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

Stable isotope signatures in soil‐derived NO 3 − 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 NO 3 − To study this, we measured net nitrification and ammonification rates and the δ 15 N and δ 18 O of NO 3 − 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 NO 3 − 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 NO 3 − detectable within 1 h after sampling. The δ 15 N and δ 18 O of soil NO 3 − changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in δ 15 NO 3 − became more depleted after incubation, as much as 13.5‰ lighter. Soils that were initially relatively depleted in δ 15 NO 3 − became more enriched after incubation, as much as 16.5‰ heavier. The latter had high net nitrification rates and low final NH 4 + concentrations, probably causing the enrichment of 15 N in NO 3 − due to a diminishing substrate pool. The δ 18 O of soil NO 3 − showed little change during incubation and changes were not significantly related to changes in the 15 N of NO 3 − , suggesting that denitrification was not a primary mechanism. Added enriched 15 NH 4 Cl (111.9‰) was rapidly incorporated into the soil NO 3 − pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil NO 3 − , and the isotopic signature of extracted NO 3 − may not be a reliable environmental tracer.