Nitrogen partitioning among microbes and two plant functional groups in the shortgrass steppe

Background/Question/Methods Most of the biomass and biological activity of grasslands occurs belowground, and we are just starting to understand complex plant-microbial interactions in these systems. In particular, semi-arid grassland are known to be primarily water and N limited, therefore, plants and microorganisms are adapted to tolerate very low soil resources. The objective of our work was to determine nitrogen partitioning among two dominant plant functional groups (C3 and C4 grasses), and microbes in the short grass steppe. To accomplish this, we measured a number of N pools during the 2006 growing season, including plant biomass, plant shoot and root N,  soil inorganic N, and microbial biomass N and C in soils underneath patches dominated by the two plant functional groups.

Results/Conclusions We expected peaks in N uptake by C3 grasses during the cooler months, and peaks of C4 grasses N during the warmer months. We expected a peak in microbial biomass N during the warm wet month (June to August in the short grass steppe). Preliminary results showed peaks in microbial biomass C and N in July, following a peak of rainfall above the precipitation historical average. Microbial biomass C and N was constant for the rest of the growing season, and did not differ between soils under C3 or C4 plants Nitrogen in C3 grasses showed two peaks, in July and September; C4 grasses also showed two peaks but in June and August 2006. We attributed the peak in June to the use of N stored in plant biomass from previous growing seasons. The results of this study will allow us to assess the potential for competition between microbes and plants in the short grass steppe.