Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands.

Judith Sitters ,Jonathan D Bakker ,Anita C Risch ,Yann Hautier ,Lori A Biederman ,E.r.j Wubs ,Sumanta Bagchi ,Elizabeth T Borer ,Thomas W Crowther ,Eric W Seabloom ,Nicole Hagenah ,Martin Schütz ,Carly J Stevens ,Julia Siebert ,Brent Mortensen ,Elsa E Cleland ,Pablo Luís Peri ,Jennifer Firn ,Johannes M H Knops ,Elisabeth S Bakker ,Charlotte E Riggs ,Joslin L Moore ,Nico Eisenhauer ,Peter B Adler ,Suzanne M Prober ,Laureano A Gherardi ,Rebecca L Mcculley ,Sarah E Hobbie ,Andrew S Macdougall ,G.f Veen

doi:10.1111/gcb.15023

Abstract

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.

Highlights

Grasslands cover 30% of the terrestrial earth surface (White, Murray, & Rohweder, 2000) and their soils are important reservoirs of carbon (C) and nitrogen (N; Jobbagy & Jackson, 2000)
We found no significant impact of treatment duration on the responses of soil C and N pools to herbivore exclusion (LMM, F1,20 = 0.65, p = .431 for C and F1,20 = 1.30, p = .268 for N), allowing us to pool the data across treatment years, even though the sites differed in the number of years that the treatments were applied
Nutrient availability explained most strongly the responses of soil C and N pools to the exclusion of mammalian herbivores: under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply pools were smaller upon herbivore exclusion (Figure 3)

Summary

| INTRODUCTION

Grasslands cover 30% of the terrestrial earth surface (White, Murray, & Rohweder, 2000) and their soils are important reservoirs of carbon (C) and nitrogen (N; Jobbagy & Jackson, 2000). Studies on the effect of herbivores on soil C and N pools have not accounted for the effect of human-induced increases in grassland soil nutrient availability, that may arise due to atmospheric N deposition or the use of artificial fertilizers (Asner et al, 2001; Galloway et al, 2004) This makes it difficult to incorporate the role of herbivores in global models predicting land–atmosphere interactions under future climate change. We quantify the responses of soil C and N pools to the exclusion of mammalian herbivores in 22 grasslands distributed across the globe, under both ambient and elevated nutrient supply These sites are part of the Nutrient Network (NutNet) distributed experiment, which was established to examine the combined effects of nutrient addition and herbivore exclusion on ecosystem processes in grasslands worldwide (Borer, Grace, Harpole, MacDougall, & Seabloom, 2017). The sites encompassed a wide range of environmental gradients including mean annual temperature (MAT; 0.1–18.2°C), mean annual precipitation (MAP; 246–1,877 mm) and total soil N concentration (0.06%–1.2%)

| METHODS

Findings

| DISCUSSION