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- Title
Abiotic immobilization of nitrate in two soils of relic Abies pinsapo-fir forests under Mediterranean climate.
- Authors
Torres-Cañabate, Patricia; Davidson, Eric; Bulygina, Ekaterina; García-Ruiz, Roberto; Carreira, Jose
- Abstract
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.
- Subjects
NITROGEN &; the environment; NITROGEN cycle; SPANISH fir; ATMOSPHERIC deposition; SOILS &; climate; FLOW injection analysis; STERILIZATION (Disinfection); CHROMATOGRAPHIC analysis; BIOTIC communities
- Publication
Biogeochemistry, 2008, Vol 91, Issue 1, p1
- ISSN
0168-2563
- Publication type
Article
- DOI
10.1007/s10533-008-9255-y