Metadata of agricultural long-term experiments in Europe exclusive of Germany

Agricultural long-term experiments (LTEs) are an important research infrastructure for agriculture, plant and soil sciences. The aim of this metadata compilation is to make LTEs easier to find and to facilitate networking. LTEs are here defined as agricultural experiments with a minimum duration of 20 years and research in the context of soil and yield. An extensive literature review was conducted to identify LTEs in Europe exclusive of Germany, because Germany's LTEs were published before. Sources were scientific papers as well as other articles, books, trial guides and websites. The following information was searched for and compiled in this dataset, if available: site and name of the LTE, start and end (if appropriate), holding institution, type of land use (e.g. field crops or grassland), research theme, website (if available), participation in networks, measured parameters, farming category (i.e. conventional or organic), size of the LTE area, longitude and latitude of the LTE, experimental setup including factors, treatments, randomization and replication, number of plots, size of the plots, crop rotation, soil type, substrate, texture, literature which was written in the context of the LTE data, and AGROVOC keywords. LTE from the following countries are included: Austria, Belarus, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, Finland, France, Great Britain, Hungary, Italy, Moldova, Norway, Poland, Romania, Serbia, Slovenia, Spain, Sweden, Switzerland, Ukraine. In total, 186 LTEs could be identified. The LTEs were classified according to the following research themes: fertilization, tillage, crop rotation, other. The majority of LTEs have the research theme “fertilization” (n = 125). Thirty LTEs have the research theme “crop rotation”, 26 LTEs have the research theme “tillage”, and 26 LTEs have “other” research themes. The following networks could be identified: 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). The oldest LTE was set up 1843, but the largest number of LTEs was established in the second half of the 20th century. Most of the LTEs are held by a scientific institution, i.e. 88 LTEs are held by a non-university scientific institution and 81 LTEs are held by a university or university of applied sciences. The link to the holding institution is provided whenever possible to facilitate contacting.

"fertilization" ( n = 125). Thirty LTEs have the research theme "crop rotation", 26 LTEs have the research theme "tillage", and 26 LTEs have "other" research themes. The following networks could be identified: 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). The oldest LTE was set up 1843, but the largest number of LTEs was established in the second half of the 20th century. Most of the LTEs are held by a scientific institution, i.e. 88 LTEs are held by a non-university scientific institution and 81 LTEs are held by a university or university of applied sciences. The link to the holding institution is provided whenever possible to facilitate contacting.   Tables  How data were acquired An extensive literature review was conducted to identify LTEs. The search terms were "long-term field experiment", "long-term experiment", "long-term field trial" and "long-term trial", as well as the German terms "Dauerfeldversuch", "Dauerdüngungsversuch", "Dauerversuch", "Langzeitfeldversuch" and "Langzeitversuch". AGROVOC ( https://agrovoc.fao.org/browse/agrovoc/en/ ) keywords were added for each LTE that best describe the respective LTE. Data format Primary (raw) and secondary data Parameters for data collection Site, name, start, end (if appropriate), duration, holding institution, type of land use (e.g. field crops or grassland), research theme, website, participation in networks, measured parameters, farming category (i.e. conventional or organic), size of the LTE area, longitude, latitude, experimental setup, soil type, substrate, texture, literature which was written in the context of the LTE data, and AGROVOC keywords. Description of data collection The information was collected out of scientific papers as well as other articles, books, trial guides and websites.

Value of the Data
• The data provide an overview of LTEs in Europe and exceed previous compilations in terms of quantity and depth of information. • The dataset makes it easier to find suitable LTEs or institutions for cooperation.
• Although it is not a complete overview, this data set allows conclusions to be drawn about which topics are being researched with LTEs in Europe and which topics may be missing.

Data Description
LTEs are here defined as agricultural experiments with a minimum duration of 20 years and research in the context of soil and yield. LTEs which will reach the 20-year threshold by 2024 ( = end of project funding phase) are also included. After a map with metadata for LTEs in Germany [1] and the complete metadata set have been published [2] , information on LTEs in other European countries is now to be published. In total, 186 LTEs could be identified. The following information was compiled in this dataset: site and name of the LTE, start and end (if appropriate), holding institution, type of land use (e.g. field crops or grassland), research theme, website of the LTE or of the holding institution, participation in networks, measured parameters, farming category (i.e. conventional or organic), size of the LTE area, longitude and latitude of the LTE, experimental setup including factors, treatments, randomization and replication, number of plots, size of the plots, crop rotation, soil type, substrate, texture, and literature which was written in the context of the LTE data. Table 1 gives an overview of the number of LTE for which the respective information is available. Table 2 indicates the number of LTEs per country which is recorded in the data set. The type of land use is specified. The LTEs are classified as fertilization LTE, tillage LTE, or crop rotation LTE, if one factor is fertilization, tillage, or crop rotation (multiple nominations possible). The majority of LTEs have the research theme "fertilization" (n = 125). Thirty LTEs have the research theme "crop rotation", 26 LTEs have the research theme "tillage", and 26 LTEs have "other" research themes ( Table 2 ).   The oldest LTE was set up 1843 ( Fig. 1 ). The largest number of LTEs was established in the second half of the 20th century. Now sixteen LTEs are finished. The total number of LTEs shows the number of established LTEs minus the number of finished LTEs ( Fig. 1 ).
The following five networks could be identified: GLTEN (Global long-term experiment network), ILTER (International long-term ecological research), IOSDV (International Organic Nitrogen Fertilization Experiments), NLFT (National Long-term Fertilization Trials, Hungary), and RetiBio 2 (Italy). Table 3 shows the number of LTEs belonging to each network.
Eighty-eight LTEs are held by a non-university scientific institution ( Table 4 ). Eighty-one LTEs are held by a university or a university of applied sciences. We could not identify the type of  holding institution for 13 LTEs. For four LTEs the holding institution is another institution than a non-university scientific institution or a University/University of applied sciences (e.g. a state authority).

Experimental Design, Materials and Methods
An extensive literature review was conducted to identify LTEs in Europe. Sources were scientific papers as well as other articles, books, trial guides and websites. The search terms were "long-term field experiment", "long-term experiment", "long-term field trial" and "long-term trial", as well as the German terms "Dauerfeldversuch", "Dauerdüngungsversuch", "Dauerversuch", "Langzeitfeldversuch" and "Langzeitversuch". Attention was given to LTEs with a minimum duration of 20 years in the context of soil research, i.e., the objects of research should at least include soil properties and yield as an important soil function. The setup of each trial should allow for statistical analyses, i.e., have clearly defined factors, replications and as much as possible a static design. Lysimeter experiments were excluded because they were considered as an own category. Some reasons for this exclusion are that soils are often transferred and not undisturbed in lysimeter experiments and tillage has to be conducted by hand instead of machines, which can bias some results.
LTEs were only included if more information than just site and start was available. Special emphasis was placed on the holding institution, whose website was linked whenever possible. Not all LTEs from other lists, e.g. in Bai et al. [3] or in Debreczeni and Körschens [4] are included in this dataset. If the duration was too short or if it was impossible to identify the holding institution or some more details, the LTE was excluded.
All information was put into an excel sheet. We partially assigned the information to categories in order to sort the LTE more efficiently. This was the case for holding institution (categories: Non-university scientific institution, university / university of applied sciences, or other institution than that) and research theme (categories: crop rotation, fertilization, tillage, or other). Each LTE was classified in one or more of these categories if one or more factors could be assigned to these research themes. The class "other" entails all research themes that could not be grouped into the first three and appeared only in a few LTE cases, so that a separate group was not justified. Two or more factorial experiments were sorted in all relevant classes, i.e., multiple nominations were possible. The current duration in 2021 of each LTE was calculated from its start year (and possibly the end if it is a finished experiment). The coordinates were either taken from the literature, if they were given there, or searched via google maps [5] . AGROVOC [6] keywords were added for each LTE that best describe the respective LTE. In addition, there are added generalizing terms of AGROVOC that make it easier to find similar LTEs on specific topics.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have or could be perceived to have influenced the work reported in this article.