Abstract
Considering the emergence of ecosystems dominated by invasive species, there is growing interest in estimating the effect of biological invasions on ecosystem processes and provision of services. African grasses are the most invasive plants in the Cerrado (Brazilian savanna), but their impact on C and N stocks is poorly known. We compared patterns of C and N stocks in the aboveground biomass, root biomass and soil in open Cerrado (campo sujo) sites, both uninvaded and invaded by the African grass Urochloa decumbens. In both sites we estimated the aboveground biomass of U. decumbens and native grasses, as well as the root biomass up to 50 cm. We obtained C and N soil content and soil bulk density up to 1 m depth, which were used to obtain the overall soil C and N stocks; we also assessed the variation in soil δ13C and δ15N. Although invasion did not affect the aboveground biomass, it did affect belowground biomass, leading to more C stocked in fine roots and soil, higher N concentration close to soil surface, and also more C along the soil profile. C and N soil stocks, soil δ13C and δ15N values did not significantly differ between invaded and uninvaded site. Even a relatively low level of invasion by U. decumbens changed the root distribution pattern and increased C and N contents in the upper soil, which may promote ecosystem changes by altering nutrient dynamics. Although our study was conducted at a single site, it clearly shows effects of the invasive species on the belowground environment, and indicates the necessity of further studies.
Highlights
Invasive exotic species may cause a wide variety of impacts on invaded environments, which often lead to biodiversity loss and changes in ecosystem processes (Simberloff et al 2013; Downey and Richardson 2016)
The invasive grass U. decumbens accounted for approximately 24% of the ground plant cover and 25% of the aboveground C stock in the invaded site, the total C stock in the aboveground biomass did not differ between the invaded and the uninvaded site (t = 0.36, p = 0.727; Fig. 1a); as expected, the C stock of native species was higher in the uninvaded site (t = 2.39, p = 0.033, Fig. 1b)
The effects caused by biological invasions in the Cerrado C and N stocks have not been reported to this moment, and this is a vital issue especially in times of global warming and changes in the natural ecological cycles.We compared C stocks aboveground, and C and N stocks belowground along the soil profile between an uninvaded open cerrado and a site invaded by the African grass U. decumbens
Summary
Invasive exotic species may cause a wide variety of impacts on invaded environments, which often lead to biodiversity loss and changes in ecosystem processes (Simberloff et al 2013; Downey and Richardson 2016). Invasive plant species often show high photosynthetic and growth rates, and many of them make associations with N-fixing organisms (Ehrenfeld 2003, 2004; Simberloff et al 2013; Vitousek and Walker 2014). As a result, they may change both above and belowground nutrient stocks – especially carbon and nitrogen – and affect biogeochemical cycles and balances, as well as other ecosystem processes and services (Ehrenfeld 2003; Vilà et al 2011). Grasses of the genus Urochloa (brachiarias) have shown great capacity to invade and rapidly dominate the herbaceous layer of open Neotropical savannas (Damasceno et al 2018) thanks to their high efficiency in producing biomass (Williams and Baruch 2000), favoured by effective N acquisition associated with symbiotic organisms (Boddey and Victoria 1986; Okumura et al 2013), as well as an effective seed regeneration strategy (Xavier et al 2021) and the release of allelopathic substances to inhibit other species germination (Gorgone-Barbosa et al 2008)
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