Project Funding: 2014 - 2017

NRA: 2013 NASA: Carbon Cycle Science   

Funded by USDA

Abstract:
Grassland ecosystems cover roughly 40% of the United States and consist of a mixture of native-dominated rangelands and human derived pasture systems. Grasslands are important to humans as a source of forage for grazing animals, alternative sources of fuel, green space in urban and suburban areas, and as wildlife and pollinator habitat. In many areas, native dominated rangeland has been converted to either cropland or grassland that is dominated by non-native (‘exotic’ or 'introduced' or 'alien' or 'invasive') plant species. Furthermore, these conversions have occurred during a time of changing climate, including altered precipitation. This conversion is hypothesized to have altered C-cycling between exotic-dominated systems and the native systems they replaced. Belowground production often exceeds aboveground production in grassland systems, and a better understanding of how human actions are impacting soil C is necessary. We propose to address a fundamental knowledge gap on how belowground C-cycling is impacted by the replacement of native rangelands with non-native communities under ambient or altered rainfall patterns in a unique long-term experiment. The experiment is in its sixth growing season at the USDA-ARS Grassland, Soil, and Water Research Laboratory (Temple, TX). Highly replicated experimental plots (64 mixtures and 144 monocultures) with identical initial plant densities, functional group proportions, and species diversity were established in 2008 to determine whether ecological differences develop over time between native and exotic plant communities with and without altered precipitation. Either all native species or all exotic species were assigned to 9-species mixtures using a paired species design, and this treatment was crossed with a summer irrigation treatment that alters rainfall patterns (n=16 mixtures in each of the four treatments) using a pool of 36 native and widely distributed non-native species. Treatment differences have stabilized into a high-diversity native system and a low diversity exotic dominated system, which matches the typical situation of fields in the region. There are also differences between native and exotic species in rooting depth, C3-C4 biomass proportions, and aboveground biomass production (peak biomass). We are measuring aboveground net primary productivity by estimating biomass by species in June and October of each year in each mixture and monoculture plot, and this sampling regime will continue for the proposed project. Here, we propose to measure the following C cycling variables: 1) belowground net primary productivity using ingrowth cores, 2) soil bacterial and fungal community structure and enzyme activities, 3) rate of infection by mycorrhizae and total fungal biomass, 4) N mineralization (NH4+ and NO3-), 5) decomposition of root and shoot litter from plots in mesh bags, and 6) soil C and root biomass in 10-cm depth increments to a 100 cm depth. We will also test whether these conversions have affected the variance in ecosystem process rates, which can be associated with unreliable production of ecosystem services. Variability in C cycling will be quantified with measures of coefficient of variation in productivity over time and with recovery from a major drought. The proposed work directly addresses Theme 3 ‘Belowground Carbon Processes and Soil Carbon’ of the Carbon Cycle Science request for proposals. Results from the study will be useful to modelers and land managers by quantifying the costs and benefits of planting native vs. non-native species under an altered climate. We have an excellent team of investigators assembled to address these issues, including two grassland ecosystem ecologists (Wilsey and Polley), and a microbial ecologist (Hofmockel).