From here to there: free-ranging large herbivores redistribute nutrients from grassland to forest soil

Abstract

Context: Large herbivores contribute to biogeochemical cycling across spatial scales. Nutrient transport by herbivores is often linked to their movements between habitats preferred for foraging (nutrient intake) and resting (nutrient excretion). Objectives: We investigated whether movement decisions of free-ranging red deer (Cervus elaphus) lead to changes in plant-available soil nutrients. We hypothesised that concentrations of soil phosphorus and potassium would increase in forest relative to grassland with increasing red deer use. Additionally, we tested whether soil nutrient concentrations are positively related to the intensity of deer resting behaviour. Methods: Our study was conducted in the Grafenwöhr military training area, Germany. We collected 200 soil samples from semi-natural grasslands and forests in three zones with varying red deer use. Using GPS telemetry and accelerometer data from 25 red deer, we classified their relocations into active (foraging, travelling) and passive (resting) and calculated a utilisation distribution for resting behaviour. Results: Phosphorus and potassium were significantly enriched in forest compared to grassland soil in the zone with the highest red deer use, and potassium also in the intermediate zone. Nutrient concentrations in forests, where red deer spent more passive than active time contrary to grasslands, increased with resting intensity. This suggests that the differences in nutrient concentrations between forest and grassland are related to deer resting behaviour in forest and associated excrement deposition. Conclusion: By combining data from soil tests, telemetry and activity sensors, we showed that movement decisions of wild ungulates result in nutrient transfer from grassland to forest, contributing to nutrient-poor conditions in grassland favourable to biodiversity. Ungulate effects in forests go beyond browsing as nutrient subsidies could increase forest soil fertility.