The Nmin method in Switzerland: potential for improvement  and development in the context of digitalization
Creators: name: Caflisch, Nicolin
Publisher: name: ETH Zurich
Year: 2022
Subjects: Nitrogen fertilization
Nmin method
Site-specific N management
Fertilization recommendations
Digitalization
Vegetation indices
Sustainable intensification
Switzerland
Europe
info:eu-repo/classification/ddc/630
Agriculture

description: Nitrogen (N) fertilization is of paramount importance for crop productivity and thus for nurturing an ever-growing world population. However, untargeted N fertilization can result in damage to natural ecosystems and threats to human health. Notably, the contamination of groundwater used as drinking water with excessive amounts of nitrate (NO₃⁻) constitutes a serious problem in the Swiss Central Plains and worldwide. The Nmin method constitutes a promising approach to tackle such negative side-effects of N fertilization without compromising crop productivity through a more targeted, site- and year-specific N fertilization based on the measurement of plant-available N (Nmin) in the soil before N fertilizer application. The overall scope of this thesis was to identify aspects of the Nmin method as it is currently used in Switzerland that may be improved during the current routine revision of the official Swiss fertilization recommendations, the GRUD, with a particular focus on the opportunities offered by digitalization.

For this purpose, the Swiss Nmin method was compared to Nmin-based and other N fertilization recommendations from Switzerland, Germany, Denmark and the United Kingdom, including the associated soil sampling and analysis methods. This notably included the calculation of the “optimum” N fertilization rate for field experiments with winter wheat conducted by the Swiss Confederation’s center of excellence for agricultural research Agroscope and its partner institutions. Furthermore, the relationship between soil organic matter and clay content and N availability in winter wheat and winter barley was studied using linear mixed models. The feasibility of predicting Nmin in early spring based on 23 vegetation indices calculated from multispectral UAV (unmanned aerial vehicle) imagery was assessed for winter wheat using bootstrappingbased correlation analysis and Passing-Bablok regression.

On average, the Swiss N fertilization recommendation yielded appreciably lower “optimum” N fertilization rates than their German and British counterparts, but an undifferentiated increase of N fertilization rates in Switzerland would likely exacerbate the existing NO₃⁻ leaching problems. However, a site- and year-specific flexibility of maximum allowable N fertilization rates based on the Nmin method or similar approaches may enable an increase in crop productivity while reducing the negative side-effects of N fertilization. Furthermore, the lack of a reasonable correction for differences in yield among fields as well as certain aspects of the associated Nmin measurement procedures were identified as the most important points that should be improved in the Swiss Nmin method.

The NDVI and the functionally equivalent NIR / R ratio were identified as the vegetation indices with the highest correlation with Nmin in the 0 - 30 cm soil horizon in early spring (ρ = 0.6675, p < 0.001). Yet, the subsequent Passing-Bablok regression clearly showed that a prediction of Nmin at the field scale based on a vegetation index alone is not sufficiently reliable to be meaningfully integrated in a fertilization recommendation. However, vegetation indices remain useful for the site- and year-specific optimization of N fertilization. In the short term, digitalization eliminates the high need for simplification inherent in non-digitalized fertilization recommendations, allowing us to make the most advanced scientific models of the N cycle usable for farmers. This way, digitalization can contribute to the site- and year-specific optimization of N fertilization even without developing something fundamentally new.