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.

Orig­i­nalar­tikel: Ver­min­derung von Nitratauswaschun­gen und Lach­gase­mis­sio­nen durch Biochar (Reduc­tion of nitrate leach­ing and nitrous oxide emis­sions through biochar)
Autor: Claudia Kammann
Pub­lished in: Lecture abstract for the con­fer­ence “Quality assur­ance and envi­ron­men­tal effects of biochar”, BUND Region Han­nover, 2014