Agriculture activities are a significant contributor to human emissions of greenhouse gases. When new fields are opened for farming, cultivation often results in the loss of 40 to 60% of the organic matter from soils, lowering their fertility and releasing carbon dioxide to the atmosphere. Among other greenhouse gases, the largest human emissions of methane stem from grazing cattle and the cultivation of rice. Similarly the largest human emissions of nitrous oxide (N2O) are associated with the microbial processing of nitrogen fertilizers and manure applied to agricultural soils.
We ask agriculture to feed a global population of more than 7 billion humans, with more than 70 million new mouths to feed each year. New lands are being brought into cultivation in Africa and South America to help feed these peoples. Agriculturalists have taken note of their emissions and made strides to reduce the impact on our planet’s climate, which ultimately affects the success of agriculture. With the ongoing discussion of policies to subsidize farmers to sequester carbon in their soils, it is worth giving a careful look to what really works.
One of the largest changes in agriculture has been the advent of no-till and conservation tillage practices. Here crops are grown without the usual cultivation, which is associated with the loss of organic matter from soils. Genetically modified crop varieties that are resistant to certain herbicides are coupled with the use of those herbicides to control the growth of weeds. Cultivation at traditional frequencies is unnecessary, and we avoid a lot of ancillary fossil fuel emissions from tractors. No-till practices can restore organic matter in the surface layers of the soil, but this carbon storage is often balanced by losses of organic matter from the lower soil profile. Overall the lower use of fossil fuels is probably more influential than the increased storage of organic matter in soils under no-till production.
The increasing popularity of organic agriculture has also played a role, since it can restore soil organic matter when fields are converted to organic practices. Problem is: organic agriculture has lower crop yields, as much as 34 percent lower, so a greater area of land must be brought into cultivation, often losing its native soil organic matter. My earlier blog post concluded that organic agriculture is largely neutral with respect to reducing greenhouse gas emissions to the atmosphere and does not offer significant mitigation of CO2 emissions from fossil fuels.
Some other practices that are often discussed are even less successful. Farmers have long recognized that the amount of organic matter in the soil can be increased with a greater production of crop residues that are left in the field or plowed into the soil. Fertilization is well known to increase crop growth, yield, and residues, so we might assume that it would increase the storage of organic matter in soils. Problem is: a lot of fossil fuels are used to produce fertilizer and to spread it on the landscape. When these carbon “costs” of fertilizer are considered, using more fertilizer is often a losing proposition– more carbon dioxide is emitted to the atmosphere than is stored in fertilized soils.
Similar net balancing casts doubt upon irrigating semi-arid lands to increase crop growth and store organic matter in soils. And similar conclusions are reached with the observation that manure increases soil organic matter. Where manure is applied, organic matter is certain to increase, but we must balance that effect by the organic matter that does not enter the soil elsewhere because it is fed to cattle, which respire a fraction of the carbon as carbon dioxide to the atmosphere. There is no “free lunch” when it comes to animal feed. Furthermore, both fertilizer and manure applications are sources of nitrous oxide (N2O) to the atmosphere, often negating the benefits of these practices on soil organic matter. Nitrous oxide has about three hundred times the global warming potential as CO2.
Recently, agriculturalists have suggested that the production and application of biochar (charcoal) to soils might be used to increase their storage of organic matter, since charcoal has long been known to decompose more slowly than fresh residues. Widespread production and application of biochar might potentially mitigate two percent of current anthropogenic carbon dioxide emissions to the atmosphere—a helpful, but small, contribution to a massive global problem.
I have just read a paper that recommends the application of ground basaltic rock to help mitigate the loss of carbon dioxide from soils, because carbon dioxide would react with the silicate minerals. The consumption of CO2 by weathering of basalt must be balanced against the CO2 released using fossil fuels to mine, transport, and apply the ground silicates to the soil, which is estimated to discount the net removal of CO2 from the atmosphere by up to 30 percent. Globally, the natural weathering of silicate minerals is estimated to consume the equivalent of only 1.3 percent of the annual human emissions of CO2 to the atmosphere. The current, massive human perturbation of soil and rocks at the Earth’s surface has roughly doubled the background rate of soil erosion and sediment transport to the oceans. Thus, it would seem most unlikely that purposeful human intervention could increase the global rock weathering process enough to provide more than a sink for about 1% of fossil fuel emissions. I am skeptical of the idea of hauling enough ground rock to agricultural fields to make a difference.
We can applaud farmers for feeding so much of the world’s population so well, and we can rejoice in their efforts to minimize the impacts of the agricultural endeavor. At some point we need to realize that if seven billion people are to eat well, there will be emissions of greenhouse gases to the atmosphere, even with the best farming practices.
As we enter an era when it appears that not much money will be available to deal with the global warming problem, let’s focus on practices that provide proven and significant reductions of these emissions and not subsidize ineffective efforts with trivial effects.
References
Gattinger, A. and 10 others. 2012. Enhanced top soil carbon stocks under organic farming. Proceedings of the National Academy of Sciences 109: 18226-18231.
Herath, H.M.S.K., M. Camps-Arbestain, M.J. Edley, M.U.F. Kirschbaum, T. Wang, and R. Van Hale. 2015. Experimental evidence for sequestering C with biochar by avoidance of CO2 emissions from original feedstock and protection of native soil organic matter. Global Change Biology-Bioenergy 7: 512-526.
Kantola, I.B., M.D. Masters, D.J. Beerling, S.P. Long and E.H. DeLucia. 2017. Potential of global croplands and bioenergy crops for climate change mitigation through deployment of enhanced weathering. Biology Letters doi: 10.1098/rsbl.2016.0714
Kopittke, P.M., R.C. Dalal, D. G.E. Hilley. 2014. New estimates of silicate weathering rates and their uncertainties in global rivers. Geochimica et Cosmochimica Acta 134: 257-274.
Owen, J.J., W.J. Parton and W.L. Silver. 2015. Long-term impacts of manure amendments on carbon and greenhouse gas dynamics of rangelands. Global Change Biology doi: 10.1111/gcb/13044
Powlson, D.S., C.M. Stirling, M.L. Jat, B.G. Gerard, C.A. Palm, P.A. Sanchez, and K.G. Cassman. 2014. Limited potential of no-till agriculture for climate change mitigations. Nature Climate Change 4: 678-683.
Schlesinger, W.H. 2000. Carbon sequestration in soils: Some cautions amidst optimism. Agriculture, Ecosystems, and Environment 12: 121-127.
West, T.O., and G. Marland. 2003. Net carbon flux from agriculture: Carbon emissions, carbon sequestration, crop yield and land-use change. Biogeochemistry 63: 73-83.
West, T.O. and 12 others. 2010. Cropland carbon fluxes in the United States: Increasing geospatial resolution of inventory-based carbon accounting. Ecological Applications 20: 1074-1086.
Zhou, M., B. Zhu, X. Zhu, H. Vereecken and B. Bruggemann. 2017. Stimulation of N2O emission by mature application to agricultural soils may largely offset carbon benefits: A global meta-analysis. Global Change Biology doi: 10.1111gcb.13648
