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Can soil carbon storage really make a difference to our climate? Do we have the right data to answer?

Posted by Chad Kruger | October 31, 2012

Three studies on the relationship between agriculture and global carbon dynamics were released in the past couple of weeks that are relevant to Pacific Northwest agricultural systems. Two are studies published in a special issue of the Journal of Soil and Water Conservation from CSANR’s Climate Friendly Farming Project (Brown and Huggins; Stöckle et al.) focused on soil carbon sequestration in PNW cropping systems, and the third is a study published in the Proceedings of the National Academy of Sciences (Gattinger, et al.) on soil carbon stocks under organic farming.

Each of these studies uses a different type of scientific methodology to explore the question of whether agriculture can contribute a “climate solution” by storing atmospheric carbon in soils. The Brown and Huggins study is a regional review of soil carbon data-sets (both published and unpublished) for cereal-based cropping systems in the Pacific Northwest (e.g. wheat), the Stöckle, et al. study is a model simulation of representative PNW cropping systems, and the Gattinger, et al. study is a literature review of published studies on “organic vs. non-organic managed” soils. While there are several interesting findings in each of the studies, all three of them contribute to the consensus that, yes, agriculture can provide a modest but important contribution to solving the global carbon challenge through the transfer and storage of atmospheric carbon in soils – a process we agricultural scientists call carbon sequestration. Furthermore, all three of these studies indicate that improving farm management practices can positively influence soil carbon sequestration.

Digging a little deeper into the studies, we see at the outset that the more productive a given soil is, the more likely that it can hold more carbon.  This relationship is a factor of carbon inputs – you can’t sequester carbon if you don’t put it in the system in the first place.  We find a few key agri-climatic factors influence productivity, and therefore influence the degree to which we can store carbon in soils.  The factors include precipitation and fertilization (more of both = more total biomass production), crop rotation (some crops or cover cropping produce more biomass), use of perennial crops, tillage (which reduces carbon inputs by oxidizing crop residues), and the application of organic soil amendments (e.g. manure, compost, etc.).

Productivity Factors Carbon Sequestration
High precipitation Increased
High fertilization Increased
High biomass production (due to crop selection) Increased
High tillage Decreased
Organic soil amendments Increased

 

So, this is all good news, right? Conservation farming can increase soil carbon sequestration and contribute to reducing the global carbon imbalance. And, because organic farming contributes more to increasing soil carbon than non-organic it is even better for the climate, right?

Well, not so fast … we still have a number of questions we need to answer before we bet the farm (and the global carbon cycle) exclusively on a soil carbon sequestration strategy. One of the key factors (above) driving increases in soil carbon is fertilization, particularly the use of nitrogen fertilization. Increasing soil carbon stocks means increasing nitrogen use – and from Stöckle et al. we can also see that increasing nitrogen use increases nitrous oxide (N2O) emissions, a very potent greenhouse gas. At this point, the differences between systemic farm management strategies (e.g. no-till, organic, conventional, etc.) are very challenging to parse out with the available science, but we do know that the nitrous oxide emissions from ALL farming systems are significant. From Stöckle et al., we see that on balance across farm management strategies N2O emissions can counteract the positive benefit of soil carbon sequestration. This doesn’t mean that improving carbon sequestration isn’t a good thing – we emit the N2O regardless of whether we’re managing soil carbon well or not – it just means that there is more to the picture that we need to measure than just the soil carbon level.

Also, none of these studies assess the “indirect” emission consequences of the various production systems. For instance, in Stöckle et al. we see that a crop rotation of continuous cereals sequesters more carbon than a crop rotation that utilizes a legume (a crop that fixes its own nitrogen). This is because legumes allocate energy to fixing nitrogen instead of producing a lot of biomass. The catch is that the legume rotation uses less synthetic nitrogen fertilizer – an input that has a huge off-farm emissions consequence. So, on balance, is it better to create more up-stream emissions in fertilizer production to increase soil carbon levels or not?

Similarly, most of the world’s organic farming systems are still heavily dependent on tillage to control weeds – and those tillage operations both speed oxidation of carbon inputs and use a lot more fossil-fuels than no-till farming systems. The moral is there are still a lot of “trade-offs” we make between farming system strategies that have indirect or consequent impacts on greenhouse gas emissions – and before we promote a given system or practice as “good for the climate” we need to more fully quantify the total impact.

Finally, the last question that I think needs to be raised is whether comparing one farming system to another is the right approach to determining a “climate friendly farming” system. We know, at this point, that there are a variety of mechanisms by which we can affect soil carbon sequestration (positive and negative) as well as consequent emissions associated with nitrogen and fuel use. The most climate friendly farm is likely one that blends features from several different current farming systems. Ultimately, in our quest for a sustainable and climate friendly farm – we’re still searching…

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