Hydrological influences on spatiotemporal variations of δ¹⁵N and δ¹⁸O of nitrate in a forested headwater catchment in central Japan: Denitrification plays a critical role in groundwater

Abstract

− is that nitrification and denitrification occur concurrently in groundwater and that denitrification is more important than nitrification. The amount of denitrification in the groundwater body was controlled primarily by groundwater residence time. The combination of hydrological flow path from groundwater to stream water and denitrification in the groundwater critically control the amount of leaching of NO 3 − and temporal distribution of NO 3 − concentration in stream water. The possibility exists that denitrification in groundwater strongly influences nitrogen leaching in forests because denitrification occurred even in this catchment, which consists of weathered granitic bedrock and predominantly typic udipsamment, with low organic matter content in the soil.

Isotopic signals of summer denitrification in a northern hardwood forested catchment

“…These δ 15 N values averaged 1.2 ± 0.4‰ in the surface organic horizons (Oi/Oe) and increased to 5.2 ± 1.1‰ in the Oa horizon and 6.0 ± 1.1‰ in Bh and Bhs horizons at 10-65 cm depth, similar to prior measurements in WS3 (48). Mineralization causes negligible 15 N fractionation (14,49) and so δ 15 N NH4 from mineralization of this soil organic N should also have this range of 15 N values (0.5-7.5‰). Isotopic fractionation during nitrification should be negligible when NH 4 + supply rate limits the rate of nitrification, but fractionation can occur when nitrification is slow or incomplete relative to the NH 4 + supply, producing δ 15 N NO3 values ∼2-20‰ lighter than those of the NH 4 + substrate (14,20,49).…”

“…Hydrological disconnection between perched saturated zones and stream channels and denitrification rates may both broadly covary seasonally with the warm, drying conditions of summer (42), creating fragmented patches of saturation (40) at the time when denitrification in these zones may be particularly active (36). Although it is possible that this NO 3 − -depleted water is later exported to the stream (15), these data show concentrations of NO 3 − in groundwater equal to or higher than those of streamwater, suggesting that this was not the case at the time of sampling. If these episodes of fragmented denitrification have increased over time, for example, with the increases in summer temperature and precipitation that have been observed at this site (10,43), then denitrification may have significantly contributed both to the amount of "missing N" in the ecosystem N balance and to the observed changes in this balance over time.…”

“…Groundwater Denitrification Is Consuming Soil NO 3 − The isotopic composition of NO 3 − in the six groundwater wells upgradient and away (≥4 m) from perennial streamflow showed substantial enrichment in both 15 N and 18 O, falling far outside the expected ranges of NO 3 − sources from deposition or nitrification (Table 1 and (Fig. 3B) and serves as a diagnostic of denitrification (14,(20)(21)(22).…”

“…Measurement of the dual isotopic composition of NO 3 − (δ 15 N NO3 and δ 18 O NO3 ) provides a powerful tool to identify NO 3 − sources and to infer its loss to denitrification (13)(14)(15)(16) (Table S1), especially during snowmelt, when catchments often release large quantities of NO 3 − (7,8). However, past δ 18 O NO3 measurements show that nitrificationnot precipitation-supplies the vast majority of NO 3 − in streamwater at the HBEF (10,17,18) and other forested catchments (refs.…”

Microbial denitrification dominates nitrate losses from forest ecosystems

“…In particular, we used the concentration-weighted δ 15 N integrated to a depth of 1 m to estimate the δ 15 N of the initial NO 3 − available for denitrification (i.e., δ 15 N NO3−input ; SI Text). However, our data suggest that much of this NO 3 − was already consumed by denitrification (22) and biological uptake (Fig. 1C and Fig.…”