Soil persistence

An important new dimension to soil health, and some questions

When does a soil activity become a burden for soil persistene (or does it ever?) (Pic: M. Rillig).

Recently, we published a paper on the concept of soil persistence (it’s open access so feel free to check it out). I had been thinking about this topic off and on for a number of years (also see some older posts here). Basically, the starting point was that terrestrial ecosystems started out being just soil, no higher plants were there at the beginning. This made me think about soil being the entity that persists (meaning dynamic persistence). This is a fascinating thought, but it wasn’t enough to spin a story that we should look at soils differently.

There were two new ideas that came together in this paper, building on some of the earlier thoughts.

The first idea: soil persistence has basically two sides. There is the loss of soil (like through erosion), which ends the existence of the soil. That one is relatively obvious, and we have been measuring soil aggregation as an important indicator of soil stability for several decades. But there is also the other side: the formation of new soil. This happens through the weathering of minerals and via colonization of new material. While we measure relatively routinely response variables related to loss of soils, for example soil aggregate water-stability, we never measure aspects related to the formation of soils. Meaning, we basically don’t know the consequences of many different factors or treatments on the soil formation side. Maybe this makes sense, since soil loss can be episodic and quick, while soil formation takes a long time. But I think it’s important to also think about this aspect.

The second idea is very interesting to me. When are soil activity and soil persistence actually at odds with each other? What we measure routinely at the end of experiments, to indicate some version of soil health, are soil process rates and activities, like decomposition rates, enzymatic activities and so on. What if the interpretation of these readouts changed? What if too high a decomposition rate may actually be bad for the soil, because this means at some point also a loss in organic binding agents for soil aggregates. The paper goes through a couple of examples, where the interpretation would flip from “this is positive activity” to “this may be bad for soil persistence”. Here is the thing: I don’t have a clear set of observations or experiments in mind to try to identify these critical thresholds: when is a decomposition rate too high, when are enzymatic activities too high? And is this even ever a problem? Is there really such a trade-off between persistence and activity?

This is the point where I would be very curious about your ideas. Do you have anything in mind that could identify these breakpoints when “activity” becomes a negative for soil persistence, and how this could be clearly identified by data synthesis or a dedicated experiment. So far, I have been drawing a blank.

That question not withstanding, I would also be very grateful for any other general input on this idea of soil persistence, as we are also still thinking about it. Thanks!