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 15N pulse labelling. The impacts of elevated carbon dioxide (eCO2), 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 15N 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 15N across treatments (recovery of the amended 15N in the pool). The fate of the applied 15N was mainly stabilization in soil, with 36% 15N recovery in soil, while the plant compartment and microbial biomass each retained only 1%–2% of the added 15N. 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 15N retention in the soil under eCO2 treatments suggests that eCO2 promoted processes that immobilize N in soil, while warming counteracted this when combined with eCO2. 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 eCO2 did not differ in 15N recovery from the control, suggesting unchanged long‐term consequences of climate change on retention of pulse added N in this ecosystem.

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