Environmental Impacts of Converting Cropland to Short-Rotation Woody Crops

Dev Joslin, Tennessee Valley Authority, Norris, TN

[Transcribed from tape of presentation]

Abstract

A few years ago the TVA and the Oak Ridge National Laboratory Biofuels Feedstock Development Program decided to look at the feasibility of growing short rotation woody crops to co-fire with coal in TVA’s (Tennessee Valley Authority) power plant. Such an effort would require converting thousands of hectares of crop land to grow short rotation woody crops. An environmental impact of converting the land to SRWC would need to be considered first.

Comparing non-woody crops to woody crops was the major focus, but also evaluating an entire rotation of short rotation woody crops to quantify sediment production, nutrient runoff, wildlife impact, groundwater impact, and soil quality impact was also an important concern. This is not only a comparison between crops and trees but a quantitative look at what is happening to a rotation.

The program is sponsored by TVA, the University of Tennessee, Alabama A&M University, Mississippi State University, the Oak Ridge Biofuels Feedstock Development Program, and Agenda 2020, which is funded by DOE along with AF&PA and some of the forest products companies in AF&PA.

Three sites were chosen based on portions of the Tennessee Valley Region that economic analysis showed short rotation woody crops might, at some point, be able to compete with other crops. At each site, species were chosen that were appropriate for that particular site given soil resources, the land, whether it was bottom land or upland, and compared to an agriculture row crop that was typical to that region.

Cultural practices that were typical to the region included no-till for corn, silage corn, sycamore, sweetgum, and cottonwood, and till for cotton. Herbicides were used on all sites. Nitrogen/ phosphorus fertilizer was applied to the corn and cotton during the first year. The second year all crops received nitrogen/phosphorus fertilization.

Experimental setups consisted of pentagons pointing down slope to a flume so the amount of flow of runoff water could be measured and water samples collected. Four hand lysimeters were installed 4.5 ft below the surface to collect gravitational water moving into the groundwater table. Water flow was monitored in the flumes. At each site there are berms around the plots to make sure the water was diverted to the flumes. Soil samples were collected before initialization of the study to evaluated the baseline chemistry and physical properties of the soil.

For the goundwater portion at 4.5 ft, which is considered to be approximately the bottom of the rooting zone, lysimeters were installed to catch water percolating down, which is collected in a bottle and pumped back to the surface.

Erosion crops are generally more erodible than tree crops, with spring and fall being a vulnerable period for erosion. Some surface protection is important on any amount of slope that exists. Cover crop strips of 4 feet in width were effective on sweetgum in controlling erosion and they do not compete for moisture and nutrients the first two years.

Runoff of nitrate, ammonium and bio available phosphorus occurred after fertilization in the spring. Nitrate runoff was higher under the agricultural crops and ammonium runoff was higher under the trees. Phosphorus runoff varied with each site. There were large peaks of groundwater nitrate losses following spring fertilization and was greatest under agricultural crops.

The hypothesis for the future is that trees will make more efficient use of the fertilizer than agricultural crops, which take longer to get established. Also, trees will require smaller third and fourth year fertilizer additions. These conditions will result in less runoff and less groundwater contamination for trees as compared to agricultural crops. A buildup of the litter layer with some minimal weedy ground cover will further reduce erosion under short rotation woody crops in later years.

File posted on March 17, 1998; Date Modified: February 21, 1999