Soil and tree biomass carbon sequestration potential of silvopastoral and woodland-pasture systems in North East Scotland

Managing agricultural land for carbon sequestration becomes more important with rising needs for greenhouse gas mitigation measures. Woodland establishment in upland grasslands can be a carbon sink, but soil carbon losses have also been observed. Dedicated woodland plots and silvopasture are two contrasting strategies to achieve increased carbon stocks. We compared the carbon sequestration potential of the two approaches with three tree species (Hybrid Larch, Scots Pine and Sycamore) planted on permanent pasture on an upland farm in North East Scotland, 24 years after planting. Soil organic carbon was measured in the A (similar to 0-30 cm) and B (similar to 30-50) horizons. The soil carbon was also fractionated into labile, protected and resistant pools. Litter layers were measured and tree biomass carbon was estimated using allometric equations. We found that total soil carbon stocks (A plus B horizon) were similar and did not differ significantly between treatments, but for both coniferous species silvopasture tended to have the greater soil carbon stock followed by woodland, whereas Sycamore had the greater stock in the woodland treatment; pasture had the least carbon stock. Woodland stored more carbon in the labile fractions and litter layer than both pasture and silvopasture of all treatments, which had similar levels of stabilized carbon. Biomass per tree was significantly greater in the silvopasture treatments for all species, but on a per hectare basis woodland stored significantly more carbon. Land management comparison shows that large proportions of grassland would need to be converted to woodland to provide similar carbon benefits as the integrative silvopastoral system.