Pushing soil animals to the extreme
Dr. OLAF SCHMIDT
University College Dublin, Ireland
Dr. CÉLINE PELOSI
INRAE, Avignon Université & Paris-Saclay Université, France
Figure 1: The unique 42-plot experiment initiated in 1928 and located in the gardens of the ‘Palace of Versailles’. Nowadays it is managed by the ‘French National Research Institute for Agriculture, Food and Environment’ (INRAE). Photo credit: INRAE.
Ecologists can answer a lot of the scientific questions that they grapple with about organisms from ‘extreme’ environments where plants, animals, or microbes barely cling on to life. Perhaps that sounds counter-intuitive because, surely, one should study living things where they are most abundant and diverse? But that also makes everything very complicated! In extreme environments, the number of organisms is small, their interactions and food webs are few, and the factors that control life or death are limited. For example, scientists have studied microscopically small nematode worms in the soils of extremely dry deserts, both in very hot and very cold climates, to learn how these animals survive and respond to environmental change such as total drought (Wall and Virginia, 1999).
Our team of scientists from France, Hungary and Ireland (Pelosi et al. 2020) recently had an opportunity to study a different group of worms in soils of a unique long-term experiment that has created extreme chemical soil conditions. Located in the world-famous ‘Palace of Versailles’ outside Paris, this experiment consists of small plots of land that have been receiving different fertilizers each year since 1928 but have no plants or crops whatsoever (see Figure 1). Some fertilizers make the soil very acidic or alkaline (measured as pH values), neither of which is suitable for soil animals. Some have added lots of nitrogen, others lots of phosphorus. One treatment (horse manure) visibly keeps the soil much ‘richer’, and other treatments have created various shades of a lighter brown, all reflecting a different soil humus (carbon) content (Figure 1).
Figure 2: Number of enchytraeid worms according to fertilizers, from left to right: basic amendments, organic amendments, ammonium-based nitrogen fertilizers, nitrate-based nitrogen fertilizers, phosphate fertilizers, potassium fertilizers, and control (without amendments). n is the number of plots sampled per treatment group. Graph redrawn from Pelosi et al. 2020.The inset picture shows an enchytraeid worm. Photo credit: INRAE.
The worms we studied in this experiment (Pelosi et al. 2020) are smaller relatives of earthworms and are called Enchytraeidae, or potworms (Figure 2). These worms are important decomposer animals that help maintain soil fertility. In our study we found 13 different species of enchytraeid worms, which is remarkable given the fact that no plants grow on these plots, and thus the soils do not receive any carbon or nutrient inputs (through roots or as litter) from plants. The two plots with horse manure had very large numbers of enchytraeid worms, about as many (>10,000 worms per m2) as one would expect from a normal agricultural soil (Figure 2). Soils with alkaline fertilizers (soil pH about 8.4) also harboured respectable worm numbers, whilst soils with acidifying fertilizers (soil pH about 3.8) had almost no worms at all. The five ‘control’ plots – plots that had not received any fertilizer for almost 90 years – had the lowest carbon content (only about 0.7% carbon), and yet they still had enchytraeid worms (about 1,000 worms per m2)! That probably means that these worms can live on very old soil carbon, and on very small amounts of it.
Figure 3: Bait-lamina sticks with some of the ‘tasty’ bait removed by soil animals. Photo credit: Stephan Jaensch – ECT.
We also measured the general feeding activity of soil animals in this experiment (Pelosi et al. 2020), with a method called bait-lamina sticks that uses little strips holding ‘bait’ that soil animals like to eat (Figure 3). Amazingly, all soils had some feeding activity, which means they all had some animal life. This is remarkable because it means that even bare and degraded soils still have some biodiversity and food web functions. In other words, soils organisms are resilient; they can cope with long spells of impoverishing treatment and some residual biodiversity will persist in spite of it all. Hopefully, these findings also mean that even highly degraded soils still have some residual biodiversity that can thrive again once soil management improves, for example when frequent ploughing is replaced with reduced tillage practices or manure is applied (Briones and Schmidt, 2017).
So, next time you stroll around the magnificent gardens of Versailles – or any other garden for that matter – remember that, although you see lush, ornate, pretty plants aboveground, this is all made possible by the soils beneath and the resilient life they harbour in their underground world!
Publication: Pelosi C, Boros G, van Oort F, Schmidt O (2020) Soil Oligochaeta communities after 9 decades of continuous fertilization in a bare fallow experiment. Soil Organisms 92(2), 129–142. DOI 10.25674/so92iss2pp129 [Open Access]
Other References:
Briones MJI and Schmidt O (2017) Conventional tillage decreases the abundance and biomass of earthworms and alters their community structure in a global meta-analysis. Global Change Biology 23, 4396–4419. DOI 10.1111/gcb.13744
Wall DH and Virginia RA (1999) Controls on soil biodiversity: insights from extreme environments. Applied Soil Ecology 13(2), 137–150. DOI 10.1016/S0929-1393(99)00029-3