Biosphere-atmosphere-exchange of C and N trace gases and microbial N turnover processes in irrigated agricultural systems of the Aral Sea Basin, Uzbekistan

Land-use and agricultural practices affect the soil microbial carbon (C) and nitrogen (N) turnover and hence the biosphere-atmosphere exchange of greenhouse gasses (GHG), namely N2O, CH4 and CO2. In view of the global importance of irrigated agriculture, it is crucial to understand how and to which extent this land-use system interferes with the terrestrial N and C cycles and contributes to the global source strength of atmospheric GHG. Up to now, knowledge of trace gas exchange and N turnover from irrigated agriculture in arid and semiarid regions is much less developed than in other climate zones. Therefore, this study aims at providing more detailed insights into the biosphere-atmosphere exchange of trace gases and the underlying soil microbial transformation processes of the irrigated agricultural systems in the Aral Sea Basin (ASB), Uzbekistan. A two-year field study was carried out to quantify and compare emissions of N2O and CH4 in various annual and perennial land-use systems dominating in the study region Khorezm in western Uzbekistan: irrigated cotton, winter wheat and rice crops, a poplar plantation as well as a natural Tugai (floodplain) forest. Irrigated agricultural production in the ASB was shown to be a relevant source of GHG especially due to high emissions of N2O during the annual cropping of wheat and cotton. Average N2O emissions ranged from 10 to 150 µg N2O N m 2h-1 with highest N2O emissions in the cotton fields, covering a similar range described in previous studies of other irrigated cropping systems. Seasonal variations in N2O emissions were principally controlled by fertilization and irrigation management. Very high N2O emissions of up to 3000 µg N2O-N m-2 h-1 were measured in periods directly following N fertilizer application in combination with irrigation events. These “emission pulses” accounted for 80-95% of the total N2O emissions over the cropping season for cotton and wheat. Cumulated emissions over one season varied from 0.5 to 6.5 kg N2O-N ha-1. The unfertilized poplar plantation showed high N2O emissions over the entire study period (30µg N2O N m 2h-1), whereas only negligible fluxes of N2O (2O N m 2h-1) occurred in the natural Tugai forest. Observations of significant CH4 fluxes were restricted to the flooded rice fields, with mean flux rates of 32 mg CH4 m 2d-1 and a seasonal total of 35.2 kg CH4 ha-1. The global warming potential (GWP) of the N2O and CH4 fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO2 eq.ha-1. The biennial cotton-wheat-rice crop rotation commonly practiced in the region averaged a GWP of 2500 kg CO2 eq.ha 1 year-1. In addition, laboratory incubation studies were conducted to assess the aggregated gaseous N losses composed of NO, N2O, and N2 from fertilized and irrigated agricultural fields in the ASB. NO3- fertilizer and irrigation water were applied to the incubation vessels to assess its influence on the gaseous N emissions. Under the soil conditions, naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main gaseous product of denitrification. Based on the results of these laboratory incubation studies, the magnitude of N2 emissions for the different field research sites of irrigated cotton could be estimated to be in the range of 24±9 to 175±65 kg-N ha-1season-1, while emissions of NO were only of minor importance (between 0.1 and 0.7 kg-N ha-1 season-1). The findings demonstrate that under the current agricultural practices in the irrigated dryland soils of the ASB, denitrification is a major pathway of N losses and that beside N2O extensive amounts of N fertilizer are lost as N2 to the atmosphere. Moreover, the experimental design of this study allows assessing the potential for reducing GHG emissions from these land-use systems. It is argued that there is wide scope for reducing the GWP of this agroecosystem by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands.Biosphäre-Atmosphäre Austausch von C/N Spurengasen und mikrobielle N Umsetzungsprozesse in bewässerten, landwirtschaftlichen Produktions-systemen des Aralseebeckens, Usbekistan Die mikrobiellen Umsetzungsprozesse von Kohlenstoff (C) und Stickstoff (N) in Böden und der damit verbundene Austausch von Treibhausgasen zwischen Biosphäre und Atmosphäre werden maßgeblich von der Landnutzung und den landwirtschaftlichen Methoden beeinflusst. Angesichts der weltweiten Bedeutung von bewässerter Landwirtschaft ist es äußerst wichtig zu verstehen, in wie weit diese landwirtschaftlichen Systeme die globalen N und C Kreisläufe beeinflussen und zu den globalen Treibhausgasemissionen beitragen. Im Gegensatz zu den landwirtschaftlichen Systemen der temperaten Klimazonen ist über N und C Spurengasemissionen aus bewässerter Landwirtschaft in ariden und semiariden Gebieten nur sehr wenig bekannt. Um einen wesentlichen Beitrag zur Schließung dieser Forschungsdefizite zu leisten, konzentrierte sich diese Studie auf den Austausch von strahlungsaktiven Spurengasen zwischen Biosphäre und Atmosphäre und die hiermit assoziierten mikrobiellen N Umsetzungsprozesse in den Böden der bewässerten landwirtschaftlichen Systeme im Aralsee-Becken (ASB) von Usbekistan. Dafür wurde über einen Zeitraum von zwei Jahren in verschiedenen einjährigen und mehrjährigen Landnutzungssystemen die Emissionen der Treibhausgase Lachgas (N2O) und Methan (CH4) untersucht. Ausgewählt wurden Landnutzungsysteme die typisch für das Untersuchungsgebiet Khorezm, in West-Usbekistan, sind: bewässerter Baumwoll-, Winter Weizen- und Reisanbau sowie eine Pappel-Plantage und der natürliche „Tugai“ Auenwald entlang des Amu Darya Flusses. Es konnte festgestellt werden, dass der bewässerte Landbau im ASB insbesondere aufgrund von hohen N2O Emissionen aus dem Baumwoll- und Weizenanbau eine maßgebliche Quelle von Treibhausgasen darstellt. In den einjährigen Anbausystemen wurden mittlere N2O Emissionsraten zwischen 10 und 150 µg N2O N m-2h-1 festgestellt, wobei die höchsten Emissionen in Baumwollfeldern gemessen wurden. Über die gesamte Saison wurden die N2O Emissionen hauptsächlich von Düngung und Bewässerung beeinflusst. Dabei traten extrem hohe N2O Emissionen (bis zu 3000 µg N2O-N m-2 h-1) auf, wenn mineralischer N-Dünger direkt vor der Bewässerung appliziert wurde. Diese „Emissionsspitzen“ hatten einen Anteil von 80-95% an den Gesamtemissionen von N2O bezogen auf die Vegetationsperiode von Baumwolle und Weizen. Insgesamt variierten die N2O Emissionen über eine Saison von 0,5 bis 6,5 kg N2O-N ha-1. In der ungedüngten Pappel-Plantage wurden über den gesamten Messzeitraum hohe N2O Emissionen (30 µg N2O N m-2h-1) gemessen, wohingegen in dem Tugai Wald lediglich äußerst kleine Flüsse von N2O (2O N m-2h-1) festgestellt wurden. Bedeutende CH4 Emissionen traten nur in den gefluteten Reisfeldern auf, mit einer durchschnittlichen Flussrate von 32 mg CH4 m-2d-1 und einer Gesamtemission über die Vegetationsperiode von 35,2 kg CH4 ha-1. Das Treibhauspotenzial der N2O und CH4 Flüsse, dargestellt als CO2-Äquivalent, war am höchsten für den Reis- und Baumwollanbau, wobei auf den verschiedenen Messflächen die Gesamtemission einer Saison von 500 bis zu 3000 kg CO2 eq.ha-1 variierte. Für eine zweijährige Rotation von Baumwolle-Weizen und Reis, wie sie typisch für das Untersuchungsgebiet ist, konnte ein durchschnittliches Treibhauspotenzial von 2500 kg CO2 eq.ha-1 Jahr-1ermittelt werden. Zusätzlich wurden im Labor Inkubationsversuche an intakten Bodensäulen durchgeführt um die gasförmigen Stickstoffverluste, bestehend aus NO, N2O, und N2, der gedüngten und bewässerten Anbausysteme des ASB zu erfassen. Ammoniumnitrat Dünger wurde zusammen mit Wasser auf die Bodensäulen appliziert, um den Einfluss von gleichzeitiger Düngung und Bewässerung zu simulieren. Es konnte gezeigt werden, dass nach synchroner Düngung und Bewässerung Denitrifikation der vorherrschende Prozess in den Böden ist, und dass der größte Teil des Nitrats vollständig zu molekularem Stickstoff (N2) denitrifiziert wird. Aufgrund dieser Ergebnisse war es möglich für Baumwolle die Größenordnung der gasförmigen N Verluste von den verschiedenen Messflächen abzuschätzen. Demnach wurden von den einzelnen Baumwollfeldern zwischen 24±9 und 175±65 kg-N ha-1Saison-1 als N2 emittiert, während nur geringe Mengen von NO freigesetzt wurden (zwischen 0,1 und 0,7 kg-N ha-1 Saison-1). Diese Studie konnte somit zeigen, dass unter den gegenwärtigen landwirtschaftlichen Methoden im ASB, erhebliche Mengen von Stickstoff durch Denitrifikation als N2 an die Atmosphäre abgegeben werden. Ferner erlaubte das experimentelle Design dieser Studie Möglichkeiten einer Reduktion des Ausstoßes von Treibhausgasen aus diesen Anbausystemen abzuschätzen. Abschließend kann festgestellt werden, dass durch (i) eine Optimierung der Dünge- und Bewässerungsmethoden und (ii) einen Wechsel von einjährigen Feldfrüchten auf mehrjährige Baumplantagen, insbesondere auf unrentablen, marginalen Boden, das Treibhauspotential dieses landwirtschaftlichen Produktionssystems wesentlich reduziert werden kann.</p

A global dataset for the production and usage of cereal residues in the period 1997–2021

Crop residue management plays an important role in determining agricultural greenhouse gas emissions and related changes in soil carbon stocks. However, no publicly-available global dataset currently exists for how crop residues are managed. Here we present such a dataset, covering the period 1997–2021, on a 0.5° resolution grid. For each grid cell we estimate the total production of residues from cereal crops, and determine the fraction of residues (i) used for livestock feed/bedding, (ii) burnt on the field, (iii) used for other off-field purposes (e.g. domestic fuel, construction or industry), and (iv) left on the field. This dataset is the first of its kind, and can be used for multiple purposes, such as global crop modelling, including the calculation of greenhouse gas inventories, estimating crop-residue availability for biofuel production or modelling livestock feed availability

Estimating global terrestrial denitrification from measured N2_{2}O:(N2_{2}O + N2_{2}) product ratios

The use of nitrogen (N) fertilizers and cultivation of N-fixing crops has grown exponentially over the last century, with severe environmental consequences. Most of the anthropogenic reactive nitrogen will ultimately be returned by denitrification to the atmosphere as inert N$_{2}$, but the magnitude of denitrification and the ratio of N$_{2}$O to (N$_{2}$O + N$_{2}$) emitted (R$_{N_{2}O}$) is unknown for the vast majority of terrestrial ecosystems. This paper provides estimates of terrestrial denitrification and R$_{N_{2}O}$ by reviewing existing literature and compiling a N budget for the global land surface. We estimate that terrestrial denitrification has doubled from 80 Tg-N year$^{-1}$ in pre-industrial times to 160 Tg-N year$^{-1}$ in 2005 with a mean R$_{N_{2}O}$ of approximately 0.08. We conclude that upscaling of R$_{N_{2}O}$ can provide spatial estimates of terrestrial denitrification when data from acetylene inhibition methods are excluded. Recent advances in methodologies to measure N$_{2}$ emissions and R$_{N_{2}O}$ under field conditions could open the way for more effective management of terrestrial N flows

From research to policy: optimizing the design of a national monitoring system to mitigate soil nitrous oxide emissions

Nitrous oxide (N2O) emissions from agricultural soils are a key source of greenhouse gas emissions in most countries. In order for governments to effectively reduce N2O emissions, a national inventory system is needed for monitoring, reporting and verifying emissions that provides unbiased estimates with the highest precision feasible. Inventory frameworks could be advanced by incorporating experimental research networks targeting key gaps in process understanding and drivers of emissions, with a multi-stage survey to collect data on agricultural management and N2O fluxes that allow for development, parameterization and application of models to estimate national-scale emissions. Verification can be accomplished with independent estimation of fluxes from atmospheric N2O concentration data. A robust monitoring system would provide accurate emission estimates, and allow policymakers to develop programs to more sustainably manage reactive N and target mitigation measures for reducing N2O emissions from agricultural soils

Strategies for mitigating N2O and N2 emissions from an intensive sugarcane cropping system

In sugarcane cropping systems, high rates of N fertiliser are typically applied as sub-surface bands creating localised zones of high mineral N concentrations. This in combination with high levels of crop residue (trash) retention and a warm and humid climate creates conditions that are known to promote soil denitrification, resulting in high emissions of the potent greenhouse gas N2O. These losses illustrate inefficient use of N fertilisers but total denitrification losses in the form of N2 and N2O remain largely unknown. We used the 15N gas flux method to investigate the effect of cane trash removal and the use of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N2 and N2O emissions on a commercial sugarcane farm at Bundaberg, Australia. High gaseous N losses were observed under the standard grower practice where cane trash retention and N fertiliser application (145 kg N ha−1 as urea) resulted in N2 and N2O emissions (36.1 kg N ha−1) from the subsurface N fertiliser band, with more than 50% of these losses emitted as N2O. Cane trash removal reduced N2 emission by 34% and N2O emission by 51%, but had no effect on the N2O/(N2 + N2O) ratio. The use of DMPP lowered N2 and N2O emission by 35% and 98%, respectively, reducing the percentage of these losses (N2 + N2O) emitted as N2O to only 4%. We conclude that the use of DMPP is an effective strategy to reduce N losses, minimise N2O emissions, while keeping the benefits of cane trash retention in sugarcane cropping systems.</p

Measuring denitrification and the N2_{2}O:(N2_{2}O + N2_{2}) emission ratio from terrestrial soils

Denitrification, a significant pathway of reactive N-loss from terrestrial soils, impacts on agricultural production and the environment. Net production and emission of the denitrification product nitrous oxide (N$_{2}$O) is readily quantifiable, but measuring denitrification\u27s final product, dinitrogen (N$_{2}$), against a high atmospheric background remains challenging. This review examines methods quantifying both N$_{2}$ and N$_{2}$O emissions, based on inhibitors, helium/O$_{2}$ atmosphere exchange, and isotopes. These methods are evaluated regarding their capability to account for pathways of N$_{2}$ and N$_{2}$O production and we suggest quality parameters for measuring denitrification from controlled environments to the field scale. Our appraisal shows that method combinations, together with real-time monitoring and soil-gas diffusivity modelling, have the potential to significantly improve our quantitative understanding for denitrification from upland soils. Requirements for instrumentation and experimental setups however highlight the need to develop more mobile and easily accessible field methods to constrain denitrification from terrestrial soils across scales