Water-limited grasslands may be exceedingly vulnerable to changes in the timing or amount of precipitation which may result in shifts in the magnitudes and patterns of carbon cycling. Shifts in CO2 exchange may lead to alterations in carbon sequestration or net losses and could accentuate the rising CO2 concentrations in the atmosphere or ameliorate the increases. The objective of our project was to quantify how changes in winter, summer, and combined winter and summer precipitation may alter rates of ecosystem C cycling in the mixed-grass prairie of the United States. Three replicated 50 m snow fences were installed to increase winter snow while summer precipitation was manipulated by either increasing (+50%) or decreasing (−50%) precipitation amounts. Measurements of net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (ER) and plant biomass were conducted throughout the snow-free period. Deeper snow in winter increased summer ER by an average of 27%, GEP by 45%, NEE by 90% and plant biomass by 50% compared to ambient snow conditions. Average plant biomass increased 44% with the addition of summer rain and decreased by 18% where rainfall was excluded under ambient snow conditions. Average NEE was also lower where rain was excluded. Our findings indicate that seasonality changes of precipitation may be important in regulating the future extent of C sequestration and C cycling in one of the most extensive, intact grasslands of North America. Copyright © 2010 John Wiley & Sons, Ltd.