content of all species, and the P content in P. australis was about one fifth of the mean P
Decomposition rates of dominant macrophytes
The values of decomposition rates (k) in this study varied with a range between 0.0019 and 0.0072 day−1, and the values differed with species. The six macrophytes can be grouped into three broad categories, i.e., submerged (C. demersum and P. pectinatus), floating (L. minor and N.nucifera), and emergent (P. australis and T. angustifolia) species. Similar to studies by Battle and Mihuc (2000), C. demersum was an easily decomposed submerged species, whose decomposition rates were between 0.0054
Conclusions
In this study, the effects of plant species on macrophyte decomposition were studied by combining field experiments with a mathematic decomposition model in Lake Baiyangdian. The control of plant species on decomposition processes was stronger than site, although both effects were significant. Decomposition rates were species specific. In this study, emergent plants showed the lowest decomposition rates compared to floating and submerged plants. P-related detritus quality indicators were well
Acknowledgements
The authors would like to thank Karianne L., Janice D., and Rebecca J. for constructive comments and early revision. This study has been financially supported by Major Science and Technology Program for Water Pollution Control and Treatment (2009ZX07209-008); and China National Funds for Distinguished Young Scientists (51125035).
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