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 N₂O 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&thinsp;×&thinsp;δ18O-N2O and SP&thinsp;×&thinsp;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&thinsp;+&thinsp;alternate wetting and drying (DS-AWD) treatment relative to water-seeded&thinsp;+&thinsp;alternate wetting and drying (WS-AWD) and water-seeded&thinsp;+&thinsp;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

Isotopic techniques to measure N2O, N2 and their sources

GHG emissions are usually the result of several simultaneous processes. Furthermore, some gases such as N2 are very difficult to quantify and require special techniques. Therefore, in this chapter, the focus is on stable isotope methods. Both natural abundance techniques and enrichment techniques are used. Especially in the last decade, a number of methodological advances have been made. Thus, this chapter provides an overview and description of a number of current state-of-theart techniques, especially techniques using the stable isotope 15N. Basic principles and recent advances of the 15N gas flux method are presented to quantify N2 fluxes, but also the latest isotopologue and isotopomer methods to identify pathways for N2O production. The second part of the chapter is devoted to 15N tracing techniques, the theoretical background and recent methodological advances. A range of different methods is presented from analytical to numerical tools to identify and quantify pathway-specific N2O emissions. While this chapter is chiefly concerned with gaseous N emissions, a lot of the techniques can also be applied to other gases such as methane (CH4), as outlined in Sect. 5.3

Micrometeorological methods for greenhouse gas measurement

Micrometeorological techniques are useful if greenhouse gas (GHG) emissions from larger areas (i.e. entire fields) should be integrated. The theory and the various techniques such as flux-gradient, aerodynamic, and Bowen ratio as well as Eddy correlationmethods are described and discussed. Alternativemethods also used areEddy correlation, mass balance techniques, and tracer-based methods.The analytical techniques with current state-of-the-art approaches as well as the calculation procedures are presented

Greenhouse gases from agriculture

The rapidly changing global climate due to increased emission of anthropogenic greenhouse gases (GHGs) is leading to an increased occurrence of extreme weather events such as droughts, floods, and heatwaves. The three major GHGs are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The major natural sources of CO2 include ocean-atmosphere exchange, respiration of animals, soils (microbial respiration) and plants, and volcanic eruption; while the anthropogenic sources include burning of fossil fuel (coal, natural gas, and oil), deforestation, and the cultivation of land that increases the decomposition of soil organic matter and crop and animal residues. Natural sources of CH4 emission include wetlands, termite activities, and oceans. Paddy fields used for rice production, livestock production systems (enteric emission from ruminants), landfills, and the production and use of fossil fuels are the main anthropogenic sources of CH4. Nitrous oxide, in addition to being a major GHG, is also an ozone-depleting gas. N2O is emitted by natural processes from oceans and terrestrial ecosystems. Anthropogenic N2O emissions occur mostly through agricultural and other land-use activities and are associated with the intensification of agricultural and other human activities such as increased use of synthetic fertiliser (119.4 million tonnes of N worldwide in 2019), inefficient use of irrigation water, deposition of animal excreta (urine and dung) from grazing animals, excessive and inefficient application of farm effluents and animal manure to croplands and pastures, and management practices that enhance soil organic N mineralisation and C decomposition. Agriculture could act as a source and a sink of GHGs. Besides direct sources, GHGs also come from various indirect sources, including upstream and downstream emissions in agricultural systems and ammonia (NH3) deposition from fertiliser and animal manure

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 ¹⁵N 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&thinsp;h chamber closing is always significant, with values up to &gt;50&thinsp;% 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&thinsp;% 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&thinsp;%. 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