The study revealed for the first time that interactions between biochar and soil that occur over time are important when assessing the influence of biochar on nitrogen losses from soil. The scientists subjected soils samples to three wetting-drying cycles, to simulate a range of soil moistures during the five-month study period, and measured nitrous oxide emissions and nitrogen runoff.
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%, and reduced ammonium leaching by up to 94%. 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."
In addition to the three wet-dry cycles, the soil samples also received glucose and nutrient applications to supply of carbon and inorganic nutrients for optimal microbial activity. The research team tested biochar from two different sources, wood waste and poultry litter. Biochar is made when organic material is burned at high temperatures in the absence of oxygen.
Research is on-going at Industry and Investment NSW to investigate the causes of the reductions in nitrous oxide emissions by biochars, especially under field conditions, and to determine optimal rate and timing of biochar and fertiliser applications to agricultural soils to maximize the greenhouse mitigation value of biochar.
This study was funded by the New South Wales Department of Environment, Climate Change and Water, and the biochars were supplied by Pacific Pyrolysis (previously known as Best Energies, Australia).