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

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 (f_BNPP) 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.  f_BNPP is defined as f_BNPP=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), f_BNPP and RUE_BNPP 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 RUE_ANPP much. Double precipitation stimulated ANPP, BNPP and NPP but suppressed RUE_ANPP, RUE_BNPP and RUE_NPP while half precipitation decreased ANPP, BNPP and NPP but increased RUE_ANPP, RUE_BNPP and RUE_NPP. Clipping interacted with altered precipitation in impacting RUE_ANPP, RUE_BNPP and RUE_NPP, 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 f_BNPP. 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).