by Russell Salazar
The Emissions Gap Report 2013 published by the United Nations Environment Programme (UNEP) may be providing a misleading emphasis on the conversion to no-till agriculture. Powlson et al. (2014) argues that, while there are data to support a correlation between no-till practices and increased carbon sequestration at certain soil depths, many other findings have been overlooked or understated, potentially skewing the focus of climate change mitigation initiatives. The UNEP and the agricultural sector may need to revise their action plans for the coming years.
Traditional cultivation methods, such as ploughing and discing, involve the upheaval of large amounts of soil, causing a release of carbon-containing compounds into the atmosphere. No-till agriculture and reduced tillage are arguably the greener alternatives, keeping the carbon compounds in the soil where they may contribute to the growth of crop. The practice has resulted in ‘healthier’ soil: greater biological activity, rainfall infiltration and hence productivity. But how significant is this alternative as a tool for climate change mitigation? The authors examine the metrics and pose several significant concerns.
The main evidence in support of further conversion to no-till agriculture is somewhat undermined by comparisons of organic carbon content at different depths in the soil. While no-till supposedly results in higher organic carbon levels closer to the surface, the total organic carbon taking into account deeper soil remains close to unchanged. Additionally, the published measurements focus on soil characteristics at particular depths, neglecting the differences in soil density between tilled and non-tilled land. Overall, the contributions of these practices toward climate change mitigation are miniscule. This begs the question: is the conversion to no-till agriculture worth promoting?
More worrying is the risk of increase in emissions of nitrous oxide, a more ‘powerful’ greenhouse gas than carbon dioxide. The correlation between no-till agriculture and the level of nitrous oxide is uncertain; nitrous oxide emissions may increase or decrease depending on the conditions. Careful evaluation of the effect of no-till practices on particular areas of land must be made.
The authors’ insightful observations bring risks and practicality into question. The potential for misconception and oversight with the UNEP-published data is troublesome, to say the least.
Powlson, D., Stirling, C., Jat, M., Gerard, B., Palm, C., Sanchez, P., & Cassman, K. (2014). Limited potential of no-till agriculture for climate change mitigation. Nature Climate Change 4, 678–683.