The functional role of petrocalcic horizons in desert ecosystems: Spatial and temporal dynamics of plant water availability

Petrocalcic horizons occur in most desert soils around the world, often within the plant rooting zone. Little is known, however, about water holding characteristic or water availability in horizons indurated with carbonates. Soil profile characteristics can control plant community composition and production by altering spatial and temporal patterns of plant available water, patterns that are important for understanding the causes and consequences of woody shrub encroachment into historic desert grasslands. A series of replicated experiments at multiple spatial scales were conducted to investigate petrocalcic horizon water retention and dynamics. Sampling and field studies were conducted in a mixed shrub-grass community in southern New Mexico, USA. A laboratory study was conducted to define the soil-water release curve for a variety of petrocalcic material from field capacity to <-10 MPa. To evaluate high carbonate horizon temporal water availability and dynamics, two multiyear field studies were conducted: a pasture scale study comparing water availability across a chronosequence of carbonate horizon development and a companion patch-interspace scale study investigating soil-water dynamics associated with woody shrub encroachment in a petrocalcic soil. Petrocalcic horizon plant available water holding capacity for desert species ranged from 0.26 m3 m-3 in plugged to 0.06 in some laminar horizons. Calcic and petrocalcic horizons retained much greater amounts of available soil water during a winter with above-normal precipitation than similar depths in the non-carbonate sand (0.12 to 0.14 m3 m -3 versus 0.08 m3 m-3) and retained soil water at plant available tensions a greater number of days during the following spring and summer. The companion study in the petrocalcic soil showed that unvegetated interspaces absorbed significantly more soil water during a wet winter and retained more available soil water into the spring than soils under shrubs. In contrast, soils under shrubs absorbed greater quantities of water following summer rains. Wetting and drying dynamics indicate petrocalcic horizons release stored water but it is unclear if plants access petrocalcic water directly. Patterns of water availability, however, indicate soils with shallow petrocalcic horizons are potentially less susceptible to dominance by deep rooted woody shrubs and beneficial to establishment and persistence of grasses.