Does a C3-C4 Forage Mix Simultaneously Improve Forage Production and Carbon Sequestration?

University of Wisconsin-Madison graduate student, Herika Kummel, conducted an experiment in two restored prairies in southern Wisconsin to assess their carbon sequestration potential under a gradient of warm-season grass (C4) to cool-season (C3) pasture grass ratios. The sites were the Bison Ridge Ranch in Marquette County and the Wisconsin Integrated Cropping Systems Trial (WICST) at the University of Wisconsin–Madison’s Arlington Agricultural Research Station in Columbia County. 

The project’s goal was to improve understanding of ecosystem support, provisioning, and regulating services provided by pasture ecosystems in the Upper Midwest. This work intended to provide much needed understanding about cool-season pastures (C3) and the benefits of re-storing native grasses to working lands.

Materials/Methods

At each site, Kummel chose thirty ~100-m2 plots for their respective C3:C4 grass ratio. At each plot, a centralized quadrat was permanently marked to monitor soils, environmental conditions, nutrient content, and respiration over time. Plant species cover was estimated with the line-point method performed on permanently marked soil monitoring stations. The quadrat area to be sampled was divided by 5 horizontal and vertical lines forming 25 intersections where the first intercept of a sharpened rod with any part of herbaceous vegetation at each intersection was recorded.

For each sampling event, total species cover was calculated as total species hits divided by the total possible hits for each quadrat. Plants were identified to the species level and grouped into functional groups. Kummel mirrored the classification method used in previous prairies studies. Plant cover data per species and functional groups were taken three times during the season and aggregated to calculate an annual average of cover for each experimental unit.

The two main dominant C4 grass species were the same for both sites, e.g. Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash; however, there were differences in the plant communities worth noting. For instance, Bison Ridge Ranch had other species of C4 grasses present. The dominant C3 grass species in both sites were also quite similar. Yet, only Bison Ridge Ranch had the winter annual Bromus tectorum L. and only WICST had Elymus canadensis L. and Phleum pratense L. present.

Kummel used linear regression to address potential relationships between net ecosystem production (NEP) and vegetation cover by functional group, by plant species cover, and then by plant species richness. Cook’s distance test, which measures the influence of individual observations on the regression coefficients, was assessed to identify potential outliers. Furthermore, the fit of a quadratic function was compared to a linear fit using the generalized least squares algorithm in S-plus. Models were compared with likelihood ratio tests. When significant differences were determined, the model with the lowest Akaike’s information criterion values was chosen, otherwise the simpler model was determined to be the better fit.

Results/Outcomes

There are several conclusions in this project that can guide land managers and policy makers regarding mixed C4-C3 grasslands.

At the management level, the July C4 grass cover can guide land managers in assessing the productivity of the grassland before the end of the season when the grasses are very tall and the heat strong. To Kummel, it seems also that low-diversity grasslands that are mainly dominated by productive warm-season grasses, such as Conservation Reserve Program lands, may prove to be a great alternative to improve soil organic carbon (SOC). At the same time, a low cover of cool-season grasses in restored prairies may not threaten the potential of the restoration to rebuilt SOC if the C4 grasses dominate.

Kummel’s results also show that the higher carbon sequestration in consequence of a higher cover of warm-season grasses is not general across sites and nutrientpoor permanent grasslands may need to be moderately intensified either by increasing organic carbon input or moderate fertilization. In a positive note, many forage species such as Panicum virgatum L. have been studied as good sources of bioenergy, and according to this study, more attention could also be given to the potential of Andropogon gerardii.

This project shows that research at the farm level, across various soil types and managements still necessary if the carbon sequestered by mixed C4-C3 grasslands is to be considered as an effective strategy to long-term carbon storage and a key part to climate stabilization.

Publications/Outreach

The information gathered during this multi-year study has been presented at two winter field days, one at the Lancaster Research Station and another at the WICST annual meeting. Posters summarizing the field data collection were presented at the 93rd Annual Meeting of the Ecological Society of America, the Wisconsin Ecology Group at the Fall Symposium in Madison, Wisconsin in 2008, and the Farming with Grass meeting sponsored by The Soil and Water Conservation Society in 2008.

Want more information? See the related SARE grant(s) GNC07-077, Does a C3-C4 forage mix simultaneously improve forage production and carbon sequestration?.