The role of legumes as a component of biodiversity in a cross‐European study of grassland biomass nitrogen

Abstract

To investigate how plant diversity loss affects nitrogen accumulation in above‐ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above‐ground biomass was quite invariable at 1.66±0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above‐ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non‐legume species taking it up. Nitrogen‐fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N2 fixation. Therefore, loss of an efficient N2‐fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.

Novel forests maintain ecosystem processes after the decline of native tree species

“…On Hawai'i, for example, introduced grass species may result in the transformation of a forest to a grassland, with implications for ecosystem function that have little to do with diversity (Hughes et al 1991, D'Antonio and Vitousek 1992, Litton et al 2006. By contrast, in this comparative study, as in several experimental tests (e.g., Tilman et al 2001, Spehn et al 2002, high N limitation created an environment in which the addition of N 2 -fixing species had an enhanced effect on ecosystem function. Given the complexities of diversity change (Wardle et al 2011), future monitoring will be essential to determining how diversity changes lead to functional outcomes.…”

“…In experimental work, the functional outcomes of diversity shifts are influenced not only by the richness and evenness of species, but also by the relative changes in plant functional traits (Hooper and Vitousek 1997, Lavorel and Garnier 2002, Spehn et al 2002. Introduced species can alter the biogeochemistry of ecosystems in a similar way (Ehrenfeld 2003), particularly when they possess plant functional traits not represented in the native flora (Versfeld andvan Wilgen 1986, Vitousek et al 1987).…”

Plant diversity increases soil microbial activity and soil carbon storage

“…This was partly unexpected as legumes have been reported to increase the availability of nitrogen in soils 35,36 and are therefore suggested to increase plant biomass production (above and below ground) 24 , resulting in increased metabolic activity of soil microorganisms 37 . Figure 2 | Conceptual model on the expected causal relationships between plant diversity and soil carbon storage.…”

Implementing large-scale and long-term functional biodiversity research: The Biodiversity Exploratories

“…To date, a number of model experiments have addressed the functional consequences of changes of plant species composition and diversity (Hector et al 1999;Tilman et al 2001;Roscher et al 2005). While overall effects of plant species composition, such as presence or absence of legumes or grasses, on biogeochemical cycles and other ecosystem processes are widely acknowledged (Spehn et al 2002), very little is known about the role of individual species. It has been shown repeatedly that ecosystem processes and services depend not only on the species composition of ecosystems but also, and mostly positively, on their species richness (e.g., Naeem, Thompson, Lawler, Lawton, & Woodfin 1994;Hooper et al 2005;Balvanera et al 2006).…”