Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2004
Publication Date: 6/3/2005
Citation: Gollany, H.T., Allmaras, R.R., Copeland, S.M., Albrecht, S.L., Douglas, C.L. 2005. Tillage and nitrogen fertilizer influence on carbon and silica distribution in a mollisol of the Pacific Northwest. Soil Science Society of America Journal 69:1102-1109. Interpretive Summary: Crop production systems that promote carbon storage and improve soil properties, such as increased infiltration, are desirable. Long-term experiments are ideal for evaluating the influence of agricultural practices on soil organic carbon accumulation. A long-term fallow-wheat experiment with two nitrogen rates (40 and 160 lb/acre) and two tillage systems (moldboard plow and sweep) were established in 1940. The objectives of this study were to: 1) determine the effect of tillage and nitrogen fertilizer on soil organic carbon accumulation in a Walla Walla silt loam soil, and 2) examine the impact of fine organic matter on soil acidification and silica movement in these soils. After 44 years of nitrogen fertilizer addition, soil organic carbon storage for the 160 lb/acre rate had increased 3% compared to the 40 lb/acre rate. In the top 24 inches of soil, fine organic matter for the sweep tillage was 14% higher than moldboard plow with the 160 lb/acre nitrogen application rate. The higher nitrogen application rate acidified the topsoil and increased silica solubility and leaching from the topsoil for both tillage systems. This movement and concentration of silica below the topsoil results in the development of a hard layer below the plow depth that reduces water infiltration. Total water-soluble silica below the plow depth was 34% and 39% greater than in the topsoil for the moldboard plow and sweep systems, respectively. Soil organic carbon accumulation increased with increased application of nitrogen fertilizer. However, long-term use of nitrogen fertilizer increased soil acidity. Interaction of tillage and nitrogen fertilizer with water-soluble silica suggests that soil organic carbon provides a mechanism to suppress silica solubility in topsoil and could improve water movement and infiltration in these soils.
Technical Abstract: Long-term experiments are ideal for evaluating the influence of agricultural practices on soil organic carbon (SOC) accretion. Little is known about the influence of tillage and nitrogen (N) fertilization on SOC distribution and silica (Si) movement in a soil. This study: i) determined the effect of tillage and N fertilizer on SOC accretion in a Walla Walla silt loam soil (coarse-silty, mixed, superactive, mesic Typic Haploxeroll), and ii) examined the subsequent influence of fine organic matter (FOM) on silica movement. A long-term fallow-wheat (Triticum aestivum L.) experiment with two tillages (moldboard plow, MP; and sweep, SW) and two N rates (45 and 180 kg N/ha) was established in 1940, in a randomized block with split-plot design and three replications. Soil cores (2-cm depth increments) were used to measure coarse organic matter (COM), FOM, pH, bulk density, water-soluble C, and water-soluble Si (Siws). The FOM fraction for the SW (6.6 kg C/m^2) was 14% higher than the MP (5.8 kg C/m^2) for the 180 kg N/ha in the 0- to 60-cm depth. After 44-yr of N addition, the SOC storage for 180 kg N ha-1 rate (6.2 kg C/m^2) had increased only 3% compared to 45 kg N/ha rate (6.0 kg C/m^2). Total Siws in the B horizon were 34% and 39% greater than in the Ap horizon for the MP and SW systems, respectively. Interaction of tillage and N with Siws suggests that SOC provides a mechanism to suppress Si solubility.