Sequestering soil organic carbon by planting hedgerows in agricultural landscapes

Recent policy initiatives have placed a strong focus on the use of agricultural soils for atmospheric CO2 removal by adopting practices for sequestering and storing SOC. In the UK, changes in agricultural land use, such as the integration of woody species in the form of hedgerows--lines of regularly trimmed shrubs commonly used to delimit agricultural fields--, have been recommended for climate change mitigation. The Climate Change Committee has proposed a 40% increase in hedgerow length across the country as a key contribution to net-zero targets. In England, this would equate to 193,000 km of newly planted hedgerows. However, the contribution of hedgerow planting to reaching net-zero goals remains unclear due to a lack of data on the rate at which CO2 is taken up and stored in the soil beneath them. In our study, seventy-eight hedgerows across six different pedo-climatic conditions in England were classified into four age categories. Soil organic carbon (SOC) stocks were quantified at 10 cm intervals for the top 50 cm of soil beneath hedgerows and in adjacent grassland fields. Moreover, we examined the distribution of SOC among particle-size fractions to investigate how hedgerow planting may influence SOC dynamics by affecting the quality and long-term stability of organic matter in soils, particularly to illustrate why hedgerow-associated SOC stocks are rapidly lost after hedgerow removal. SOC stocks beneath hedgerows were higher than adjacent fields for all age categories and hedgerows stored an average additional 40% SOC stock in the top 50 cm of soil compared to adjacent fields and 30% in the top 30 cm of soil. The additional SOC stock beneath hedgerows was 40.9 Mg C ha-1 at 0-50 cm depth, or 6.1 Mg C km-1. We used a 37-year-old SOC sequestration rate to show that if England were to reach its goal of 40% increase in hedgerow length, 6.3 Tg of CO2 will be sequestered and stored in the soil over 40 years (9.9 Tg with aboveground biomass). However, it will take ~200 years to reach this target with current rates of planting in national public agri-environment schemes. These results contribute measurable outcomes towards the estimate of targets for net-zero 2050 and the extent of ecosystem services provision by hedgerow planting in agricultural landscapes.