Comparing the fate of N from fertilizer treatments and root litter turnover in a Mediterranean Savanna

Nutrient imbalances induced by anthropogenic N deposition may fundamentally alter plant activity and consequently, their role in biogeochemical cycling. Mechanistic understanding of N cycle processes is commonly informed by ¹⁵N tracers in fertilizer applications, but over the long term most N is obtained by plants is from litter mineralization rather than ‘new’ deposition N or mineral fertilizer applications. In many ecosystems this litter pool is dominated by root remains.

Here, we will compare results between two experiments: a ¹⁵N-labelled root litter experiment, and an associated conventional ¹⁵N-mineral fertilizer experiment, both located in a typical, spatially heterogeneous, seasonally-arid Spanish dehesa amended with N and NP fertilizers to induce nutrient imbalances. We show that recovery of the litter tracer in soil and plants was substantially higher than the fertilizer N, especially in microsites under trees, which are rich in organic inputs. In contrast, recovery of mineral tracers was strongest in more resource-poor grassland areas.  Plant acquisition of N from the organic source was also affected by the concurrent P addition treatment while we found little evidence for a similar effect in mineral additions.

Our results imply that scaling nitrogen cycle processes informed by isotope tracers from experiments to ecosystems depends heavily on appropriateness of methodology, especially in interpreting short-term traces applied as fertilizer N.