The Development of a Bioindicator System for Soil Acidity Based on Arthropod pH Preferences

Abstract

1. Changes in ecological communities may reveal information on the environment that is not easily obtained from abiotic sampling. This paper aims to develop a bioindicator system that may support the interpretation of acidity-related changes in soil chemistry. 2. An experimental system was developed to test the preferences of soil arthropods (springtails, mites, woodlice) by their tendencies to settle in a gradient of soil pH from 2 to 9. The system consisted of a circular walkway, the bottom of which was divided into 16 compartments filled with purified sand; the pH of the sand was adjusted using buffer solutions. 3. An extensive series of experiments was conducted to investigate the reproducibility of preference distributions observed for Orchesella cincta (Collembola) and Platynothrus peltifer (Cryptostigmata), using six different buffer variants and four different layouts of buffers in the walkway. Iso-osmolar phosphate-citrate buffers produced a response similar to the one observed using pH-adjusted extracts of leaf litter. 4. Chemical analysis of the water phase in the sand/buffer system showed that the concentrations of K, Na, Ca, Fe and Al co-varied with pH, making it difficult to decide the direct chemical stimulus for arthropod preference behaviour. 5. There was a great variety of responses observed among 20 arthropod species tested. Based on the frequency distributions observed, each species was assigned a median preferred pH and an 'indicator value', to denote the specificity of the response. 6. Many species had a broadly dispersed preference distribution, but the median preferred pH varied from pH 2.9 to pH 7.6. Three mites (Nothrus silvestris, Rhysotritia duplicata and Odontocepheus elongatus) did not react to pH. The collembolan Foisomia candida showed a weak and variable response, but avoided pH 2. Three other Collembola (Orchesella flavescens, O. cincta, Lepidocyrtus cyaneus) and three isopods (Trichoniscus pusillus, Porcellio scaber, Philoscia muscorum) preferred the subneutral range (median preference between pH 5.6 and pH 6.1). 7. Species with a more or less pronounced preference for one or other end of the gradient were one collembolan (Tomocerus flavescens), two mites (Hypochthonius rufulus and Adoristes ovatus) and one isopod (Oniscus asellus), which were classified as acidophilous, and two Collembola (Isotoma notabilis, Entomobrya corticalis), two mites (Pelops occultus, Platynothrus peltifer) and one isopod (Armadillidium vulgare), which were classified as alkalophilous. The species Tomocerus minor showed a bimodal preference distribution. 8. An 'arthropod acidity index' is proposed which allows the median preferred pH of an arthropod community to be estimated from the indicator values, in conjunction with abundance scores in the field. Preliminary calculations show that the index corresponds to differences between forests regarding soil pH, but the absolute pH indicated is one unit above litter pHs. 9. When calibrated to field data, the proposed bioindicator system may be used in monitoring programmes for the analysis of ecological effects of long-term trends in soil acidity.