Application of hydrochar and pyrochar to manure is not effective for mitigation of ammonia emissions from cattle slurry and poultry manure

Nitrogen (N) loss as ammonia (NH3) from agricultural systems is one of the major sources of atmospheric pollutants and is responsible for more than 50% of global NH3 emissions. Ammonia volatilization from animal manures may be altered by amendment with chars derived from pyrolysis (pyrochars) or hydrothermal carbonization (hydrochars) by providing exchange sites for ammonium (NH4+) or changing the pH of manure. Pyrochar and hydrochar differ in chemical and structural composition, specific surface area, and pH and therefore may affect NH3 volatilization differently. In a laboratory incubation experiment, we investigated the effect of pyrochar (pH 9.0) and hydrochar (pH 3.8) from Miscanthus on NH3 emission after addition to poultry manure and cattle slurry. We analyzed manure treatments with and without char addition and acidification and determined the effect of char addition on immobilization of manure-derived NH4+. Ammonia emission from pure poultry manure amounted 84% of the applied NH4+-N, while 67% of the applied NH4+-N was lost as NH3 from cattle slurry. Addition of pyrochar or hydrochar had no or only marginal effects on NH3 emissions except for a reduction in NH3 emissions by 19% due to hydrochar application to CS (p < 0.05), which seems to be primarily related to the char pH. Sorption of NH4+ by admixture of chars to manure was generally small: between 0.1- and 0.5-mg NH4+-N g−1 chars were sorbed. This corresponds to between 0.1 and 3.5% of the NH4+ applied, which obviously was not strong enough to reduce emissions of NH3. Overall, our results do not provide evidence that addition of pyrochar or hydrochar to cattle slurry and poultry manure is an effective measure to reduce NH3 volatilization.