Global Change and Ecosystem H2O and CO2 Fluxes in the Mojave and Negev Desert
Investigators (most current known information)
The results of our field study in which we experimentally manipulated rainfall and N inputs clearly indicate that water availability – i.e., precipitation, remains the strongest modulator of biological and ecological activity in both deserts and consequently is the primary driver of net ecosystem and net soil surface CO2 and water vapor fluxes. Natural rains, and artificial rains or water added during "fetrigation" (N-solution application), first seem to stimulate ecosystem and soil respiration resulting in net CO2 effluxes from the land surface to the atmosphere. However, this effect is not sustained and appears to be over compensated in the Mojave Desert study by the occurrence of longer and drier periods of net ecosystem CO2 uptake. Higher water availability in irrigated plots led to and overall increase in plant productivity (NPP), relative to that observed in unwatered plots, and this seemed to be responsible for higher rates of NEE observed during the study period. Surprisingly, no interactive effects of water and N additions were observed. Persistently high mean 24 h CO2 uptake values over the measurement period in the Mojave Desert suggest that these ecosystems may represent a surprisingly large CO2 sink, as has been observed nearby sites.
Responses of crust-covered soil surface responses to pulses of water at the neighboring site suggest that the strong net release of CO2 from ecosystems in the summer measured in all plots of the main experiment, regardless of irrigation or nitrogen treatment, are largely due to immediate responses of both heterotrophic and autotrophic soil micro- and macroorganisms. Although we were unable to collect parallel rainpulse response data for whole ecosystems during cooler months, high rates of net CO2 uptake measured immediately and up to several days after water pulses suggest that soil surfaces remain moister longer during late fall through early spring than they do in warmer months when high air temperatures and high VPDs result in rapid surface drying and rapid inactivation of autotrophic crust organisms. The absence of any significant responses to N additions, even in the presence of additional moisture in "irrigated"plots, was somewhat surprising. These results indicate, though, (a) that water availability provided at levels that may occur under conditions of some global change scenarios can strongly influence both soil and ecosystem net CO2 uptake and release; and (b) that anticipated increases in atmospheric N deposition from expanding urban areas may not significantly impact the ecology of Mojave Desert organisms in a way that causes changes in net CO2 fluxes. Finally, the relatively poor correlations observed between NEE and ET fluxes and air temperature and light (PAR) suggest that, at the time scales and temporal frequency at which we sampled, fluxes in arid shrubland ecosystems are controlled more by other factors. Certainly, both short- and long-term water availability – expressed as soil water content/potential or leaf area index or plant greenness (NDVI) – should be evaluated specifically. Thus, the development of equations and models aimed at calculating NEE and ET from measured environmental variables – thus enabling interpolation between measurement points – turned out to be difficult.
Articles in Journals
Jasoni, R.L., J.D. Larsen, L.F. Fenstermaker, E. Knight, J. Grünzweig, and J.A. Arnone III. 2009. “Effects of increased rainfall and atmospheric N deposition on net ecosystem CO2 exchange and evapotranspiration in a Mojave Desert ecosystem.” Global Change Biology. (to be ubmitted).
Arnone III, J.A., R.L. Jasoni, L.F. Fenstermaker, J.D. Larsen, and J. Grünzweig. 2009. “Seasonal and diurnal variation in net CO2 fluxes of Mojave Desert soil surfaces covered with cryptobiotic crusts: differential impacts of water pulses in winter and summer.” Oecologia. (to be submitted)
Grünzweig J. et al. 2009. “Environmental and biotic controls on soil and ecosystem net CO2 fluxes in the Negev Desert.” Global Change Biology. (to be submitted)
Grünzweig, J. 2008. Seminar at the Weizmann Institute of Science. May. Rehovot, Israel.