Changes in soil organic carbon pools along a chronosequence of land abandonment in southern Spain

Abstract

Land abandonment is the dominant land use change in the Mediterranean and the resulting vegetation recovery has a strong influence on soil organic carbon (SOC). Yet, the gradual changes in SOC pools during secondary succession remain poorly studied. As SOC is a mixture of pools with distinct functional properties, isolating these pools may provide a better understanding of the decadal SOC dynamics. Topsoil samples were collected along a chronosequence of cropland abandonment in the region north of Málaga (Spain). Five fractions were isolated: particulate organic matter (POM), dissolved organic carbon (DOC), SOC bound to silt and clay (s+c), SOC attached to sand particles or occluded in aggregates (S+A) and a chemically resistant fraction (rSOC). The significant increase in the S+A mass over time indicated aggregation processes along the vegetation recovery stages. Carbon concentrations within S+A or rSOC fractions did not change over time. In contrast, carbon in the s+c fraction significantly increased after three decades from 7.9gC·kg−1 to 20.4gC·kg−1 at the final stage of the chronosequence. Hence, in addition to the frequently reported increases in POM, carbon was also allocated to the more stable SOC fractions. Taking active carbon (DOC+POM; high turnover rate) and intermediate carbon (s+c, S+A; low turnover rate) as indicators for carbon turnover, the proportion of active carbon increased from 17% to 39% along the chronosequence. Consequently, the proportion of slow cycling carbon decreased from 72% to 54%. These results show that the proportion of labile carbon increases along the secondary succession while, at the same time, the slow cycling carbon fractions sequester carbon (from 0.6 to 1.12kgC·m−2).