Combined biochar and manure addition to an agricultural soil benefits fertility, microbial activity, and mitigates manure-induced CO₂ emissions

The importance of blending biochar into manure prior to its application as a soil amendment was investigated by this laboratory and field study, hypothesizing an (i) increase in soil fertility; (ii) improvement in microbial activity; (iii) reduction in manure-induced CO2 emissions after combining amendments. Biochar was mixed with manure (50:50 V/V) and aged in field conditions for 6 months whereafter pristine biochar, manure, and manured biochar (50:50 V/V) were incorporated into an agricultural drought-prone Regosol. All treatments and soil mixtures were incubated, with laboratory measurements of CO2 emissions taken using a coupled column respirometer set up, while nutrient availabilities as well as enzymatic activities were also tested. Demonstrable benefits of the biochar when blended to manure were exemplified in reduced CO2 emissions, by 60% compared with manure alone, and improved C and N acquisition by microorganisms. When applied to soil, blended biochar manure did not enhance CO2 emissions, but did significantly improve soil fertility by the avoidance of leaching of non-plant consumed nutrients from the soil (e.g. NO3- by 58%). A significant increase in functional diversity indices in the presence of manured biochar highlights the added value of the blended approach. Field application yielded reduced effects; manure induced a higher carbon and nitrogen mineralization (thus potentially higher C and N emissions via CO2, CH4 and N2O) as well as a higher microbial functional diversity. Further studies should verify the effects observed here in a range of soil types and climates.