Drastic short-term changes in the isotopic composition of soil Nitrate in forest soil samples

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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.

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