|Climate “smart” enthusiasts claim that damaging effects of climate change can be reversed by agricultural practices that promote sequestration of atmospheric carbon in soil. What is the evidence?|
In my February 7, 2017 palaver Climate “Smart” or Climate Semantics? I gave an unflattering review of climate smart agriculture (CSA), now in heavy rotation by international donors USAID, FAO, The World Bank and research centers like CGIAR. Since then, it has been covered by Crops, Soils, Agronomy News magazine, a monthly review published jointly by the Crop Science, Soil Science, and Agronomy Societies of America. Nothing I have read there changed my mind about CSA.
Central to the climate smart push is the “triple win”, the idea that adopting CSA enables farm operators, primarily (but not necessarily) in the developing nations, to simultaneously boost productivity, leverage risk, and mitigate climate change all in one nifty, if rebooted, package of “sustainable” know-how. The mitigation potential of CSA rests primarily upon efficient capture and storage of atmospheric CO2 in soil organic matter, a process known as carbon sequestration. I should point out that carbon sequestration is not the same as soil organic matter, which is different from soil organic carbon.
The term “carbon sequestration” has waxed brightly in climate change circles. In its ruling context, carbon sequestration refers to the procedure for removing CO2 from the atmosphere and storing it in one of four major “pools”: oceanic, geologic, biotic, and soil. Organic matter is the main repository of carbon in soil, and the final product of decayed, once-living biomass. Regardless of whether plant or animal, all living biomass is directly or indirectly, the product of carbon transfers from the atmosphere via photosynthesis. Cropping systems that favor soil organic matter accumulation include reduced or zero tillage, cropping intensification, and permanent or semi-permanent soil cover (mulching) via plant residue. Collectively, such techniques are known as conservation agriculture (CA) whereas conservation tillage (CT) emphasizes reduced tillage and concomitant soil cover.
Curiously, time scale is rarely mentioned in the annals of carbon sequestration. Is carbon fixed in soil organic matter for one year “sequestered” by global climate change models? Would this be considered “successful” mitigation for CSA accounting purposes? What about a decade, or a century? For example, soil organic carbon can be divided into two fractions: “labile” and “stable” based on predicted residence time in the soil. For the labile fraction, consisting of surficial or buried plant litter, and particulate organic matter, decomposition occurs over a time scale of days to years. Humus, and resistant products like charcoal and biochar, in contrast, decompose over years to decades. What’s a climate-smart operator to do?
In the real world, farmers adjust soil and crop practices due to changing markets and prices for commodities, pest control, and related agronomic reasons. Such changes may be neutral, carbon negative, or involve net transfer of CO2 back to the atmosphere (carbon positive). The point is, decisions regarding tillage, pests, fertilization, and harvest and post-harvest handling may change year to year depending on circumstances. Carbon that was sequestered in the farmer’s fields last year, or the year before, may suddenly “gas off” to the atmosphere. Suffice to say, the terrestrial carbon cycle is not a simple input/output device, like some hard drive container manipulated by pushing a sequence of buttons. It is a grave misunderstanding to treat it as such; however, I leave the temporal ambiguity surrounding carbon sequestration in soil unresolved for now.
A key point in the climate-smart debate is how to evaluate the performance of CA in reducing greenhouse gases (GHGs) particularly CO2. In that vein, several pertinent questions call out: How much carbon is sequestered in the soil under conservation tillage compared with conventional plow tillage management? Is carbon sequestration in the soil predictable? Are carbon sequestration rates high enough to offset global CO2 emissions from fossil fuels in a manner that would put the brake on climate change? What follows is a deliberate if somewhat esoteric, vetting of claims and counter-claims, framed against the backdrop of experimental data from numerous, well characterized research plots including long-term tillage plots maintained by NC State University. The esoteric part refers to units of measurement which are not commonly encountered in daily life even for those well versed in the metric system. Hang in there, though. The Internet is at your fingertips, feel free to use it. See also my End Notes for further help.
First, we need some idea of how much carbon must be sequestered, and the period of time for getting the job done. Here, we’ll follow the Intergovernmental Panel on Climate Change (IPCC) 2007 guidance that to limit global temperature rise since the industrial revolution to 2 degrees Celsius (C), considered a prudent limit to avoid dangerous interference with the climate system, CO2 emissions must be cut to 15% of the year 2000 levels. This target may seem rather drastic, even unattainable, but it provides a benchmark for calculating how much carbon must be sequestered. Regardless, a further 0.5 degrees C increase may be “baked in the cake” due to thermal inertia of the oceans even if GHG emission were eliminated today (see “B1” scenario in Meehl et al. 2007). So, the task of stabilizing earth’s temperature is not simply a matter of reducing and/or removing GHGs. Still, the 2-degree limit has been adopted by more than 100 countries (IPCC, 2007) so it’s not just some arbitrary figure.
Global carbon emissions in the year 2000 were approximately 6.3 Gt C1 (World Bank, 2004). Thus, a reduction to 15% level would place 0.9 Gt C per year as the limit. The estimated anthropogenic (=from human activity) carbon emissions in 2015, by contrast, was 41.9 ± 2.8 Gt CO2 (Quere et al., 2016), or 11.4 ± 0.76 Gt C so we are forging ahead quite bravely on the emissions front. Even so, that figure could surge to 58 Gt CO2 yr-1 (15.8 Gt C yr-1) by 2050 without measures in place to reduce GHG emissions (IEA, 2016). In this scenario potential 2000-2049 cumulative GHG emissions could reach 2,000 Gt CO2 (546 Gt C). The task before us is daunting, without question.
Saying that, the task is not beyond human reach. The pertinent question is: What is agriculture’s “climate smart” potential for mitigating GHG emissions?
Let’s begin by examining claims published in the Emission Gap Report 2013 from the United Nations Environment Program (UNEP). The report contends that direct GHG emissions from agriculture could be reduced by 1.1 to 4.3 Gt CO2e yr-1 by 2020, with 89% of this potential realized through improved management practices including conversion to no tillage cropping, more efficient use of fertilizers, and biochar addition to soil. Here, I focus on the first item in the list: improved management via no- and reduced-tillage cropping. I don’t mean to discount more efficient use of fertilizers, which would undoubtedly curb GHG emissions to some extent. The problem is, I do not have any data on this, nor am I familiar with methodology for evaluating such a claim. As for adding biochar to the soil, this is not happening, or ever likely to happen, on large enough scale to make a difference in GHG emissions. Industrial-scale biochar production is elaborate and costly, while the availability of potential feedstock (organic debris) is severely limited by competition for feed, fuel, soil cover, among others. It’s moot, in my opinion.
Back in 2014 David S. Powlson, with a group of leading scientists, published an article with the provocative title “Limited potential of no-till agriculture for climate change mitigation” which cast doubt on the UNEP Emission Gap Report 2013 claims. They argued that emission reductions of 1.1 to 4.3 Gt CO2e yr-1 by 2020 are over-optimistic. Powlson’s group calculated that the most realistic no-tillage scenario was 0.6 Gt CO2e yr-1, applying an average sequestration rate of 0.3 t C ha-1 yr-1 over a global cereal crops area of 559 million hectares. Limiting their calculation to include only the remaining corn, rice, and wheat cropping area reduced the sequestration rate to 0.4 Gt CO2e yr-1. Even the 0.4 Gt CO2e yr-1 figure, they contend, is rosy.
Turns out, we can test the Powlson and UNEP claims using soil carbon data from NC State University’s long-term tillage plots in Reidsville, NC. These plots have been maintained in continuous corn, or corn-soybean rotation since 1984, providing a unique platform for evaluating land management technology on soil properties and crop response in the southern Piedmont and similar environments, globally. Briefly, the Reidsville plots comprise nine tillage treatments varying in timing and intensity: fall or spring moldboard plowing, fall or spring chisel plowing with or without disking, spring disking only, no-tillage, and no-tillage with shallow under-row subsoiling. Here, we focus on soil and crop data from four tillage plots: no-tillage, spring disk, spring chisel plow, and spring moldboard plow + disk tillage. These four technologies have consistently exhibited differences with respect to grain yield year over year.
Back in 2006 under the flush (and kindly) auspices of North Carolina’s Corn Growers Association, I measured soil carbon (among other soil properties) in the Reidsville plots through 30 cm (12 inches) deep, in 10 cm increments, twenty-three years after they were established. Twenty years is the time period adopted by IPCC (2006) for assessing carbon retention in soil, so our sampling may be considered in good, if not perfect, alignment with this benchmark. Sampling was limited to 30 cm because this is the maximum working depth of the tillage tools. One could argue that some roots, particularly corn, extend deeper than 30 cm. However, beneath 30 cm the environment for root extension becomes increasingly hostile (see further comments on this topic below). As such, the surface 30 cm is considered the prime soil-plant-tillage interaction zone, “the staging ground” where the action is. In addition, I measured soil dry (bulk) density in the same plots, also in 10-cm deep increments. Dry density is needed to calculate soil carbon stocks on a mass per unit area basis, an essential piece of information for estimating carbon changes over time.
Unfortunately, soil carbon was not measured before establishing the plots, so the pre-1984 30 cm carbon stock has been made equal to the conventional moldboard plow tillage, i.e. the “check” or “baseline” measurement. This is justified because moldboard plow tillage was widely practiced across similar soil-climatic conditions in the southern Piedmont prior to the advent of no-tillage planting technology. It also aligns with the method applied by Aguilera et al. in their 2013 meta-analysis testing long-term soil carbon sequestration in Mediterranean cropping systems. Here also, I have applied Aguilera’s formula for calculating carbon sequestration rate in the Reidsville plots. The soil data were compiled and subject to Analysis of Variance (AOV) procedures in SAS 9.4.
Results from the 2006 campaign at Reidsville are summarized in the chart panels below.
Panel (A) shows that soil carbon stock under no-tillage increased 42% compared with conventional moldboard plow tillage. Chisel plow tillage, consisting of spring-loaded shanks mounted 30 cm apart on the toolbar (Figure C), accumulated only 8% more carbon compared with moldboard plow tillage, a rather poor performance given that chisel plows are “non-inversion”, meaning they do not bury surface residues in the same manner as disk and moldboard plows.
Panel (B) portrays the information we are after: change in soil carbon accumulation over time, i.e. the carbon “sequestration” rate. Here, solid blue dots indicate the average annual soil carbon accumulation over 23 years. The horizontal lines are confidence intervals for the mean conditioned on the AOV, indicating the relative uncertainty in soil carbon measurements. Confidence interval lines that overlap, mark no difference in soil carbon accumulation even though the mean values (represented by the relative position of the blue dots along the horizontal axis) may be numerically different.
What does this information reveal about carbon sequestration in the Reidsville plots?
First, annual carbon accumulation under no-tillage, on average, was 0.51 t C ha-1 yr-1 compared with 0.25 and 0.06 t C ha-1 yr-1 respectively under disk and chisel plow tillage. Thus, on average, our no-tillage outperformed Franzluebbers’ (2010) 0.45 t C ha-1 yr-1 multi-site conservation tillage estimate for the southeastern United States, and performed considerably better than 0.3-0.4 t C ha-1 yr-1 reported by Aguilera et al. for Mediterranean cropping systems. Incidentally, the Aguilera rates were applied by Powlson et al. in their calculations, and generally conform with measurements from other soil and climatic regions (Sanderman et al. 2010; VandenBygaart et al. 2008). Applying a global cereal crops area of 559 million hectares (per Powelson et al. 2014), soil under no-tillage at Reidsville would accumulate, annually, 0.28 Gt C, equal to 1.0 Gt CO2e yr-1. This estimate falls short of the UNEP 2020 emissions reduction figures, but not fatally.
Second, the long, overlapping confidence interval lines for disk and chisel plow, and no-tillage indicate high cross-plot variance in soil organic carbon accumulation within less than 1 hectare (2.47 acres) of a uniform soil mapping unit (Wedowee sandy loam). For example, the AOV model confidence interval in Panel (B) is ±0.34 t C ha-1 yr-1 whereas the mean value for disk plow tillage is one-half that of no-tillage, and a fraction for chisel plow! Further, the coefficient of variation (CV), a relative statistical measure of data precision, was 68% for no-tillage; above 100% for chisel and disk plow tillage; and several disk and chisel plow tillage plots had negative rates of soil carbon accumulation. As such, the data are not very useful predictors of no-tillage performance at Reidsville let alone other locations with different soil, crop, cropping rotation, and management. Given that tillage plots are cropped across a ~5% slope (Figures D and E), we surmise that redistribution of crop residue and particulate organic matter has occurred over the years (simulations measuring runoff and sediment deposition have been conducted in these plots so we know it is happening). This could explain some of the plot-to-plot variation in carbon stocks.
None the less, sloping topography is a common feature in the southern Piedmont even down through the upper Coastal Plain. We should, therefore, expect to find variations in soil carbon accumulation at least as great, if not greater than, those measured at Reidsville. On the opposite end, in one of the longest running, well characterized tillage trials located in northern France (Dimassi et al. 2014), there has been, astonishingly, no increase in soil organic carbon under no-tillage after 41 years! Similar findings were reported by Loke et al. (2012) under wheat in South Africa, and Young et al. (2009) under long-fallow and continuous cereal cropping in Australia. No wonder then, Dimassi et al. conclude “there is still no consensus on the importance of sequestration which can be expected from reduced tillage”. I would concur.
Following the null soil carbon reports above, Baker et al. (2007) make the point in their commentary “Tillage and soil carbon sequestration – What do we really know?” that where sampling beneath 30 cm deep has been done in conservation tillage there has been no consistent accumulation of soil organic carbon. What is happening, they claim, is redistribution of carbon through the soil profile, with more carbon accruing deeper in the profile under conventional plow tillage, and higher concentrations near the surface under no-tillage. Luo et al. (2010) reached similar conclusions when sampling down through 40 cm deep. Soil mixing and root extension were reasons given for the apparent carbon redistribution.
Lastly, and by way of “proof in the pudding”, Reidsville corn grain yields are shown below in Panel (F).
Here, no-tillage has outperformed chisel, disk, and moldboard plow tillage, hands down. The relatively resilient yield under chisel plow tillage is interesting, given chisel’s low soil carbon status, and negligible 23-yr carbon accumulation rate. This seems to contradict the notion, popular among soil health and CSA enthusiasts, that soil carbon is the prime factor, the linchpin if you will, driving terrestrial ecosystem productivity. The Reidsville tillage plots beseech otherwise: corn and soybean productivity is, likely, related to differences in infiltration following precipitation, and the conservation of profile soil moisture via residue cover, i.e. once-living plant mulch. In fact, we have measured percentage residue cover multiple times and found that it was inversely related to tillage intensity, and directly related to grain yield. The high land surface roughness obtained after chisel plowing has also helped concentrate precipitation where it can infiltrate better. It is questionable, therefore, whether carbon is the prime driving force impacting agro-productivity in our southern Piedmont mineral soils. In fact, carbon as a rooting media constituent is not needed at all for plant production (hydro-, aero-, and aquaponic systems certainly stand as proof of concept); rainforests are among the highest in net primary productivity of all terrestrial ecosystems whereas the soil beneath them stores very little carbon. To my knowledge, there is no “optimal” soil carbon content that has been established, anywhere. This should be a clue.
A final word about corn grain yields in Panel (F) because inevitably someone will point this out: Why are they so low? There are two principle reasons.
First, about two-thirds of the mapped Piedmont soils in North Carolina are eroded “Typic Kanhapludults”, the name soil scientists have given old, weathered mineral soils with strongly developed horizons (horizons are layers, as in a cake). Soil series like Cecil, Pacolet, Rion, Vance, and Madison occupy millions of upland acres throughout the region. The Typic Kanhapludults have sandy to sandy clay loam surface horizons (the “A” horizon or “Ap” if under cultivation to about 8 inches deep) over loamy- to clayey-textured subsoil (the “B” horizon). Unlike Midwest prairie soil developed beneath sod (“Mollisols”), the Typic Kanhapludults are not endowed with high nutrient and water holding capacity. Micronutrient supplies are often low, thus to prevent deficiencies we adjust soil to pH ~ 6.0, which is considered slightly acidic. Root extension is often limited by subsoil acidity and shallow dense saprolite (decomposed rock).
Second, the Reidsville plots are all “rain fed” i.e. without supplemental irrigation. Rainfall during the critical period for corn and soybean in North Carolina usually lags well below crop evapotranspiration. While corn and soybean grain yields at Reidsville have, on average, risen over the years, stress from short-term summer droughts is common during the critical crop development stages. Few years have seen bumper crops, while most have been mediocre, and a few disastrous. The result is that corn and soybean grain yields at Reidsville are, on average, low compared with state and U.S. averages: 129 bu/acre in North Carolina, and 174.6 bu/acre nationally for all production categories, in 2016. Still, the long-term trend is irrefutable: conservation tillage practices have built yields in the southern Piedmont despite irregular rainfall, and negative, or relatively low, variable soil carbon accumulation.
In summary, and reaching back to the pivotal question: How much carbon is sequestered in the soil under conservation tillage compared with conventional plow tillage, the answer from Reidsville is: 0.28 Gt C annually, equal to 1.0 Gt CO2e yr-1 on a global cropping area basis. The wide variances (±) depicted in Panel (B) provide a clue as to the inherent unpredictability of carbon sequestration under three representative conventional and conservation tillage planting systems. In some cases, experimental data has shown no net profile carbon accumulation reported whereas gains, on average, of ~0.45 t C ha-1 yr-1 were reported by Franzluebbers for the southeastern US, and ~0.57 t C ha-1 yr-1 reported by West and Post (2002), worldwide. Shallow sampling depth (< 30 cm) in many studies has hampered unbiased, accurate portrayal of soil carbon cycles and attendant sequestration.
What about climate change mitigation? Are carbon sequestration rates associated with conservation tillage practices high enough to offset global CO2 emissions from fossil fuels in a manner that would put the brake on climate change? Again, we are forced to ponder the magnitude of the problem: CO2 emissions in 2015 were 41.9 ± 2.8 Gt (billions of metric tons), a figure that must be cut to 0.9 Gt C per year (15% of the year 2000 emissions) to limit global temperature rise above a pre-industrial 2 degrees Celsius. Cumulative GHG emissions could reach 2,000 Gt CO2 by 2100. In contrast, the Reidsville plots indicate that mean carbon sequestration rate under conservation tillage could potentially reach 1.0 Gt CO2e yr-1 using the global cereal crop area of 559 million hectares applied by Powlson et al. (2014).
The Reidsville no-tillage rate is invariably over optimistic due to the limitations for “across the board” land conversions, annual cropping cycle changes and consequently, changes in land management by operators; spatially variable carbon sequestration rates; limited saturation potential in coarse-textured, e.g. sandy, soil (Hassink, 1997; Chen et al. 2019); misunderstanding of carbon flows in terrestrial ecosystems and, the potential for elevated CO2 emissions from increasing plant and soil microbial respiration as temperature rises. Carbon sequestration in soil and biomass are natural processes, and obviously the most practical, cost-effective solutions. However, the annual potential under conservation agriculture is small relative to global CO2 emissions.
Where does this position CSA? I would argue that the impact of CSA practices on climate change is immeasurable. That doesn’t mean conservation agriculture, and soil- and water-conserving tillage practices, should not be encouraged. They should, for reasons of operational efficiency, cost savings, and productivity unrelated to climate change mitigation.
In the meantime, I’ll stick with the time tested, CSA-free blueprint for agronomic progress and global food security: developing stable, stress tolerant germplasm capable of exploiting local environments, improved soil management, post-harvest technology, and infrastructure for delivering seed, pest control, irrigation, fertilizer, and timely information to farmers regardless of what climate change brings.
If we sequester some carbon along the way, all the better.
1Greenhouse gases involved in carbon sequestration are expressed herein two different ways: (1) “carbon dioxide equivalent per unit time”, written as “CO2e yr-1” (read: “carbon dioxide equivalent per year”), the negative exponent is mathematical notation for the reciprocal, i.e. 1/yr; and (2) mass of elemental carbon (C) per unit area and/or unit time, written as “Gt C ha-1 yr-1 [read: “gigatons (one billion metric tons) per hectare per year”] or “t C ha-1 yr-1” [read: metric tons (1,000 kg) per hectare per year]. Hectare is a metric unit of area = 10,000 m2, or 2.47 acres. The conversion from elemental carbon (C) units to carbon dioxide equivalent (CO2e) is given by:
C x 3.66 = CO2e.
This is based on the ratio of the molecular weight of carbon dioxide to that of one atom of elemental carbon: 44/12. Thus, each kilogram (symbol: kg) of elemental carbon equals 44/12 = 3.66 kg CO2 equivalent. Conversely, each kg of CO2 equivalent contains 12/44 or 0.27 kg C. In all cases I use “pure” CO2e equivalent, where no other GHGs are bundled in the expression. The reader should, however, know that CO2e figures encountered elsewhere may force mixtures of methane, nitrous oxide, hydrofluorocarbons, among others, into calculations expressed interchangeably as a common unit CO2e or CO2eq.
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Author’s note 11 Feb 2019: References to Hassink (1997) and Chenu et al. (2019) were added; some minor editorial changes in the text body are reflected.
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