Agronomy and Horticulture, Department of


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A THESIS Presented to the faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agronomy, Under the Supervision of Professors Daniel T. Walters (deceased), Charles A. Shapiro, and Stephen C. Mason. Lincoln, Nebraska: December, 2010
Copyright 2010 Agustin Martellotto


Soil organic carbon (SOC) stock is controlled by many factors, but for given conditions it is the long term summation of the balance between inputs and outputs. Management practices will alter this balance by affecting the system’s productivity and the speed of residue and soil organic matter decomposition. Given that annual changes in SOC are generally small, compared with the large and variable SOC background, long-term experiments, and soil samples taken at the start of experiments (archived samples) are necessary to determine SOC trends over time. Changes in SOC were analyzed for two long-term experiments; one an irrigated site at Mead, NE (1997) with continuous corn (CC) (Zea mays L.) and corn-soybean [Glycine max (L.) Merr.] (CS) rotation) and 3 nitrogen (N) application rates (0, 100 and 300 kg ha-1); and the other a rainfed site at Concord, NE, (1985) with three tillage treatments (no-till, NT, disk, DK, and moldboard plow, MP), two crop sequences (CC and CS) and three N application rates (0, 80 and 160 kg ha-1). Soil samples taken at early stages of the experiments were used as a benchmark to analyze carbon trends over time. Results showed that in spite of considerable differences, C inputs did not affect changes in SOC. Nitrogen fertilizer application resulted in a 3% increase in SOC, but required 24 years to generate detectable differences in the surface 400 kg of soil m-2. Over the last 12 years, MP lost 1.52 ±0.4 kg of C m-2 while NT lost 0.73 ±0.4 and DK lost 0.76 ±0.4 kg of C m-2 in the 1200 kg of soil m-2 profile. None of the evaluated treatments under the conditions of these experiments were able to sequester atmospheric C since all lost SOC over time. The use of archived samples made possible the determination of the rate of SOC change over time, and allowed an accounting for natural soil variability.