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United States Department of Agriculture

Agricultural Research Service

2007 Annual Report

1a.Objectives (from AD-416)
Quantify the impact of agricultural practices and environmental changes on surface/atmosphere exchange of greenhouse gases (GHG) in order to develop farming systems that reduce global warming potential (GWP) and promote soil C sequestration; Develop farming systems that permit the removal of biomass for energy production while protecting soil resources; Identify and overcome agronomic impediments to the adoption of farming practices, such as reduced tillage, cover crops, and companion crops, that are developed to reduce GWP and permit stover harvest.

1b.Approach (from AD-416)
We will participate in a multi-location effort to identify farming practices that will help slow the increase in atmospheric concentrations of the greenhouse gases CO2, N2O, and CCH4. Our approach will include continuous, field-scale measurement of the surface/atmosphere exchange of all three gases in three adjacent fields under different management. Parallel plot-scale studies will be conducted with chamber-based gas exchange measurements to permit testing of a broader variety of tillage, nitrogen (N) fertility, and rotation strategies. In the second principal area of inquiry, we, again in cooperation with other ARS locations, will examine the soil sustainability of harvesting corn stover for ethanol production. Our goal is to test the hypothesis that cover or companion crops can fill the role of the removed stover in supplying carbon (C) compounds to maintain soil organic matter. We will explore the use of forage digestibility analyses to characterize the quality and quantity of C compounds contained in corn stover and in cereal rye, kura clover, and selected other cover crops. The third component of this project will focus on identifying and correcting practical, agronomic impediments to adoption of the practices mentioned above. In the upper Midwest, the major hindrance to wider use of cover crops, companion crops, and reduced tillage has been the perception that they will reduce the yield of the subsequent crop, due to such factors as cold, compacted spring seed bed conditions and adverse effects on N availability. We will test and refine theories describing near-surface heat and water flow and develop sensors to more easily measure soil bulk density. We will also conduct plot-scale studies of the effects of reduced tillage and cover crops on N losses by leaching and gaseous emissions. The results of this research will facilitate the development of better reduced tillage and cover crop systems for northern soils.

3.Progress Report

Biennial tillage maximizes C storage in corn/soybean systems in the Upper Midwest. Climate change and soil quality considerations mandate identification of farming systems that favor retention and buildup of soil carbon, while maintaining profitability. We conducted deep sampling (60 cm) of long-term tillage trials in Minnesota and found that the system resulting in greatest C storage in corn/soybean rotations was one with biennial tillage, i.e. chisel plowing in the fall following corn harvest, with no tillage following soybeans. This system also produces the highest overall yields. These results, demonstrating that a tillage practice of intermediate intensity was able to accumulate more soil carbon compared to both more intensive and less intensive tillage, have important implications for scientists and policy-makers involved in the development of strategies for minimizing the contribution of agriculture in the upper midwest to atmospheric CO2 levels. NP202, Problem Statement 5: Adoption and Implementation of Soil and Water Conservation Systems.

Does reduced tillage consistently sequester C? Policy makers have been trying to identify and encourage farming practices that sequester C, and the most frequently mentioned practice is conservation tillage. We examined the issue of tillage impacts on soil carbon sequestration and found that in nearly all reported cases where conservation tillage was found to store C, soils were only sampled to a depth of 30 cm or less, despite the fact that crop roots often extend much deeper. In the relatively few studies where sampling extended to a depth greater than 30 cm, conservation tillage has shown no consistent C benefit, with near-surface C gains offset by subsurface C losses, relative to conventional tillage. This may be caused by tillage-induced differences in soil physical properties, such as temperature and bulk density, that cause shallower rooting in reduced-tillage fields than in conventionally-tilled fields. While there are good reasons to promote reduced tillage, particularly to control erosion and reduce fuel use, it does not appear to be a consistently effective means for sequestering C. NP202, Problem Statement 3: Soil Carbon Measurement, Dynamics and Management.

An improved method for measuring near surface soil heat flow. The design of improved reduced tillage, cover crop, and companion crop systems would be facilitated by more accurate measurements of heat and water flow near the soil surface. Our experiments showed that the conventional method for measuring soil heat flow often resulted in underestimates of ~25%. We also demonstrated that commonly accepted methods for determining soil heat storage could lead to gross errors under some conditions. The impact of these accomplishments lies in their potential to improve the accuracy of soil heat flow measurements in agronomic and ecological experiments. NP202, Problem Statement 5: Adoption and Implementation of Soil and Water Conservation Systems.

New Isotope Techniques for Studying Carbon Processes. One of the difficulties in identifying C sequestration opportunities is separating the measurement of photosynthesis and respiration. Stable isotope measurements offer promise here, because plants discriminate against the heavier 13-C isotope in photosynthesis, but not in respiration. We applied a new technique, tunable diode laser spectroscopy to obtain the first continuous measurements of field-scale exchange of both 12- and 13-CO2 simultaneously. In this way we were able to partition between photosynthesis and respiration in a corn/soybean rotation. This method will provide valuable data for constraining global scale models of carbon cycling. NP202, Problem Statement 3: Soil Carbon Measurement, Dynamics and Management.

5.Significant Activities that Support Special Target Populations

6.Technology Transfer

Number of web sites managed1
Number of non-peer reviewed presentations and proceedings14
Number of newspaper articles and other presentations for non-science audiences2

Review Publications
Venterea, R.T., Baker, J.M., Dolan, M.S., Spokas, K.A. 2006. Carbon and nitrogen storage are greater under biennial tillage in a Minnesota corn-soybean rotation. Soil Science Society of America Journal. 70:1752-1762.

Heitman, J.L., Horton, R., Ren, T., Ochsner, T.E. 2007. An improved approach for measurement of coupled heat and water transfer in soil cells. Soil Science Society of America Journal. 71:872-880.

Zhou, J., Heitman, J.L., Horton, R., Ren, T., Ochsner, T.E., Prunty, L., Ewing, R.P., Sauer, T.J. 2006. Method for maintaining one-dimensional temperature gradients in unsaturated, closed soil cells. Soil Science Society of America Journal. 70:1303-1309.

Olmanson, O.K., Ochsner, T.E. 2006. Comparing ambient temperature effects on heat pulse and time domain reflectometry soil water content measurements. Vadose Zone Journal. 5:751-756.

Ochsner, T.E., Sauer, T.J., Horton, R. 2006. Field tests of the soil heat flux plate method and some alternatives. Agronomy Journal. 98:1005-1014.

Venterea, R.T. 2007. Nitrite-driven nitrous oxide production under aerobic soil conditions: Kinetics and biochemical controls. Global Change Biology. 13:1798-1809.

Griffis, T.J., Zhang, J., Baker, J.M., Kljun, N., Billmark, K. 2006. Determining carbon isotope signatures from micrometeorological measurements: implications for studying biosphere-atmosphere exchange processes. Boundary Layer Meteorology. 127:295-316.

Baker, J.M., Ochsner, T.E., Venterea, R.T., Griffis, T.J. 2007. Tillage and carbon sequestration: what do we really know? Agriculture, Ecosystems and Environment. 118:1-5.

Russelle, M.P., Morey, R., Baker, J.M., Porter, P.R., Jung, H.G. 2007. Comment on "Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass". Science. 316:1567b.

Last Modified: 10/13/2015
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