Alley width and slope position influence soil carbon storage, nutrient dynamics and hydrology at a mature silvoarable site, SW England

Abstract

Optimising benefits from agroforestry requires better understanding of spatial factors such as alley width and slope position. We sampled soil (0–50 cm) from a mature organic silvoarable site in SW England with tree rows at 12 and 24 m spacing to determine the impact of these factors on soil physical properties, carbon (C) storage and fertility. We consider how functioning differs in cropped alley and tree-row components, and how alley width influences trade-offs in ecosystem benefits. Benefits from rows extended into alleys which were 8.8 % less compacted and contained 70 % more available P than an adjacent, treeless control. Competition for nutrients and moisture was observed at the row-alley boundary, with lower subsoil concentrations attributable to tree root uptake. Agroforestry mitigated soil erosion despite being parallel to slope: in the control area 0.8 % more soil organic matter and a 3.5 % higher clay fraction was observed downslope than upslope, with no equivalent effect under agroforestry. Fertility traded off with alley width, with more N and P stored in 12 m alleys. Soil and tree-biomass C differences (700 kg C ha⁻¹ year⁻¹) compared with the control were only significant in the 12 m system (110 stems ha⁻¹) and three times lower than estimated silvoarable contributions to future UK C budgets. Moreover, planting at lower densities (∼50 stems ha⁻¹) is likely due to constraints of modern farm machinery. Assessment of silvoarable contributions to temperate ecosystem service provision must therefore consider additional benefits beyond C sequestration if agroforestry is to contribute to future landscape resilience.