Table of Contents
Podcast’s Essential Bites:
[5:43] ES: "There are a couple of main ways that agriculture has altered soil and its role in the carbon cycle. So plants are fixing carbon and they're adding it to the soil through a number of ways, through their leaves, and to a large extent via their roots and also through carbon compounds that leak out of the roots. [...] The sort of natural baseline condition is that there's an ecosystem on the surface, there are plants [that] are fixing carbon, they're introducing it, there are microbes that are constantly breaking it down and releasing it back to the atmosphere. But those two processes ultimately come into some kind of balance that allows carbon to build up to a certain level in the soil."
[7:01] ES: "Agriculture involves harvesting biomass, replacing forests with cropland potentially, converting ecosystems that were composed of deeply rooted plants like perennial grasses with ecosystems that are managed by humans and are composed of our crop plants, which tend to be fairly short lived, and hence shallow rooted, and therefore add less carbon to the soil, particularly at depth. And so we're managing the plant communities in agro ecosystems, when we practice agriculture, and therefore controlling the amount of carbon that actually makes it into the soil and at some level, choking it off."
[8:07] "Agriculture often, or almost always, involves disturbing the soil, originally by hand or plowing a field with oxen [...] and nowadays using machinery. And that disturbance and also the amount of time the soil spends bare without vegetation on it, increases the rate at which soil is eroded. And we think humans have increased the rate of soil erosion massively in agricultural land since the advent of agriculture. And that means the carbon in that soil leaves the system, although it doesn't necessarily get readmitted to the atmosphere the way it does if microbes eat it and then respire it, it ends up in water bodies essentially, in rivers and lakes, or at the bottom of the ocean."
[11:03] ES: "There's been an ongoing debate about whether erosion of soil and transport of that carbon amounts to a source or a sink of CO2 to the atmosphere. [...] It's very hard to actually predict what's going to happen [with soil] in any particular location, even with our best of current models. [...] Our conceptual understanding of what goes on in soil and why carbon sticks around as long as it does in soil has been evolving really rapidly over the last couple of decades. And it's by no means settled. [...] We can't calculate anything with high precision. We're still figuring out the basics of how things work at some level."
[17:00] ES: "There are a whole range of agricultural practices that were originally developed for perfectly good agronomic reasons that aren't actually related to sequestering atmospheric CO2. And that would include cover cropping, [no-till or agroforestry] [...]. What's happened over the last couple of decades is that there's been an increased interest in using these practices to also to fight climate change. There are difficulties though. All of these agricultural practices yield increases in the amount of organic carbon in soil. And organic carbon is not necessarily a long lived sink for atmospheric CO2. [...] One of these practices might succeed in building up some carbon, but the amount of time it sticks around is unclear, in that it will depend on future land use and future climate. [...] Conservation agriculture [...] shouldn't be treated as equivalent to a more durable or permanent carbon removal strategy."