31 research outputs found
Effect of soil saturation on denitrification in a grassland soil
Nitrous oxide (N2O) is of major importance as a greenhouse gas and precursor of ozone (O3) destruction in the stratosphere mostly produced in soils. The soil-emitted N2O is generally predominantly derived from denitrification and, to a smaller extent, nitrification, both processes controlled by environmental factors and their interactions, and are influenced by agricultural management. Soil water content expressed as water-filled pore space (WFPS) is a major controlling factor of emissions and its interaction with compaction, has not been studied at the micropore scale. A laboratory incubation was carried out at different saturation levels for a grassland soil and emissions of N2O and N2 were measured as well as the isotopocules of N2O. We found that flux variability was larger in the less saturated soils probably due to nutrient distribution heterogeneity created from soil cracks and consequently nutrient hot spots. The results agreed with denitrification as the main source of fluxes at the highest saturations, but nitrification could have occurred at the lower saturation, even though moisture was still high (71% WFSP). The isotopocules data indicated isotopic similarities in the wettest treatments vs. the two drier ones. The results agreed with previous findings where it is clear there are two N pools with different dynamics: added N producing intense denitrification vs. soil N resulting in less isotopic fractionation
Dem Stickstoff auf der Spur: N2O Prozesse und Nmin Dynamik nach Grünlanderneuerung
Eine weit verbreitete Maßnahme des Grünlandmanagements, die zur Beseitigung von Narbenschäden und zur Steigerung der Futterqualität in unproduktiven Grünländern angewendet wird, ist die Grünlanderneuerung. Die mechanische Bearbeitung von Grünlandböden und die dadurch gesteigerte Mineralisation durch den Abbau organischer Bodensubstanz und der alten Grasnarbe kann zu hohen N-Verlusten in Form des klimarelevanten Treibhausgases Lachgas (N2O) und/oder Nitratauswaschung (NO3-) führen. Bisher gibt es jedoch über die Dauer des beschriebenen Effektes, sowie den Einfluss unterschiedlicher Grünlanderneuerungstechniken nur wenige Informationen. Insbesondere für die nationale Treibhausgasbilanzierung ist es jedoch von Bedeutung, die Prozesse der N2O Umsetzung und ihre Quellen zu kennen und zu erfassen, da sich nur so Maßnahmen zur Emissionsminderung ableiten lassen. Zu diesem Zweck wurde ein Parzellenversuch (2013-2015) auf zwei Standorten (Plaggenesch, Anmoorgley) in der Nähe von Oldenburg (Niedersachsen) mit unterschiedlichen Erneuerungsvarianten etabliert. Als Referenzvarianten dienten: Grünlandumwandlung in Ackerland (Mais) und langjähriges Dauergrünland. Die N2O Flüsse und die Dynamik des mineralischen N (Nmin) wurden über einen Zeitraum von zwei Jahren untersucht. Zusätzlich wurden Nmin Profile (0-90 cm) genutzt, um den N-Verlust über Winter zu quantifizieren und das Risiko einer möglichen NO3- Auswaschung abzuschätzen. Obwohl die N2O Flüsse für einen kurzen Zeitraum (2 Monate) nach der Bearbeitung erhöht waren, konnte kein Jahreseffekt festgestellt werden. Im ersten Winter nach dem Aufbrechen der alten Grasnarbe trat jedoch für den Plaggenesch ein erhöhtes Risiko für NO3- Auswaschung auf. Die Untersuchung der N2O-Produktionswege und der N2O-Reduktion zu N2 (dem Endprodukt der Denitrifikation) erfolgte unter Nutzung stabiler Isotope. Hierzu wurde die 15N-Gasflussmethode im Sommer 2014 angewendet (1). Zusätzlich wurden natürlich vorhandene stabile Isotopensignaturen im bodenbürtigen N2O (δ15NbulkN2O, δ18ON2O und δ15NSPN2O = intramolekulare Verteilung von 15N im N2O Molekül) genutzt, um Quellen der N2O-Bildung im ersten Jahr nach Grünlanderneuerung (2013-2014) zu ermitteln. Auf dem Anmoorgley wurden große N-Verluste durch den Prozess der Denitrifikation bestimmt, wobei N2 die Emissionen dominerte. Für den Plaggenesch konnten generell geringere gasförmige Verluste festgestellt werden
Early season N<sub>2</sub>O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile
Soil moisture strongly affects the balance between nitrification, denitrification
and N2O reduction and therefore the nitrogen (N) efficiency and N
losses in agricultural systems. In rice systems, there is a need to improve
alternative water management practices, which are designed to save water and
reduce methane emissions but may increase N2O and decrease nitrogen
use efficiency. In a field experiment with three water management treatments,
we measured N2O
isotope ratios of emitted and pore air N2O
(δ15N, δ18O and site preference, SP) over the
course of 6 weeks in the early rice growing season. Isotope ratio
measurements were coupled with simultaneous measurements of pore water
NO3-, NH4+, dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths.
We then used the relationship between SP × δ18O-N2O and
SP × δ15N-N2O in simple two end-member
mixing models to evaluate the contribution of nitrification, denitrification
and fungal denitrification to total N2O emissions and to estimate
N2O reduction rates. N2O emissions were higher in a
dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to
water-seeded + alternate wetting and drying (WS-AWD) and
water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD
treatment the highest emissions were associated with a high contribution from
denitrification and a decrease in N2O reduction, while in the WS
treatments, the highest emissions occurred when contributions from
denitrification/nitrifier denitrification and nitrification/fungal
denitrification were more equal. Modeled denitrification rates appeared to be
tightly linked to nitrification and NO3- availability in all
treatments; thus, water management affected the rate of denitrification and
N2O reduction by controlling the substrate availability for each
process (NO3- and N2O), likely through changes in
mineralization and nitrification rates. Our model estimates of mean
N2O reduction rates match well those observed in 15N
fertilizer labeling studies in rice systems and show promise for the use of
dual isotope ratio mixing models to estimate N2 losses.</p
Elucidating the impact of nitrate and labile carbon application on spatial heterogeneity of denitrification by 15N modelling
Mineralogical and oxygen isotope composition of inorganic dust-fall in Wrocław (SW Poland) urban area – test of a new monitoring tool
We have analysed the mineralogical and oxygen isotope composition of solid inorganic atmospheric particles (SIAP) in Wrocław (SW Poland) to determine potential natural and anthropogenic sources of deposited dust. The mineralogical compositions of SIAP and local soils are very similar and quite typical. Dust sources were attributed to high emission sources (two large coal-fired power generation plants, i.e., "Wrocław and "Czechnica") and low emission sources (mostly small furnaces for home heating). A mullite phase was confirmed in the non-magnetic fraction of high emission dust. The δ1818181
Underestimation of denitrification rates from field application of the <sup>15</sup>N gas flux method and its correction by gas diffusion modelling
Common methods for measuring soil denitrification in situ
include monitoring the accumulation of 15N-labelled N2 and
N2O evolved from 15N-labelled soil nitrate pool in closed chambers
that are placed on the soil surface. Gas diffusion is considered to be the
main transport process in the soil. Because accumulation of gases within the
chamber decreases concentration gradients between soil and the chamber over
time, the surface efflux of gases decreases as well, and gas production rates
are underestimated if calculated from chamber concentrations without
consideration of this mechanism. Moreover, concentration gradients to the
non-labelled subsoil exist, inevitably causing downward diffusion of
15N-labelled denitrification products. A numerical 3-D model for
simulating gas diffusion in soil was used in order to determine the
significance of this source of error. Results show that subsoil diffusion of
15N-labelled N2 and N2O – and thus potential underestimation
of denitrification derived from chamber fluxes – increases with chamber
deployment time as well as with increasing soil gas diffusivity. Simulations
based on the range of typical soil gas diffusivities of unsaturated soils
showed that the fraction of N2 and N2O evolved from
15N-labelled NO3- that is not emitted at the soil surface
during 1 h chamber closing is always significant, with values up to
>50 % of total production. This is due to accumulation in the
pore space of the 15N-labelled soil and diffusive flux to the
unlabelled subsoil. Empirical coefficients to calculate denitrification from
surface fluxes were derived by modelling multiple scenarios with varying
soil water content. Modelling several theoretical experimental set-ups
showed that the fraction of produced gases that are retained in soil can be
lowered by lowering the depth of 15N labelling and/or increasing the
length of the confining cylinder.
Field experiments with arable silt loam soil for measuring denitrification
with the 15N gas flux method were conducted to obtain direct evidence
for the incomplete surface emission of gaseous denitrification products. We
compared surface fluxes of 15N2 and 15N2O from
15N-labelled micro-plots confined by cylinders using the closed-chamber method with cylinders open or closed at the bottom, finding 37 %
higher surface fluxes with the bottom closed. Modelling fluxes of this experiment
confirmed this effect, however with a higher increase in surface flux of
89 %.
From our model and experimental results we conclude that field surface
fluxes of 15N-labelled N2 and N2O severely underestimate
denitrification rates if calculated from chamber accumulation only. The
extent of this underestimation increases with closure time. Underestimation
also occurs during laboratory incubations in closed systems due to pore
space accumulation of 15N-labelled N2 and N2O. Due to this
bias in past denitrification measurements, denitrification in soils might be
more relevant than assumed to date.
Corrected denitrification rates can be obtained by estimating subsurface
flux and storage with our model. The observed deviation between experimental
and modelled subsurface flux revealed the need for refined model evaluation,
which must include assessment of the spatial variability in diffusivity and
production and the spatial dimension of the chamber.</p
Quantifying N2O reduction to N2 during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis
The last step of denitrification, i.e. the reduction of N2O to N2, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for N2O.However, assessing N2 production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical “mapping approach” model using the δ15Nsp/δ18O slope that has been proposed to potentially be used to indirectly quantify N2O reduction to N2 at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He–O2 atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N2O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N2O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N2O isotope signatures