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Title:   Author  item
                                            Baker, Johnitem
                                            GRIFFIS, TIMOTHY - UNIV. OF MINNESOTA
  Submitted to: European Geophysical Society; American Geophysical Union; European Union of Geosciences
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 2/15/2003
                                    Publication  Date: 4/2/2003
                                    Citation: Baker, J.M., Griffis, T. 2003. A paired flux approach to study the carbon balance of corn/soybean rotation. European Geophysical Society; American Geophysical Union; European Union of Geosciences. EAE03-A-13131.Interpretive Summary:
                                Technical Abstract: There is intense interest in finding ways to damp projected increases in atmospheric CO2 by adopting strategies that alter local rates of surface/atmosphere carbon exchange in ways that favor increased surface storage. It is generally accepted that row crop agriculture has historically been a contributor to the debit side of the terrestrial carbon ledger. However, changes in farming practice, primarily reduction of tillage, appear to have arrested this decline. The very fact that so much soil carbon was lost is now viewed positively, in the sense that it may represent a reservoir that can be refilled through the adoption of appropriate farming practices, but verification of carbon gain is a difficult problem.  Soil sampling is the accepted standard, but it has a low signal to noise ratio and poor temporal resolution so it offers little insight into causes and effects or underlying processes. Micrometeorological methods address these shortcomings, replacing them with a new set of problems. Temporal resolution is superb, but determination of net carbon gain or loss requires integration of short-term (e.g. half-hourly) flux measurements over at least one full cycle of the system under test- typically a year or more. Unfortunately, data are missed due to instrument failures and power outages, and other data must be discarded because theoretical assumptions of turbulent transport are not met. As a consequence, the end sum of net carbon exchange from these sites depends substantially on the gap-filling strategies that are used. Nonetheless, valuable information can be obtained by measuring differentially, i.e.- making simultaneous flux measurements in two adjacent fields that are subjected to the same weather conditions, but with specific differences in farming practice. Insights into the differences in carbon balance between the systems, even in the face of the inevitable data gaps, can be obtained by considering only those time periods in which both fields are producing acceptable data. We used this approach to examine a common crop rotation in the Midwestern United States, alternation of corn and soybean. One field was farmed conventionally, while in the other we employed two practices expected to favor carbon gain: reduced tillage and the use of a spring cover crop prior to soybean. Carbon exchange rate and latent heat flux were measured by eddy covariance in both fields for an entire year, dating from harvest of corn to harvest of soybean. Data were screened to remove all time periods when either of the two fields did not meet acceptance criteria. Differential analysis of the remaining data showed that the alternative practices did indeed have a discernible impact.  With respect to carbon balance, the effect of reduced tillage was subtle but positive, in the form of lower respiration losses during fall and early spring. The impact of the spring cover crop (oats) was much more significant. As for water balance, there was almost no difference in cumulative latent heat flux; higher offseason evaporation from the conventionally-tilled field was nearly offset by the higher ET from the spring cover crop, relative to the bare soil in the conventional treatment. Soybean yields were also nearly identical for the two fields. We conclude that these practices can improve carbon balance, with little or no impact on yield or annual water use.


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