Provision of metadata of European agricultural long-term experiments through BonaRes and EJP SOIL collaboration

Agricultural Long-Term Experiments (LTEs) are crucial agricultural research infrastructures for monitoring the long term effects of management and environment on crop production and soil resources. We have compiled the meta-information of 616 LTEs from 30 different countries across Europe with a duration of typically 20 years, including clustered information of the European LTEs in different categories (management operations, land use, duration, status, etc.). It consists of the updated version of the dataset published by Grosse et al., (2020) but is extended by further LTE metadata, categories and research themes. Each set of metadata consists of up to 49 different attributes (categorical or numeric). Collected attributes were analyzed according to several research themes, including fertilization, crop rotation and tillage treatments. The collection of individual metadata was enlarged by the recent agreement between the BonaRes (www.bonares.de) and EJP SOIL (www.ejpsoil.eu) groups into the most comprehensive dataset in Europe, providing access to LTE and other, shorter running experiments. This dataset centralized past and existing information usually dispersed across several national actors. As such, it provides an extensive database that can be used by decision-makers, scientists, LTE owners and the public. The dataset can be updated in the future to foster networking and information exchange continuously.


a b s t r a c t
Agricultural Long-Term Experiments (LTEs) are crucial agricultural research infrastructures for monitoring the long term effects of management and environment on crop production and soil resources. We have compiled the meta-information of 616 LTEs from 30 different countries across Europe with a duration of typically 20 years, including clustered information of the European LTEs in different categories (management operations, land use, duration, status, etc.). It consists of the updated version of the dataset published by Grosse et al., (2020) but is extended by further LTE metadata, categories and research themes. Each set of metadata consists of up to 49 different attributes (categorical or numeric). Collected attributes were analyzed according to several research themes, including fertilization, crop rotation and tillage treatments. The collection of individual metadata was enlarged by the recent agreement between the BonaRes ( www.bonares.de ) and EJP SOIL ( www.ejpsoil.eu ) groups into the most comprehensive dataset in Europe, providing access to LTE and other, shorter running experiments. This dataset centralized past and existing information usually dispersed across several national actors. As such, it provides an extensive database that can be used by decision-makers, scientists, LTE owners and the public. The dataset can be updated in the future to foster networking and information exchange continuously. ©

Value of the Data
• The data comprises the most comprehensive metadata set of LTEs and other experiments in Europe through the combination of independent datasets of the BonaRes and EJP SOIL projects. • The dataset primarily offers a wide selection of the LTE locations, research themes, and sources that the scientists and decision-makers can use for possible collaborations, even with LTE holders, intending to analyze the soil ecosystem services. • It is a significant source of information about various LTEs for soil and environmental researchers, research institutions, local authorities, along with access to relevant information for future studies and cooperation. • This LTE data collection established a new basic framework in this study to make data from different LTEs com parable and appraisable, even though there are no standard procedures for LTE set-up, treatment designs, minimum information and data management.

Data Description
Agricultural long-term experiments (LTEs) are experiments for monitoring plant and soil parameters such as yield under different environmental conditions and management practices. These experiments were set up on a large number of different soil textures and soil types to reveal the effects of management practices and environment on crop production and soil resources. They offer a unique look at the change in several soil properties over the long-term effect of investigated management practices. Under changing climate conditions, representative data and time series from LTEs will help decision-makers, environmental organizations, authorities, local governments and scientists to develop innovative (research) activities to mitigate climate deterioration on agricultural productivity Since the LTEs are essential infrastructures for sustainable soil use and yield, information produced from this LTEs-based information attracted attention from many different research institutions and organizations. For instance, LTE-based data were acquired and managed by various national, international or global initiatives and networks, including 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), RetiBio 2 (Italy), and the projects BonaRes and EJP SOIL. While each organization has its data management plan, the variety of actors makes that LTE-related information was dispersed across different databases, data holders and publications, which makes making it challenging to access and reuse the data.
Since 2016, an extensive metadata collection on LTEs has been compiled within the BonaRes project. The basis of this collection was the BonaRes factsheet and LTE metadata template for the structured collection of metadata and the BonaRes definition of which field experiments have the status long-term experiment. The BonaRes definition is as follows: "field experiments with a minimum duration of 20 years and a static design" [1] . EJP SOIL data has been built very closely on the Factsheet and metadata template developed and proven in BonaRes to collect metadata. However, there are differences in the definition. EJP definition: "running field experiments focusing on (sustainable) soil management and a minimum duration of 5-10 years with a statistically sound design, a control treatment and regular monitoring of crop/soil parameters. We have not filtered this dataset using the strict BonaRes defaults but also integrated LTE meeting the EJP definition. However, we offer simple filtering possibilities based on the duration of the experiment, and thus every user has the possibility to decide for himself. In this case, we found that expressing the differing LTE definitions of both BonaRes and EJP SOIL would be relevant for researchers to select the experiment duration used in their own research.
As the leading research initiatives, the BonaRes and EJP SOIL research groups have recently merged their LTE metadata databases of the European LTEs. Several LTE parameters and features for a time span of more than 150 yrs (1843-2022) were aggregated and published. The dataset comprises an updated number of the LTEs and revised research categories compared to the data published by Grosse et al. [1 , 3] . Recently the complete dataset consists of metadata for 616 LTEs from 30 countries in Europe ( Fig. 1 ), that can be divided into five groups; (i) trial information (i.e. site, country, duration, status), (ii) land use (i.e., arable land, grassland), (iii) research theme, (iv) management operations (i.e. tillage, fertilization, crop rotation), and (v) basic soil parameters (i.e. types, texture).
Collected LTE data were merged and visualized through an online overview map ( https: //lte.bonares.de ) running under the BonaRes Repository ( https://datenzentrum.bonares.de ). The overall dataset comprises the complete list of the LTEs and their attributes. The file includes 616 rows compatible with the listed LTEs and 49 columns of attributes in various categories. Besides the LTE-related attributes, the file consists of the cited literature and source information for each LTE. A comprehensive EXCEL sheet is provided as supplemental information along with the published dataset: https://doi.org/10.20387/bonares-40kc-2661 .   The article displays two tables; (1) Table 1 includes the list, and description of the categories used for displaying into the Bonares LTE overview map, (2) Table 2 offers the ranges of values or the choices of the LTEs for each country and main categories.
While merging the BonaRes and EJP SOIL LTE sets, 19 doubles became visible. Fifteen of these LTEs are located in Germany, and one is from Switzerland, Belgium, Denmark, and Italy. Duplications of these LTEs were excluded from the dataset to prevent redundancies. Following the exclusion of duplicates, the final number of the LTEs were defined. The majority of the LTEs in Europe are located in Germany ( n = 299), Sweden ( n = 51), Austria ( n = 31), Great Britain ( n = 27). A considerable number of LTEs are also located in Mediterranean countries, including Italy (18), France (18), Spain (14) and Turkey (3), respectively. The world's oldest and still running LTE is the Broadbalk experiment (Rothamsted Research, Harpenden, UK), which started in 1843. Fig. 2 shows the number of LTE according to their starting date and current status.
The majority of LTEs ( n = 508) are still ongoing, where 82 LTEs have been finished (after running times of at least a typical 20 years). By far, most LTEs are categorized as crop rotation (65), fertilization (139) or tillage experiments ( n = 322, Fig. 3 ).
The soil information of LTEs was presented in nine attributes representing the soil texture, types, bulk density, organic carbon and further soil information. These parameters were assigned for each LTE, if applicable. Where the data could not be inferred, we included "unknown" in the respective fields of the LTE tables.

Experimental Design, Materials and Methods
The study comprised the LTEs in Europe and merged the data of three datasets. Additional data were identified through a comprehensive bibliographic review and personal communication with numerous LTE holders. Articles from scientific journals, books, research notes and websites were selected addressing the research themes and parameters of the agricultural LTEs. At first, we conducted a systematic, keyword-based search in Web of Science (WoS) Core Collection ( https://apps.webofknowledge.com/ ) and SCOPUS ( https://www.scopus.com/ ) as well as the Google Scholar databases to create an overview of available LTE information. The bibliographic