The response of ecosystem CO2 exchange to small precipitation pulses over a temperate steppe

In water-limited grassland ecosystems, discrete and occasional precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation pulses may constrain biogeochemical transformations at the ecosystem scale. We examined the short-term dynamics of the whole ecosystem response to small precipitation events during 2003 and 2004 in a steppe on the Inner Mongolia Plateau. The results indicate that changes in soil moisture occur, with a 1–2 day time lag, only when the amount of precipitation exceeds 3 mm (from day of year [DOY] 120 to DOY 180) or 5 mm (after DOY 180). The interception of the developing plant canopy is a primary reason for the different temporal precipitation threshold. The lower threshold of effective rain is different between Net Ecosystem Exchange (NEE, 3 mm), Ecosystem Respiration (Re, 3 mm) and Gross Ecosystem Production (GEP, 5 mm). The NEE reached a maximum 4–5 days after the end of effective rain events and dropped to 60–70% of the original fluxes after 10 days. However, the drop in GEP was greater than that of NEE and reached 30–50% of the original fluxes after 10–15 days without “effective rainfall.” The characteristics of the response time can be attributed to the variation in soil water content and the time of readjusting for the ecological processes after the effective rainfall. In addition, the independent responses of photosynthesis, respiration and evapotranspiration to precipitation probably contributed to this time lag. The results support the hypothesis that the concept of an ecologically significant rainfall event can be developed for an ecosystem.