Poster's note : text is full of links to relevant papers. Read online if poss. I've stitched two articles together to make one 'Frankenarticle'
http://blog.ucsusa.org/andrea-basche/soils-to-reverse-climate-change-what-do-we-know-about-carbon-farming-practices?utm_content=bufferf19c3&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer http://blog.ucsusa.org/andrea-basche/soils-to-reverse-climate-change-carbon-farming-and-the-untapped-potential-in-ecological-approaches?utm_content=buffereeef6&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer Soils to Reverse Climate Change: What Do We Know about “Carbon Farming” Practices? ANDREA BASCHE, KENDALL SCIENCE FELLOW | MAY 11, 2016, 4:45 PM EDT Extract I specifically mention cover cropping and reduced plowing because of the wide use and general acceptance of these solid conservation agriculture practices by scientists and industry. But, what do we know from actually measuring their effect on soil carbon? Cover crops Cover cropping refers to the practice of producing a crop that is not primarily intended for harvest. This is particularly apt when the soil would otherwise be bare, and in principle this has the potential to increase carbon in the soil. From a carbon “budget” standpoint, adding a plant that you do not remove can be a potential net carbon input. Asynthesis of field research found that cover cropping practices increased topsoil carbon to an average level that—if applied to 25% of current cropland globally—could offset approximately 8% of agricultural emissions. The authors of that analysis recognize this to be a rough estimate, but it does represent a start on soil-based climate mitigation. Agricultural emissions, keep in mind, make up approximately 11% of overall global emissions, so even if this one practice could make a difference within the agricultural sector, a lot more work would be needed to offset as much as possible of the remainder. A winter rye cover crop protects the soil (left) this spring at a research site near Iowa State University. This experiment also investigates the impacts of not plowing (no-till, right) leaving crop residue (like these corn stalks) on the soil surface. These are two approaches commonly discussed in how improved agricultural management offers climate change mitigation. Photo: Aaron Price Soil tillage (plowing) Disturbing soil, as with plowing (to prepare seedbeds) or cultivation (to control weeds), is the main purpose of soil tillage. This disturbance can expose carbon to decomposition, with a significant fraction of this eventually emitted as greenhouse gases. However, there are planting and weed control techniques that do not require these modes of soil disturbance. Just how much can the reduction (or elimination) of soil disturbance increase soil carbon? Scientists have found that such “no-till” practices may increase topsoil carbon while simultaneously reducing it in the subsoil, where plowing would normally move plant residue from the soil surface to a deeper layer (see here, here and here: these are “meta-analyses,” which summarize the outcomes of numerous field studies, and are a powerful way to understand separate experiments). For this reason there is much scientific debate about tillage and carbon sequestration; many studies do not measure carbon beyond the topsoil (soil surface layer.) It has also been found that just one year of plowing after multiple years of not plowing can negate years of carbon accumulation. Such a practice is often necessary after a sequence of years without plowing or cultivation. As a result,some researchers believe the overall carbon storage potential from reduced tillage alone may be overstated. It is also worth noting that on many farms, reduced- or no-till practices are made feasible by replacing tillage with herbicides, most commonly paired with genetically engineered herbicide-resistant crops. Farmers constantly need to evaluate trade-offs like these. This calls for the further development of agroecological techniques that suppress weeds without forcing farmers into the false choice between disturbing soil (and the consequent greenhouse gas emissions) or contributing to the development of herbicide-resistant weeds. These are useful practices—but we need to dig deeper The bottom line is that the best soil management techniques in an annual cropping system optimized to remove large amounts of carbon as grain or forage can have at best a minor role in reversing climate change. Ultimately, an agricultural system redesign with more complexity and featuring perennial vegetation will likely be necessary for soil and crop management to reverse net carbon losses. However, there are additional sound agricultural reasons to reduce tillage and increase soil cover. Plowing the soil can degrade soil structure and reduce its ability to maintain healthy nutrient and water cycling (as is vividly seen in this demonstration). Further, given the many co-benefits of topsoil carbon, such as greater ability to store water, small increases of soil carbon can have large impacts. In a companion post, I look at a series of other agricultural practices that might offer more on the carbon storage front “Carbon Farming” and the Untapped Potential in Ecological Approaches ANDREA BASCHE, KENDALL SCIENCE FELLOW | MAY 12, 2016, 12:00 PM EDT Are there agricultural practices that might offer more potential than the ones commonly discussed in the “carbon farming” conversation? In a companion post, I wrote about what the science tells us about cover cropping and reduced tillage, two practices getting a lot of attention in what I’ve called the “carbon farming” rage. Here I want to address some more agroecological practices, those that incorporate ecological principles, and what is known from field research about their ability to add carbon to the soil. Cattle graze here on a ranch in the Sand Hill region of North Central Nebraska. Grasslands cover a large percentage of the planet and research demonstrates greater potential with improved management (such as compost additions and plant composition) to increase soil carbon. What do we know about soil carbon potential beyond the basic conservation practices? There is less research on the relationship of agroecological practices, such as crop rotations, agroforestry, and improved grazing-based systems, with soil carbon—but what is known is very positive. A synthesis paper of global field studies found that crops in rotation plus cover crops increased total soil carbon by 8.5%. Some estimates have suggested that when land is shifted from use for growing agricultural crops to pasture for livestock, carbon increases significantly (by 19%, according to one study synthesizing several research sites). The same analysis found that returning some cropland to forest could lead to even larger soil carbon gains (up to 53%). On grazing lands, which cover around 25% of lands globally, changes to management (for example, compost additions orimproved grazing practices such as fertilizer additions or the use of native plant communities) can increase soil carbon such that small changes make a big difference when scaled up. Further, on the option of integrating of trees into agricultural lands, scientists haveestimated the potential of agroforestry systems to increase soil carbon to be approximately 95 times greater than the conservation practices of no-till, cover crops and crop rotations (as estimated by one analysis that looked at many studies in the tropics). This is quite impressive untapped potential, particularly when considering that many of these ecological practices receive a fraction of public research dollars. Of course, some of these practices require taking a small amount of land out of production, but if done strategically on low yielding unprofitable sections of a field,such diversification practicescould offer a “win-win” in the form of cost savings for producers and environmental benefit for all. An alfalfa crop next to a plowed bare soil at a research farm near Iowa State University. Alfalfa is a forage crop that puts down roots for multiple seasons, leading to a greater potential for soil carbon increase relative to bare soil or annual crops. Photo credit: Aaron Price Even with complexities in the science, carbon farming deserves the attention Carbon farming is a topic of growing interest for both the research and policy communities. The science is complex. Researchers in the fields of ecology, agronomy, forestry, soil science and biogeochemistry, among others, dedicate entire careers toward this area. It can take a lot of soil sampling to detect changes, which is expensive and potentially difficult. Soils are highly variable from one location to another. The initial carbon content of soil heavily influences how much carbon can be increased. It is important to note that there are other emissions linked to agricultural management decisions, so any net change in the carbon sink is just one piece of the overall greenhouse gas emissions picture. One example of this frommy own work is that cover crops may lead to small increases in nitrous oxide emissions, depending upon management. Finally, something else I found inmy research was that the warmer temperatures resulting from climate change may drive increased carbon decomposition—all the more reason to get started sooner rather than later growing the soil carbon sink. It is important to conclude explicitly that not all carbon farming practices are equal. Many equate to working around the edges rather than systemically changing our design of agricultural systems and landscapes. As I’ve discussed here and in a companion post, the current science indicates that the approaches offering the greatest opportunities to increase soil carbon are those which 1. increase the amount of crops covering the soil in both space and time (which would increase the total carbon input to the soil “budget”) and/or 2. incorporate multiple carbon farming principles. If the agriculture sector is going to get serious about soils as a sink for carbon (and agriculture as a net sink of greenhouse gas emissions), more comprehensive research, combined with appropriate policyand economic incentives, will be necessary -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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