PS 6-54
Net primary productivity and rain use efficiency as affected by warming, altered precipitation and clipping in a mixed grass prairie

Monday, August 5, 2013
Exhibit Hall B, Minneapolis Convention Center
Xia Xu, Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA
Background/Question/Methods     Grasslands in response to climate change and land use are of global concern. In order to explore the effects of climate change and land use on net primary productivity (NPP), NPP partitioning (fBNPP) and rain use efficiency (RUE) of NPP, we conducted a field experiment with warming, altered precipitation, and annual clipping in a mixed grass prairie in Oklahoma, USA. Infrared heaters are used to elevate soil temperature by approximately 3 ℃  since July 2009. We use a rainfall-collection-redistribution (RCR) device to double precipitation and a rainout-shelter to halve precipitation. Annual clipping was to mimic hay harvest. ANPP, separated into C3 and C4 species, was directly measured by annual clipping at peak biomass in the clipped subplots and indirectly estimated by pin-contact method in the unclipped subplots. The root ingrowth-core method (5.2 cm in diameter, 0-90 cm) was applied to estimate BNPP.  fBNPP is defined as fBNPP=BNPP/(ANPP+BNPP). Rain use efficiency (RUE) of ANPP, BNPP, and NPP is calculated as ANPP, BNPP and NPP, respectively, divided by the amount of actual precipitation received by specific plots under different treatments.

Results/Conclusions     Across the years, warming significantly increased belowground NPP (BNPP), fBNPP and RUEBNPP by an average of 11.6 %, 2.8 % and 6.6 %, respectively. This indicates that BNPP was more sensitive to warming than aboveground NPP (ANPP) since warming did not change ANPP and RUEANPP much. Double precipitation stimulated ANPP, BNPP and NPP but suppressed RUEANPP, RUEBNPP and RUENPP while half precipitation decreased ANPP, BNPP and NPP but increased RUEANPP, RUEBNPP and RUENPP. Clipping interacted with altered precipitation in impacting RUEANPP, RUEBNPP and RUENPP, suggesting land use could confound the effects of precipitation changes on ecosystem processes. Soil moisture was found to be a main factor in regulating variations in ANPP, BNPP and NPP while soil temperature was the dominant factor in influencing fBNPP. These findings suggest that BNPP is a critical point for future research. Additionally, results from single-factor manipulative experiments should be treated with caution due to the non-additive interactive effects of warming with altered precipitation and land use (clipping).