Document: DAV-3-59-12

Immobilization of nitrate and nitrite in temperate forest soils: Microbial and abiotic processes and potential interaction.

DAIL, D.B.* ¹, E.A.DAVIDSON ² and J.CHOROVER ¹

The Pennsylvania State University, University Park, PA 16802 USA ¹
The Woods Hole Research Center, Woods Hole, MA USA ²

Abstract:
Biotic and abiotic mechanisms by which inputs of inorganic-N are converted to organic-N and retained in soil were investigated using ¹⁵N in sterilized and live soils. Samples of the organic and mineral soil horizons were analyzed immediately after ¹⁵N addition and 24 hours later to determine ¹⁵N enrichment of nitrate/nitrite, ammonium, and organic-N pools. Efficacy of sterilization was confirmed by monitoring soil respiration during the incubation period and serial plating of soil extracts of sterilized and non-sterilized soil on various solid media. Up to 20% of ¹⁵Nitrate was not recoverable as nitrate immediately after addition to organic soils, while all nitrate was recovered as such in mineral soils. About 30% of the nitrate added to organic soils moved into an organic fraction during the 24 hour incubation in sterilized soils, indicating that an abiotic process may be responsible for this immobilization. Although sterilization by autoclaving or GAMMA irradiation did not significantly affect the percentages of label recovered in inorganic and organic pools, autoclaving reduced overall recovery of ¹⁵N by as much as 50% and did so immediately upon addition. Autoclaving, and to a lesser degree, GAMMA irradiation of soils drastically reduced the total recovery of labeled nitrite; 20% and 44% respectively. Sterilization of soils may lead to changes in soil properties that enhance the conversion of nitrite (and nitrate) to nitrogen gasses. Hence, artifacts of sterilization are a concern, but nevertheless, these results show that an unidentified process of abiotic immobilization of nitrate to form organically bound N may be important in organic layers of temperate forest soils.

Keywords: 15N, N immobilization, sterilized soils

This abstract is being presented at: 1:30 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.