Effect of land abandonment on soil organic carbon fractions along a Mediterranean precipitation gradient

• We study SOC fractions after land abandonment along a precipitation gradient.
• We compare SOC under different vegetation covers.
• Transition grasslands after abandonment promote a quick stable SOC gain.
• Colonisation of woody vegetation triggers POM accumulation.
• In the wet site there is a stable SOC loss after grassland–forest transition.

Land abandonment has been the main land use change in rural Mediterranean areas over the last decades. The secondary succession following land abandonment is strongly affected by precipitation, which in turn determines the change of soil organic carbon (SOC) and other soil properties. However, SOC consists of different fractions with contrasting resistance to decomposition that are differently affected by land abandonment. The aim of this study is assessing the evolution of different carbon fractions after land abandonment along a West–East precipitation gradient in southern Spain (Gaucín (GAU): 1080.5 mm y− 1–Almogía: 650 mm y− 1–Gérgal (GER): 350 mm y− 1). In each site, samples from a chronosequence of land abandonment were size fractionated, yielding three fractions with increasing degrees of carbon stability (coarse, intermediate, fine). Following land abandonment, there was a transitional grassland state in GAU and ALM that promoted a quick SOC gain of 0.87 and 0.42 kg m− 2, respectively. The majority of this gain, i.e. 79.2% in GAU and 71.4% in ALM, occurred in the most stable fractions. SOC was rapidly stabilised due to the presence of litter with a low C:N ratio. The colonisation of grasslands, in GAU and ALM, or croplands, in GER, by woody vegetation triggered SOC accumulation of 0.07, 0.78 and 0.74 kg m− 2. However, in GAU there was at the same time a SOC loss in the most stable fractions (0.23 kg m− 2). In ALM and GER a reverse trend was observed: SOC accumulated in the intermediate and fine fractions. SOC accumulation in the coarse fraction was related to the low quality of the litter from woody vegetation, that had a higher C:N ratio than litter from grasslands. The largest effect of precipitation was found in the fine fraction, whereas SOC in the coarse fraction was mostly controlled by land abandonment and related factors, such as vegetation, litter quality and soil conditions.