The isotopic composition of nitrate produced from nitrification in a hardwood forest floor

Abstract

Dual isotopic analysis of nitrate ( 15N/ 14N and 18O/ 16O) is increasingly used to investigate the environmental impacts of human-induced elevated atmospheric nitrate deposition. In forested ecosystems, the nitrate found in surface water and groundwater can originate from two sources: (1) atmospheric deposition, and (2) nitrate produced from nitrification in forest soils (microbial nitrate). Application of the dual nitrate isotope technique for determining the relative importance of nitrate sources in forested catchments requires knowledge of the isotopic composition of microbial nitrate. We excluded precipitation inputs to three zero-tension lysimeters installed below the F-horizon (Oe) at the Turkey Lakes Watershed (TLW) in order to measure the isotopic composition of microbial nitrate produced in situ. To our knowledge, this is the first in situ study of the isotopic composition of microbial nitrate in forest soils. Over a 2-week period, nitrate produced by nitrification was periodically flushed to the lysimeters by watering the area with a nitrogen-free solution. Nitrate produced in the forest floor had δ 18O values ranging from +3.1‰ to +10.1‰ with a mean of +5.2‰. These values were only slightly higher than from the expected value of +1.0‰ calculated for chemolithoautotrophic nitrification, which depends on the δ 18O of available O 2 and H 2O. In addition to nitrate, we also collected soil gas to determine if soil respiration and O 2 diffusion affected soil gas δ 18O–O 2, which is typically assumed to be identical to atmospheric O 2 (+23.5‰) when calculating microbial nitrate δ 18O values. No significant difference in δ 18O–O 2 from atmospheric O 2 was found in forest soils to a depth of 55 cm, and therefore 18O-enrichment of soil gas O 2 could not explain the modest enrichment of nitrate 18O. Evaporative 18O-enrichment of soil water available to nitrifiers in the forest floor is a plausible mechanism for slightly elevated nitrate δ 18O values. However, the observed nitrate δ 18O values could also be explained by a minor contribution of nitrate from heterotrophic nitrifiers. The δ 15N of nitrate produced ranged from −10.4 to −7.3‰ and, as expected, was depleted in 15N relative to soil organic nitrogen. Microbial nitrate produced in the forest floor was also significantly depleted in 15N relative to microbial nitrate exported in groundwater and headwater streams at the TLW. We hypothesize that 15N-depleted forest floor nitrate is not detected in groundwaters largely because of: (1) the immobilization of forest floor nitrate in the mineral soil and (2) the mixing of the remaining forest floor nitrate with nitrate generated in the mineral soil, which is expected to have higher δ 15N values. This study demonstrates that current methods of calculating a priori the δ 18O of microbial nitrate provide a reasonable value for nitrate produced by nitrification at the TLW.