Carbon sequestration potential and fractionation in soils after conversion of cultivated land to hedgerows

Abstract

Hedgerows supply multiple ecosystem services in agricultural landscapes and have been advocated to provide a relevant carbon sink for climate change mitigation. Using a space-for-time approach, we investigated carbon sequestration and fractionation in soils beneath hedgerows 1 to 70 yr after planting on cultivated land in the humid continental climate of Eastern Austria. We sampled 54 pairs of hedgerow and adjacent cultivated soil volumetrically to 40 cm depth and analysed them for bulk soil (< 2 mm) mass, SOC fractions of differential stability and related soil properties. Total SOC stocks to 40 cm depth increased significantly (p < 0.05) by 34.7 ± 4.70 Mg C ha−1 to 119 ± 6.77 Mg C ha−1 beneath 31–70 yr old hedgerows, and by 15.6 ± 5.94 Mg C ha−1 to 106 ± 8.98 Mg C ha−1 beneath 1–30 yr old hedgerows. Carbon sequestration was limited to the organic and mineral topsoil layer (0–20 cm), SOC changes (ΔSOC) at 20–40 cm depth were small and insignificant (p < 0.05). Linear regression of ΔSOC on hedgerow age (1–70 yr) yields a mean SOC sequestration rate of 0.65 ± 0.10 Mg ha−1 yr−1 down to 40 cm depth. Combining our data with published work we found hedgerow age explaining ∼65 % of the variation of ΔSOC in topsoils, and ∼68 % in shallow subsoils; ΔSOC increases with hedgerow age in topsoils, but declines in subsoils. Annual sequestration rates decline with hedgerow age in both soil layers. Up to 30 years after conversion, SOC is preferably sequestered in labile particulate organic matter (POM; > 20 µm), and stabilised in the mineral-associated organic matter (MAOM; < 20 µm fraction) thereafter. In the bulk soil and MAOM fraction, SOC strongly increases with oxalate-extractable Al (Alo). We conclude that SOC sequestration beneath hedgerows offers a relevant CO2 sequestration potential which is mainly limited to topsoils and primarily controlled by hedgerow age and Alo.