Hedgerow effects on CO2 emissions are regulated by soil type and season: Implications for carbon flux dynamics in livestock-grazed pasture

In this study we assess the potential for farmland hedgerows to provide climate mitigation via carbon (C) storage, using soil carbon dioxide (CO2) efflux to improve upscaling validity. Two contrasting sites, freelydraining (FD) versus seasonally-wet (SW), situated in mixed-livestock farms (Conwy, Wales, UK), were selected. We measured soil CO2 efflux associated with three field boundaries: hedgerow on SW soil; hedgerow on FD soil; stone wall (abiotic control) on FD soil, quantifying the influence of distance from field boundary and grazing occurrence (grazed pasture versus un-grazed zone adjacent to hedgerows) on annual C budgets based on soil CO2 flux and net primary productivity. For the FD site, the annual C budget showed that pasture was a net source of C emissions (11 +/- 1.5 t CO2 ha(-1) yr(-1)) and the un-grazed zone adjacent to the hedgerow a net sink (-0.9 +/- 2.2 t CO2 ha(-1) yr(-1)). For the SW site, pasture acted as a small net sink of C (-0.1 +/- 1.3 t CO2 ha(-1) yr(-1)) and the hedgerow zone a net source (5.8 +/- 0.8 t CO2 ha(-1) yr(-1)), due entirely to a spike in soil CO2 efflux associated with a relatively unusual summer drought. To investigate the effect of this observed summer drought on more typical (for the UK maritime climate) annual C source-sink dynamics, we modelled soil CO2 efflux for a summer-drought-excluded year for both FD and SW soils. With greater hedgerow cover (modelled prediction compared with a baseline of no hedgerows), annual CO2 flux became more negative (greater net sink) in fields on FD soil (by 1 t CO2 ha(-1) yr(-1) at 8% hedgerow cover), with drought limiting the effect size. In SW soils, greater hedgerow cover also led to a more negative annual CO2 flux (by 0.4 t CO2 ha(-1) yr(-1) at 8% hedgerow cover) when drought was excluded, but a more positive flux (net C source) with drought included (by 0.5 t CO2 ha(-1) yr(-1) at 8% hedgerow cover). This study illustrates the importance of the interaction between soil type and seasonal events such as drought on the ability of hedgerows to act as a net C sink.