Soil carbon of hedgerows and ‘ghost’ hedgerows

Abstract

Agroforestry can contribute significantly to carbon sequestration in agricultural lands, as carbon accumulates both in tree biomass and the soil. One of the oldest, yet declining, forms of agroforestry in Europe are hedgerow-bordered fields. An analysis of historical maps of our study area in Belgium shows that 70% of the hedgerow network was cleared since 1960, creating a large number of ‘ghost’ hedgerows. We selected arable fields next to hedgerows, 'ghost' hedgerows and grass strips to study how hedgerow trees influence SOC stocks and how much of these are still present after hedgerow clearing. SOC stocks to a depth of 23 cm reached up to 81.7 ± 28.8 Mg C ha⁻¹ in hedgerows, storing a considerably larger amount of soil carbon compared to grass strips (56.6 ± 14.5 Mg C ha⁻¹). These built-up stocks were completely gone in 'ghost' hedgerows (57.9 ± 14.1 Mg C ha⁻¹). In the fields adjacent to hedgerows, SOC stocks were only slightly (and insignificantly) increased compared to stocks in fields with grass strips (56.4 ± 6.3 vs 55.6 ± 5.0 Mg C ha⁻¹) with an exponential decay up to 30 m from the margin. This trend was still limitedly detectable in 'ghost' hedgerowbordered fields, however stocks were not elevated anymore (53.9 ± 6.1 Mg C ha⁻¹). Since 1960, 4 957 ± 1 664 Mg C from the soil alone were released back into the atmosphere due to hedgerow removal in the study area. The implementation of a strict hedgerow conservation policy would thus be a highly effective climate change mitigation measure in agricultural landscapes.