Compost vs biochar amendment: a two-year field study evaluating soil C build-up and N dynamics in an organically managed olive crop

Background and aims Despite the high interest in biochar as soil amendment, the number of field studies is still limited, being experiments with perennial crops in arid lands particularly lacking. This study evaluated the application of compost and biochar in a drip-irrigated organic olive crop in a calcareous soil in South-East Spain. Methods During two consecutive years, changes in soil total organic C (TOC), dissolved organic C (DOC) and water soluble N (WSN), mineral N (NH₄⁺ and NO₃⁻), N₂O emissions, denitrifying enzyme activity (DEA) and number of amoA gen copies were monitored. Results Biochar increased TOC compared to the rest of treatments, whereas DOC and WSN significantly increased in plots amended with compost and the compost-biochar mixture. DEA, amoA-encoding genes and N₂O emissions were highest with the compost-biochar mixture, but results were not always significant. Conclusions Our results show that, in these N-limited and deficit irrigated semi-arid agro-ecosystems, compost amendment has an impact on soil microbiological activity with a link to N availability. Biochar applied alone does not alter the N dynamics, but markedly builds-up soil C. In both cases only during the first year these effects were statistically significant. When applied in combination a synergistic effect was observed and the highest values of DEA, amoA gene copies and N₂O emissions were detected.