## 2 Material and Methods
### Microarthropod Sampling and pH Measurement  
In 1959, samples were taken at three subsequent dates: 11 September, 9 October, and 30 October. Samples in 1987 were taken on one date, 9 October, just as on 30 October 2019. Samples were taken following a standard procedure, developed at the Institute for Applied Biological Research in Nature, Armhem, the Netherlands (later merged into the Research Institute for Nature management, Institute for Forestry and Nature Research and Alterra resp., now known as Wageningen Environmental Research); this procedure has been published by [PERSON] and [PERSON] in 1988 [20]. Each mineral soil sample has 100 cc: a volume of 5 cm diameter and 5 cm depth plus litter on top. In 1959, two samples per date were taken on each plot, making a total of 6 samples (only pooled data are available); in 1987 and in 2019, 4 and 5 samples were taken for each plot, respectively (data per sample available).  
Soil cores were put on a Tullgren funnel for 1 week, during which temperature was increased from 35 to 45 \({}^{\circ}\)C, and then, microarthropods were collected in 70% alcohol and later put into 20% lactic acid for clarification and identification [20]. The Tullgren funnel used for extraction [21] has been used ever since 1936 and efficiency has not changed as the tool and protocol was the same all over the years.  
Identification was done to the species level as much as possible using at present the keys for Orbitadio [22], for Gamasina [23], for Uropodina [24], and for Collembola [25]. Material from the extractions of 1959 and 1987 was re-examined as far as possible to check the correct species identification. In the 1959 and 1987 samples, only obriatid mites were identified to the species level, whereas in 1959, all species of Quadroppiidae, Oppiidae, and Suctobelbidae were pooled. In 2019, all microarthropods were identified to the species level. Sorting and identification of the 1959 microarthropods was carried out by an experienced acarologist ([PERSON]), in 1987, this was done by a student ([PERSON]) and completed and checked by the second author. For the 2019 samples, we decided to demonstrate the potential difference in picking out the microarthropods from the extraction fluid into the slides for identification as part of the experiment: the first author made a first series of slides including all distinguished animals (dataset 2019 a), while the second author made an extra set of slides with the animals missed by the first (dataset 2019 b). The first author did know since the beginning that the second author would check all samples after her sorting session. In this way, we intended to demonstrate the potential difference in this crucial part of the procedure by a starting and an experienced professional. In the analysis, we compare dataset (2019 a) with (2019 a + b), in order to highlight the difference between a starting and an experienced acarologist. Nomenclature adopted was updated according to current standards, following, e.g., the checklists for Orbitadio [26], for Astigmatina [27], and for Mesostigmata [28]. Values of pH-KCl were measured in the core material after the extraction of the microarthropods, both in 1959, 1987, and 2019.  
Figure 1: Location of the Hackfort site in the Netherlands (**left**), more in detail on the topographic map (**middle**) and aerial photo of the experimental area (**right**).  
Figure 2: Soil profiles: typic haplaqull ((**a**), at plot I), psammaquentic haplorthod ((**b**), plot III), and humaqueptic spodic psammaquent ((c), plot V) (see spade for scaling).