## 4 Discussion  
### The Relation between pH with Soil Microarthropods  
The aim of this study was to monitor a pH gradient in a forest soil over a 60 years' time period using the microarthropod community, initially ribatids only, but extended to all microarthropods later. The distribution and abundance of soil microarthropods are determined by a large number of factors. Among the abiotic factors, soil pH is of high importance [29]. Our results showed a significant negative trend of soil microarthropod communities in 1959 and 2019 with pH (see Supplementary S2), as 1959 and 2019 have indeed similar pH gradients, whereas the 1987 series showed a mixed pattern, maybe due to the decrease in pH values, that not all species could follow equally. Several authors have proven that the number of soil biota strongly depends on the state of acidification of the soils [30; 31; 32; 33]. Soil acidification induces a mobilization of potentially toxic Al and heavy metals [34; 35; 36; 37; 38; 39] and a decrease in base nutrients such as K, Ca, and Mg, which make some species not normally absorb water at a low pH [40; 41]. That microarthropods may directly recognize the acidity of the pore water was suggested by [PERSON] and [PERSON] in 1984 [42]. Studies about pH preferences highlighted difference among species and habitats. Some species appear to prefer the lower pH range, such as _Hypochthonous rufulus_, shown by [PERSON] and [PERSON] in 1997 [43], which in our data also showed a negative correlation with pH (preference for low pH). However, referring to _Platynothrus peltier_, we found a preference for low pH, whereas [PERSON] in 1990 [21] found the species also in the less acidified habitats. [PERSON] and [PERSON] in 2004 [44] suggested that the most informative groups of microarthropods are ribatids and collembolans because of their high species diversity as well as their relatively well-developed taxonomy compared to other groups of mites. This study shows that soil microarthropod communities could be an interesting tool in order to monitor soil acidity (pH).  
Although in acid forest soils, soil-f fauna activity is strongly reduced, differences between species are striking. _Oppiella nova_, for instance, has been reported to be strongly reduced in abundance from older forest lining experiments [45], which makes sense as in our results, it is negatively correlated with pH, while some species show an opposite correlation. For example, limiting in a Lodgepole pine forest gave increased abundance of the potworm _Enchytronia parva_ and a corresponding tendency for _Friesea mirabilis_, while artificial acid rain in a spruce forest gave a nonsignificant decrease in the abundance of _Parisotoma notabilis_[46; 47]. Our data evidenced a positive correlation of _Friesea mirabilis_ with pH and a negative one for _Parisotoma notabilis_, which corresponds to these findings. _Isotomiella minor_ is, according to [PERSON] in 1943 [48], quite independent of soil pH, which indicate that this species is well represented in all soil types; in our data, we found the species also having no correlation with pH, although the incomplete dataset 2019 (a) gave a false correlation result.  
One of the most interesting observations made in this study was the remarkable opposite relationship with pH of two couples of species belonging to the same genera, one among the orbiatids _Hypochthonous luteus_ and _H. rufulus_, where _H. luteus_ filled the gap at the higher pH values. In 1959, nor in 1987, the latter species has been found in the gradient, although it was known from the area as could be confirmed from slides in the institutes collection. The other couple is among the Mesostigmata (_Rhodacarus calcarulatus_ and _R. coronatus_). It is plausible to expect that these congeneric species have about the same food preferences and that their segregation is caused by abiotic preferences such as pH or humidity [49]. Thus the \"realized pH niche\" might be a combination of many factors.