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Nitrogen enrichment potential of biochar in relation to pyrolysis temperature and feedstock quality

Jassal, Rachhpal S., Johnson, Mark S., Molodovskaya, Marina, Black, T. Andrew, Jollymore, Ashlee, Sveinson, Kelly
Journal of environmental management 2015 v.152 pp. 140-144
adsorption, ammonium nitrate, ammonium nitrogen, biochar, carbon, cation exchange capacity, dairy manure, ecological value, electrical conductivity, feedstocks, hydrogen, leaching, nitrate nitrogen, nitrogen, nitrogen fertilizers, pH, potassium chloride, poultry, poultry manure, pyrolysis, softwood, temperature, urea ammonium nitrate
Nitrogen (N) enrichment of biochar from both inorganic and organic waste N sources has the potential to add economic and environmental value through its use as a slow release N fertilizer. We investigated the sorption of N by, and its release from, biochar made at pyrolysis temperatures of 400, 500 and 600 °C from three feedstocks: poultry litter (PL with a carbon (C) to N ratio (C:N) of 14), softwood chips of spruce-pine-fir (SPF with a C:N of 470), and a 50:50 mixture of PL and SPF (PL/SPF). The prepared biochars were enriched with ammonium nitrate (AN) and urea ammonium nitrate (UAN). PL biochars had the lowest C content (50–56% C), but the highest pH (9.3–9.9), electrical conductivity (EC, 780–960 dS m−1), cation exchange capacity (CEC, 40–46 cmol kg−1), and N content (3.3–4.5%). While N content and hydrogen (H) to C atomic ratio (H:C) decreased with increasing pyrolysis temperature irrespective of the feedstock used, both pH and EC slightly increased with pyrolysis temperature for all feedstocks. The PL and SPF biochars showed similar H:C and also similar N sorption and N release at all pyrolysis temperatures. These biochars sorbed up to 5% N by mass, irrespective of the source of N. However, PL/SPF biochar performed poorly in sorbing N from either AN or UAN. Biochar H:C was found to be unrelated to N sorption rates, suggesting that physical adsorption on active surfaces was the main mechanism of N sorption in these biochars. There were minor differences between N sorbed from NO3–N and NH4–N among different biochars. Very small amounts of sorbed N (0.2–0.4 mg N g−1 biochar) was released when extracted with 1 M KCl solution, indicating that the retained N was strongly held in complex bonds, more so for NH4–N because the release of NO3–N was 3–4 times greater than that of NH4–N. NH4–N sorption far exceeded the effective CEC of the biochars, thereby suggesting that most of the sorption may be due to physical entrapment of NH4+ in biochar pores. The results of this study suggest that biochar can be used to remove excess N from poultry and dairy manure and be a good mitigation option for reducing N leaching and gaseous losses.