“Biochar is neither black pixie dust nor a magic silver bullet – but in my opinion an incredibly multi-faceted instrument in the fight against climate change,” writes Claudia Kammann, Professor of Climate Impact Research on Special Crops at Geisenheim University, in her lecture abstract on the reduction of nitrate leaching and nitrous oxide emissions. The results presented on yield increases in agricultural crops and the reduction of undesirable environmental effects (such as nitrate leaching into groundwater or emissions of the climate-damaging greenhouse gas nitrous oxide) are based on a number of different studies in greenhouses, climate chambers, in the laboratory and outdoors.
Composting changes the properties of biochar
The composting of biochar (in this case pyrolyzed/carburized wood chips) increased plant growth to 305 % of the control planting in case of nutrient deficiency (sandy soil). If the nutrient supply was already good, the promoting effect was lower, but still significant (138% of the control). If only biochar (pure) was added, growth-reducing or neutral effects could be observed.
According to Kammann, the co-composted biochar also showed surprisingly high nitrogen loading and release. Significantly more than, for example, woodchip biochar which has aged for 2 years in a field test and which has never come into contact with compost. Another study showed that biochar “preloaded” with compost absorbs more nitrogen (nitrate) in the sandy soil than an initially nitrogen-free biochar (pure addition to the soil). After 3 days, up to 60% of the added nitrogen was bound by the composted biochar, although it only accounted for 2% by mass of the soil mixture. According to Kammann, this indicates an active uptake and binding mechanism. The nitrogen-filled plant coal particles acted as exchangers: They took up nitrogen, but also released it again. The composted biochar also acted as an emergency reservoir for bad times: Differences in plant growth only became significant when soil nitrogen was exhausted. Despite the high nitrogen load, the total nitrous oxide emissions of both plant carbon variants (pure and co-composted) were reduced by more than 60%.
Nitrogen leaching is most significantly reduced by a mixture of compost and biochar
In addition, a large container study with Riesling showed that the combination of biochar and compost reduces significantly more effective nitrogen leaching than pure biochar (as much as 60%). With a pure compost addition (without biochar), a stronger nitrogen release even occurred shortly after mixing than in the control. This effect did not occur once biochar was part of the mixture.
Less nitrous oxide emissions thanks to nitrate binding
According to Kammann, the binding of nitrogen or nitrate observed could be one of the reasons why nitrous oxide emissions in soils enriched with biochar are significantly lower. This has already been demonstrated in numerous other studies. The microorganisms that metabolise the nitrogen to nitrous oxide are about 1 micrometre (diameter) in size. However, since biochar has multiple pores with considerably smaller diameters (nanometre range), the nitrate bound in the pores is longer be available for the microorganisms.