In a newly published paper in the European Journal of Agronomy, authors María Sánchez-García, Miguel Ángel Sánchez-Monedero, Diego A. Moreno, Danilo E. Bustamante, Martha S. Calderon, and María Luz Cayuela investigated how a decade of repeated organic additions alters agricultural systems. The researchers focused on a mature organic olive orchard in Jumilla, a region located in Spain, to evaluate long-term environmental and physiological responses. Over an eleven-year period, the team tracked changes in soil organic matter, evaluated the soil microbiome through high-throughput genetic sequencing, and measured both the nutrient levels and specialized phytochemicals found inside the olive leaves and fruits. The study utilized four distinct experimental field treatments applied every two years, which included a non-treated control plot, a compost-only application, a biochar-only application, and a combined compost-biochar mixture mixed at a ninety to ten dry weight ratio.

The scientific investigation revealed that adding these organic materials significantly altered the chemical composition of the calcareous topsoil over ten years. All three organic amendment options succeeded in increasing the total soil organic carbon concentration in the upper twenty centimeters of the soil profile. Specifically, compared to the unamended control plots, the soil organic carbon rose by zero point seventy-eight percent in compost plots, one point zero four percent in the mixture plots, and one point twenty-four percent in the biochar plots. Total soil nitrogen also shifted based on the type of amendment used, registering notably higher values in the compost and mixture treatments due to direct nutrient addition, whereas the biochar-only treatment remained comparable to the control because of its stable, unreactive structure. Additionally, the soluble fractions of carbon and nitrogen were consistently higher in plots treated with compost and the organic mixture across all seasonal samplings.

Despite these clear, long-term changes in available soil nutrients, the soil microbiome displayed a remarkable level of structural stability and resistance to permanent change. Genetic testing of bacteria, archaea, and fungi ten years after the experiment started showed no lasting alterations in alpha diversity indices compared to the unamended control soil. When the researchers conducted high-resolution seasonal sampling immediately following a fifth application of the amendments, they did capture temporary shifts in the microbial groups, particularly among the soil fungi. The nutrient-rich compost inputs temporarily decreased fungal diversity and evenness by favoring a few competitive decomposers that outcompeted rarer microbes. However, these community shifts quickly dissipated, and the soil microbiome returned to its baseline state, demonstrating that established Mediterranean agricultural soils harbor a core microbiome highly resilient to human interventions.

In stark contrast to the stable soil biology, the secondary metabolites within the olive fruits proved to be highly sensitive to the different soil management strategies. The long-term differences in soil nutrient accumulation directly influenced the accumulation of oleuropein, which is the most prominent individual phenolic component responsible for stress responses and flavor in olive berries. Olives harvested from plots treated with the compost and the compost-biochar mixture exhibited significantly lower oleuropein concentrations, dropping by up to twenty-four percent compared to the unamended control units. This decline happens because increased nutrient availability from compost relaxes plant stress, causing the trees to allocate less energy toward defensive secondary metabolites and more toward primary growth and lipid synthesis. Consequently, the organic amendments enhanced the overall lipid nutritional value of the fruits by increasing desirable monounsaturated fatty acids.

Importantly, the study demonstrated a clear decoupling between secondary fruit chemistry and overall agricultural productivity. While the phytochemical profiles shifted dramatically, the actual crop yields remained stable and statistically unchanged across all four field treatments. The olive production during the final harvest campaign ranged uniformly between seven point two and seven point eight metric tons per hectare across the entire orchard. Leaf nutrient and phenolic concentrations were also scarcely affected by the soil treatments, showing that leaves remain structurally buffered while the developing fruits act as highly plastic metabolic sinks. Ultimately, these findings indicate that while long-term biochar and compost applications do not permanently disrupt soil microbial ecosystems or reduce total crop yields, they provide farmers with a practical management tool to naturally tailor the health-promoting compounds and flavor profiles of olive oils.

Source: Sánchez-García, M., Sánchez-Monedero, M. A., Moreno, D. A., Bustamante, D. E., Calderon, M. S., & Cayuela, M. L. (2026). Soil microbiome and phytochemical responses to a decade of compost and biochar amendments in an olive orchard. European Journal of Agronomy, 179, 128175.