Belowground effects of porous pavements—Soil moisture and chemical properties

Impermeable pavements cover a considerable land area in cities. Their effect on the hydrological cycle is clear; as a barrier in the soil–atmosphere continuum they minimise rainfall infiltration and evaporation. Porous pavements are beginning to replace impermeable alternatives because of perceived hydrologic benefits. The impact of porous pavements on soil moisture and chemistry as they relate to urban vegetation was investigated in Christchurch, New Zealand. An experiment was established comprising 25 plots evenly distributed amongst controls (no pavement, exposed soil) and four different pavement treatments: a factorial combination of pavement type (porous, impervious) and pavement profile design (including or excluding a greywacke gravel base). Results indicate that pavements altered soil pH from moderately acidic (pH=5.75) to more neutral levels (pH=6.3). The effect on pH was greater beneath porous pavements, and also when a gravel base was included. Concentration of soil Al, Fe, and Mg decreased, while Na increased beneath pavements. Soil moisture was consistently higher beneath pavements than control plots, except following periods of heavy rainfall where high soil moisture muted all treatment effects. Throughout most of the study period, soil moisture content was lower beneath pavement profiles designed with the gravel base, presumably due to the gravel acting as a capillary break to a distillation process, whereby soil moisture migrates upwards to the soil surface. In early autumn, when soil moisture content was lowest for all treatments, precipitation recharged soil moisture in control plots and beneath porous pavements. But impervious pavements prevented infiltration resulting in significantly lower soil moisture content beneath these pavements. Pavements can alter soil moisture and chemical characteristics, but the effects differ depending on pavement porosity and profile design. Implications of the results pertain to stress physiology of urban vegetation, in particular drought stress avoidance.