Grasslands are highly responsive to inter- and intra-annual variability in precipitation and temperature, which interact to affect soil water dynamics and plant and soil processes. In the U.S. Central Plains mean temperatures are expected to increase and rainfall patterns are predicted to become more variable and extreme, with increased frequency of large rainfall events and extended inter-rainfall droughts. Increased temperatures may intensify or moderate the impacts of altered rainfall timing on key ecosystem processes, such as plant productivity, soil CO₂ flux, and soil N availability. We have been assessing the consequences of these climate changes for 12 years, using field-scale rainfall manipulation plots to alter the timing of growing season rainfall events (since 1998) and infrared heaters to increase mean temperature (since 2003) in intact plots of native tallgrass prairie at the Konza Prairie Biological Station. The long-term nature of this study and inherently high inter-annual climatic variability of these grasslands allows us to begin assessing the conditions under which altered rainfall timing and warming are likely to have their largest effects.

Results/Conclusions

More extreme patterns of growing season rainfall, with no change in total rainfall amount, significantly reduced the mean and increased variability in soil water content in most years. Warming further reduced mean soil water content. Across all years, annual total ANPP was positively related to mean soil water content and negatively related to within season soil moisture variability, and grasses were more responsive than forbs to soil moisture dynamics.  The effect of altered rainfall timing on ANPP varied across years, but when significant it reduced ANPP by 13-22%. Warming accelerated early season plant growth, but this effect did not persist through the growing season. When significant, warming reduced ANPP by 10%, with forbs generally being more responsive than grasses. Mean annual soil CO₂ flux was generally reduced by 11% under altered rainfall timing, while the effects of warming were contingent on rainfall amounts and differed for growing and non-growing seasons. Elevated temperatures decreased growing season soil CO₂ by 9% flux in dry years, but stimulated soil CO₂ flux in wet years and in the non-growing season (by 11%). Altered rainfall timing increased resin-bag collected nitrate by up to two-fold, with no apparent increase in plant uptake.  In contrast, warming did not affect soil N availability. These results suggest that the effects of warming on mesic grasslands will depend on changes in amounts and timing of rainfall.