## 4 Discussion
### Methodological Aspects  
We started this study with two goals: the study of a pH gradient in a period of increasing and decreasing acid deposition as reported above and review of methodological aspects concerning real long time series. We have not found time series, equally long, dealing with soil properties in relation to soil animals in literature as the one presented here. So, this series gives a nice opportunity to evaluate the needs and potential pitfalls for such a study. The frequency of measurements in this study is rather low, only three points in time: 1959, 1987, and 2019. It is, however, questionable whether a higher frequency would have added more results. It would have had a more disturbing effect on the gradient, as room to take undisturbed samples becomes limited rather soon on such a small-distance gradient. As we know from other shorter time series, which indicate a rather constant soil fauna [16], the necessity to increase the frequency is not present as well. Given the situation of a long series with a low frequency, other challenges appear: the time series transcends normal project periods and even researchers' life times. A good description of the site, with exact coordinates of plots and samples, becomes evident. Our gradient has a total length of 20 m, resulting in interplot distances of 5 m. Thanks to the present labelled pickets, it was possible to reconstruct the entire grid and the exact location of the plots sampled in 1959 and 1987. Small deviations would have given large disturbances, as the change of the population area of, for instance, _Hypocthonius rufulus_ (occupying almost the whole gradient, when that suffered from acidification in 1987 and retracting towards the acid part in 2019) would have been unnoticed or seen as spatial noise. Sampling itself has been subject to a constant institute's protocol [20]; small variations therein would have led also to noise, now in a temporal way. In addition, the use of the same equipment for the extraction procedure could be kept constant in this study as differences in extraction method or equipment can give different results [55]. We used the Tullgren funnels of the institute that originate from 1936 and efficiency has not changed as the tool and procedure was the same all over the years. A next, very important step in the procedure is the preparation of the slides from the alcohol material the specimens were collected in. In this study, we treated this step as an experiment, where the first author, as part of her training, made the slides initially: dataset 2019 (a). A thorough re-examination of the rest material by the second author resulted in an extra set of slides making the dataset complete: 2019 (a + b). Dataset 2019 (a) had on average 50.8% of all specimens. Strikingly, this rather big deviation gave no differences in the analysis of the group totals, as data appeared rather robust here. As some inconspicuous species were not or hardly present in dataset 2019 (a) (for instance, 13.4% in _Microppiia minus_ and 2.5% in _Nanorchestes arboriger_ and none in _Spelecchestes pratensis_), analysis at the species level did show discrepancies. Deviations of this kind are hard to discover in literature, although in some cases, the efficiency of this step in the procedure can be questioned. Finally, it proved to be useful to store at least some specimens from every species and sampling period for later examination. We could rectify a few identification mistakes in this way (one from 1959 and one from 1987). Moreover, species can be detected later. In our 2019 (a + b) dataset _Quadroppia galaica_ occurs, a species that has been described in 1985, much later than the start of our time series. Last but not least, the necessity of proper checklists helps to translate older synonyms to current valid names.