I finished my PhD!
I’m providing my written dissertation and presentation slides. Obviously there are many edits. Nonetheless, I figured providing the deep thinking I’ve done so far is more helpful than waiting another year before the peer-review proccess has finished. As someone passionate about SciComm I wanted this to be available sooner rather than later.
- My defense on YouTube
- My presentation notes and slide deck (with citations/links in the notes section)
- My written dissertation.
- Inspiration Dissemination homepage & podcast archive
You might ask yourself, “how can you spend an entire PhD focusing on dirt?!” A few surprising facts about soils captured my curiosity. First, soils hold about twice as much carbon compared to the carbon dioxide content of our atmosphere. If your calculation include permafrost soils (permanently frozen soils in the cold regions of the world that are quickly warming up), then soils hold 2-3 times more carbon than both the atmosphere and all vegetation combined. That’s a lot of carbon under our feet! Second, soils use to hold nearly 10% more organic carbon 12,000 years ago (when the last ice age ended), but most of those carbon losses came within that last 200 years (due to wide-spread industrial agriculture). Finally, carbon in soils has a wide age distribution; some plant-derived carbon can enter the soil and be released back to the atmosphere within hours, but other soil carbon can persist for thousands of years. We’re beginning to understand how/why this occurs, but this remains a highly contentious research area for the soil science community. To recap, soils used to hold more carbon than they do today, our land management practices resulted in a loss of soil carbon, and if we can get more carbon back into soils some of it could stick around for thousands of years! Surely we could manage our way back towards the way things used to be; even if us humans can increase global soil carbon content by just a tiny-tiny bit, it could have climatic-scale benefits.
That is a major reason why I pursued graduate school, because I wanted to learn how soils can be a natural climate solution through changes in land management practices. But soils are so unique across ecosystems - and really expensive to consistently measure - which is probably why there were hardly any continental-scale research studies. My PhD project looked at almost 40 representative ecosystems across North America and put soils through a battery of tests and analysis to try and learn their secrets to hopefully predict how soils will respond to a changing climate. My experience with NEON had many collaborations that have produced over 12 peer-reviewed publications and dozens of presentations, so it’s tough to summarize.
Briefly, we did find that soils can be a natural climate solution, but only if we can convince humans to change land management practices in perpetuity. For example, when farmers change their practices to minimize soil tilling, or adding cover crops, those do over time increase soil carbon. But the soil carbon increases are tiny each year and difficult to pinpoint relative to the inherently noisy ecological data. Observable increase in soil carbon takes 5-10 years, but the on-farm benefits are seen much sooner than that (less water & fertilizer required, more drought resistant crops, etc…). However, one major flood, a few extra tillage passes, or one major windstorm that removes soils can undo a decade of soil conservation practices. Additionally, if the commodities markets change and different crops become highly valuable, then farmers are likely to expand their plowing footprint (e.g. farmland that was under a voluntary conservation program to leave untouched was quickly plowed and planted when Russia invaded Ukraine eventually spiking wheat demand. See reporting in March 2022 where USDA denied the idea of allowing soil conservation areas to be planted, then read the USDA statement in May announcing the exact opposite.).
Soils are complicated enough to predict on their own - I know because I just spent an entire PhD trying to do it - but how we treat soils today and thirty years from now depends much more on individual farm solvency, global commodity prices, turnover of land ownership, and lobbying through the farm bills among many other things. Soils can be a natural climate solution, but the solutions we have are almost exactly the same recommendations as what scientists recommended after the Dust Bowl (even today, we’re making the same exact mistakes as we did in the 1930’s). Many prominent soil scientists are evangelizing that soils are such a big solution that we can sell soil carbon credits; I’m personally weary of over promising the global potential of soils to return to a higher carbon state, let alone mitigate our excess emissions in the form of carbon credits. We can and should expect soils to provide us the basics: clothing, food, and clean water. As a professor once told me, without soils we’d be cold, hungry, and worse of all sober. Those are still plenty of reasons to appreciate and fight for soils!
Post defense notes
I smashed a lot of content into 44 minutes of presentation time, and of course I missed a few things!
First, I should have ended my NEON discussion with the caveat that because I only looked at plant-derived organic matter, maybe the microbial world has something to offer I can’t examine with my soil chemistry method. As always, more research needs to be done.
Second, as reiterated in my closed door session from my committee, I should have said/written the more cumbersome “lignin contributions to the soil organic carbon pool” instead of “lignin content”. The former more accurately reflects what I measured, the latter is ambiguous. Just another lesson learned in how SciComm shorthand conflicts with the need for scientists to be necessarily precise in our language.
Finally, managing soils as a natural climate solution for carbon credit schemes is a technically complex problem, but it’s not the only problem worth grappling with! In the closed door session my committee noted how the October Biogeochemistry special issue is dedicated to bringing reality closer to what we know about soil science (see here). If you follow me on twitter, you know I’ve been vocal about how awesome soils are, but also that trying to quantify changes in soil carbon for carbon offsets is extremely challenging to do, especially at scale. Nonetheless, if we’re serious about soil carbon credit schemes ‘sequestering’ our excess emissions, I would argue the more difficult issue is predicting how people will treat soil for the long-term. I’m a soil scientist who has read more social science literature than I’m willing to admit, and understanding how people will behave in the future is infinitely more complicated than predicting soil.
- Big Data for Big Problems - The National Ecological Observatory Network (NEON)
- Moisture-driven divergence in mineral-associated soil carbon persistence
- Lignin and fungal abundance modify manganese effects on soil organic carbon persistence at the continental scale
- Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry
- Key predictors of soil organic matter vulnerability to mineralization differ with depth at a continental scale