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Soil & Water Management & Conservation

Tillage and Nitrogen Fertilizer Influence on Carbon and Soluble Silica Relations in a Pacific Northwest Mollisol

H. T. Gollany^a,*, R. R. Allmaras^b,, S. M. Copeland^b, S. L. Albrecht^a and C. L. Douglas, Jr.^a,

^a USDA-ARS, Columbia Plateau Conservation Research Center, P.O. Box. 370, Pendleton, OR 97801
^b USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author (hero.gollany{at}oregonstate.edu)

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 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 (coarse-silty, mixed, superactive, mesic Typic Haploxeroll), and (ii) examined the subsequent influence of fine organic matter (FOM) on Si movement. A long-term wheat (Triticum aestivum L.)–fallow experiment was established in 1940, in a randomized block with split-plot design. Soil cores (2-cm increments) from two tillages (moldboard plow, MP; and sweep, SW) and two N rates (45 and 180 kg N ha^–1) were used to measure coarse organic matter (COM), FOM, pH, bulk density (_b), water-soluble C (C_ws), and water-soluble Si (Si_ws). The FOM fraction (6.6 kg C m^–2) in SW was 14% higher (5.8 kg C m^–2) than in MP for the 180 kg N ha^–1 rate. After 44 yr of N additions, the SOC storage (6.2 kg C m^–2) for the 180 kg N ha^–1 rate increased 3% above that for the 45 kg N ha^–1 (6.0 kg C m^–2). Total Si_ws 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 Si_ws suggests that SOC provides a mechanism to suppress Si solubility, which impacts siliceous pan formation, reduces soil mechanical resistance, and enhances drainage and plant growth.

Abbreviations: _b, bulk density • C_ws, water soluble carbon • COM, coarse organic matter • D, soil depth • FOM, fine organic matter • K_sat, saturated hydraulic conductivity • MP, moldboard plow tillage • Si_ws, water soluble Si • SOC, soil organic carbon • SON, soil organic nitrogen • SW, sweep tillage

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