Soil and crop management under conservation agriculture in Zimbabwe

Carbon mitigation strategies and mechanisms have been globally implemented in an attempt to prevent catastrophic climate change. Kyoto targets require reductions in greenhouse gas emissions and increases in carbon sequestration. Although a number of carbon neutral technologies have been adopted such as bio-fuel and alternative energies, these currently fail to meet global requirements. An innovative and rediscovered carbon negative strategy is biochar production and soil incorporation. Biochar production is a variant of bio-energy production, which yields energy and a biochar (charcoal) product which when added to soil locks away carbon for millennia. Biochar incorporation into soil has also been shown to double crop yields and decrease greenhouse gas emissions in low fertility soils. Scenario analysis has suggested that biochar sequestration could offset up to two thirds of the global annual net CO2 accumulation (15 Pg C a-1). Ironically ancient Amazonian people knew of the benefits of biochar, cultivating highly fertile Terra Pretas (Black Soils) by incorporation of char into their soils (Lehmann 2007). Biochar is formed by partial combustion or pyrolysis of plant derived biomass or wastestream products, yielding a continuum of black carbon (BC) compounds. It is biochar’s aromatic-macromolecular structure that renders it more recalcitrant to microbial decomposition than uncharred organic matter and makes it a potential long-term carbon sink. Evidence from ancient Terra Preta and carbon dating suggest that biochar has a mean residence time in the range of 1000-10 000 years. To put this in perspective, when un-charred organic matter is added to the soil it is mineralized within months with a small portion of the carbon stabilized for hundreds of years. Another problem is that the application of large amounts of un-charred carbon to the soil can cause immobilisation of valuable inorganic nitrogen, leading to reductions in crop yield due to nitrogen deficiency. Biochar addition to soil does not cause significant nitrogen immobilisation because it is not assimilated by the soil microbial biomass. Biochar addition has been shown to improve soil fertility, substantially increasing biomass production in tropical soils (Lehmann 2007, Steiner et al., 2006) probably due to the associated increase in soil cation exchange capacity (Liang et al., 2006). Preliminary greenhouse experiment results suggest that the presence of biochar may reduce emissions of two potent greenhouse gases, nitrous oxide and methane (Rondon 2005). The benefits of adding Biochar to low cation exchange capacity tropical soils are clear from the success of the fertile Terra Pretas and recent experiments (Glaser et al., 2002, Steiner et al., 2006). However for Biochar application to become an acceptable routine agronomic practice and have a major impact on global carbon budgets, a more detailed scientific understanding of the consequences of Biochar addition to soil is required. Stable isotopes have and will play a key role in unravelling the complex chemical and physical interactions and benefits of Biochar. Stable isotope techniques allow the direct tracing of the pathways and processes that account for the environmental and nutrient benefits of Biochar. For example using 15N labelled fertilizers, it is possible to directly trace the fate of applied nitrogen and calculate nitrogen use efficiency on addition of fertilizer and Biochar. It is also possible to study the effect of Biochar addition on the fundamental soil processes of nitrogen mineralization and nitrification using 15N isotope dilution techniques. In addition it is possible to study the mechanisms associated with the reduction in potent greenhouse gas emissions such as nitrous oxide. Carbon isotopes enable us to unravel the carbon side of the story; recent work using carbon isotopes has shown that the carbon added as Biochar is highly resistant to soil microbial attack and therefore acts as an extremely long term carbon sink in soils. Using the isotopes of hydrogen and oxygen it will also be possible to determine the impact of Biochar on soil-water interactions. The beauty of isotopic techniques is that they enable us to study soil processes in policy relevant time frames over seasons rather than decades, which means policy makers are armed with sufficient knowledge to make informed decisions about national and international carbon policies, which will ultimately determine the future of the planet. Soils Newsletter, Vol. 32, No. 2, January 2010 14 AquaCrop for Agricultural Water Management