Net ecosystem productivity (NEP) was measured on shortgrass steppe (SGS) vegetation at the USDA Central Plains Experimental Range in northeastern Colorado from 2001 to 2003. Large year-to-year differences were observed in annual NEP, with >95% of the net carbon uptake occurring during May and June. Low precipitation during the 2002 April to June time period greatly reduced annual net carbon uptake. Large precipitation events (>10 mm day−1) promoted carbon uptake, while small precipitation events (<10 mm day−1) enhanced heterotrophic respiration and resulted in a net loss of carbon from the system. Large precipitation event enhanced carbon uptake was attributed to increased soil water content (SWC), which promotes plant photosynthesis. The large precipitation events which occurred from July to October have lower increases in daytime net CO2 uptake (NEPd) due to the presence of low live plant biomass compared to earlier in the growing season. Live aboveground plant biomass (AGB), solar radiation, and SWC were the major variables that controlled NEPd, while AGB, SWC, and relative humidity control nighttime respiration losses (NEPn). Aboveground plant biomass is the most important variable for controlling both NEPd and NEPn dynamics. These results suggest that the major factor controlling growing season NEPn is the amount of carbon fixed via photosynthesis during the day. Heterotrophic soil respiration is greatly enhanced for one to 2 days following rainfall events with daily rainfall events >5 mm having a similar increase in respiration (>3.00 g m Cm−2 day−1). In addition, the size of the heterotrophic respiration pulse is independent of both the amount of time since the last rainfall event and the time of occurrence during the growing season.
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