Summer Rain Pulses may Stimulate a CO2 Release rather than Absorption in Desert Halophyte Communities

In arid and semiarid ecosystems, annual net primary productivity is limited by the availability of water and correlates strongly with total annual precipitation. However, in these ecosystems, precipitation often arrives in discrete, episodic, unpredictable precipitation pulses, which can instigate a complex cascade of physical, chemical and biological processes. When evaluating the response of arid and semiarid ecosystems to precipitation, it is thus pertinent to focus on the implications of these pulses, rather than solely seasonal or yearly totals.

Since halophyte desert ecosystems cover a large area of global terrestrial surface, it is important to understand the responses of these systems to episodic precipitation pulses. However, only few studies have assessed the carbon flux sensitivity to precipitation pulses in these ecosystems. A more complete understanding of the influence of precipitation on ecosystem carbon exchange and the differential response of plant- and soil microbe-mediated processes after rainfall will improve regional predictions of the impact of climate variability in water-limited ecosystems.

Therefore, LIU Ran et al. selected two desert halophyte sites that share similar environmental conditions in the southern periphery of the Gurbantonggut Desert, to assess how the ecosystem carbon exchange (NEE) of desert halophyte communities of different plant functional-types responds to summer precipitation pulses in Tamarix and Haloxylon communities.

The results showed that plant water status and photosynthetic assimilation did not differ before and after summer precipitation pulses in either community. In contrast, soil respiration and NEE responded strongly to summer precipitation events in both communities. At the ecosystem level, precipitation pulses induced a pulse of CO₂ release, rather than absorption. The NEE response to summer precipitation was less in the deep-rooted Tamarix community, compared to the shallow-rooted Haloxylon community, which was even converted into a carbon source after summer precipitation inputs. As a result, the effect of summer precipitation inputs on soil respiration was more important than the plant (carbon assimilation) response in determining the ecosystem response to episodic precipitation pulses. The result was published in Plant and Soil in December 2013.