Open Access

Injection of slurry or digestate below maize seeds is a relatively new technique developed to improve nitrogen use efficiency. However, this practice has the major drawback of increasing nitrous oxide (N2O) emissions. The application of a nitrification inhibitor (NI) is an effective method to reduce these emissions. To evaluate the effect of the NI 3,4‐dimethypyrazole phosphate (DMPP) on N2O emissions and the stabilization of ammonium, a two‐factorial soil‐column experiment was conducted. PVC pipes (20 cm diameter and 30 cm length) were used as incubation vessels for the soil‐columns. The trial consisted of four treatments in a randomized block design with four replications: slurry injection, slurry injection + DMPP, digestate injection, and digestate injection + DMPP. During the 47‐day incubation period, N2O fluxes were measured twice a week and cumulated by linear interpolation of the gas‐fluxes of consecutive measurement dates. After completion of the gas flux measurement, concentration of ammonium and nitrate within the soil‐columns was determined. DMPP delayed the conversion of ammonium within the manure injection zone significantly. This effect was considerably more pronounced in treatment digestate + NI than in treatment slurry + NI. Regarding the cumulated N2O emissions, no difference between slurry and digestate treatments was determined. DMPP reduced the release of N2O significantly. Transferring the results into practice, the use of DMPP is a promising way to reduce greenhouse gas emissions and nitrate leaching, following the injection of slurry or digestate.

Easy and inexpensive methods for measuring ammonia emissions in multi-plot field trials allow the comparison of several treatments with liquid manure application. One approach that might be suitable under these conditions is the dynamic tube method (DTM). Applying the DTM, a mobile chamber system is placed on the soil surface, and the air volume within is exchanged at a constant rate for approx. 90 s. with an automated pump. This procedure is assumed to achieve an equilibrium ammonia concentration within the system. Subsequently, a measurement is performed using an ammonia-sensitive detector tube. Ammonia fluxes are calculated based on an empirical model that also takes into account the background ammonia concentration measured on unfertilized control plots. Between measurements on different plots, the chamber system is flushed with ambient air and cleaned with paper towels to minimize contamination with ammonia. The aim of this study was to determine important prerequisites and boundary conditions for the application of the DTM. We conducted a laboratory experiment to test if the ammonia concentration remains stable while performing a measurement. Furthermore, we investigated the cleaning procedure and the effect of potential ammonia carryover on cumulated emissions under field conditions following liquid manure application. The laboratory experiment indicated that the premeasurement phase to ensure a constant ammonia concentration is not sufficient. The concentration only stabilized after performing more than 100 pump strokes, with 20 pump strokes (lasting approximately 90 s) being the recommendation. However, the duration of performing a measurement can vary substantially, and linear conversion accounts for those differences, so a stable concentration is mandatory. Further experiments showed that the cleaning procedure is not sufficient under field conditions. Thirty minutes after performing measurements on high emitting plots, which resulted in an ammonia concentration of approx. 10 ppm in the chamber, we detected a residual concentration of 2 ppm. This contamination may affect measurements on plots with liquid manure application as well as on untreated control plots. In a field experiment with trailing hose application of liquid manure, we subsequently demonstrated that the calculation of cumulative ammonia emissions can vary by a factor of three, depending on the degree of chamber system contamination when measuring control plots. When the ammoni background values were determined by an uncontaminated chamber system that was used to measure only control plots, cumulative ammonia emissions were approximately 9 kg NH3-N ha􀀀1. However, when ammonia background values were determined using the contaminated chamber system that was also used to measure on plots with liquid manure application, the calculation of cumulative ammonia losses indicated approximately 3 kg NH3-N ha􀀀1. Based on these results, it can be concluded that a new empirical DTM calibration is needed for multi-plot field experiments with high-emitting treatments.

Ammonia emissions caused by liquid manure application affect human life expectancy and threaten natural ecosystems. However, other associated concerns like greenhouse gas emissions and nitrate leaching are equally relevant. Thus, German legislation severely restricted autumn application of liquid manure, since most crops have low nutrient demand at this development stage, so that much of the applied nitrogen would be lost to the environment. Therefore, liquid manures have to be applied in spring into growing crops. However, immediate incorporation into the soil to minimize ammonia emissions is not possible under these circumstances. Moreover, biogas digestate has become an increasingly popular organic fertilizer over the last three decades, since anaerobic fermentation is a climate friendly energy source. However, it might be associated with increased ammonia emissions due to its comparatively high pH and ammonium content. Therefore, liquid manure should be applied using optimized techniques for growing crops to mitigate ammonia emissions. Those techniques are based either on reducing the contact of fertilizer and atmosphere or on acidifying liquid manure. To evaluate optimized techniques, ammonia emissions have to be quantified in multi-plot field trials. Unfortunately, standard micrometeorological methods require large field areas and expensive equipment, making them difficult or even impossible to apply. Thus, other approaches adjusted to those specific requirements are used in multi-plot field trials. Calibrated passive sampling uses acid traps placed in the center of each plot to absorb ammonia, which enables a relative comparison of emissions. Subsequently, acid trap samplings are scaled by simultaneous measurements with the dynamic tube method, which uses a mobile chamber system to quantify ammonia emissions. The first objective of this study was therefore to evaluate calibrated passive sampling in multi-plot field trials with liquid manure application. However, ammonia drift between plots as well as chamber system contamination might be a particular challenge in such an experimental set-up. Therefore, the first step was to analyze the potential influence of the ammonia background on acid trap samplings and dynamic tube measurements. In a second step, the best practice to scale relative differences between plots obtained from acid trap samplings was assessed. In order to reduce costs and to minimize chamber system contamination, dynamic tube measurements are only performed on a few selected plots. Thus, characteristics of a well-suited treatment to perform simultaneous measurements with both methods were evaluated. However, the transfer coefficient (cumulated qualitative emissions divided by cumulated acid trap samplings) required to scale emissions might depend on the level of data aggregation. It can be calculated based on individual plots, treatment means or all plots of a field experiment. Therefore, it was evaluated which amount of data aggregation is sufficient. The second objective of this study was to evaluate optimized techniques to apply liquid manure in growing winter wheat in a series of field experiments in Germany. Calibrated passive sampling was used to assess ammonia emissions and yield and nitrogen uptake were measured as well to allow an agronomical evaluation of those techniques. Nitrogen fertilizer were applied at a total rate of 170 kg N ha−1 split into two equal dressings. Each experiment consisted of several techniques to apply cattle slurry and biogas digestate: i) trailing hose application using untreated and ii) acidified liquid manure, as well as iii) a combination of open slot injection for the first dressing and trailing shoe application for the second dressing. Furthermore, ammonia emissions, yield and nitrogen uptake of organically fertilized treatments were put into perspective by also implementing a treatment with mineral fertilization (broadcast calcium ammonium nitrate) and an unfertilized control. Furthermore, the unfertilized control was crucial to assess the influence of ammonia drift between plots. Acid trap samplings differed significantly between control plots, indicating that cumulated samplings of each individual plot depend not only on the ammonia emissions of the respective plot, but also on its specific background. Hence, many replications are necessary to obtain valid treatment means and those mean values show high standard deviations. However, there is no evidence, that passive sampler results are generally biased. Therefore, they are an easy way to obtain relative comparisons between treatment means. For the dynamic tube method, ammonia drift between plots had only a minor impact. However, we showed that chamber system contamination has a profound effect on calculating cumulated ammonia emissions in multi-plot field trials. The on field cleaning procedure using paper towels was not sufficient to reduce contamination. The relative influence of background ammonia was higher in treatments with low emissions for both methods. Therefore, scaling of acid trap samplings by simultaneous dynamic tube measurements should be performed in a treatment with high ammonia emissions. Regarding the amount of data aggregation required to scale emissions, this thesis showed that calculating a transfer coefficient based on individual plots is not sufficient, due to the influence of the fluctuating ammonia background. Therefore, acid trap samplings were scaled based on mean values in a treatment with high ammonia emissions. In this series of winter wheat field trials, the highest ammonia emissions (on average 24 kg N ha−1) occurred following trailing hose application. Applying biogas digestate lead to approximately 60 % higher emissions than cattle slurry application. Overall, acidification reduced emissions by 64 % for both liquid manure types. On average, the combination of slot injection and trailing shoe application resulted in 23% lower ammonia emissions compared to trailing hose application. However, decreasing ammonia emissions did not increase yield and nitrogen uptake. All treatments with liquid manure application led to similar crop yield (approximately 7 t ha−1 grain dry matter yield) and aboveground biomass nitrogen uptake (approximately 150 kg ha−1). Yield (8 t ha−1) and nitrogen uptake (approximately 190 kg ha−1) were significantly increased for the minerally fertilized treatment, while for the control, yield (approximately 4.5 t ha−1) and nitrogen uptake (approximately 90 kg ha−1) were significantly reduced. In summary, our results show that the mitigation of ammonia emissions originating from liquid manure application to growing crops is possible by using optimized application techniques. For this series of field trials, acidification was the technique with the highest ammonia mitigation potential. Future studies using calibrated passive sampling should address the importance of ammonia drift and chamber system contamination. Therefore, the use of separate dynamic tube chamber systems for each plot is recommended. Furthermore, increasing the plot size might reduce ammonia drift.

There is a great need for simple and inexpensive methods to quantify ammonia emissions in multi-plot field trials. However, methods that meet these criteria have to be thoroughly validated. In the calibrated passive sampling approach, acid traps placed in the center of quadratic plots absorb ammonia, enabling relative comparisons between plots. To quantify ammonia emissions, these acid trap samplings are scaled by means of a transfer coefficient (TC) obtained from simultaneous measurements with the dynamic tube method (DTM). However, dynamic tube measurements are also comparatively costly and time-consuming. Our objective was to assess the best practice for using calibrated passive sampling in multi-plot field trials. One particular challenge in such experiments is to evaluate the influence of ammonia drift between plots. In a series of eight multi-plot field trials, acid traps and DTM were used simultaneously on all plots to measure ammonia emissions caused by different slurry application techniques. Data obtained by both methods were correlated, and the influence of the ubiquitous ammonia background on both methods was evaluated by comparing net values, including the subtraction of the background with gross values (no background subtraction). Finally, we provide recommendations for calculating a TC for calibrating relative differences between plots, based on simultaneous acid trap and dynamic tube measurements on selected plots. Treatmentmean values obtained by bothmethods correlatedwell. For most field trials, R2 values between 0.6 and 0.8were obtained. Ammonia background concentrations affected both methods. Drift between plots contributed to the background for the acid traps, whereas the contamination of the chamber system might have caused the background for the DTM. Treatments with low emissions were comparatively more affected by that background. For a robust application of calibrated passive sampling, we recommend calculating the TC based on a treatment with high ammonia emissions, reducing the relative influence of the ubiquitous ammonia background.