Abstract
Site-specific nitrogen (N) management in precision agriculture is used to improve nitrogen use efficiency (NUE) at the field scale. The objective of this study has been (i) to better understand the relationship between data derived from an unmanned aerial vehicle (UAV) platform and the crop temporal and spatial variability in small fields of about 2 ha, and (ii) to increase knowledge on how such data can support variable application of N fertilizer in winter wheat (Triticum aestivum). Multi-spectral images acquired with a commercially available UAV platform and soil available mineral N content (Nmin) sampled in the field were used to evaluate the in-field variability of the N-status of the crop. A plot-based field experiment was designed to compare uniform standard rate (ST) to variable rate (VR) N application. Non-fertilized (NF) and N-rich (NR) plots were placed as positive and negative N-status references and were used to calculate various indicators related to NUE. The crop was monitored throughout the season to support three split fertilizations. The data of two growing seasons (2017/2018 and 2018/2019) were used to validate the sensitivity of spectral vegetation indices (SVI) suitable for the sensor used in relation to biomass and N-status traits. Grain yield was mostly in the expected range and inconsistently higher in VR compared to ST. In contrast, N fertilizer application was reduced in the VR treatments between 5 and 40% depending on the field heterogeneity. The study showed that the methods used provided a good base to implement variable rate fertilizer application in small to medium scale agricultural systems. In the majority of the case studies, NUE was improved around 10% by redistributing and reducing the amount of N fertilizer applied. However, the prediction of the N-mineralisation in the soil and related N-uptake by the plants remains to be better understood to further optimize in-season N-fertilization.
Highlights
In Swiss agriculture, farm size is considered medium to small scale averaging around 20 ha (Swiss Federal Statistical Office FSO 2019)
This study indicates that the use of NDVI is less sensitive for in-season fertilization support, despite being a commonly used solution in other studies (Tremblay et al 2009; Walsh et al 2013; Wood et al 2003)
Under Swiss conditions, NDRE showed a better differentiation of the N-status of the plants, than NDVI
Summary
In Swiss agriculture, farm size is considered medium to small scale averaging around 20 ha (Swiss Federal Statistical Office FSO 2019). 20 ha (EUROSTAT 2016), and partly to village farm systems in the North China Plain (Chen et al 2019; Zha et al 2019) composed of family households managing 0.3–0.5 ha At this scale, monitoring and management of in-field variability are confronted with additional obstacles compared to large-scale precision farming operations. The inaccurate calculation of fertilizer requirements is often the main cause of environmental problems such as eutrophication or pollution of water bodies and emission of the potent greenhouse gas N 2O and for additional societal costs (Lassaletta et al 2014) It is, of practical and political concern to increase nitrogen use efficiency (NUE) and reduce high N input in the agricultural sector. In 2014, the efficiency of N used in Swiss agriculture was around 30% (FSO 2019), which is lower compared to the Danish agriculture, for instance, depicting a NUE of 41% in 2012 (Hansen et al 2017)
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