Effects of Landuse Change from Grassland and Wheat to Switchgrass in the Southern Great Plains.D.P. Billesbach1, M.S. Torn2, J.A. Bradford3, S.A. Gunter3, M.L.

Fischer2, 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 initiated an experiment to explore the environmental effects of switchgrass (Panicum vergatum L.) production on two Southern Great Plains ecosystems. One initial aspect of our program is to examine the impacts of

landuse change (from current wheat and pasture systems to switchgrass production) on the carbon and water cycles. As our switchgrass stands develop and mature, we will also gain a better understanding of the carbon and water dynamics of production-scale systems at equilibrium, and how this new crop and management regime may change longer term water

recharge dynamics 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 would be 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 immediate killing of the emerging wheat and re-planting to switchgrass. However, with the emergence of the switchgrass (and some hearty 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 steep, linear source term between DOY 220 and DOY 240 may be due to inadequate gap filling during an extended instrument failure. We also note that late in the season, the cumulative NEE leveled off. There may be several reasons for this. First, the soil at this site is very low in organic content, and substrate for respiration may have become exhausted. Also, the relatively warm autumn seemed to stimulate some cold season C3 grasses and weeds to re-emerge, possibly balancing the small respiratory component.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.