Carbon storage and soil property changes following afforestation in mountain ecosystems of the Western Rhodopes, Bulgaria

Abstract

Abstract: Land-use changes and afforestation activities are widely recognized as possible measures for mitigating climate change through carbon sequestration. The present study was conducted to evaluate the effect of afforestation on (i) soil physical and chemical properties and soil carbon stocks in four mountain ecosystems and (ii) whole ecosystem carbon storage. The four experimental sites, situated in the Western Rhodope Mountains (Bulgaria) were characterized by typical forest-related land-use conversions. The four sites were a Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) plantation (Rd1) established on former cropland, a mixed black pine (Pinus nigra Arn.) with Scots pine (Pinus sylvestris L.) plantation (Rd2) established on former cropland, a cropland (RdA1) and an abandoned land with uncontrolled extensive grazing (RdA2) historically used as cropland. Soil parameters, i.e., sand content, pH, organic C and N contents, C/N ratio and soil organic carbon (SOC) stocks, were significantly affected by land use and land-use history. Conversion from cropland into forestland significantly reduced soil bulk density and coarse fragments at 0-10 cm depth. Compared with adjacent cropland and abandoned land, soils in coniferous plantations were acidified in their upper layers. Sites Rd2 and RdA2 contained the least SOC owing to the previous long-term arable cultivation (>100 years). Analysis of the ecosystem C stock distribution revealed that most of C in forests was stored in the aboveground tree biomass. Our study confirmed that afforestation of cropland turned the soil into a C sink for the selected mountain region, but showed conflicting results when afforestation occurred on abandoned cropland.