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

Abstract

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 δ15N and δ18O 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 δ15N and δ18O of soil NO3− changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in δ15NO3− became more depleted after incubation, as much as 13.5‰ lighter. Soils that were initially relatively depleted in δ15NO3− 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 δ18O 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.