Former colleagues

Dr. Ivan Guzman Bustamante

PhD dissertation "Gaseous N emissions from a loamy soil as affected by N fertilization strategies, and by the use of nitrification and urease inhibitors - Results from field and incubation experiments"
Summary

Agricultural activities are responsible for a substantial share of anthropogenic greenhouse gases. At the same time, agricultural production must feed a growing world population under a changing climate. In the case of wheat, the use of nitrogen (N) fertilizers is needed in order to insure grain yield and quality. Nevertheless, its use is associated with reactive N losses, which are detrimental for the environment and human health. Among the gaseous N species emitted after N fertilization we find nitrous oxide (N2O), a potent greenhouse gas, and ammonia (NH3) that after its deposition can be oxidized to N2O.
Chemical compounds such as nitrification and urease inhibitors (NIs and UIs, respectively) are a useful tool, able to raise the fertilizer nitrogen use efficiency, by retarding the nitrification of ammonium based fertilizer in the case of NIs and by retarding the hydrolysis of urea in the case of UIs. A side benefit of the use of NIs is the reduction of N2O emissions. The use of UIs reduces the NH3 volatilization. One of the most used NIs in Europe is 3,4-dimethylpyrazol phosphate (DMPP) which can be applied with ammonium sulfate nitrate (ASN). The relatively new NI, 3,4-dimethylpyrazol succinic acid (DMPSA), acts similarly to DMPP but has a different time of action and can be applied to several fertilizers, unlike DMPP. N-(n-butyl) thiophosphoric triamide (NBPT) is an effective UI that provenly reduces NH3 volatilization by inhibiting the urease enzyme.
In a two-year field experiment with winter wheat several fertilizer strategies were tested, including splitting strategies, use of NIs and reduction of N amount. Reducing N amount reduces the amount of soil mineral N, which is the substrate for N2O producing microbiological processes, nitrification and denitrification. Splitting of N fertilizer might reduce soil mineral N as well because N fertilizer applications are better suited to the physiological needs of the wheat plants. Applying NIs in splitting schemes may further mitigate emissions. The relationship between N amount and N2O losses in a wheat production system was investigated by applying lower and higher N amounts than the recommended N application rate.
Use of DMPP was able to reduce N2O emissions in both years, not only on an annual basis (by 21 %: 3.1 vs 2.5 kg N2O-N ha-1 a-1 average for both years) but also during winter, when up to 18 % of total annual emissions occurred. A change of the soil microbial community due to DMPP could be the reason for the reduction of winter emissions 8 to 12 months after DMPP application. An economic assessment of N fertilizer amount showed that DMPP applied with suboptimal N fertilizer amounts can maintain yield and at the same time decrease yield scaled N2O emissions compared to an optimal N fertilizer rate without NI.
Using CAN together with the NI DMPSA reduced N2O emissions only during the vegetation period. On an annual basis, DMPSA did not significantly reduce N2O emissions. Because DMPSA and DMPP were applied with different N fertilizers with different ammonium and nitrate shares, a direct comparison between these two NIs cannot be made.
A traditional threefold split fertilization did not reduce annual emissions compared to a single application of ASN or CAN. Nevertheless, the use of DMPP in twofold split applications reduced annual emissions significantly by 33 % and increased protein content by 1.6 %.
Because N2O flux peaks were not as high as expected after N fertilization during the first year, a short experiment investigating the effect of soil moisture, N and C application on N2O fluxes was conducted. A C limitation of the field was found, which explained high N2O emission events when C was available, e.g. after rewetting of dry soil and incorporation of straw after harvest. In this context we tested the removal of wheat straw – which should reduce the organic substrate supply for denitrifiers – as a possible mitigation strategy. Nevertheless, the removal of straw had no effect on N2O emissions.
Furthermore, the effect of DMPP on microorganisms was studied in an incubation experiment: the copy number of bacterial amoA genes (nitrifiers) was lowered by the use of DMPP, while the number of archaeal amoA genes was increased by DMPP. Gene copy number of denitrifiers was unaffected by DMPP, nevertheless, soil respiration was reduced when DMPP was applied. It seems as DMPP has an inhibiting effect on heterotrophic organisms, nevertheless, the investigated variables did not support this hypothesis, so that further investigation is needed.
The effect of NBPT and straw residues on NH3 and N2O emissions was studied in a two-week incubation experiment with a slightly alkaline soil. NBPT reduced NH3 volatilization and N2O fluxes from urea fertilization almost completely. Incorporation of straw residues significantly increased N2O emissions. In a further four-week incubation experiment, the effect of NBPT in two concentrations and DMPP was studied. A higher NBPT concentration as the recommended rate, reduced NH3 emissions by 53 %; DMPP on the other hand increased NH3 volatilization by 70 %. Regarding N2O, DMPP reduced emissions to the same level as the unfertilized control; NBPT only shifted the emission peak so that by the end of the experiment no difference in the cumulative N2O emission was found between urea and NBPT treatments. These results show that UI can lead to a reduction of N2O emissions, but the ammonium formed by the urea hydrolysis should be used by crops, otherwise it serves as a substrate for N2O production in soils.
In the final incubation experiment, the combined application of a NI (DMPSA) and a UI (NBPT) was studied. Lower concentrations than the recommended doses were applied in order to assess synergistic effects. The combined application of DMPSA and NBPT did not lead to synergistic effects in the analyzed variables (soil urea amount, soil mineral N, ammonia volatilization, soil respiration and N2O emission). The higher the NBPT concentration, the slower urea was hydrolyzed and the higher the reduction in NH3 volatilization. A third of DMPSA application rate was enough to reduce N2O emissions; however, the use of NI increased NH3 losses.
Our results highlight the importance of annual datasets when assessing mitigation strategies for N2O. For wheat production, a reduction of the N fertilizer amount when a NI is used should be taken into consideration. When elite wheat cultivars are grown split application with NI fertilizers could ensure high protein content and simultaneously reduce N2O emission. Urea fertilizer should be applied with NI and UI so that NH3 volatilization and N2O emission is reduced. Nevertheless, long-term effects of these compounds on soil microbiology must be monitored to avoid unseen ecotoxicological effects. Since some of these compounds or their metabolites might be absorbed by plants and end up in food and feed more research is needed to protect consumers.

Dr. Christina Herr

PhD dissertation "Effects of nitrification inhibitors and application technique on trace gas fluxes from a maize field after cattle slurry fertilization"
Summary

In a time of climate change and against the background of intensive animal husbandry and biogas production in Germany, strategies for mitigation of greenhouse gas (GHG) release and Nitrogen (N) losses from silage maize production become increasingly important, especially for organic fertilizers. Consequently, the main objective of this study was to determine the height of GHG release from silage maize production on a medium textured soil which is typical for this region in Southwest Germany and to evaluate useful fertilization opportunities to mitigate carbon dioxide (CO2) footprint per yield unit. To identify management factors improving GHG budget from silage maize, annual nitrous oxide (N2O) and methane (CH4) measurements were carried out during maize growth and subsequent black fallow at least weekly. Investigations were conducted over two years on two adjacent fields (one for each study year). Amounts of ammonia (NH3) volatilizations after fertilization and nitrate (NO3-) leaching losses were also included in GHG balances. In dependence on available data, determined or estimated values were used. Additionally, yield and N removal from maize plants were quantified. The basic treatments of this study which investigated impact of fertilizer form and application techniques, were an unfertilized control (CON), a mineral fertilization (MIN), a banded cattle slurry application by trailing hose and subsequent incorporation (INC) and a cattle slurry injection (INJ). As confirmed repeatedly, in contrast to broadcast slurry incorporation, slurry injection efficiently reduced the risk of NH3 losses by direct slurry placement into the soil, but simultaneously provoked N2O formation more strongly, probably due to the anaerobic conditions in the injection slot favoring denitrification. For reducing N2O release from slurry injection, the applicability of six single or combined nitrification inhibitors (NIs) concerning potential GHG reduction were investigated. This N2O reduction should be reached through the desynchronized availability of carbon (C) and NO3-, derived from nitrified slurry ammonium (NH4+). Thus, in the period after slurry application, N2O losses from denitrification as well as from nitrification should be reduced through NIs. For final evaluation, collection of measured and estimated data (including direct and indirect N2O losses (NH3, NO3-), CH4 budget, pre-chain emissions from mineral fertilizer and fuel consumption) were converted into CO2 equivalents and summarized as area- or yield-related GHG balances. Except for one of the INJ treatments with NI (exclusively investigated in the first year) and one INC treatment with NI (exclusively investigated in the second year), all remaining treatments were tested in both experimental years. The height of NH3 emissions from INC treatment (12-23 % of applied NH4+-N) was more weather-dependent than those from INJ treatment (12-15 % of applied NH4+-N). In mean over both years, cumulative N2O emission from INJ treatment (13.8 kg N2O-N ha-1 yr-1), was significantly higher than from CON, MIN, and INC which recorded 2.8, 4.7, and 4.4 kg N2O-N ha-1 yr-1. NIs decreased the fertilization-induced N2O emissions from injection by 36 % (mean over all NIs and years) by an order of magnitude comparable to slurry incorporation. The NIs investigated tended to be categorized in inhibitors with prior and delayed inhibitory maximum. Whether low persistence, or poor biological degradability was an advantage, depended on environmental conditions. A combination of two NIs, one with putative prior and one with delayed release behavior reached the highest N2O reduction. In the additional INC treatment, this NI combination tended to reduce annual N2O release by 20 % in comparison to incorporation without inhibitor. Beside the potential of reducing fertilization-induced N2O emissions, NIs might also help to improve CH4 budgets in silage maize production. In general, CON, MIN and INC were net CH4 sinks in both years with mean uptakes of 460, 127, and 793 g CH4-C ha-1 yr-1, respectively. Conversely, slurry injection resulted in net CH4 emissions of 3144 g CH4-C ha-1 yr-1 (mean over both years). However, NIs tended to reduce CH4 emissions from injection by around 48 % and increased CH4 consumption from slurry incorporation by 20 %. Across all treatments and years, direct N2O emissions were the major contributor to total GHG balance. Yield-related GHG budgets from both years were lowest for CON, followed by INC or MIN treatment and significantly highest for sole slurry injection. NIs decreased fertilization-induced GHG release from injection in mean over both years by order of magnitude comparable with slurry incorporation. Consequently, alongside slurry incorporation and broadcast mineral fertilization, slurry injection combined with recommended NIs was evaluated as an equally appropriate fertilization strategy in terms of the atmospheric burden for livestock farmers.

Dr. Katharina Kesenheimer

PhD dissertation "Nitrous oxide emissions and mitigation strategies in winter oilseed rape cultivation"
Summary

After carbon dioxide and methane, nitrous oxide, is the third most important greenhouse gas in the atmosphere. Nitrous oxide contributes to the greenhouse gas effect as well as to ozone depletion. The major portion of anthropogenic N2O emissions are stimulated by the use of nitrogen fertilizers in agriculture. The main processes for N2O production in soils are nitrification and denitrification. Various environmental and management factors such as precipitation, soil type, tillage, and crop residues affect these processes. N2O emissions can occur substantially in the post-harvest period. In Germany, approximately 50 % of the annual N2O emissions can occur during winter. This exhibits the importance and necessity of annual data sets which prevent misinterpretations instigated by investigations limited to the vegetation period.

Winter oilseed rape is the most important raw material for biodiesel in Germany. As of 2018, the framework of the European Renewable Energy Directive requires that the use of biofuels achieve GHG savings of at least 50 % compared to fossil fuels. Feedstock production for biodiesel contributes more than half of the total GHG emissions. To close the nutrient cycle with renewable energy, digestate from biogas plants can be used as a substitute for mineral N fertilizer permitting the reduction of GHG emissions in the production process of synthetic fertilizers. When compared to other crops, OSR has a high N demand. The low N removal by the seeds results in inefficient use of nitrogen and therefore a high N surplus in the soil which is susceptible to gaseous or leaching losses to the environment. Another potential risk for N2O losses are crop residues after harvest. The type of soil cultivation can have both positive and negative implications on N2O emissions which depend, among other things, on tillage depth, soil type and moisture. Results from studies measuring N2O emissions from different tillage systems are contradicting and site dependent.

This study aims to investigate the effect of (a) N fertilization (mineral and organic), (b) nitrification inhibitors, (c) crop residues and (d) tillage on direct N2O emissions and, inter alia, yield and soil nitrogen dynamics in OSR production. N2O emissions were monitored for three years over a range of N fertilization levels at five study sites chosen so as to best represent typical winter oilseed rape production in Germany. Furthermore, the effect of the nitrification inhibitor (NI) TZ+MP (1H-1,2,4-triazole and 3- methylpyrazole) with digestate is investigated. Additional experiments for 15N labelled crop residues, nitrification inhibitor DMPP (3,4-dimethylepyrazole phosphate) with mineral fertilizer and soil tillage were implemented.

A high spatial and temporal variability in N2O fluxes over all sites was observed. At each site, increased N2O fluxes were often detected after N fertilization in conjunction with rainfall events. During the first six weeks after harvest we frequently observed increased N2O fluxes following rainfall. In this postharvest period of winter oilseed rape, nitrate contents in the top soil were generally elevated. There were no considerable N2O pulses observed during thawing of frozen soil. Winters were mild without any severe frost periods in all three surveyed years which could be a reason for the generally low N2O winter fluxes observed in this study. On all examined sites, increasing N fertilization significantly enhanced N2O flux rates. Data obtained during the study were used to augment an existing model, wherefrom a new emission factor for OSR can be calculated. Assuming a quantity of 200 kg N ha-1 the global fertilizer-related emission factor derived from the exponential model was 0.6 %. This factor is within the uncertainty range of the EF1 IPCC emission factor (0.3 % - 3.0 %), but about 40 % lower than the 1 % IPCC default. The nitrification inhibitor (NI) TZ+MP combined with digestate mitigated the N2O fluxes significantly across all study sites and experimental years. As already noted in the fertilizer experiment, a high spatial and temporal variability in N2O fluxes over all sites was observed. The magnitudes of the N2O fluxes also showed similar trends. Over the entire investigation, the application of the NI significantly reduced annual N2O emission by a factor of three. During the fertilization period this mitigation effect was six times significant. This clearly emphasizes the importance of annual data sets to avoid overestimating NI effects.

Dr. Helena Pfab

Doctoral thesis "Nitrous oxide emissions and mitigation strategies - Measurements on an intensively fertilized vegetable cropped loamy soil"

Summary of doctoral thesis

Nitrous oxide emissions and mitigation strategies: measurements on an intensively fertilized vegetable cropped loamy soil

Nitrous oxide (N2O) is a potent greenhouse gas which is also involved in stratospheric ozone depletion. There is consensus that a reduction in N2O emissions is ecologically worthwhile. Agricultural soils are the major source of N2O emissions in Germany. It is known that high N-fertilization stimulates N2O emissions by providing substrate for the microbial production of N2O by nitrification and denitrification in soils. However, outside the vegetation period, winter freeze/thaw events can also lead to high N2O emissions. Winter emissions constitute about 50% of total emissions in Germany. Therefore, annual datasets are a prerequisite for the development of N2O mitigation strategies in regions with winter frost.

Many studies have investigated mitigation strategies for N2O emissions from agricultural soils. However, N2O release from vegetable production has seldom been studied. None of the existing trace gas measurements on intensive vegetable production is representative for the climatic conditions of Southern Germany. Due to the high fertilizer N-input (resulting in high levels of mineral N in the soil) and N-rich residues in late autumn, high annual N2O emissions are to be expected.

N2O fluxes were measured from a soilcropped with lettuce and cauliflower in Southern Germany by means of the closed chamber method, at least weekly, for two years. An additional study was conducted using 15N labeled ammonium sulfate nitrate (ASN) fertilizer and exchange of labeled and unlabeled residues to obtain information about the sources (fertilizer, residues, soil internal mineralization) of N2O emissions.

Different mitigation strategies such as fertilizer reduction, addition of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) and banded fertilization were evaluated with respect to their reduction potential on an annual base. Fertilizer reduction is supposed to decrease the soil mineral N level, reducing the available substrate for N2O producing microorganisms. DMPP is a chemical compound which inhibits nitrification enzymatically. In banded fertilization, ammonium rich fertilizer is applied in a depot. This high concentration is also supposed to inhibit nitrification as it is toxic to microorganisms. N2O emissions should be firstly reduced directly by this inhibition of nitrification and secondly, by a lower nitrate content in soil resulting in less N2O release due to denitrification.

A high temporal variability in N2O fluxes was observed with emission peaks after N-fertilization, after the incorporation of crop residues (especially in combination with N-fertilization), after rewetting of dry soil and after thawing of frozen soil in winter. Total cumulative annual emissions were 8.8 and 4.7 kg N2O-N ha-1 a-1 for the first and second experimental year in the conventionally (broadcast) fertilized treatment. This treatment was fertilized according to the German Target Value System. N2O emission factors were 1.6 and 0.8%. This is within the range of 0.3 - 3% which is cited in the Guidelines for the Calculation of National Greenhouse Gas Inventories proposed by the Intergovernmental Panel of Climate Change (IPCC).

A positive correlation was found in both years between the mean nitrate content of the top soil and the cumulative N2O emissions of all treatments (r2=0.44 and 0.68) as well as between the N-surpluses and the cumulative N2O emissions of the different fertilizer levels during the first year (r2=0.95). Fertilizer reduction from fertilization according to good agricultural practice following the recommendations of the German Target Value System reduced annual N2O emissions by 17% in the first experimental year without yield reduction. For the second year, the reducing effect was 10%, but statistically not significant. Another fertilizer reduction of a further 20% reduced N2O emissions, but also resulted in lower lettuce yields in the first year. Therefore, an additional fertilizer reduction is not recommendable.

This work provides, for the first time, annual datasets on the effect of DMPP-application on N2O emissions. Addition of DMPP significantly reduced annual N2O emissions by > 40% during both years, there was also a pronounced effect, both during the vegetation period and winter. The reason for the reducing effect in winter is not yet clear because the degradation of the active agent DMPP is temperature dependent and should take about 6 to 8 weeks under summer climatic conditions. However, we still observed significant reductions in N2O emissions in winter, about 3 months after the application. Furthermore, a reduction in CO2 release was observed indicating a possible influence on heterotrophic activities or at least on their C-turnover. Due to its high N2O mitigation potential, further investigations concerning the functional and structural changes in microbial biomass after DMPP application are needed. Banded fertilization with ASN did not result in the expected reduction in N2O emissions on an annual base. Even when exchanging the ASN fertilizer by nitrate-free ammonium sulfate, N2O emissions were not diminished. We assume that the high emissions were derived from the microbially intact surroundings of the depots, where nitrification was not inhibited and nitrate concentrations were probably very high, creating ideal conditions for denitrification.

After one year, the major part of the fertilizer-15N was found in the soil. Only between 13 -15% of the fertilizer was taken up by the marketable plant parts. 1.4% of the 15N was lost as N2O-N. Total 15N recovery was 70% after one year. The losses of non-recovered N were probably caused by nitrate leaching or as gaseous compounds such as N2 or NOx. Compared to cereal production systems, the N use efficiency of this vegetable production system is much lower, even with an optimized fertilization strategy. The measurement of 15N abundances in the N2O revealed that the most significant part of the emissions (38%) was derived from the fertilizer-N which had been taken up by cauliflower residues. N2O emissions directly derived from lettuce and cauliflower fertilizer contributed 26% and 20% respectively while N2O emissions from soil internal N pools accounted for 15%. The contribution of lettuce residues was negligible due to their low amount of C and N.
The reason for the high importance of the cauliflower residues was ascribed to the temporarily C-limitation of the system and the provision of electron donators by organic material. Furthermore, O2 is consumed during their degradation leading to the formation of anaerobic microsites when soil moisture is high. These sites offer ideal conditions for denitrification. Especially the combination of mineral N-fertilization and input of organic substance was found to increase N2O emissions. Therefore, the influence of a de-synchronization of the incorporation of crop residues and the mineral N-fertilization by waiting periods of up to 3 weeks was tested in an additional field trial during the cultivation of chard. The longer the waiting time between incorporation of crop residues and N-fertilizer application was, the lower were the N2O emissions. However, the effect was not statistically significant on an annual base.

In an additional microcosm incubation model study, the effect of reduced and increased input as well as of different C/N-ratios of cauliflower residues was analyzed. It was shown that due to the high nitrate level in the microcosms only the amount of residue input has an effect on the N2O emissions. The N2O emissions increased with increased amount of cauliflower residues.

Although the emission factors were within the range given by the IPCC, the absolute annual N2O emission was high in intensive vegetable production due to the high N-input. Further research is required in order to fully understand the effect of DMPP on the processes of N2O production in the field. Our study underlines the importance of avoiding N-surpluses and of strategies for residue management to reduce N2O emissions in intensive vegetable production.

Perik Seiz

  • PhD student in project "Data-based N-balancing in open field vegetable production: Impact of measures to reduce N-balance surpluses on N2O emissions from soils used for vegetable production" (Datengestützte N-Bilanzierung im Freiland-Gemüsebau: Einfluss von Maßnahmen zur Verminderung von N-Bilanzüberschüssen auf die N2O-Emission gemüsebaulich genutzter Böden)

  • Publications:

    Seiz, P.; Guzman-Bustamante, I.; Schulz, R.; Müller, T.; Ruser, R. Effect of crop residue removal and straw addition on nitrous oxide emissions from a horticulturally used soil in South Germany. Soil Sci. Soc. Am. J. 2019, 83, 1399–1409. https://doi.org/10.2136/sssaj2018.11.0448

    Budhathoki, R.; Panday, D.; Seiz, P.; Ruser, R.; Müller, T. Effect of Broccoli Residue and Wheat Straw Addition on Nitrous Oxide Emissions in Silt Loam Soil. Nitrogen 2021, 2, 99-109. https://doi.org/10.3390/nitrogen2010007

Lisa Stecher

  • Technische Assistentin im Projekt Minderung der Treibhausgasemissionen im Rapsanbau unter besonderer Berücksichtigung der Stickstoffdüngung