Mar 7, 2010

Does 'zero-tillage' help in carbon sequestration?

Some 25 years back or may be still more, I remember the first time I reported about 'zero tillage' (at that time I worked with the Indian Express). The news report I wrote was based on a study conducted by Punjab Agricultural University, Ludhiana, on 'zero-tillage' in maize. The PAU study ofcourse was all ga-ga over the virtues of no-tillage.

I wasn't convinced then, nor am I now. Although the World Bank supported Conservation Agriculture talks profusely about the merits of 'no-tillage', and the subject has assumed importance in the ongoing debate on climate change, I would rather ignore the suggestion than to add onto the existing agrarian crisis. But then, knowing that Indian scientists only replicate what American universities experiment with, I am sure they will try to push 'zero-tillage' to the unsuspecting farmers in the days to come.

However, I always wonder how can scientists talk of 'zero-tillage' in Indian agriculture when nature had bestowed us with the best tiller we know -- the lowly earthworm. Each earthworm in its lifespan upturns some 6 tonnes of soil, and therefore is called nature's tiller. If this wasn't useful, I am sure earthworms would have disappeared from the tropical soils much before chemical fertilisers decimated its numbers.
 
The Soil Association (in UK) had recently come out with a report, entitled: 'Soil Carbon and Organic Farming', which made the following two points:
 
Ploughing: A concern that the common use of deep cultivation in organic farming could be a weakness are answered by a number of trials in Europe that show that the depth of cultivation has no effect on the overall soil carbon levels of organic farming. Ploughing is used to incorporate organic matter into the soil; with sufficient inputs, increases in ploughing depth can even increase topsoil depth.
 
'Min-till’ and ‘no-till’: Reduced soil cultivation is the main non-organic farming solution commonly put forward for raising soil carbon levels, but its benefits have greatly been exaggerated. According to government scientific advice, the soil carbon benefits are minimal in the UK. Reduced tillage is effective in maintaining soil carbon storage in semi-arid regions where carbon is being lost by erosion and by the use of fallow periods, but otherwise there is no clear scientific evidence that it increases carbon levels over the whole soil profile, and certainly not to the extent of organic farming. Moreover, the carbon is then in a relatively unstable form, and any soil carbon gains may be offset by higher soil N20 emissions.
 
The report says very conclusively that there is no scientific evidence of enhanced carbon levels from 'zero-tillage' or 'no-tillage.' And yet, ICAR scientists (and also from ICRISAT and other international agricultural research centres) are swarming into the villages to draw farmers into practicing 'zero-tillage', and like what they did to establish the gains of Green Revolution technologies, they are hyping the results to show how effective 'no-tillage' is as a Conservation Agriculture technology.
 
What they are ofcourse not telling is how does (and how much) the promotion of 'no-tillage' helps in the sale of herbicides and some related farm equipments.
 
Meanwhile, Doug Gurian-Sherman and his colleagues in the Union of Concerned Scientists, Washington DC, have come out a two-page fact sheet on Agricultural Practices and Carbon Sequestration (published Oct 1, 2009). They have clearly established that a number of biological and soil-based integrated systems have a greater potential to sequester carbon. I am reproducing below some relevant sections from the fact sheet:

One of agriculture’s major opportunities to help mitigate the effects of climate-warming gases lies in management of soil to increase organic content, thereby removing carbon from the atmosphere. Many scientists are conducting studiesto determine which agricultural practices will in fact sequester carbon. Recent studies, summarized below, demonstrate that a number of biological, soil-based practices employed in integrated systems have great potential to sequester carbon. In contrast, recent studies suggest that no-till, a form of conservation tillage, has environmental benefits such asreducing soil erosion, but may not sequester more carbon than conventional tillage (plowing).

Integrated soil-based practices The most promising systems for carbon sequestration in soil combine crop rotation and low or no inputs of pesticides,herbicides, and industrial fertilizers. Long-term studies done by the Rodale Institute and others suggest that such systems build (not simply conserve) significant quantities of soil organic carbon through a variety of mechanisms such as enhanced abundance of mycorrhizal fungi. Several studies, including some done over long periods of time, have compared carbon accumulation in organic (plowed) and conventional (plowed) systems and demonstrate that organic systems sequester more carbon than conventional chemical-intensive systems.

In a head-to-head comparison between conventional no-till and organic plowed systems, organic plowed systems sequestered more carbon even though the sampling was restricted to shallow soil, where no-till tends to show carbon accumulation.

Although more studies are needed, there are good reasons to believe that organic systems would do atleast as well as conventional systems deeper in the soil. Current organic systems typically employ plowing to control weeds, and conventional plowed systems generally sequester more carbon at greater soil depths than no-till. Systems that use crop rotations and green and animal manure have shown higher biodiversity by foregoing chemical pesticides, supplying more diverse habitats, and reducing nitrogen pollution. Systems that integrate livestock and crops, employ perennial pastures, and adopt many of the practices used in organic production (e.g., long crop rotations, leguminous crops and cover crops, manure produced by livestock as fertilizer) also have shown potential for improved greenhouse gas balance, reduced pollution, and higher profitability.

Further research on these promising approaches will help optimize their benefits and determine their applicability across geographic regions. In summary, available data suggest that organic and near-organic farming systems achieve greater carbon sequestration and other benefits compared with conventional systems. Further work, supported by adequate research funding, is needed to realize the promise of these biologically sophisticated production systems.

Summary of the science The current scientific literature does not support favoring no-till over plowing for carbon sequestration. The emerging consensus from numerous studies and reviews is that under a variety of environmental conditions no-till sequesters nomore carbon than plowing. The apparent advantage for no-till in previous studies of carbon sequestration was an artifact of sampling carbon only near the soil surface.

2 comments:

J. D. Etchevers said...

READ ALAN J. FRANZLUEBBERS PAPER ON SSSAJ 74(2). 347-357

Devinder Sharma said...

Hi Etchevers !

How does one access this paper? Can you point me to a link?

Regards
Devinder Sharma