Spatial and temporal variability of plant-available water in calcium carbonate-cemented soils and consequences for arid ecosystem resilience.

Increased variability in precipitation, including frequency of drought, is predicted for many arid and semiarid regions globally. The ability of soils to retain water can increase resilience by buffering vegetation communities against precipitation extremes. Little is known, however, about water retention by carbonate-cemented soil horizons, which occur extensively in arid and semiarid ecosystems. It has been speculated that they may significantly modify vertical and temporal distribution of plant-available water (PAW). To investigate this hypothesis, PAW was monitored at three sites in a mixed shrub-grass community in southern New Mexico, USA, across soils with differing degrees of carbonate horizon development: no carbonate horizon, a horizon partially cemented with carbonates (calcic), and a horizon continuously cemented with carbonates (petrocalcic). Results are presented from 3 years that included extremely dry and wet periods. Both carbonate-cemented horizons absorbed and retained significantly greater amounts of PAW for several months following an extremely wet winter and summer compared to the non-carbonate soil. Following a wet summer, continuously cemented horizons retained very high PAW (16-18% volumetric or approximately 72-80% of soil water holding capacity) through early spring of the following year, more than double the PAW retained by similar depths in the non-carbonate soil. Drying dynamics indicate both carbonate-cemented horizons release stored water into the grass rooting zone during growing seasons following extreme wet events. Water dynamics of these horizons during extreme events provide a mechanism to explain previous observations that perennial grasses exhibit greater resilience to drought when carbonate-cemented horizons occur at shallow depths (<50 cm). Water holding capacity of the entire profile, including horizons cemented with carbonates, should be considered when evaluating the potential resilience of vegetation communities to disturbance, including the increased variability in precipitation expected to occur as a result of global climate change.

Scale and the isotopic record of C4 plants in pedogenic carbonate: from the biome to the rhizosphere.

The 13C/12C ratio in pedogenic carbonate (i.e., CaCO3 formed in soil) is a significant tool for investigating C4 biomes of the past. However, the paleoecological meaning of delta13C values in pedogenic carbonate can change with the scale at which one considers the data. We describe studies of modern soils, fossil soils, and vegetation change in the Chihuahuan Desert of North America and elsewhere that reveal four scales important for paleoecologic interpretations. (1) At the broadest scale, the biome scale (hundreds to millions of km2), an isotopic record interpreted as C3 vegetation replacing C4 grasslands may indicate invading C3 woody shrubs instead of expanding C3 forests (a common interpretation). (2) At the landscape scale (several tens of m2 to hundreds of km2), the accuracy of scaling up paleoclimatic interpretations to a regional level is affected by the landform containing the isotopic record. (3) At the soil-profile scale (cm2 to m2), soil profiles with multiple generations of carbonate mixed together have a lower-resolution paleoecologic record than soil profiles repeatedly buried. (4) At the rhizosphere scale (microm2 to cm2), carbonate formed on roots lack the 14-17 per thousand enrichment observed at broader scales, revealing different fractionation processes at different scales. A multi-scale approach in dealing with delta13C in pedogenic carbonate will increase the accuracy of paleoecologic interpretations and understanding of soil-geomorphic-climatic interactions that affect boundaries between C4 and C3 vegetation.