Four Conjectures on Soil Microarthropods and Ecological Restoration

In what follows I conjecture about soil organisms in the context of restoration projects.  This are listed below as C1-C4.  I concentrate here on soil microarthropods (primarily free-living soil mites and springtails), since these are the groups that my lab are most interested in.  I also have Midwestern systems in mind, but the remarks can probably be generalized.  These are for the most part empirically-based conjectures (a "empirijecture, if you will!)": there is not enough work done to be emphatic, but there is data emerging that supports each contention.

C1.       Soil microarthropods are hyperdiverse at most restoration sites.  This may be true even those that are considered to be in poor ecological health.  The number of described mite species globally is 45,000 or so and this may represent less than 10% of the total diversity.  To put this is perspective: if mite diversity got proportionate attention there would be over 100 consecutive "Mite Weeks" on Discovery for every one 'Shark Week”.  In the coming years we will get some real numbers at a variety of sites.  Expect no fewer than 200 species per hectare.

C2.       Factors that negatively affect plant diversity will also have adverse affects on soil organismal diversity.  Invasive species, fragmentation, nitrogen deposition, altered hydrology, climate change and so forth have implications for the soil environment.  In particular, factors that elevate decomposition rates may have devastating implications for soil animals.  This is because the decomposing litter, hosts the greatest diversity of soil arthropods.  I conjecture that in habitats where the litter layer has been reduced diversity is greatly diminished.  This may represent a vast unnoticed local extinction crisis.

Should We Care about the Conservation and Restoration of Decomposer Assemblages?

Soil organisms are phylogenetically diverse, trophically heterogeneous, highly variable in size, dissimilar in longevity, variegated in morphology, behaviorally divergent, adapted to different soil horizons, disparately pigmented, but united in their reliance on death.

By this, I mean to imply that an adequate study of soil ecology calls for interdisciplinarity on a scale that we are not especially good at.  That being said, the fact that dead things provide a foundation for these complex foodwebs has been enough for them to be functionally lumped together despite their multifarious attributes.  For the purpose of examining the fate of detritus this makes good sense.  Collectively the action of detrital-based foodwebs results in the breakdown of dead organic matter and the mineralization of organic compounds making them key nutrient available to the living. 

Restorationists need to pay attention to soil organisms both because they are a very large component of the diversity of most sites, and because the regulation of nutrient availability exerts a large influence on the diversity of plants and other components of the biotic community.  We all live in the shadow of the kingdom of decay.  Concern for the conservation of soil communities is made all the more urgent because soils are vastly affected by global change.  A warmer earth implies generally more rapid decomposition rates (when other factors such as moisture content of the soil remain constant).  Invasive species can also have dramatic implications for soils, either directly when soil animals (earthworms and isopods, for instance) are moved around, or indirectly when plants invade.

Reflections on Experimental Soil Manipulations to Reduce European Buckthorn Invasion, (with observations on soil lead mobilization)

What follows are some reflections on the results of our MOLA restoration experiment.  Please note that I post these to provoke conversation and get your thoughts on soils and restoration.  The reflections are very preliminary and have not been peer-reviewed.  For ease of read (or quick skim) I have not included literature references or a bibliography.  In the coming weeks we will be sending a manuscript in for publication on the topic.  Lauren Umek is the project manager and is a senior scientist on the project. 

For more background on the project see here and here.

The analysis of soil nitrogen availability is a focus of many studies examining the efficacy of incorporating material of low carbon quality (e.g. wood mulch) into soils as a potential restoration tool.  This is because these treatments are hypothesized to reduce nitrogen, elevated in soil as a consequence of anthropogenic atmospheric input or as a legacy of fertilizer use in agricultural land.  Nitrogen elevation is known to be a factor in some invasions and in a concomitant loss of native species.  Defertilization (N reduction) in circumstances where land is managed for the protection of biodiversity may lead to a reduction in invasion and an increased prevalence of species of conservation significance.  Our treatments were designed to reduce the availability of soil nutrients, especially nitrogen, and thereby modify the structure of the plant community in revegetating plots in this manipulative field experiment.  We anticipated that the soil environment modified in this way would support a lower density of Rhamnus cathartica (hereafter “buckthorn”) a dominant shrubby invaded of Midwestern Woodlands. 

We found that for some of the nutrients measured in our study, including nitrogen, availability was indeed reduced.  However, these effects were not consistent over the two years that we measured them with the important exception of phosphorus availability which increased in plots that had received a mulching treatment.  Consistent with our expectations buckthorn seedlings and saplings density was significant reduced in all plots compared with our controls (where buckthorn was not cut).  Additionally, those plots that had received wood mulch, either of buckthorn or commercial mulch, had lower buckthorn seedling or sapling density than all other treatments.