High nitrogen isotope fractionation of nitrate during denitrification in four forest soils and its implications for denitrification rate estimates

Abstract

Denitrification is a major process contributing to the removal of nitrogen (N) from ecosystems, but its rate is difficult to quantify. The natural abundance of isotopes can be used to identify the occurrence of denitrification and has recently been used to quantify denitrification rates at the ecosystem level. However, the technique requires an understanding of the isotopic enrichment factor associated with denitrification, which few studies have investigated in forest soils. Here, soils collected from two tropical and two temperate forests in China were incubated under anaerobic or aerobic laboratory conditions for two weeks to determine the N and oxygen (O) isotope enrichment factors during denitrification. We found that at room temperature (20°C), NO was reduced at a rate of 0.17 to 0.35μgNgh, accompanied by the isotope fractionation of N (ε) and O (ε) of 31‰ to 65‰ (48.3±2.0‰ on average) and 11‰ to 39‰ (18.9±1.7‰ on average), respectively. The N isotope effects were, unexpectedly, much higher than reported in the literature for heterotrophic denitrification (typically ranging from 5‰ to 30‰) and in other environmental settings (e.g., groundwater, marine sediments and agricultural soils). In addition, the ratios of ΔδO:ΔδN ranged from 0.28 to 0.60 (0.38±0.02 on average), which were lower than the canonical ratios of 0.5 to 1 for denitrification reported in other terrestrial and freshwater systems. We suggest that the isotope effects of denitrification for soils may vary greatly among regions and soil types and that gaseous N losses may have been overestimated for terrestrial ecosystems in previous studies in which lower fractionation factors were applied.

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi‐site dual nitrate isotopes

“…Isotopic effects have been described ranging from 5‰ to 30‰ for pure culture (Granger et al, 2008;Mariotti et al, 1981) and 6‰ to 65‰ for natural soils Wang et al, 2018). The apparent average isotopic effect of denitrification determined with our field data was ~5‰ (Figure 4), which is at the low end of the global reported range.…”

“…While isotopic effects for N uptake and leaching show little variation (Evans, ; Granger et al, ), isotopic effects of denitrification itself vary over large ranges. Isotopic effects have been described ranging from 5‰ to 30‰ for pure culture (Granger et al, ; Mariotti et al, ) and 6‰ to 65‰ for natural soils (Houlton & Bai, ; Wang et al, ). The apparent average isotopic effect of denitrification determined with our field data was ~5‰ (Figure ), which is at the low end of the global reported range.…”

δ15N of Nitric Oxide Produced Under Aerobic or Anaerobic Conditions From Seven Soils and Their Associated N Isotope Fractionations

“…Under the anaerobic condition, the 15 ε of NO production from forest soils was significantly smaller than agriculture and grassland soils (Table 3). Previous studies reported that 15 ε was highly correlated with denitrification rates for the studied soils, with 15 ε exponentially decreasing with an increasing NO 3 − consumption rates (expressed as rate constant k 1 ) (Mariotti et al, 1988; Wang et al, 2018). Similar to these findings, across forest and grassland soils in the present study, 15 ε was also found to exponentially decrease with the denitrification rates (Figure 9).…”

A streamlined method to quantify the fates of 15N in seawater samples amended with 15N‐labeled organic nitrogen

“…Second, the fractionation factor for 15 N is generally on the level of part per thousand (ppt, or ‰). For instance, the fractionation factor ( 15 ε = δ 15 N A ‐ δ 15 N B ) for denitrification ranged from 5‰ to 40‰ in open ocean system (Wang et al ). However, this number is negligible compared with the δ 15 N in our work (generally ≥1000), because the 15 N compound was spiked at high levels as the incubation substrate ( 15 N: 14 N usually ≥1).…”