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FOREST, RANGE & WILDLAND SOILS

Drastic Short-Term Changes in the Isotopic Composition of Soil Nitrate in Forest Soil Samples

^a Dep. of Environmental Conservation, 103 S. Main St., Waterbury, VT 05671
^b Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05405

^* Corresponding author (heidi.hales{at}state.vt.us).

Stable isotope signatures in soil-derived NO₃^– provide an opportunity for environmental source tracing, but rapid changes in N transformation rates caused by sampling disturbance may create an artifact in the signature of extracted NO₃^–. To study this, we measured net nitrification and ammonification rates and the ¹⁵N and ¹⁸O of NO₃^– in soils from a watershed in Camels Hump State Forest, Vermont, known to be sensitive to sampling disturbance. Eleven Oa and one A horizon samples (C/N ratio 15–25, pH 3.3–4.2) had NO₃^– extracted for isotope analysis approximately 30 min after sampling and again after a 2- to 3-d incubation period at 10°C. Net nitrification rates during incubation were rapid and linear (0.9–15.3 µmol L^–1 soil h^–1), with increases in NO₃^– detectable within 1 h after sampling. The ¹⁵N and ¹⁸O of soil NO₃^– changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in ¹⁵NO₃^– became more depleted after incubation, as much as 13.5 lighter. Soils that were initially relatively depleted in ¹⁵NO₃^– became more enriched after incubation, as much as 16.5 heavier. The latter had high net nitrification rates and low final NH₄⁺ concentrations, probably causing the enrichment of ¹⁵N in NO₃^– due to a diminishing substrate pool. The ¹⁸O of soil NO₃^– showed little change during incubation and changes were not significantly related to changes in the ¹⁵N of NO₃^–, suggesting that denitrification was not a primary mechanism. Added enriched ¹⁵NH₄Cl (111.9) was rapidly incorporated into the soil NO₃^– pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil NO₃^–, and the isotopic signature of extracted NO₃^– may not be a reliable environmental tracer.

Abbreviations: DI, deionized distilled

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