Which is worse for the soil – combine harvesters or dinosaurs?

Words I didn’t expect to read in this week’s scientific paper: “The similarity in mass and contact area between modern agricultural vehicles and sauropods raises the question: What was the mechanistic effect of these prehistoric animals on Earth’s productivity?” The paper, from Thomas Keeler and Danny Orr, raises what may be a major concern: Agricultural compounds have grown over the past few decades, to the point where they may compress the subterranean soil where the roots of crops extend. This poses a threat to agricultural productivity.

The paper then compares the threat of compression to that posed by the largest animal ever to roam our land: sauropods.

The big crisis

We think the ground is solid, but the gaps and channels within the soil are essential for plant life, as they allow air and water to reach the roots. Soil compaction, in its extreme form, gets rid of all these spaces, making the land less suitable for plants. pressure is difficult to reverse; It can take decades of plant and animal activity to break up compacted soil again and re-establish a healthy ecosystem.

There is a lot of agricultural equipment dedicated to plowing the soil, breaking the ground and making it more permeable to air and water. But this device does not reach all that depth. The “subsurface” area remains under the plowed soil, but below the surface remains well within the area used by the roots of agricultural plants.

Obviously, having something heavy rolling over the soil is an easy recipe for compacting it. Agricultural equipment is also getting increasingly heavy, because larger equipment has become more efficient. In 1958, the weight of a typical combine harvester was about 4000 kg. Today, this average weight has grown to more than 35,000 kg. To avoid crushing soil under this size, tires are made larger, and run at lower pressure, allowing the tire to spread over a larger area to reduce topsoil pressure.

But forces from the surface translate below the surface in a lesser way depending on the area in which the pressure force spreads. Instead, at some depths, the stresses are highly dependent on the mass of each wheel. As such, the increased mass of combines (and other agricultural equipment) increases the risk of the subsurface soil being subjected to compaction. This effect will be difficult to detect and correct, but it can lead to a loss of agricultural productivity.

global risks. Plus the dinosaurs!

Compression risks are not evenly distributed. Drier regions have less groundwater pressure on them and are therefore less at risk of pressure. Some countries also farm from small plots that do not take advantage of the efficiencies offered by large machines. However, a lot of areas are at risk of compaction, including the eastern United States, eastern Australia, Argentina, and much of Europe. In general, these areas correspond well with those suggested by a separate study to have lower productivity due to soil compaction. Overall, about 20 percent of important agricultural areas are at risk.

Which brings us back to the case of dinosaurs. Sauropods became much larger than even the largest harvesters – perhaps close to 80,000 kilograms. Their weight was distributed on only four ends, with footprints almost identical to those of modern tires (by contrast, reapers often have six tires). Finally, in order to move, the sauropod had to raise at least one limb at a time. All these factors combined meant that sauropods had to put more pressure on the soil.

If farming equipment now posed a risk of compression, dinosaurs almost certainly were causing problems. But at the same time, animals of this size required a thriving ecosystem to support them. “The potential for significant soil compaction by foraging for sauropods appears to be incompatible with the productive lands that supported renewed vegetation to feed these prehistoric herbivores,” say the researchers.

How do we explain this apparent contradiction? Keeler and Orr, both agricultural studies specialists, resorted to an idea that had been popular in palaeontological circles for a while: Anything this size was almost semi-aquatic, because it would struggle to support its size. But rather than talking to the paleontologist for more ideas, the authors simply declared that “resolving this paradox is outside the scope of this study.”

PNAS2022. DOI: 10.1073/pnas.2117699119pnas.org (About DOIs).