Impact of precipitation dynamics on net ecosystem productivity

Abstract

Net ecosystem productivity ( NEP ) was measured on shortgrass steppe ( SGS ) vegetation at the USDA C entral P lains E xperimental R ange in northeastern C olorado from 2001 to 2003. Large year‐to‐year differences were observed in annual NEP , with >95% of the net carbon uptake occurring during M ay and J une. Low precipitation during the 2002 A pril to J une time period greatly reduced annual net carbon uptake. Large precipitation events (>10 mm day −1 ) promoted carbon uptake, while small precipitation events ( −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 J uly to O ctober have lower increases in daytime net CO 2 uptake ( NEP d ) 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 NEP d , while AGB , SWC , and relative humidity control nighttime respiration losses ( NEP n ). Aboveground plant biomass is the most important variable for controlling both NEP d and NEP n dynamics. These results suggest that the major factor controlling growing season NEP n 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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