Increase of fast nutrient cycling in grassland microcosms through insect herbivory depends on plant functional composition and species diversity

Abstract

Nutrient cycling in terrestrial ecosystems is aff ected by various factors such as plant diversity and insect herbivory. While several studies suggest insect herbivory to depend on plant diversity, their interacting eff ect on nutrient cycling is unclear. In a greenhouse experiment with grassland microcosms of one to six plant species of two functional groups (grasses and legumes), we tested the infl uence of plant species richness (diversity) and functional composition on plant community biomass production, insect foliar herbivory, soil microbial biomass, and nutrient concentrations in throughfall. To manipulate herbivory, zero, three or six generalist grasshoppers ( Chorthippus parallelus ) were added to the plant communities. Increasing plant species richness increased shoot biomass and grasshopper performance, without signifi cantly aff ecting root biomass or insect herbivory. Plant functional composition aff ected all of these parameters, e.g. legume communities showed the highest shoot biomass, the lowest grasshopper performance and suff ered the least herbivory. Nutrient concentrations (dissolved mineral N, PO 4 -P, SO 4 -S) and pH in throughfall increased with herbivory. PO 4 -P and pH increases were positively aff ected by plant diversity, especially under high herbivore pressure. Plant functional composition aff ected several throughfall variables, sometimes fully explaining diversity eff ects. Increasing plant diversity tended to increase soil microbial biomass, but only under high herbivore pressure. Faeces quantities strongly correlated with changes in pH and PO 4 -P; frass may therefore be an important driver of throughfall pH and a main source of PO 4 -P released from living plants. Our results indicate that insect herbivory may signifi cantly infl uence fast nutrient cycling processes in natural communities, particularly so in managed grassland

The Stoichiometry of Nutrient Release by Terrestrial Herbivores and Its Ecosystem Consequences

“…However, in terrestrial ecosystems it is hard to isolate the effect of herbivoredriven nutrient recycling, as the "net effect" of herbivores on nutrient cycling depends not only on direct effects of nutrient release through waste products, but also on indirect effects through modification of plant litter quantity and quality, and in the case of vertebrates, by alteration of soil physical properties through trampling (Ritchie et al, 1998;Belovsky and Slade, 2000;Hunter, 2001;Bardgett and Wardle, 2003;Schrama et al, 2013). Therefore, most empirical studies addressing how terrestrial herbivores shift plant assemblages between N-or P-limitation examined the "net effect" of herbivores (e.g., Carline et al, 2005;Frank, 2008;Zhang et al, 2011;Bai et al, 2012;Nitschke et al, 2015;Sitters et al, 2017) and not on the effects of nutrient release per se.…”

Trophic and non‐trophic interactions influence the mechanisms underlying biodiversity–ecosystem functioning relationships under different abiotic conditions

“…This increase in complementarity may be driven by the influence of herbivores and detritivores on nutrient cycling, which has been found to influence complementarity effects in experimental grasslands (Craven et al ). Herbivores may accelerate nutrient cycling, changing nutrient availability for soil microbial communities and plants (Hunter , Nitschke et al ). Detritivores and herbivores not only modify the quantity of nutrients in the system, but also increase the number of plant‐available forms of nutrients by diversifying the types of inputs, e.g.…”

“…Belowground herbivores decreased plant community productivity, but the negative effect did not vary with plant species richness. Therefore, belowground herbivores did nutrient availability for soil microbial communities and plants (Hunter 2001, Nitschke et al 2014. Detritivores and herbivores not only modify the quantity of nutrients in the system, but also increase the number of plant-available forms of nutrients by diversifying the types of inputs, e.g.…”

Nutrient fluxes from insect herbivory increase during ecosystem retrogression in boreal forest

“…Herbivores play an important role in terrestrial ecosystems by creating feedbacks between plant community properties and soil processes Wardle 2003 , Hartley andJones 2004 ). However, much of our current understanding of the ecosystem effects of herbivores has been shaped by studies on large ungulates (e.g., Pastor et al 1988, Augustine and McNaughton 1998, Olofsson et al 2004 ) and on insects in grassland systems (e.g., Belovsky and Slade 2000, Ritchie 2000, Nitschke et al 2015. By comparison, the ecosystem-level effects of insect herbivores across the ~30% of the Earth ' s land surface covered by forests (Lindquist et al 2012 ) remain relatively poorly understood.…”