The group, led by Professor David Powlson at Rothamsted Research, advocate use of no-till under certain conditions but believe too much emphasis is directed towards its potential for reducing global GHGs.
“Over-stating the climate change benefits of no-till is serious as it gives a falsely optimistic message of the potential to mitigate climate change through altered agricultural practices,” Powlson told Milling & Grains.
“If the climate change mitigation achievable through adopting no-till is far less than claimed, there is even more pressure to decrease GHGs from other aspects of agriculture.”
Consider the variables
A report published by the United Nations Environment Programme (UNEP) recommends no-till practices to sequester soil organic carbon (SOC) and offset the effects of carbon dioxide released through conventional ploughing.
But Powlson says there are too many variables involved to justify sweeping statements about the benefits of no-till.
“There are numerous reasons to be cautious including misunderstanding of carbon (C) flows and the likelihood that organic C in soil will be subject to more rapid decomposition at elevated temperatures resulting from climate change.
“In addition, the extra carbon under no-till is predominantly in labile forms that would certainly be decomposed if no-till practices ceased and a farmer reverted to conventional tillage”
No-till is practised extensively in South America and Australia. It involves sowing seeds directly under the mulch layer left by the previous crop and can minimize mechanical soil disturbance, thus improving soil structure, reducing the risk of erosion and improving the extent to which rainfall infiltrates the soil and is stored for later use by crops.
“In many situations and soil types (though not all) no-till is beneficial agronomically and for soil quality. No-till agriculture can deliver significant benefits for farmers and sustainability in many situations,” explained Powlson.
According to the UNEP’s Emissions Gap Report 2013, wheat grown under no-till in Kazakhstan was more resilient and produced yields three times higher than crops cultivated conventionally. It has also reduced fuel costs by over 50%, in some cases, in Argentina, where one liter of fuel under conventional tillage produced 50kg of grain, compared to 123kg under no-till.
“Time and labor is saved by using no-till or reduced till compared to conventional ploughing followed by one or more secondary cultivations to prepare a seedbed. The redistribution of organic matter towards the soil surface can give better soil aggregate structure near the surface. This can be good for seedling emergence and also for water infiltration,” Powlson added.
Nevertheless, he still urges caution about the limited potential of no-till for climate change mitigation.
A report published by Powlson in Nature Climate Change, highlights the fact that evidence supporting no-till to sequester soil organic carbon (SOC) is ambiguous and is often misinterpreted. Increased SOC near the soil surface is often counteracted by less deeper in the soil, though there is often a small net increase.
“Reduced GHG emissions are a small but important additional benefit of no-till, not the key policy driver for its adoption,” he said.
“There are some genuine opportunities for mitigating climate change in the agricultural sector, largely through improved management of water and nutrients — especially nitrogen from fertilizer and manure — and through improved feeding practices and management of ruminant livestock.”