Increasing plant species richness by seeding has marginal effects on ecosystem functioning in agricultural grasslands

Abstract

Experimental evidence shows that grassland plant diversity enhances ecosystem functioning. Yet, the transfer of results from controlled biodiversity experiments to naturally assembled ‘real world’ ecosystems remains challenging due to environmental variation among sites, confounding biodiversity ecosystem functioning relations in observational studies. To bridge the gap between classical biodiversity‐ecosystem functioning experiments and observational studies of naturally assembled and managed ecosystems, we created regionally replicated, within‐site gradients of species richness by seeding across agricultural grasslands differing in land‐use intensity (LUI) and abiotic site conditions. Within each of 73 grassland sites, we established a full‐factorial experiment with high‐diversity seeding and topsoil disturbance and measured 12 ecosystem functions related to productivity, and carbon and nutrient cycling after 4 years. We then analysed the effects of plant diversity (seeded richness as well as realized richness), functional community composition, land use and abiotic conditions on the ecosystem functions within (local scale) as well as among grassland sites (landscape scale). Despite the successful creation of a within‐site gradient in plant diversity (average increase in species richness in seeding treatments by 10%–35%), we found that only one to two of the 12 ecosystem functions responded to realized species richness, resulting in more closed nitrogen cycles in more diverse plant communities. Similar results were found when analysing the effect of the seeding treatment instead of realized species richness. Among sites, ecosystem functioning was mostly driven by environmental conditions and LUI. Also here, the only functions related to plant species richness were those associated with a more closed nitrogen cycle under increased diversity. The minor effects of species enrichment we found suggest that the functionally‐relevant niche space is largely saturated in naturally assembled grasslands, and that competitive, high‐functioning species are already present. Synthesis: While nature conservation and cultural ecosystem services can certainly benefit from plant species enrichment, our study indicates that restoration of plant diversity in naturally assembled communities may deliver only relatively weak increases in ecosystem functioning, such as a more closed nitrogen cycle, within the extensively to moderate intensively managed agricultural grasslands of our study.

The importance of multi‐species grassland leys to enhance ecosystem services in crop rotations

“…This results from a combination of the yield reduction, which indicates that each added species increases the chance of including a species with lower yields than previously included species (Roscher et al, 2005), and the saturation curve that indicates that similarities in occupied ecological niches between species increase and reduce the yield gain from each additional species (Lüscher et al, 2022; Tilman et al, 1997). The saturation of ecological niches occurs with comparably few species in productive grasslands, compared to semi‐natural grasslands (Freitag et al, 2023).…”

Relationships between species richness and biomass production are context dependent in grasslands differing in land-use and seed addition

“…We identify four possible explanations. First, BEF-relationships are often asymptotic in grassland experiments 50 , with biomass gains levelling off when richness levels exceed approximately 10 species, possibly due to a saturation in resource use 8,51 . Only 8.7% of our managed grassland plots had less than 10 species, making them more diverse than most grassland communities in biodiversity experiments 52 .…”

“…2). This was done by scarifying the reduced land-use + seed addition plot, whereby scarifying is a minor manual mechanical disturbance of the top soil surface down to 5 cm using lawn scarifier to increase seedling establishment by promoting seed-soil contact as well as by reducing light competition 37,48,49,51 , and then sowing new species (twice: in autumn 2019 and spring 2020, ranging from 40 to 74 species depending on deviance of the species pool at the site to the regional species pool). In the Schorfheide-Chorin region, only species originally absent in the plot (but present in the region) were sown, while in Hainich-Dün and the Schwäbische Alb, in addition to new species, unintentionally also species already present at the site were sown.…”

A set of ecosystem service indicators for European grasslands based on botanical surveys

“…Some of the previous studies on plant diversity and productivity (biodiversity experiments) showed that species-rich grassland communities are associated with high-level delivery of aboveground biomass (Hector et al, 1999). However, these relationships are less established and still under-debated under more realistic grassland management conditions (Freitag et al, 2023). More recent studies have focused on the diversity of the functional traits of different plant species within the grassland plant community (de Bello et al, 2010;Gross et al, 2017).…”