Soil organic carbon storage in forest restoration models and environmental conditions

The scale of forest and landscape restoration is expected to increase during the UN Decade on Ecosystem Restoration and its contribution to the provision of critical ecosystem services to society. Several models of forest restoration may improve ecosystem services, including soil organic carbon (SOC) storage. A review was carried out to access: (1) the variability of SOC storage between worldwide forest restoration models and, (2) the effects of climate, soil class, soil texture, and vegetation type on SOC storage. We reviewed 119 primary studies with information on SOC and soil texture for different forest restoration models. The restoration models were grouped into four categories: natural regeneration, monocultures, agroforestry, and mixed forest. SOC data was extracted from these four restoration models, other land uses (cropland, bare land, grassland, and natural forest), climate regimes, and soil properties. The SOC storage in the forest restoration models and other land uses at a global scale ranged between 0.1 to 514 Mg ha(-1). The overall mean value for SOC storage was higher for natural regeneration (112 Mg ha(-1)), followed by agroforestry (74 Mg ha(-1)), mixed forest (73 Mg ha(-1)), and monocultures (68 Mg ha(-1)). However, the average SOC storage was similar among the four restoration models in the moist warm climate zone. The SOC storage mean value in the moist cool zone was 23% higher than the dry cool zone (81 and 62 Mg ha(-1), respectively), and 50% higher for the moist warm zone when compared to the dry warm climatic zone (74 and 38 Mg ha(-1), respectively). The SOC storage of the restoration models was positively related to soil depth (0.59; p < 0.01), clay content (0.29; p < 0.01), and stand age (0.17; p < 0.01). Globally, the mean values of SOC storage were 26, 66, and 139 Mg ha(-1) at zero - 10, zero - 30, and zero - 100 cm depths, respectively. In addition, sandy soils showed smaller mean values of SOC storage than medium to clay soils, especially in deeper soil layers. Furthermore, SOC storage was positively related between restoration models and other land uses (0.93; p < 0.01), suggesting a prominent effect of climate and soil properties on SOC. Forest restoration models showed larger SOC storage when compared to croplands and bare land, but in general it was smaller or similar when compared to pasture and natural forest.