Soil organic matter and nutrient improvement through cover crops in a Mediterranean olive orchard

•K is released from residue in a higher proportion than C, N and P.•Soil organic matter, N and K are improved by studying cover crops at 0−40 cm depth.•Sinapis can enhance nutrients at depth of 0−40 cm more than Brachypodium.•Soil organic matter increased by cover crops between 10.9-14.3 Mg ha−1 in 4 seasons.•All used cover crops fulfilled thoroughly the 4perMille initiative goal. Cover crops (CCs) are herbaceous plants established in the inter-rows of woody crops. CCs have multiple purposes, including improving soil organic matter and soil nutrients. In the present research, a permanent grass, Brachypodium distachyon (BRA); two annual crucifers, Eruca vesicaria (ERU) and Sinapis alba (SIN); and the natural flora of the area (SPV) were studied during four seasons in a Mediterranean olive orchard. C, N, P and K released from residue were measured. Likewise, soil organic matter (SOM), soil N, P and K were assessed up to a depth of 40 cm. C was released in higher amounts from the residue. The final percentages of remaining K in the plant residues from the mowing were the lowest, indicating the fastest release of this nutrient. The different precipitation recorded between seasons influenced both the dynamics of the CCs in the developing and decomposition stages as well as of the SOM and soil nutrients at the surface. Overall, SOM and soil K were higher at the surface (0−5 cm) after each decomposition period. BRA increased the SOM at the surface to the greatest extent. No significant differences between CCs were observed in SOM in the whole profile. In the global balance, SIN increased soil N over 1.5 Mg ha−1 in the first 40 cm. SIN also improved soil K by 2.7 Mg ha−1 and ERU by 2.5 Mg ha−1, with significantly higher amounts in both crucifers than in BRA. Soil P was only enhanced in the first 10 cm by CCs, as it decreased over the whole profile (0−40 cm). From the soil fertility perspective, BRA improved the SOM and soil N at surface (0−5 cm) to a greater extent than SVP, which is currently the most used CC by farmers.