Long-Term Tillage Studies: What's New and What's Surprising?.

Conservation tillage practices (e.g. reduced or no-tillage) have been promoted for mitigating atmospheric CO₂ levels by enhancing soil organic carbon (SOC) storage in agricultural landscapes.  Conservation tillage can lead to higher SOC in surface soils (<30 cm), but when the whole soil profile is considered, the net difference in SOC stocks is often equivocal.  Six different tillage practices ranging in intensity from no-till to moldboard plow have been evaluated since 1981 in continuous corn, continuous soybean, and corn-soybean systems at a long-term field experiment in eastern Nebraska.  During the first 22 years of management (1981-2004), SOC accrual occurred in surface soils under all tillage treatments, with greatest gains under continuous corn, then corn-soybean, and smallest gains under continuous soybean.  On a whole soil basis (0-150 cm), SOC accrual was evident for soils under no-till and disk tillage, with no change occurring under the most aggressive practice of moldboard plow.  Since 2004, SOC stocks declined in surface soils, though levels remained significantly above 1981 baseline values.  Changes in farm equipment (e.g. method of N application, updated tillage implements) may have contributed to recent SOC losses.  While results here are specific to this site, coordinated data sets from multiple long-term studies are necessary to evaluate overall management impacts and agroecosystem potential to sequester C.  Data networks such as USDA-ARS’s Greenhouse gas Reduction through Agricultural Carbon Enhancement Network (GRACEnet) help fill this critical need by (1) providing empirical data to the scientific community at large, and (2) facilitating the calibration and validation of computer models used to simulate regional and national impacts of agricultural management practices.