This LTE overview map contains information about 499 agricultural experiments with a duration of at least 20 years. The following networks are included: EJP SOIL (European Joint Programme), GLTEN (Global long-term experiment network), ILTER (International long-term ecological research), IOSDV (International Organic Nitrogen Fertilization Experiment), NLFT (National Long-term Fertilization Trials, Hungary), RetiBio 2 (Italy).
(You can get easy access to your LTE of interest by using the categories as a filter criteria.)
 "Metadata of agricultural long-term experiments in Europe exclusive of Germany" https://doi.org/10.1016/j.dib.2021.107322
 "Long-term field experiments in Germany: classification and spatial representation" https://doi.org/10.5194/soil-6-579-2020
 "Provision of Metadata of European Agricultural Long-Term Experiments through BonaRes and EJP SOIL Collaboration" BonaRes. Repos., v1 (2022) https://doi.org/10.20387/bonares-40kc-2661
 "Agricultural long-term experiments in Europe outside Germany", Bona Res. Repos., v1 (2021) https://doi.org/10.20387/bonares-eff3-0mb4
 "Long-term Field Experiments in Germany", Bona Res. Repos., v1 (2019) https://doi.org/10.20387/BonaRes-3TR6-MG8R