Abrupt climate change (ACC) is an unprecedented global threat. Cost effective strategies to mitigate ACC include offsetting anthropogenic emissions through terrestrial carbon (C) sequestration. Cultivated tropical soils may contain 40 % less soil organic carbon (SOC) than their potential capacity. Consequently, the conversion into appropriate land use and management practices could increase the SOC pool and mitigate ACC. The identification of appropriate soils and land use options for establishment of terrestrial C offsets necessitate establishing what proportion of the total C pool is securely stable within the soil aggregates and adsorbed to the silt and clay particles of the soil.
A field study was designed to understand mechanisms of C sequestration in soils of the Neotropics and their capacity to mitigate ACC under different management schemes. This study focuses on characterizing the SOC pool up to 1-m depth in 12 land uses, distributed in 3 contrasting ecoregions of Costa Rica. Specific objectives were to: (i) to determine the effect of land use on SOC pool, (ii) to establish the role of climatic parameters on SOC pool and, (iii) to establish the role of primary and secondary soil particles on the physical protection of SOC. The hypothesis tested was that SOC pool is significantly influenced by climate and land use, and its stability is strongly dependent on the physical protection provided by soil aggregates and primary particles.
Results show that the mean SOC concentration was higher in the order Montane > Pacific Dry > Atlantic Moist ecoregion. The total SOC pool ranged from 114 – 150 Mg C ha-1 in the Atlantic Moist ecoregion, 76 – 165 Mg C ha-1 in the Pacific Dry ecoregion and 166 – 246 Mg C ha-1 in the Montane ecoregion. The estimated C sink capacity was of 18.1 - 36.7 Mg C ha-1, 14.1 – 88.6 Mg C ha-1 and 9.4 – 80.7 Mg C ha-1 in the Atlantic, Pacific and Montane ecoregions, respectively. The SOC was significantly correlated to bulk density, field moisture content, mean annual temperature, mean annual precipitation and altitude. There was a physical protection capacity of 6 to 60% in the silt plus clay fraction of some of the land uses studied. The land uses with heavier texture had a larger SOC storage capacity than those with lighter texture, which had already attained their estimated capacity. However, some soils were significantly deviated from their capacity level, which indicated that given the high SOC contents and greater proportion of silt plus clay, the models suggested by the literature are not reflective of the real attainable SOC storage capacity. Finally, the aggregate properties were significantly related to SOC and differed significantly among land uses and ecoregions. It was concluded that the ultimate SOC sink capacity is determined by the protective capacity in the silt and clay fraction, rather than the SOC sink capacity on total soil basis.