Evaluating coal char as an alternative to biochar for mitigating nutrient and carbon loss from manure-amended soils: Insights from a greenhouse experiment

Animal manure has been increasingly adopted as a more sustainable substitute for synthetic fertilizers but might result in increased dissolved organic C (DOC) and phosphate (PO43-) leaching and elevated greenhouse gas emissions from soil. Biochar may reduce nutrient loss from manure-amended soils, but large-scale application has been hindered, in part, by its high cost. Minimum cost alternatives, such as incomplete coal combustion residue (char), may provide a more viable option to farmers, but char needs to be analyzed in comparison to high-temperature pine biochar before recommendations can be made. We valuated losses of soil C, N, and P, as well as plant yields and changes in microbial biomass, in two contrasting soils amended with dairy slurry or swine lagoon wastewater and with biochar or coal char over 105 d. Dissolved organic C leaching decreased with addition of biochar or char (0.6-27% or 1.6-36%), independent of soil texture and manure type. Leaching of PO43- was reduced by biochar (15-24%) and char (38-50%) in the silt loam. Soil N leaching increased after char application (likely due to our high application rate) but was unaffected by biochar. Char reduced CO2 emissions from the sandy loam by 9.7-54%, whereas both biochar and char increased CO2 emissions in the silt loam by 38-48% during plant root senescence. Depending on soil characteristics, char may outcompete biochar with respect to reduction of PO43- and DOC leaching. Unlike biochar, some char-N is available, and this should be accounted for when considering application rates.