Higher fluxes of C, N and P in plant/soil cycles associated with plant invasion in a subtropical estuarine wetland in China
Graphical abstract
Introduction
Plant invasion has been associated with differences in litter decomposition and in nutrient use, uptake and cycling between invasive and native plants (Liao et al., 2007, Liao et al., 2008a, Liao et al., 2008b; Holly et al., 2009; Martina et al., 2016; Duke et al., 2015; Zhang et al., 2016; Sardans et al., 2017). Plant invasion in nutrient-rich ecosystems has usually been associated with higher rates of litter decomposition of invasive than native species (Allison and Vitousek, 2004; Arthur et al., 2012; Hickman et al., 2013; Aragon et al., 2014). Invasion in nutrient-poor environments, however, has been related with low litter nutritional quality linked to higher nutrient resorption efficiencies and accumulation of plant biomass (Godoy et al., 2010; Mincheva et al., 2014; Sardans et al., 2017) and thus generally with a more conservative use of nutrients.
The fall and spread of litter in communities containing both invasive and native species can change the chemical and physical conditions of the soil, e.g., by changing decomposition rates, nutrient release and the plant-soil nutrient cycling (Zhang et al., 2014; Sardans et al., 2017). The decomposition rates of litter of native species can also been affected by the presence of litter of invasive species (Zhang et al., 2014).
The divergent results for the rate of litter decomposition become even more complex when litter of invasive and native species are mixed. Mitchell et al. (2011) reported an increase of litter decomposition rates of native plants when mixed with the litter of invasive plants. Interactions could occur when the litter mixture differ in various aspects of litter quality (Wardle et al., 1997). Positive interactions are likely to occur if one of the component species is relatively rich in nutrients, because nutrients can be translocated from one litter type to another by diffusion (through a water film) and/or active transport through the hyphae of fungi that could connect the two litter types (McTiernan et al., 1997).
The decomposition of aboveground and underground litter is a process widely studied for its key role in carbon cycling. Litter decomposition during plant invasion is, however, much less known, especially in wetland ecosystems, and even less in estuary wetlands submitted to the effects of periodic tides. We studied litter decomposition occurring in Chinese tidal estuarine wetlands widely invaded by invasive plant species such as Spartina alterniflora (Liu et al., 2015) and Phragmites australis (Wang et al., 2015a). Phragmite australis also spread into the China wetland areas (Wang et al., 2015a). Therefore, remains to further study of the key patterns such as plant litter decomposition occurring during plant invasion in Chinese tidal estuarine wetlands.
The changes in C and nutrient fluxes and stocks in plant/soil systems during plant invasion in wetland ecosystems are thus not clearly understood. Plant invasion is common in Chinese tidal estuarine wetlands (Wang et al., 2015a), and there are several open questions, such as the effect of alien litter fall on the rate of litter decomposition of native species. There are two hypotheses underlying these differences in litter decomposition rates of invasive and native species. The home-field advantage (HFA) hypothesis claims that decomposition occurs more rapidly when litter is placed beneath the plant species from which it had been derived than beneath different plant species (Ayres et al., 2009). This is based on the fact that the decomposer communities are adapted to the specific litter matrix of their own species. On the other hand, the substrate quality-matrix quality interaction (SMI) hypothesis implicitly assumes that in a multispecies ecosystem with a composite litter matrix, the local decomposer community should be adapted to decompose the litter of many species present in this ecosystem (Freschet et al., 2012). In all these situations the decomposer community composition influenced by resource history (Keiser et al., 2011) should reflect the average quality of the litter matrix (Santiago, 2007; Grime, 1998; Garnier et al., 2004; Strickland et al., 2009). SMI claims that low-quality substrates (e.g., Pinaceae needles with high C:N ratios) will decompose slower than high quality litter (e.g., low C:N) (Santiago, 2007; Grime, 1998; Garnier et al., 2004; Strickland et al., 2009). However, they will decompose even slightly slower in intermediate quality matrices and substantially slower in matrices composed of high-quality litters (Freschet et al., 2012).
We hypothesize that the decomposition of native plant litter and nutrient release should be faster in natural mono-specific stands than when mixed with invasive species. And second, decomposition of invasive plant litter and nutrient release should be faster in mixed and under native stands than in the corresponding mono-specific stands. These fast litter decomposition of litter species under native plants would be important in early stages of invasion. For instance when wind transports litter from native plants to invasive stands, which would accelerate nutrient release from the litter, favoring the spread of the invasive species if associated with a higher capacity of nutrient uptake and a higher use efficiency than the native species, as previously described (Sardans et al., 2017). Our aims were: (1) clarify the change of litter decomposition when the litter of native and invasive species are gradually mixed, and the pattern of gradual decomposition and nutrient release from litter as a function of the proportion of litter of native and invasive species; (2) measure the absolute rates of C and nutrient release from litter during plant invasion and their relation with potential soil influencing factors; (3) study the differences in belowground litter decomposition of native and invasive species.
Section snippets
Study area
The study was conducted in the Shanyutan wetland (26°01′46″N, 119°37′31″E; Fig. 1), the largest tidal wetland (approximately 3120 ha) in the estuary of the Minjiang River. The climate in this region is warm and wet with a mean annual temperature of 19.6 °C and a mean annual precipitation of 1346 mm (Zheng et al., 2006). The soil surface is flooded in the study site beneath 10–120 cm of water column during 3–3.5 h in each tidal event (Wang et al., 2015b) depending on the season. The daily tidal
Decomposition of aboveground litter of native and invasive species
The native C. malaccensis litter decomposed more slowly when mixed with P. australis than in its mono-specific community, with more total biomass, C and N remaining in the mixed community after decomposition (Fig. 1, Fig. 2, Tables 1 and S1). C. malaccensis litter N concentration and N:P ratio were higher when decomposed under mono-specific stands of P. australis than under its mono-specific community. Litter of the two invasive species decomposed faster in communities containing the native C.
Decomposition of litter in different plant communities
Our results indicated a slower decay of the litter of the invasive plants in invasive mono-specific stands than the litter from native plants in mono-specific stands, but the litter of the invasive plants decomposed faster when mixed with native plants than in mono-specific stands and even faster than the native litter, e.g., the percentage of aboveground litter N remaining for C. malaccensis (45.6 ± 0.71%), 75% C. malaccensis/25% P. australis (38.2 ± 2.04%), 25% C. malaccensis/75% P. australis
Conclusions
Our results provided evidence that the decomposition of litter of invasive species was slower in monoculture stands. However, the decomposition rate of aboveground litter of both invasive species under native plants or when mixed with native species litter was faster, promoting larger absolute amounts of C, N and P released from litter than the litter of native species.
Furthermore, our results also highlight the possibility of a large nutrient release from the litter of invasive species, early
CRediT authorship contribution statement
Chun Wang:Project administration, Conceptualization, Methodology, Software, Validation, Investigation, Resources, Writing - original draft, Project administration, Funding acquisition.Weiqi Wang:Project administration, Conceptualization, Methodology, Software, Validation, Investigation, Resources, Writing - original draft, Project administration, Funding acquisition.Jordi Sardans:Project administration, Conceptualization, Methodology, Software, Validation, Investigation, Resources, Writing -
Declaration of competing interest
We confirm that this manuscript has not been published elsewhere and is not under consideration by another journal. All authors have approved the manuscript and agree with submission to “Science of the Total Environment”. All authors declare do not have any conflict of interest.
Acknowledgements
The authors would like to thank Ting Pan, Siang Wan, and Hui Xu for their assistance with field sampling. Funding was provided by the National Science Foundation of China (41571287), Fujian Provincial Outstanding Young Scientists Program (2017), the European Research Council Synergy grant ERC-SyG-2013-610028 IMBALANCE-P, the Spanish Government grant CGL2013-48074-P and the Catalan Government grant SGR 2014-274.
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