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Evidence for abiotic immobilization of nitrogen (N) in soil is accumulating, but remains controversial. Identifying the fate of N from atmospheric deposition is important for understanding the N cycle of forest ecosystems. We studied soils of two Abies pinsapo fir forests under Mediterranean climate seasonality in southern Spain—one with low N availability and the other with symptoms of N saturation. We hypothesized that biotic and abiotic immobilization of nitrate (NO3 −) would be lower in soils under these forests compared to more mesic temperate forests, and that the N saturated stand would have the lowest rates of NO3 − immobilization. Live and autoclaved soils were incubated with added 15NO3 − (10 μg N g−1 dry soil; 99% enriched) for 24 h, and the label was recovered as total dissolved-N, NO3 −, ammonium (NH4 +), or dissolved organic-N (DON). To evaluate concerns about possible iron interference in analysis of NO3 − concentrations, both flow injection analysis (FIA) and ion chromatography (IC) were applied to water extracts, soluble iron was measured in both water and salt extracts, and standard additions of NO3 − to salt extracts were analyzed. Good agreement between FIA and IC analysis, low concentrations of soluble Fe, and 100% (±3%) recovery of NO3 − standard additions all pointed to absence of an interference problem for NO3 − quantification. On average, 85% of the added 15NO3 − label was recovered as 15NO3 −, which supports our hypothesis that rates of immobilization were generally low in these soils. A small amount (mean = 0.06 μg N g−1 dry soil) was recovered as 15NH4 + in live soils and none in sterilized soils. Mean recovery as DO15N ranged from 0.6 to 1.5 μg N g−1 dry soil, with no statistically significant effect of sterilization or soil type, indicating that this was an abiotic process that occurred at similar rates in both soils. These results demonstrate a detectable, but modest rate of abiotic immobilization of NO3 − to DON, supporting our first hypothesis. These mineral soils may not have adequate carbon availability to support the regeneration of reducing microsites needed for high rates of NO3 − reduction. Our second hypothesis regarding lower expected abiotic immobilization in soils from the N-saturated site was not supported. The rates of N deposition in this region may not be high enough to have swamped the capacity for soil NO3 − immobilization, even in the stand showing some symptoms of N saturation. A growing body of evidence suggests that soil abiotic NO3 − immobilization is common, but that rates are influenced by a combination of factors, including the presence of plentiful available carbon, reduced minerals in anaerobic microsites and adequate NO3 − supply.

Stable isotopes and their potential for detecting various and complex ecosystem processes are attracting an increasing number of scientists. Progress is challenging, particularly under global change scenarios, but some established views have been challenged. The IX meeting of the Spanish Association of Terrestrial Ecology (AAET, Úbeda, 18–22 October 2009) hosted a symposium on the ecology of stable isotopes where the linear mixing model approach of partitioning sinks and sources of carbon and water fluxes within an ecosystem was challenged, and new applications of stable isotopes for the study of plant interactions were evaluated. Discussion was also centred on the need for networks that monitor ecological processes using stable isotopes and key ideas for fostering future research with isotopes.