Soil carbon storage in plantation forests and pastures: land-use change implications

Afforestation may lead to an accumulation of carbon (C) in vegetation, but little is known about changes in soil C storage with establishment of plantation forests. Plantation forest carbon budget models often omit mineral soil C changes from stand-level C budget calculations, while including forest floor C accumulation, or predict continuous soil C increases over several rotations. We used national soil C databases to quantify differences in soil C content between pasture and exotic pine forest plantations dominated by P. radiata (D. Don), and paired site studies to quantify changes in soil C with conversion of pasture to plantation forest in New Zealand. Overall, mineral soil C to 0.10 m was 20–40% lower under pine for all soil types (p < 0.01) except soils with high clay activity (HCA), where there was no difference. Similar trends were observed in the 0.1–0.3 m layer. Moreover, mineral soil C to 0.1 m was 17–40% lower under pine than pasture in side-by-side comparisons. The only non-significant difference occurred at a site located on a HCA soil (p < 0.08). When averaged across the site studies and the national databases, the difference in soil C between pasture and pine was about 16 t C ha^-1 on non-HCA soils. This is similar to forest floor C averaged across our individual sites (about 20 t C ha^-1). The decrease in mineral soil C could result in about a 15% reduction in the average C sequestration potential (112 t C ha^-1) when pasture is converted to exotic plantation forest on non-HCA soils. The relative importance of this change in mineral soil C will likely vary depending on the productivity potential of a site and harvest impacts on the forest floor C pool. Our results emphasize that changes in soil C should be included in any calculations of C sequestration attributed to plantation forestry.