Can subsoils buffer against the influence of global change on terrestrial ecosystems?
Subsoils are likely more insulated from many factors of global change compared to topsoil. Can this help terrestrial ecosystems cope?
Subsoils are typically defined as the soil deeper than 0.3 m, or in agricultural systems, the soil below the tilled or formerly tilled horizon (the Ap horizon). This realm of soil is dramatically less well studied and understood compared to the soil at the top. The reason for this lack of knowledge is simply because we can less easily access this deeper soil by sampling, often requiring special equipment. (An additional reason is perhaps that we think the topsoil just matters more, because this is where most of the roots and the microbial activity are; however, this may not be true when you look at the total volume of soil: even if there is lower activity, the subsoil still represents a potentially large volume of soil.)
Nevertheless, it is clear that subsoil is functionally important, for example in delivering nutrients and water to plants. Subsoil can contribute a rather large percentage to plant nutrients, from 10 to 80%, and this effect is particularly strong when the topsoil is nutrient poor or dry. Subsoils also store a lot of soil carbon.
We already appreciate that subsoil management may be important for mitigating effects of climate change, especially in agroecosystems. But could subsoils also buffer ecosystems from the effects of a range of factors of global change? Buffering can be understood to mean both resistance and resilience, meaning attenuating effects and facilitating recovery from impacts.
There are several points to consider in discussing this potential buffering function of subsoils:
Decreasing effects of global change factors with depth in the soil. Global change effects come from above, no matter if it’s warming or pollutants. For many pollutants, effects may be attenuated as organic compounds can be degraded as they move through soil, and inorganic pollutants (such as heavy metals) may be immobilized by integration into soil aggregates. It thus stands to reason, that given the likely decreased impact of global change factors on subsoils, these may be able to continue to function and continue to provide services to the ecosystem, thus providing resistance to global change impacts: a buffering function.
Deep-rooted plants. Plants with tap roots can explore deeper soil layers, in addition to the topsoil. By having this access to resources in the deep soil, which is likely also less strongly impacted by factors of global change, can such plant species provide buffering functions to ecosystems?
Reservoir of biological diversity in subsoils. This point is about resilience to global change impacts, i.e. about recovery. Could re-colonization of the topsoil from subsoil communities happen, and would this provide resilience for global change impacts?
These are some points that suggest that subsoils could be quite important to consider in global change impacts. Am I missing some important points? Please let me know in the comments!
Alternatively, could subsoils also be responsible for some additional ecosystem vulnerabilities under global change? Could synthetic organic chemical pollutants, once they arrive in the subsoil, stick around for longer, because of the low microbial activity, and could this have negative effects on any subsoil processes? Could the subsoil be particularly sensitive to some effects, thus failing to support ecosystem functions? For example, there is evidence that organic carbon can be lost relatively quickly from subsoils with warming.
What do you think?
Hello Matthias,
Thank you for igniting this question about subsoils. I think that the subsoils are going to experience higher microbial activity, in addition to assuming more significant role as nutrient reserves in future. For instance, with the focus on soil health, sustainability, biofuel crops (deeper roots, & multi-year life cycles) as well as the popularity of regenerative agriculture (again focus on deeper plant roots, hence higher priming etc, in subsoils), the subsoils are going 'change' substantially in future. Similarly, in permafrosts where rising temperatures are facilitating establishment of of deep rooted plants & hence exploration of subsoils, intense activity is expected in subsoils. Therefore, depending on the context of a soil, subsoil will act as a buffering or feedback agent to global changes.
Best,
Tanvir
What do you think? Can subsoils be a part of the solution or can they potentially be particularly vulnerable? And would this be a function of the factor of global change you're considering?