Biochar increases crop productivity in many tropical soils. The reasons probably include improved water retention, reduced leaching, and better availability of nutrients to plant roots. In temperate conditions, studies have been fewer in number and haven’t produced results that are as clear. A new study adds usefully to our knowledge.
Alfred Gathorne-Hardy and colleagues at Imperial College, London applied varying amounts of biochar and of nitrogen fertiliser to barley. Their research showed that biochar improved yields substantially but only in those trials when large amounts of artificial fertiliser were also applied. Adding 20 tonnes a hectare (2 kg a square metre) of biochar to a soil fertilised with 100 kg of nitrogen a hectare increased the crop yield by over 30%. Loosely put, biochar improves the effectiveness of the nitrogen. But for soils with no added fertiliser, increasingly heavy applications of biochar tended slightly to reduce the crop yield.
This is an extremely interesting result for two reasons. Firstly, it shows that it may be financially rational for UK and other temperate zone farmers to add biochar to the soil. Secondly, the improved crop yields may be arising from greater retention of nitrogen in the soil. This is important because it would probably imply reduced run-off of nitrogen into water courses. Run-off may be the most important source of nitrous oxide from agriculture. (Nitrous oxide is a potent global warming gas, three hundred times worse than CO2.)
Let’s examine these two hypotheses in turn. First, the financial incentive to add biochar. Many barley farmers have an unsaleable surplus of straw. Charring the straw will cost money but biochar will usually remain in the soil for many years. In the tropics, many biochars seem to have half-lives of several centuries. So if the result identified in the paper is replicated in the field biochar will add to crop yields for many years. Typical barley yields are about 6 tonnes per hectare and recent prices for good quality grain have averaged about £80 per tonne. A 30% increase in yields is therefore worth about £150 a hectare a year. Over ten years the undiscounted value is perhaps £1,500, a number vastly greater than the cost of charring 20 tonnes of straw. I guess that the cost of the biochar might eventually be as low as £10 per tonne.
So it may make financial sense to apply biochar to UK soils, even before considering the impact on emissions. We should therefore move on to the impact on GHGs. At today’s prices in the European Emissions Trading system, the value of 20 tonnes of pure carbon newly sequestered in the soil is more than £700. (Carbon dioxide, currently trading at more than £10 a tonne has 3.67 the weight of carbon.) Also important is the impact on nitrous oxide emissions. If biochar reduces nitrous oxide emissions by holding nitrogen better in the soil, the value will add to the impact of CO2 reduction. IPCC studies suggest that 1% of the nitrogen in fertiliser ends up as nitrous oxide gas, usually through interaction with water in fields and in drainage ditches at field edges. More recent studies suggest that the actual number may be much higher, particularly in wet climates like the UK. But even the IPCC figure suggests that 1 kg of nitrous oxide emissions per year – equivalent to about 300 kg of CO2 – will be avoided, perhaps for many years, by the use of biochar. But let’s assume the nitrous oxide reduction only persists for ten years. The value of this at £10 per tonne of CO2 equivalent is about £30.
Here’s a highly simplified summary of the benefits of biochar, at today’s crop and carbon permit prices (all numbers are per hectare):
|Cost of charring 20 tonnes||£200|
|Benefit of improved yields for 10 years||£1,500|
|Shadow value of 20 tonnes of sequestered biochar as pure carbon||£700|
|Shadow value of avoided nitrous oxide emissions||£30|
The key conclusions are that yield improvement values dwarf the monetary value of carbon emission improvements but also that it should be worthwhile sequestering biochar even if yield gains are negligible or non-existent.
Appendix: some data on straw Barley straw weighs approximately the same as the grain from a field. So each hectare produces about 6 tonnes of straw per year. A tonne of straw might produce char of about a third of the weight. So each hectare should produce char of about 2 tonnes per year of crop production. To apply 20 tonnes per hectare thus requires about 10 hectares of straw. But even if biochar lasts as little as 10 years, the straw per hectare will produce enough char to generate the crop productivity improvements. So there is no need for external inputs of biomass to produce the fertility improvements – the biochar can be generated on the farm itself.
Footnote  Alfred Gathorne-Hardy and others, ‘Biochar as a soil amendment positively interacts with nitrogen fertiliser to improve barley yields in the UK’, IOP Conference Series: Earth and Environmental Sciences, 6 (2009), 372052 http://bit.ly/Gathorne-Hardy2009 (doi:10.1088/1755-1307/6/7/372052).