2007 Annual Report
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.
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.