Variation in soil organic carbon over time in no-till versus minimum tillage dryland wheat-fallow

Interest in soil organic carbon (SOC) levels is focused on both soil quality and sequestering carbon from the atmosphere. Farmers have developed minimum tillage systems and no-till systems. This study carefully measured SOC at three sites where long-term, randomized, replicated studies compared minimum tillage and no-till ro-tations in the low-precipitation inland Pacific Northwest, USA. To overcome seasonal, annual, and rotational effects, a soil sample was taken from each plot monthly for three years. The top 250 kg m-2 dry soil mass (representing the 0-to 20-cm depth) from each core was analyzed for SOC. Averages of monthly samples revealed the tilled treatment had 7.21 g kg-1 of SOC compared to 7.04 for the no-till treatment (p < 0.004). Examining variation month-to-month, 93% of sample variation was due to sample date, and 7% due to blocks (replications) within sites. Of 77 individual site-by-date samples, only six produced significant t-tests (p < 0.05) but SOC differences were 46-31 in favor of tillage and those 46 had larger differences. Power calculations estimated 34-100 samples would be required to detect a relative difference of 5% given the data's standard deviation. Soil water and soil temperature to 30 cm was measured at the time of sampling, and while no-till more often had greater water, temperature differences were inconsistent. We conclude that no-till did not result in more soil carbon in these systems and judicious tillage is an option for sustainable production.