Rondon, Marco (Centro Internacional de Agricultura Tropical, A.A. 6713, CIAT, Cali, COLOMBIA; Phone: 57-2-4450000; Fax: 57-2-4450073; Email: m.rondon@cgiar.org)
M. A. Rondon *, J. A. Ramirez, J. Lehmann
Charcoal is a ubiquitous material that has been used in agriculture by several cultures throughout history. Increasing evidence indicates that in very low fertility soils, additions of charcoal could increase plant yield and improve several soil quality indicators. Charcoal is a very stable material in soils, with residence times in the order of thousands of years contrasting with mean residence times of decades to centuries for most other soil organic matter pools. Charcoal additions could be used as a mechanism for long-term storage of C in soils and can play a key role for mitigation of climate change and to improve naturally unfertile or degraded soils. There is no information so far, however, related to the effects of charcoal additions to soils on net fluxes of greenhouse gases and on the overall global warming potential of soils amended with charcoal. Here we present results from a glasshouse pot experiment, where very acid, low-fertility oxisols (typic haplustox) from Colombian savannas were amended with 0, 7.5, 15, and 30 g kg-1 of charcoal. Soybeans and a tropical grass (B. humidicola) were planted and allowed to grow for 50 days. Plant yield and total biomass were measured at harvest time and the soil tested for various nutrients. Monitoring of fluxes of nitrous oxide and methane was done using the closed vented chamber approach. Gas samples were collected at two-weeks intervals after planting. Analysis of methane and nitrous oxide was conducted by gas chromatography using FID and ECD detectors. Total net fluxes of methane and nitrous oxide from pots cropped to soybean and B. humidicola were significantly reduced by the addition of charcoal. Methane emissions were virtually suppressed in the grass pots already at charcoal additions of 20g kg-1 soil. Nitrous oxide emissions were reduced by up to 50% on soybean and by 80% in grass pots. At the same time, biomass production of soybean was positively affected by increasing charcoal additions, with a 60% increase after charcoal additions of 20 g charcoal kg-1 soil. In contrast, biomass of B. humidicola did not change with charcoal additions. Charcoal effectively increased soil pH, CEC and availability of various soil nutrients. This study showed that additions of moderate doses of charcoal to very acid and nutrient-limited soils not only enhance plant yields especially of legume species, but also result in drastic reductions in net emissions of methane and nitrous oxide from soils. Given that most of the added charcoal-C remains unmineralized in the soil due to its chemical recalcitrance, a clear positive effect of charcoal additions to soils was observed on net reductions of total emissions of greenhouse gases to the atmosphere. To our knowledge, this is the first study in exploring charcoal effect on GHG emissions. Further research is needed to establish the response of different charcoals in contrasting soil types and with other plants species.