Soil Organic Carbon Stocks and Saturation Deficit in Mediterranean Olive Groves: Effects of Intensification and Management Practices

Abstract

Soil organic carbon (SOC) plays a critical role in climate regulation, particularly in agroecosystems where soil management practices significantly influence carbon sequestration. In the Mediterranean Basin, olive groves represent a major agricultural system whose carbon storage potential remains insufficiently characterized at large spatial scales. This study assessed SOC stocks and their saturation status in 52 olive orchards sampled across five Mediterranean countries (Spain, Greece, Italy, Portugal, and Morocco), with 39 orchards retained for stock calculations after quality control of bulk density and coarse fragment data. SOC was quantified by both dry combustion and the Nelson and Sommers wet oxidation method. Agreement between methods was formally tested, and a carbonate‐dependent positive bias of the dry combustion method ( r  = +0.20 between ISO−NS difference and carbonate content, p  < 0.0001) led us to retain Nelson & Sommers values for all stock calculations. Bulk density measured with Kopecky cylinders was retained over a Saxton & Rawls pedotransfer estimate after a paired comparison showed poor concordance (Lin's CCC = 0.08; 95% LoA = ±0.5 g cm −3 ). The maximum SOC saturation limit (SOC_max) was estimated from the silt + clay protective fraction, with consistent (1−FG) coarse‐fragment correction applied to both current and maximum stocks so that the saturation deficit (SOC_potential = SOC_max−SOC_current) is a directly comparable, physically meaningful gap. Multivariate analyses (PCA, LDA with Wilks' Λ, two‐way PERMANOVA) revealed that olive orchards are organized along a single dominant intensification gradient (PC1, 30.5% of variance; CD1, 91.7% of canonical variance, p  < 0.001) rather than into separable agronomic management classes; the gradient is robust to the exclusion of operationally tautological variables. Mediterranean olive groves operated on average at 41% of their textural carbon‐storage capacity, with a mean saturation deficit of 64 Mg C ha −1 (59% of SOC_max). The deficit varied along the intensification axis in the direction predicted by carbon‐saturation theory (saturation level: 49% organic, 39% conventional, 33% high‐density), although per‐group differences did not reach significance with n  = 9–12 orchards per management class. Super‐intensive (hedgerow) systems with > 1000 trees ha −1 were not represented in the sample, and this is acknowledged as a limitation. These findings highlight that intensification, more than the organic‐versus‐conventional dichotomy, structures the multivariate variability of Mediterranean olive groves, and that current SOC stocks lie well below textural saturation in most systems, indicating substantial potential for additional carbon sequestration through soil‐conservation practices.