Carbon and Nitrogen Mineralization of Common Organic Amendments in Olive Grove Soils

Cover crops (CC), pruning residues (PR), and olive mill pomace (OP) are common amendments to enhance soil quality in olive groves; however, there is limited information on the dynamics of carbon (C) and nitrogen (N) during their decomposition under varying conditions. In this laboratory decomposition experiment, we assessed the C and N mineralization of CC, PR, and OP at application rates of 2 and 5 mg C g −1 in soils with varying organic C levels, both with and without the addition of NO 3 − . The results indicated that C and N mineralization patterns varied significantly between amendments, although the predicted remaining C after one year was similar for CC and PR (46.7–48.9%) and slightly lower for OP (40.0%). Soil organic carbon level did not affect remaining amendment-C. The addition of N accelerated the decomposition rate of labile C by 15% but slowed down that of the recalcitrant C another 13%, with no significant effect on remaining C. Conversely, increasing the C dose led to a 13% overall reduction in amendment-C mineralization across all combinations of factors. CC decomposition released between −8 and 31% of the amendment-N by day 130, while PR and OP showed net N immobilization. Therefore, CC residues contribute to both C sequestration and N availability, while PR and OP residues can improve N retention in the olive groves while also contributing to C sequestration.