Impact of Precipitation Dynamics On Net Ecosystem CO2 Exchange.

Anthropogenic climate change is expected to alter both the frequency and size of rainfall events. Our results suggest that this will likely affect the net carbon balance of semi-arid grasslands. Net ecosystem carbon dioxide (CO₂) exchange (NEE) was measured on shortgrass steppe (SGS) vegetation in northeastern Colorado. As expected, nighttime NEE was positive (CO₂ source) throughout the year and daytime NEE was positive during the non-growing season. But during the growing season, daytime NEE could be positive or negative (CO₂ sink), depending on the size of precipitation events in this water limited system.  Large precipitation events (> 10 mm day^-1) promoted carbon uptake because increased soil water content in sub-surface layers promoted plant photosynthesis, while small precipitation events (5 - 10 mm day^-1) that only wet the surface soil enhanced heterotrophic respiration and resulted in a loss of carbon from the system. Although the growing season extends from April - October, more than 95% of the net carbon uptake occurred during the April - June time period. Live aboveground plant biomass, solar radiation, and soil water content were the major variables that controlled daytime net CO₂ uptake and nighttime respiration losses.