4.1. Earthworm species composition
Altogether, seven endogeic and three epigeic earthworm species were detected out of the three functional groups (Csuzdi 2003), no anecic species were found. Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea rosea are typical synanthropic species. They can be observed in several land uses, e.g. grassy areas, lawns, gardens and wooded sites (Csuzdi and Zicsi 2003; Csuzdi 2007). A. chlorotica and A. rosea are common almost in any soil type, but they prefer and can be found in greater abundances in soils with higher moisture and organic matter content. A. caliginosa can survive even in sandy and highly disturbed soils. These species belong to the endogeic group, i. e. they live, burrow and feed in the mineral soil layer, usually in the top horizons. Aporrectodea georgii is also an endogeic earthworm, but it has a greater abundance in clayey soils with greater soil moisture content. P. opisthoductus, P. tuberculatus and O. lacteum species also belong to the endogeic group, thus they mostly live, burrow and feed in the topsoil layers (Csuzdi 2007).
In our research, two endogeic earthworm species (A. chlorotica and A. rosea) were found in all the examined farms, in both examined seasons mostly with the greatest proportion. There was only one exception, permaculture (September 2020) where D. veneta had the highest proportion (40.6%). A. caliginosa and O. lacteum were also very common on the examined farms.
Three epigeic species were also found on the farms. Eisenia fetida is the so-called manure worm, it is the most suitable for vermicomposting due to its high proliferation rate. It has been introduced worldwide and, thus, has high variations in its morphological characteristics (Csuzdi 2007). In our study, E. fetida was only found in permaculture and conventional farms, only in autumn.
Dendrobaena veneta is a widely distributed peregrine species with high variations in morphological characteristics. It can be mostly found in manure and compost, and it is suitable for vermicomposting. It has been spread all over Europe mainly due to vermicomposting activities (Csuzdi 2007). This species was only found in permaculture farms, probably distributed by the animal manure or compost additions to these farms.
Bimastos rubidus, an epigeic earthworm species (earlier called Dendrodrilus rubidus) can be found under logs or stones and also in manure. They have widely spread peregrine earthworms (Csuzdi 2007). They were found only on permaculture and organic farms, but not in conventional ones since they need a lot of organic debris or manure on the field to survive.
4.2. Possible effects of soil characteristics on earthworm performance
The pH of the soil samples was quite homogeneous, 6.2 was the lowest and 7.7 was the highest value, so the pH range was between the mid-range of the slightly acid to the mid-range of the slightly alkalic category. We can consider these soils as good horticultural soils for the majority of the plants produced.
The pH was most likely not an influencing factor of earthworm numbers, abundance and diversity in the investigated farms in our analyses. Our results are similar to those of Lofs-Holmin (1986) who found that pH values between 4.5 and 7.0 did not have a great effect on the presence of earthworms in permanent pastures, based on which Boag et al. (1997) also concluded that the detected soil pH range was between 4.5-7.0 on 68 of the arable fields they examined, thus, they assumed that pH had little effect on earthworm communities. However, Johnson (2009) found that high soil moisture content and close-to-neutral pH have a strong connection with earthworm populations. Prastowo et al. (2020) found that higher soil organic matter with lower pH in the topsoil might explain the higher number of earthworms, to some extent. The relation between earthworms-acidity-soil organic matter is reasonable as soil organic matter is mostly acidic, but the connection is not necessarily strict.
The CaCO3 content of the soils was in the very low (0.1–0.5%) and low (1.1–4.5) range, and some of them were in the medium range (8.7–13.3%). Bernard et al. (2009) found that adding crushed lime to the soil may increase the earthworm population. Holland et al. (2018) describe in their review that positive impacts of liming on biodiversity have been observed in many ecological studies, especially increased earthworm abundance that serves as prey for grassland birds.
According to various authors (Pfiffner and Mäder 1998, Scullion et al. 2002, Bernard et al. 2009), soil organic matter (SOM) has a great influence on earthworms and vice versa, however, there was not a great variation in SOM in the investigated areas, the lowest value was in the case of a sandy soil type (Arenosol) but even in these cases the amount of SOM reached 1.6% that is considered as a good amount for sandy soil. The highest value was 5.3% which is a normal maximum value under continental climate. The relation between the soil organic matter content and the juvenile/adult/all earthworm averages were strong in the examined soils, however, this strong relation was not found between soil organic matter vs. Shannon diversity, nor species number per site. Furthermore, interestingly, the total thickness of all humus layers (Szilágyi et al. 2021) did not have a strong correlation with any examined earthworm data.
Soil texture is also an influencing factor (Pfiffner and Mäder 1998, Scullion et al. 2002, Bernard et al. 2009). Sandy soils are known to have smaller numbers of earthworms as they tend to get dry very quickly and do not have the necessary volume of fallen litter, so do not favour earthworms. However, according to the Arany texture index, there was one farm in each farming system where texture reached the limit of coarse sand (Arany texture index below 25 is considered as coarse sand, between 25 and 30 it is sand), so we cannot consider sand content as an influencing factor when comparing farming systems. The reason for this in these horticulture farms, on one hand, that irrigation is almost always applied due to the production type, and on the other hand, there is enough organic matter as horticulture farms are normally either use organic fertilizers, or mulch, or both. Furthermore, organic and permaculture farms are often taking great care of having plant cover for a long period during the vegetation season and beyond, as long as possible. Moreover, the permaculture farms are often having other, positively influencing factors, such as bushes and trees that both help providing an extra amount of shade (helping the longer moist state of the soil that favour earthworms) and fallen leaves and branches and sometimes fruits.
There are also interesting findings that the Na-content and the total salt content had the highest correlation with the average number of adult earthworms. Furthermore, the juvenile earthworm average number was not strongly correlated with the Na-content (R2 < 0.1).
There was a negative correlation in all earthworm characteristics versus Cu- and Mn-content. Similar correlations were found by Paoletti et al. (1999) between copper and several species (including Aporrectodea caliginosa), total earthworm abundance and biomass were found severely reduced by copper input in orchards and vineyards. Eijsackers et al. (2005) also found a negative correlation between copper-containing fungicides and earthworms. Copper in soil resulted in decreased burrowing rate and avoidance of these copper-containing soils.
There were no articles found on Google Scholar, nor google.com related to the relation between Na, Cu or Mn content and earthworms on horticulture farms. The majority of the articles are in relation to soil contamination or vermicompost, simple analysis of farms is very rare if non-existent.
Permaculture farms had the highest abundance of earthworms, which was significant in May (Szilágyi et al 2021). Previous studies found higher earthworm abundance in no-tillage compared to other tillage types (Boag et al. 1997, House and Parmelee 1985, Van Capelle et al. 2012, Deekemati et al. 2019). Based on the field soil examination it is of great importance to know as much soil information as possible (i.e. soil thickness, soil organic matter content, texture, soil management, fertilisers used, soil moisture content at the time of the counting, soil cover, etc.) for considering earthworms data as a good indicator for soil quality assessment. The importance of considering multiple soil factors is also emphasised in the literature (Lofs-Holmin 1986, Nadolny et al. 2020). In our pilot study, we have found similar patterns in 2019, based on three farms (Szilágyi et al. 2019) which are in line with what we explored in soil quality during sustainability assessment of permaculture farms compared to organic and conventional farms (Szilágyi et al. 2018). The relatively low sample size (15 farms, 5–5 farms from each farm type) is an issue for the statistical analysis and our analyses showed that with a greater sample size and a more robust database we could have probably found more significant statistical results. Finally, it is worth mentioning that the suitability of earthworms as indicators changes rapidly with soil moisture content. Our future goal is to further explore connections of soil biota characteristics to ecosystem service delivery and also assess the attitude, motivation and management decisions of the farmers as their perception of biodiversity (Kelemen et al. 2013) and ecosystem services determine of what will happen on the farm management level.