The current understanding of controls of belowground net primary productivity and the fraction of productivity allocated belowground is rudimentary compared to our knowledge of controls of aboveground net primary productivity. However, belowground productivity accounts a large flow of carbon in most water-limited ecosystems (deserts to grasslands). Our work aims to answer two questions. (1) How do changes from year to year in water availability at one location affect the fraction of belowground net productivity relative to total production? (2) How does this temporal effect of water availability changes across spatial gradients of long-term mean precipitation from desert to humid grasslands? In order to test these hypotheses, we carried out a multi-site field manipulative experiment from a Desert grassland, to the Shortgrass steppe, to the Tallgrass prairie. At each site, we manipulated incoming precipitation with a combination of rainout shelters and automatic irrigation systems achieving five levels of precipitation ranging from 10th percentile to 90th percentile of historic rainfall records for two years. We estimated belowground productivity using ingrowth cores, standing root biomass and minirhizotron images and aboveground productivity through biomass harvests.
Results/Conclusions
Our results show that the fraction of primary productivity allocated to belowground components changed along a precipitation gradient. At the Desert grassland site, both belowground and aboveground productivity increased with increasing water availability. The fraction of net primary production allocated belowground did not change across rainfall manipulations treatments. At the Tallgrass prairie site, belowground productivity showed non-significant responses whereas aboveground net primary production increased with precipitation. This contrasting responses resulted in a significant response of the fraction of biomass allocated belowground decreased with precipitation suggesting that competition for other factors different from water may be driving plant allocation patterns at the wettest site of our gradient. We found intermediate responses at the Shortgrass Steppe site indicating that regional productivity allocation patterns change linearly along precipitation gradients. Our results highlight the importance of within biome differences in biomass allocation. This study contributes to the understanding of the belowground functioning of grasslands and their ability to sequester carbon.