Soil properties of a widely spaced, planted poplar (Populus deltoides)-pasture system in a hill environment

Planting poplars is an effective technology for controlling hill soil erosion in New Zealand pastureland. However, the effects of widely spaced poplars on soil properties are not documented. Soil was sampled from 3 soil depths (0-75, 75-150, and 150-300 mm) in poplar pasture (PP) and adjacent open pasture (OP) systems. Four sites were examined, 3 unreplicated sites with mature poplars (>29 years, 37-40 stems/ha) and a replicated experiment with immature poplars (5 years, 50-100 stems/ha). Pastures at all sites were dominated by low fertility species and were grazed by sheep and cattle. Pasture species were used in a glasshouse experiment to assess the production potential of the topsoil media (0-20 mm) from the PP and OP soils, and the OP topsoil amended with 5% poplar leaf litter. Soil pH was higher (0.5-1.2 units) in the PP than the OP at all depths, as were exchangeable cations. After 2 months incubation, OP topsoil amended with 5% leaf litter had higher soil pH than the PP or the OP topsoil. Organic C, total N, P or S, and plant-available P or S were similar in the PP and OP soils, or lower in the PP soil. Soil hydraulic characteristics, macroporosity, water aggregate stability, and bulk density were also similar in the PP and OP areas. The differences in the earthworm numbers and mass between the PP and OP areas were inconsistent. Pasture grown in the PP topsoil produced more over 11 months than treatments containing OP topsoil. In addition to increasing the stability and, hence, sustainability of hill soils at risk from erosion, poplar trees enhanced several other key attributes of the soil. The study suggested that poplars have a restoration role by accelerating soil development following the loss of topsoil from an erosion event.