Carbon and Water Cycling in Southern Great Plains Ecosystems Converted to Switchgrass Production.

D.P. Billesbach1, M.S. Torn2, J.A. Bradford3, S.A. Gunter3, and Chris Zou4

1University of Nebraska 2Lawrence Berkeley National Laboratory 3USDA-ARD Southern Plains Range Research Station 4Oklahoma State University

Introduction and Background

The ARM Carbon group has always had an interest in the linkages between the carbon and water cycles and land use and land management in the Southern Great Plains. This project continues the theme with an emphasis on land use change.

Biofuels in general and ethanol in particular are becoming and will continue to be an important part of our national energy strategy. It is therefore vital to understand their potential impacts on the environment; specifically on carbon and water cycles. Currently, the most heavily used feedstock for ethanol production is corn. There are, however, issues that suggest other feedstocks may be a better choice. Some of the positive aspects of corn ethanol are:

• Corn is very plentiful in the U.S. It is often a surplus commodity.• The production technology for corn feedstock is very mature.• The production technology for ethanol distillation from corn is also mature.• A large infrastructure already exists (or is developing) for both corn production and corn ethanol distilling.

While some of the negative aspects are:

• There are arguments that corn ethanol production potentially removes product from the global human food chain as well as cultivatable acres.• Corn production is relatively water-intensive and the resulting energy gain is thought by some to be minimal.• Corn production requires relatively intense land management, including cultivation, irrigation, and fertilization.

Another strategy for ethanol production is the usage of cellulosic feedstocks rather than grains. Some of the positive aspects of this approach are:

• Cellulosic feedstocks are very plentiful and cheap. Many current byproducts can be used.• Cellulosic feedstocks don’t necessarily remove land from food production.• Many cellulosic crop feedstocks are much more water efficient relative to corn.• Cellulosic feedstock crops can be successfully grown using low-impact farming methods (annual grasses, very low fertilization requirements,

minimal tillage).

There are, however some negative aspects as well:

• Cellulosic ethanol production is an immature technology that requires an extra “enzymatic” step compared to corn ethanol.• Currently, cellulosic feedstock producers would realize a very small profit margin from converted fields.• Conversion of currently cropped (or idle) land requires a transition period before full productivity is realized.

Despite the pros and cons, we believe that cellulosic ethanol production will become a significant part of future energy policy. Because of this, we are seeking a better understanding of the impacts that this emerging industry may have on the air, water, and soil. We chose switchgrass (Panicum vergatum L.) as a likely candidate crop in the southern Great Plains (SGP). Our initial interest is focused on the impacts that land use change (from current cropping systems to switchgrass) will have on carbon, water, and energy fluxes. As our switchgrass stands develop and mature, we will also gain a better understanding of the carbon and water dynamics of production-scale systems, and how this new crop and management regime may change water recharge and soil chemistry.

Experiment

To more fully understand the range of carbon and water dynamics associated with switchgrass production, we chose two different systems for our study. The first was a marginal wheat field near Ft. Supply, OK (36º 38’ N 99º 35.8’ W 645 m AMS). The second was a pasture consisting of native warm season (C4) perennial grasses and forbs located in Woodward, OK (36º 25.6’ N 99º 25.2’ W 610 m AMS) These two plots represent current land usages that would be strong candidates for switchgrass production. They also represent two different conversion strategies with the wheat field being the simpler of the two. These two fields are owned and managed by the USDA-ARS Southern Plains Range Research Station in Woodward, OK.

The conversion process for the wheat field began in March of 2009 when the field was sprayed with a glyphosate herbicide to kill the emergent wheat (from the previous fall planting). This was followed with no-till planting of switchgrass seed (variety Alamo). In late June, a weed control herbicide was applied to reduce competition with the emerging switchgrass seedlings. Spot replanting of under-germinated zones and further broad leaf control will be considered in the spring of 2010.

The pasture was burned in March of 2009. Cattle were introduced at a high stocking density in early May, and a “grazing friendly” herbicide (Grazon) was applied in early June to kill broadleaf species. This treatment was designed to reduce the original pasture species to non-competitive levels. The same variety (Alamo) of switchgrass seed will be sown in the spring of 2010 using a no-till planter followed by broad leaf control if necessary.

Flux towers were installed by University of Nebraska in each field and became operational at the end of April, 2009. At the same time, monthly biomass sampling was begun by the USDA-ARS group to characterize above ground biomass growth and leaf area index (by functional groups). In mid September, a network of soil moisture sensors will be installed by the Oklahoma State University collaborators in the former pasture. These, along with an existing flume will be used to evaluate the effects of production switchgrass on groundwater recharge and runoff.Additionally, extensive soil sampling was undertaken in September of 2009 in both fields by the Lawrence-Berkeley National Laboratory group to characterize the physical and chemical properties of the soils. These measurements will be repeated as needed to monitor effects due to the land use change.

Results

As can be seen in the graphs, cumulative net ecosystem exchange (NEE) behaved as we expected. In the pasture (blue line), the system quickly became a strong sink for carbon as the well established grasses and forbs became active in the spring. The application of herbicide and the over grazing, however quickly curtailed the net uptake of CO2 and the field became a source of carbon to the atmosphere. This phase is to be regarded as one of the carbon “costs” of converting an established pasture to switchgrass production.

The wheat field (represented by the red line) remained a carbon source for much longer. This was due to the killing of the emerging wheat and re-planting to switchgrass. However, with the emergence of the switchgrass (and some hardy weeds), the field eventually did become a CO2 sink. The trend reversal to a source near DOY 200 was due to dry soil and is thus regarded as climatically driven.

The details of these beginning states may very well be specific to the fields under study and the management techniques (and timing) used. They do however; reveal a glimpse of the carbon costs associated with the land use change necessary for cellulosic biofuel production. Future observations will reveal the trajectories that both of these fields take in evolving from their transient states to equilibrium production systems.

This research was supported by the Office of Biological and Environmental Research of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, as part of the Atmospheric Radiation Measurement Program.