When you input fresh or labile organic sub-strates into soil influence overall decomposition of native Soil organic carbon and then… the mineralization or immobilization of C, N and other nutrients in the short-term. This effect is known as the priming effect
In this study they interested in positive priming effect and negative priming effect, so after the depletion of the labile carbon fraction of the biochars the remaining highly recalcitrant carbon product a negative priming effect