Effect of abandonment of olive orchards on soil organic carbon sequestration in Mediterranean Lebanon

Abstract

As a result of rural exodus, terraced orchards are the first abandoned lands in Mediterranean Lebanon. The aim of the present study was to develop knowledge of the status of soil organic carbon (SOC) in abandoned olive orchards. Thirty pairs of abandoned–actively managed fields were sampled at two soil depths (0–0.1 and 0.1–0.3 m) and two positions (crown area and between trees). Mean SOC ranged between 20.9 and 29.1 g kg–1 soil, with the smallest values between trees in the 0.1–0.3 m depth. Abandoned plots showed a surface build up of SOC (SOC0–0.1 m > SOC0.1–0.3 m) and a significantly higher SOC0–0.1 m content than the control. As a result of abandonment, 80% of plots showed increased C stocks (+18.6 Mg C ha–1). Recently abandoned plots (6–12 years) had a C0–0.1 m stock of 6.2 Mg C ha–1, compared with 9.5 Mg C ha–1 for those abandoned for over two decades. Two chronosequences showed that when the initial C was high, one decade was sufficient to reach stock stability. The relative C stock increase reached 38% after two decades of abandonment. Particle size separation showed that 30% of C0–0.1 m in the abandoned fields occurred as particulate organic matter (>50 μm) compared with 20% in the actively managed plots.