OK. Not clueless. But today we have yet more evidence that we really don’t understand how this planet’s carbon cycle works,thanks to the latest issue of the Soil Science Society of America Journal. In “Nitrous Oxide Emissions Respond Differently to No-Till in a Loam and a Heavy Clay Soil“, a group of Canadian researchers makes it clear that our species is just plain ignorant when it comes to managing greenhouse gas emissions. It’s looking like there is one fewer tool in the box of options at our disposal for mitigating global warming.
Until now, it was generally assumed by aggies the world over that no-till, also known as zero-tillage, farming, is a good thing, as it reduces erosion and increases organic matter. Essentially, the practice avoids disturbing the soil after harvest. Forgoing tilling also means less work and potentially lower costs for farmers. And, as evidenced by the Wikipedia entry, the conventional wisdom is it cuts down on carbon dioxide emissions and therefore poses a possible method of pulling back the reins on global warming.
NASA’s James Hansen hasn’t written about no-tilling specifically, as far as I know, but when he offers “improved forestry and agricultural practices” as one way of absorbing CO2 and bringing down concentrations below 350 ppm, this is one of the kind of things he’s presumably talking about.
But along comes Philippe Rochette and his colleagues at Canada’s Department of Agriculture and Agri-Food with their analysis of gas exchange between some clay-soil farmland at the atmosphere. For some reason, their experiment was carried out several years ago, from April 2001 to November 2003, but it isn’t the timing that’s important. Here’s what they found:
Adoption of NT [no-till] practices has several benefits including reduced energy inputs and soil erosion. Soil C stocks are also often increased under NT compared with conventional tillage practices. Such C gains are a sink for atmospheric CO2, which contributes to decrease net GHG from farms. Our results indicate, however, that NT can result in incremental N2O emissions that can more than offset the soil CO2 sink during the first 5 yr after adoption of this soil conservation practice in a heavy clay soil.
So, yes, there is a net drawn-down of CO2 from the air, but a net release of nitrous oxide. And when you normalize for each gas’ heat-trapping ability, the end result is an increase in planet-warming effects.
There are lots of limitations to this study, of course. For one thing, it involves heavy clay soils, the kind found in much of Quebec and elsewhere in eastern Canada. But that doesn’t mean the same kind of nitrogen movement will be found in other soils. So it could be that no-till practices, if used everywhere on Earth they make sense, would still represent a net sink of greenhouse gases. Rochette et al. write that “the potential of NT for decreasing net GHG emissions may be limited in fine-textured soils that are prone to high water content and reduced aeration.”
On the one hand, it’s always good to know things like this. It prevents us from embracing what we thought was a good idea but turns out to be making things worse. On the other, it reminds us just how little we really know about carbon cycles, and how foolish are these geo-engineering schemes that some would have us pursue in hopes of preventing catastrophic climate change. We simply aren’t equipped with the sophisticated understanding required to go about tinkering with planetary heat and carbon budgets.
Some would argue that we have to take the chance, so close are we to irrevocable tipping points, and so unlikely are we to voluntarily stop burning the fossil fuels that are the cause of the problem. But given the choice between doing what we know with a high degree of confidence will slow global warming —;;;; shut down the coal-fired plants and so forth —;;;; and running wild with some hairbrained scheme to reset the planet’s thermostat, I’ll choose the first option, even if the political challenge involved is enormous.
P. Rochette, D. A. Angers, M. H. Chantigny, N. Bertrand (2008). Nitrous Oxide Emissions Respond Differently to No-Till in a Loam and a Heavy Clay Soil Soil Science Society of America Journal, 72 (5), 1363-1369 DOI: 10.2136/sssaj2007.0371