Moderate nitrogen retention in temperate heath ecosystem after elevated CO2, drought and warming through 7 years

Nitrogen (N) dynamic is one of the main controlling factors of responses to climate change in N-limited terrestrial ecosystems, which rely on nutrient recycling and retention. In this study we investigate the N partitioning in ecosystem compartments of a grassland heath, and the impact of multiple climate change factors on long-term N retention after N-15 pulse labelling. The impacts of elevated carbon dioxide (eCO(2)), warming and drought and the treatments in combination on ecosystem N retention were investigated in a field scale manipulation experiment. A 6-year time-course was assessed by pulse-labelling with the stable N isotope N-15 and by sampling after 1 day, 1 year and 6 years. After 6 years we observed that the total ecosystem retained 42% of the amended N-15 across treatments (recovery of the amended N-15 in the pool). The fate of the applied N-15 was mainly stabilization in soil, with 36% N-15 recovery in soil, while the plant compartment and microbial biomass each retained only 1%-2% of the added N-15. This suggests a moderate retention of N, for all treatments, as compared to similar long-term studies of forest ecosystems. A decreased ammonium and vegetation N pool combined with higher N-15 retention in the soil under eCO(2) treatments suggests that eCO(2) promoted processes that immobilize N in soil, while warming counteracted this when combined with eCO(2). Drought treatments contrastingly increased the vegetation N pool. We conclude that as the organic soil layer has the main capacity for N storage in a temperate heathland-grassland, it is important for buffering nutrient availability and maintaining a resilient ecosystem. However, the full treatment combination of drought, warming and eCO(2) did not differ in N-15 recovery from the control, suggesting unchanged long-term consequences of climate change on retention of pulse added N in this ecosystem.