Tillage Influence On Soil Organic Carbon and Silica Relations.

The Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll) is one of many Mollisols in the Pacific Northwest (PNW) that contains a high concentration of potentially mobile silica (Si). Siliceous pans induced by cropping practices have been observed in the Palouse and Columbia plateau regions of the PNW. Limited research has examined influence of tillage and organic C inputs on SOC and Si interaction. Two long-term experiment (LTE), one LTE (established in 1931) with several residue management practices (NB, no burn; SB, spring burn; and FB, fall burn), three N rates (0, 45, and 90 kg N ha^-1), and organic amendments (NB_M, 11.2 t ha^-1 yr^-1 manure; and NB_PV, 1.12 t ha^-1 yr^-1 pea vines), and a second LTE (established in 1940) with two tillages (moldboard plow and sweep) and two N rates (45 and 180 kg N/ha)  were used to examine effect of tillage, residue management, organic amendment and N fertilizer on SOC storage, and subsequent influence of SOC distribution and interaction with water soluble Si (Si_ws). The SOC storage for the sweep tillage (6.6 kg C m^-2) was 14% higher (5.8 kg C m^-2) than in moldboard plow, and for the NB_M (5.78 kg C m^-2) was 25% higher than for the FB₀ (4.62 kg C m^-2) in the 0- to 50-cm depth. Applied manure or pea vines increased Si_ws by 10% while N fertilizer decreased Si_ws by 17%. Silica solubilization and movement in response to reduced pH was greater in the absence of organic amendments or reduced crop residue returns. Increased SOC storage derived from amendments or N fertilization with retention of crop residues is important for preventing siliceous pan formation and associated impaired infiltration and internal drainage. 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.