Can char carbon enhance soil properties and crop yields in low-carbon soils?

Restoring soil carbon (C) lost due to intensive farming is a long-term endeavor under current conservation management practices. Application of coal combustion residue (293 g C kg⁻¹) from a sugar beet (Beta vulgaris L.) processing factory, hereafter referred to as char, could rapidly restore soil C and productivity in degraded croplands, but data on this potential strategy are unavailable. We assessed the impacts of char application to two relatively low-C soils (10.1 and 12.2 g C kg⁻¹) and one relatively high-C soil (17.3 g C kg⁻¹) on soil C, soil physical and fertility properties, and crop yields in no-till systems in the Great Plains after 2 yr. Char was disked to 15 cm soil depth at char-C application rates ranging from 0 to 19.7 Mg C ha⁻¹, corresponding to char application rates ranging from 0 to 67.3 Mg ha⁻¹. The highest char rate increased C concentration in all soils but increased C stock only in low-C soils. Char did not affect soil penetration resistance, available water, aggregate stability, most nutrients, and crop yields. Char application at high rates increased sulfate, Ca, Mg, and Na concentrations but did not influence other properties. Carbon recovery of the char applied at the highest rate varied among soils from 50 to 85%, but the mechanisms for such differences need further investigation. Short-term duration, low char C concentration, and low application rates may explain the limited char effects. Overall, char application at 19.7 Mg char-C ha⁻¹ (i.e., 67.3 Mg char ha⁻¹) increased soil C concentration but had negligible effects on other soil properties and crop yields after 2 yr.