Initially, biochar application produced inconsistent effects. Several early samples produced greater nitrous oxide emissions and nitrate leaching than the control samples. However, during the third wetting–drying cycle, four months after biochar application, all biochars reduced nitrous oxide emissions by up to 73 percent, and reduced ammonium leaching by up to 94 percent. The researchers suggest that reductions in nitrous oxide emissions and nitrogen leaching over time were due to “ageing” of the biochars in soil.
“The impacts of biochars on nitrous oxide emissions from soil are of interest because even small reductions in nitrous oxide emissions can considerably enhance the greenhouse mitigation value of biochar, which is already proven to be a highly stable carbon pool in the soil environment,” according to senior author Bhupinder Pal Singh. “This research highlights that impacts of biochar on nitrogen transformations in soil may change over time and hence stresses the need for long-term studies to assess biochar’s potential to reduce nitrogen losses from soil.”
http://eponline.com/articles/2010/08/03/aged-biochar-lowers-nitrogen-emissions-from-soil-study-finds.aspx