Reduction of nitrogen losses in vegetable production by optimization of the fertilization and crop residues management

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Title: Reduction of nitrogen losses in vegetable production by optimization of the fertilization and crop residues management
Other Titles: Optimierung von Düngungs- und Bewirtschaftungsstrategien im Gemüsebau zur Minderung von Stickstoffemissionen
Authors: Frerichs, Christian
ORCID of the author:
Thesis advisor: Prof. Dr. Gabriele Broll
Thesis referee: Prof. Dr. Diemo Daum
Abstract: To ensure a high yield and quality of the produce, a high fertilizer nitrogen (N) supply is often required in vegetable crop rotations. However, the fertilizer N requirement is difficult to calculate since the N supply via mineralization and actual N uptake of the crop cannot accurately be predicted in advance. Therefore, base fertilization and top dressing are usually applied before intensive N uptake starts. However, if a crop is harvested before yield expectations are reached, this strategy can result in high soil mineral N residues. In addition, high quantities of N-rich and readily mineralizable crop residues often remain in the field at harvest. In sandy soils, however, nitrate can be easily leached. Other significant N losses are caused by gaseous ammonia volatilization and nitrous oxide emission. However, the emission of these N species can directly and indirectly damage ecosystems, contribute to global warming, and contaminate sources of drinking water. The first part of this study aimed at the risk of nitrate leaching losses during the cultivation of field-grown spinach. Therefore, the effect of a reduced N base fertilization as well as a split N top dressing was examined in a series of fertilization trials. The reduced base fertilization was compensated by an increased top dressing based on the soil mineral N concentration. In a further treatment the second top dressing of 50–70 kg N/ha was applied by a frequent urea foliar spraying instead of the single application of a granulated nitrate fertilizer. A further part of this thesis focused on reducing post-harvest N losses following autumn-grown spinach during the winter leaching period. For this purpose, the depth, frequency, and timing of the incorporation of the spinach crop residues were varied. Furthermore, the effectiveness of spraying the crop residues with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) was examined. The results showed that both reducing the base fertilization and splitting the top dressing significantly decreased the nitrate concentration exposed to leaching. In addition, the splitting approach made it possible to dispense with the second top dressing in the case of harvesting at an early harvest stage. Thus, the soil mineral N residues were reduced. On the other hand, these measures diminished the crop yield at an early harvest stage by 6% on average. At a late harvest stage, spinach was less affected by the schedule of the fertilizer N supply. Due to diminished plant growth and necrosis at the leaf margin, a frequent urea foliar spray proved to be an inappropriate fertilization strategy in contrast to the application of a granulated fertilizer in combination with a subsequent irrigation. In spring-grown and winter-grown spinach, nitrogen was one of the most limiting growth factors. In contrast, in the summer and autumn seasons marketable yield appeared to be limited by bolting and decreasing irradiation, respectively. Thus, in these seasons it was possible to dispense with the second top dressing without affecting the marketable yield. Within a few weeks after the incorporation of the spinach crop residues an intensive mineralization increased the soil nitrate concentration up to 100 kg N/ha (0–90 cm). Even the treatment of the crop residues with DMPP was hardly able to delay nitrate formation. In addition, the winter cover crops only partially absorbed the soil mineral N. As a result, high quantities of nitrate shifted down the soil profile during the winter leaching period. Based on an N balance sheet, potential N losses were mostly independent of the tillage intensity. In contrast, by postponing the incorporation of the crop residues from early autumn to spring the potential N losses were reduced to ≤ 20 kg/ha. This effect was due to the N uptake and N conservation of the resprouting spinach crop residues as well as the lower net N mineralization in the soil. However, when the spinach plants partially or completely decomposed in autumn and winter, the potential N losses were considerable even without tillage. To ensure N uptake and a low N mineralization a superficial sowing of a winter hard catch crop into the resprouting spinach crop residues without soil disturbances seems to be a promising approach. Unlike open-field cultivation, nitrate leaching losses can be effectively reduced by cultivation in soilless closed-loop irrigation systems. However, organic cultivation of fast-grown crops like basil is challenging in such systems since N supply is dependent on the mineralization of organic fertilizers in a small substrate volume. To ensure a sufficient N availability, high quantities of organic N are applied to the growing media. However, depending on the mineralization rate, this can lead to plant damage and yield depressions. Based on this background, in the third part of this thesis it was examined whether pot-grown basil is affected by ammonia and ammonium exposures resulting from an organic fertilization. Therefore, basil was cultivated at different base N fertilization and substrate pH levels. The growing media consisted of pure peat or a mixture of peat and 5% (v/v) mature green waste compost. A high base fertilization as well as high substrate pH caused considerable damage to basil seedlings and diminished crop growth. These observations were mainly explained by the ammonical exposure. Thus, both a reduced base fertilization and lower initial substrate pH mitigated these effects. However, optimum plant growth was only achieved by the amendment of compost. In addition, the peat-compost mixture had to be stored for about two weeks before sowing. In this approach an early initiation of the nitrification led to a fast and effective reduction of the ammonia and ammonium exposures right from the seedling stage. Furthermore, the resulting increased nitrate supply promoted basil growth. Due to the variety of organic fertilizers and substrate components used in organically grown crops, further trials are required to make general recommendations. The optimized synchronization of N demand and N supply proved to be effective in reducing potential nitrate leaching losses in open fields as well as prevent harmful exposures to ammonia and ammonium in organically pot-grown basil. However, the development of specific plant and soil monitoring technologies seems to be necessary to better estimate whether the N fertilization can be delayed or even the total N supply can be reduced without affecting marketable yield in individual cases. Quantitatively, most N losses were observed in the off-season during the winter leaching period. A holistic approach to reducing N losses and emissions into the environment must therefore also include the post-harvest strategy.
Subject Keywords: Stickstoff; Nitratauswaschungsrisiko; Ammoniak; Ammonium; Spinat; Basilikum
Issue Date: 14-Mar-2023
License name: Attribution 3.0 Germany
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Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB01 - E-Dissertationen

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