Soil organic carbon and nitrogen in a Minnesota soil as related to tillage, residue and nitrogen management
Introduction
Global increases in mean air temperature have been observed by numerous researchers. Schlesinger (1997) outlined the suspicion of many scientists that at least part of these increases may be due to the documented increase in the concentration of so called “greenhouse gases” in the atmosphere, which trap infrared radiation and increase the atmosphere's ability to absorb heat. The gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are the most important anthropogenetically-related greenhouse gases and are of special interest to agricultural scientists. Carbon dioxide is both a product of respiration and a byproduct of the combustion of fossil fuels, while N2O losses are apparent in fertilizer and manure applications. An informed assessment of the role production agriculture plays in global climate change requires knowledge of C and N cycling.
The cultivation of prairie soils in the Cornbelt has reduced total soil N and C (Allmaras et al., 2000). Conventional plowing increases soil aeration and soil-residue contact and hastens the oxidation of soil organic carbon (SOC), leading to increased emissions of CO2 (Lal et al., 1998, West and Post, 2002). Conservation tillage practices maintain residue near the surface of soils, reducing wind and water erosion, increasing soil water storage and often reducing agricultural production costs (Allmaras and Dowdy, 1985). Lamb et al. (1985) found N losses due to erosion and leaching minimized by switching from plowing to minimum or no-tillage winter wheat (Triticum aestivum L.) systems. Lal et al. (1998) states that widespread modification of agricultural practices could result in more C sequestered in soils than agriculture generates through combustion of fossil fuels and conventional land use practices.
Tillage and related crop residue management practices have a major influence on C sequestration (Havlin et al., 1990, Kern and Johnson, 1993, Reicosky and Lindstrom, 1993, Robinson et al., 1994, Dao, 1998, Wander et al., 1998, Duiker and Lal, 1999, Deen and Kataki, 2003). However, tillage practices influence only the upper portion of the soil profile, causing stratification of residues dependent on the depth of tillage and the type of tillage tool (Allmaras et al., 1996). Potter et al. (1997) found that no-tillage increased mean total C mass in surface soils (0–20 cm) under continuous sorghum (Sorghum bicolor L.) and wheat. Wander et al. (1998) and Deen and Kataki (2003) both found increased SOC in surface soils of no-tillage treatments, but generally less SOC stored at lower soil depths, when compared to conventional tillage. These recent studies suggest conservation tillage practices may not always lead to net accumulations of SOC and N in the soil profile as a whole, as earlier studies have implied (Kern and Johnson, 1993, Lal et al., 1998, Moldenhauer et al., 1995, Doran et al., 1998).
Jenny (1933) stated that total soil N in grassland soils would reach a steady state only after decades of cultivation. More recently, West and Post (2002) described changes in soil C sequestration rates due to tillage over decades before coming to equilibrium. Both long-term studies and observations of the soil profile as a whole would seem vital to a comprehensive evaluation of the role management practices have on SOC and N storage and potential greenhouse emissions to the atmosphere. As such, the objectives of this experiment were as follows:
- 1.
Determine the effects of tillage and residue management on soil profile C and N storage.
- 2.
Determine the effects of N fertilization rates on soil profile C and N storage.
- 3.
Observe treatment effects on other soil properties, including bulk density and δ13C.
Section snippets
Materials and methods
Our study was conducted as part of long-term field experiments at the UMORE (University of Minnesota Outreach, Research and Education) Park in Rosemount, Minnesota (latitude 44°45′N, longitude 93°4′W). Nitrogen fertilization, tillage and residue management treatments were established in 1980 on the site's Waukegan silt loam (fine-silty over skeletal, mixed, superactive, mesic Typic Hapludoll). Treatment blocks were 18 m × 50 m in size and randomly selected over an area with less than 1% slope. In
SOC and N
Soil organic carbon (SOC) and soil nitrogen (N) responses to tillage, stover residue management and N fertilization treatments were essentially the same. In the top three soil depths (0–5, 5–10 and 10–15 cm), conservation tillage practices stored significantly more SOC and N than conventional tillage (p < 0.01, Table 1). Error bars shown in all figures indicate variation around the mean ± 1 S.D. Specifically, the order of stored SOC and N was NT > CH > MB for each depth to 15 cm (Fig. 1, Fig. 2). Soil N
Discussion and conclusions
Long-term (20+ years) use of conservation tillage practices influenced the distribution of SOC and N in the soil profile. Soils near the surface had more SOC and N stored in NT and CH systems as compared to the MB system. However, below 20 cm, the MB treatment retained more stored SOC and N than in conservation tillage practices. The net result of these differences in the distribution of SOC and N was that when observing the soil profile as a whole, or at least in the upper 45 cm, no differences
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