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Woods Hole Research Center, Woods Hole, MA
* Corresponding author: edavidson{at}whrc.org
Soil is a dominant long-term sink for N in forest ecosystems (1). With increasing atmospheric N deposition in forests caused by combustion of fossil fuels (2, 3), concern over effects of N loading on ecosystem sustainability is substantial. Net primary productivity is thought to be limited by N in most temperate forests (4), but much of the N from deposition appears to be immobilized in the organic matter of the soil rather than being taken up by plants (1). However, the mechanisms of N immobilization as well as the bioavailability and ultimate fate of organic N remain poorly understood. One hypothesized pathway of immobilization is abiotic reduction of nitrate to nitrite and reaction of nitrite with dissolved organic matter to produce dissolved organic nitrogen (DON) (5). Studying the fate of DON would be facilitated by the development of a 15N label in a realistic soil DON product. In the present study, we tested incubation conditions that would permit nitrate immobilization to DON during 24 h, followed by conditions that would minimize the remaining nitrate. Water content and N concentrations were varied in 1–8-day incubations of organic horizon to study the dynamic of nitrate reduction and DON formation.
Soil for these incubations was collected from the humus (Oa) layer of a forest stand dominated by oak at the Harvard Forest, Massachusetts. The stand developed after agricultural abandonment at the end of the 19th century and after a devastating hurricane in 1938 (6). The soil samples were passed through a 2-mm sieve to remove large roots. Subsamples of 10 g (n = 64) were placed in 60-ml serum bottles, which then received a combination of two treatments in a factorial design. First, 3.33 ml of either deionized water or nitrate solution (50 µg N/g dry soil as KNO3) was added. Second, 30 ml of deionized water was added to half of the samples to create saturated conditions, whereas those not receiving additional water were considered unsaturated. The bottles were sealed using rubber stoppers and tear-away metal crimping seals. Each treatment combination was replicated 16 times so that four replicates could be destructively sampled by extraction in water at 0, 1, 4, and 8 days after the start of the incubation. The "day zero" extraction was done immediately after sample treatment (addition of nitrogen or water) to measure background nitrate and DON and to measure initial recovery of added nitrate. Ambient aerobic conditions prevailed during the first day of incubation to allow for aerobic immobilization processes. After the first day, air was flushed from the serum bottles with dinitrogen gas to create anaerobic conditions in an effort to increase denitrification, thus reducing residual nitrate. At the end of each incubation period, 60 ml of water was added to the saturated samples and 30 ml to the unsaturated. All samples were shaken and immediately filtered through No. 1 Whatman filter paper. Filtrate from each incubation was analyzed for nitrate, ammonium, and total N using a Lachat 8000 flow-injection autoanalyzer with in-line persulfate digestion (7, 8). DON was calculated by subtracting the sum of nitrate and ammonium from total N.
Nitrate concentrations in soil extracts remained at low values throughout the experiment in incubations under saturated and unsaturated conditions with no added nitrate (Fig. 1). In incubations with added nitrate, concentrations increased slightly and similarly under saturated and unsaturated conditions during the first day, after which they declined for the remainder of the experiment. Incubation time, addition of nitrate, and the interaction of time by nitrate were significant (P < 0.01) in an analysis of variance. Saturation was also found to be significant (P < 0.05), as nitrate concentrations were lower in saturated incubations compared to those remaining unsaturated.
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DON concentrations in soil extracts increased over time in incubations with and without nitrate addition under saturated and unsaturated conditions. An analysis of variance shows incubation time to be the only significant (P < 0.01) factor. Also, there was not a stoichiometric conversion of nitrate to DON, suggesting that DON was produced primarily from sources other than nitrate immobilization. However, only a small amount of nitrate immobilization to DON would be necessary to create a labeled DON pool when 15N-labeled nitrate is used.
This experiment will be repeated with a 15N label in the added nitrate to determine the recovery of labeled N in the extracted DON, ammonium, and in the small remaining nitrate pool. These preliminary results using unlabeled N demonstrate that it may be possible to create a series of incubation conditions that would permit nitrate immobilization into DON while minimizing remaining nitrate. Hence, it may be possible to obtain an extract with labeled DON that is created naturally within the soil and that can be used to study DON bioavailability in subsequent experiments.
This research was funded by the NSF Research Experience for Undergraduates site Grant (OCE-0097498).
Literature Cited
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