Soil water holding capacity regulates ecosystem responses to multifactor global change: A modeling analysis

We conducted a modeling study to evaluate how soil water holding capacity (WHC) regulates ecosystem water and carbon dynamics in response to altered precipitation amount and frequency, climate warming, elevated atmospheric [CO₂]. In this study, we used a process-based terrestrial ecosystem (TECO) model, which was validated against data from two experiments with warming and clipping or doubled precipitation in Great Plains.  The validated TECO model simulates responses of soil water content, evaporation, transpiration, runoff, net primary production (NPP), ecosystem respiration (R_h), and net ecosystem production (NEP) to changes in precipitation amounts (halved, ambient, and doubled) and intensity (ambient and doubled), temperature (ambient and +2 ^oC), and atmospheric CO₂ concentration (ambient and 700 ppm) along a soil WHC gradient from 10% to 45%.

Our simulation results showed that soil WHC determines partitioning of precipitation among runoff, evaporation, and transpiration, and consequently regulates ecosystem responses to global environmental change. Fractions of precipitation that are used for evaporation and transpiration increase with WHC but decrease with WHC for runoff.  Usually, the soil with high WHC can greatly buffer water stress during long drought periods, particularly after a large rainfall event. NPP, R_h, and NEP usually increase with soil WHC in ambient and doubled precipitation scenarios but increase from 10 to 15% of WHC followed by declines under the halved precipitation amount regardless precipitation intensity. Warming and CO₂ effects on soil water content, evapotranspiration, and runoff are magnified by soil WHC.  Regulatory patterns of soil WHC on responses of NPP, R_h, and NEP to warming are complex. In general, CO₂ effect on NPP, R_h, and NEP increases with soil WHC.  Our results indicate that variations in soil water holding capacity may be one of the major causes underlying variable responses of ecosystems to global changes observed from different experiments.