Deciphering Soil Nitrogen Biogeochemical Processes Using Nitrogen and Oxygen Stable Isotopes

Description

<p>Variations
in stable isotope abundances of nitrogen (δ¹⁵N) and oxygen (δ¹⁸O)
of nitrate are a useful tool for determining sources of nitrate as well as
understanding the transformations of nitrogen within soil (Chapter 2). Various
sources of nitrate are known to display distinctive isotopic compositions,
while nitrogen transformation processes fractionate both N and O isotopes and
can reveal the reaction pathways of nitrogen compounds. However, to fully
understand the δ¹⁵N and δ¹⁸O values of nitrate sources,
we must understand the chemistry and the isotopic fractionations that occur
during inorganic and biochemical reactions. Among all N cycle processes,
nitrification and denitrification displayed some of the largest and most
variable isotope enrichment factors, ranging from -35 to 0‰ for nitrification, and -40 to -5‰ for denitrification. In this dissertation,
I will first characterize the isotopic enrichment factors of ¹⁵N during nitrification and
denitrification in a Midwestern agricultural soil, two important microbial
processes in the soil nitrogen cycle. Nitrification incubations found that a
large enrichment factor of -25.5‰ occurs
during nitrification NH₄⁺ è NO₃^-, which agrees well with previous studies (Chapter 3).
Additionally, oxygen isotopic exchange that occurs between nitrite and water
during nitrification was also quantified and found that 82% of oxygen in NO₃^- are derived from H₂O, much greater than the 66%
predicted by the biochemical steps of nitrification. The isotopic enrichment
that occurs during denitrification was assessed by measuring the change in δ¹⁵N as the reactant NO₃^- was reduced to N₂ gas (Chapter
4). The incubations and kinetic models showed that denitrification can causes
large isotopic enrichment in the δ¹⁵N
of remaining NO₃^-.
The enrichment factor for NO₂^- è gaseous N was
-9.1‰, while the enrichment factors for NO₃^- è NO₂^- were between -17 to -10‰, both of which
were within the range of values report in literature. The results demonstrated
that nitrification and denitrification caused large isotope fractionation and
can alter the presumed δ¹⁵N
and δ¹⁸O values of nitrate
sources, potentially leading to incorrect apportionment of nitrate sources.</p>

<p>The
results of the denitrification incubation experiments were applied to a field
study, where the measured enrichment factor was utilized to quantify loss of N
by field-scale denitrification (Chapter 5). Field-based estimates of total
denitrification have long been a challenge and only limited success has been
found using N mass balance, N₂O gas flux, or isotope labeling
techniques. Here, the flux of nitrate and chloride from tile drain discharge from
a small field was determined by
measuring both dissolved ions (ion chromatography) and monitoring water
discharge. The δ¹⁵N and δ¹⁸O of tile nitrate
was also measured at a high temporal resolution. Fluxes of all N inputs, which
included N wet and dry deposition, fertilizer application, and soil
mineralization were determined. The d¹⁵N and d¹⁸O values of these nitrate
sources was also determined. Using this data, I first detected shifts in δ¹⁵N
and δ¹⁸O values in the tile drain nitrate, which indicated variable
amounts of denitrification. Next, a Rayleigh distillation model was used to determine
the fraction of NO₃^- loss by field scale denitrification. This natural
abundance isotope method was able to account for the spatial and temporal
variability of denitrification by integrating it across the field scale. Overall,
I found only 3.3% of applied N was denitrified. Furthermore, this study emphasized the
importance of complementary information (e.g. soil moisture, soil temperature,
precipitation, isotopic composition of H₂O, etc.), and the evidence it
can provide to nitrogen inputs and processes within the soil.</p>

Links & Downloads

1. 10.25394/pgs.8014592.v1

Tags

Nitrogen Cycle

Stable Isotopes

Denitrification

Nitrification

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8014592
Date	15 May 2019
Creators	Benjamin P Wilkins (6612953)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/Deciphering_Soil_Nitrogen_Biogeochemical_Processes_Using_Nitrogen_and_Oxygen_Stable_Isotopes/8014592