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

and in the tillage pan was maintained at a high level only when crop residue returns increased and soil pH Long-term experiments are ideal for evaluating the influence of was maintained above 5.6 (Pikul and Allmaras, 1986). agricultural practices on soil organic carbon (SOC) accretion. Little is known about the influence of tillage and N fertilization on SOC Long-term use of N fertilizer also reduced soil pH (Rasdistribution and silica (Si) movement in a soil. This study: (i) determussen and Rohde, 1989), as a linear function of N mined the effect of tillage and N fertilizer on SOC accretion in a Walla applied, in both the MP and SW systems. Acidifying Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeffects were concentrated in the upper 7 and 22 cm of eroll), and (ii) examined the subsequent influence of fine organic the SW and MP system, respectively. matter (FOM) on Si movement. A long-term wheat (Triticum aestiThe Walla Walla silt loam is one of many Mollisols vum L.)–fallow experiment was established in 1940, in a randomized in the Pacific Northwest that contains a high concentrablock with split-plot design. Soil cores (2-cm increments) from two tion of potentially mobile Si, ranging from 40 to 100 mg tillages (moldboard plow, MP; and sweep, SW) and two N rates (45 and SiO2 kg 1 soil in the form of H4SiO4 (Douglas et al., 180 kg N ha 1) were used to measure coarse organic matter (COM), 1984). Douglas et al. (1984) showed that the concentraFOM, pH, bulk density ( b), water-soluble C (Cws), and water-soluble Si (Siws). The FOM fraction (6.6 kg C m 2) in SW was 14% higher tion of H4SiO4 in these soils increased as pH decreased, (5.8 kg C m 2) than in MP for the 180 kg N ha 1 rate. After 44 yr when base cations were leached out of the soil profile. of N additions, the SOC storage (6.2 kg C m 2) for the 180 kg N ha 1 Measurements of Siws , soil pH, amorphous Si, and total rate increased 3% above that for the 45 kg N ha 1 (6.0 kg C m 2). acidity in the same Walla Walla silt loam (Baham and Total Siws in the B horizon were 34 and 39% greater than in the Ap Al-Ismaily, 1996) indicated that Si had leached from horizon for the MP and SW systems, respectively. Interaction of tillage the upper Ap layer, illuviated into the lower Ap, and and N with Siws suggests that SOC provides a mechanism to suppress deposited in the boundary between the Ap and B horiSi solubility, which impacts siliceous pan formation, reduces soil mezons. Higher mechanical (penetrometer) resistance, eschanical resistance, and enhances drainage and plant growth. pecially in dry soils, has been reported by several investigators and was attributed to movement and deposition of Si at the 20to 30-cm depth to form a weakly cemented S of C and N in a Walla Walla silt loam has siliceous plow pan (Douglas et al., 1984; Baham and Albeen remarkably sensitive to long-term tillage and Ismaily, 1996; Wilkins et al., 2002). N fertilization in a semiarid climate (Rasmussen and Brown and Mahler (1987, 1988) hypothesized that Rohde, 1988; Rasmussen and Collins, 1991). After 44 yr acidification from long-term use of NH4 –based N fertilof the SW system, soil organic N (SON) and SOC in izers solubilized Si, which then illuviated and was rethe top 7.5 cm of soil increased 26 and 32%, respectively, tained as amorphous Si in the plow pan of Mollisols in compared with the MP system (Rasmussen and Rohde, Idaho. Water extracts of soil in well-developed plow 1988). They also noted that 18% of the applied N was pans had more Si than in poorly developed plow pans, retained in soil organic matter, and that SW retained while Si sorption occurred above and below the plow 5.7 kg ha 1 yr 1 more N than MP system. Bulk density pans. Brown and Mahler (1988) concluded that Si conand pH in the surface 30 cm were sensitive to long-term centrations in the plow pan were apparently controlled treatments (Pikul and Allmaras, 1986). Pikul and Zuzel by amorphous Si deposit. These studies did not investi(1994) reported a 12% increase in surface crust porosity gate the possible influence of SOC on soluble Si. Bloom with fertilizer addition compared with no fertilizer addiand Nater (1991) showed that complex-forming ligands, tion, possibly because of accelerated biomass decomposuch as carboxylic and phenolic acids, may cause dissolusition with N application. Over a 50-yr period, additional tion of amorphous siliceous minerals to Siws, and that biomass and organic C produced by a high N fertilization these weathering reactions are a forward dissolution. rate increased the final infiltration rate in the Walla Little or no research has examined the combined inWalla silt loam by a factor of 2.4 (Zuzel et al., 1990). fluence of tillage and N fertilization on SOC distribution, Saturated hydraulic conductivity (Ksat) in the Ap horizon acidification profiles, and Si movement in a soil profile because of the complexity of interactions between soil H.T. Gollany, S.L. Albrecht, and C.L. Douglas Jr. (retired), USDAconstituents, climate, and soil management. Through ARS, Columbia Plateau Conservation Research Center, P.O. Box. 370, Pendleton, OR 97801; and R.R. Allmaras (retired) and S.M. the use of long-term field experiments, the relation of Copeland, USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of soluble Si to both SOC and acidification were examined Minnesota, St. Paul, MN 55108. Received 24 Aug. 2004. *Correspondin the context of soil management and C sequestration ing author ([email protected]). Abbreviations: b, bulk density; Cws, water soluble carbon; COM, Published in Soil Sci. Soc. Am. J. 69:1102–1109 (2005). Soil & Water Management & Conservation coarse organic matter; D, soil depth; FOM, fine organic matter; Ksat, saturated hydraulic conductivity; MP, moldboard plow tillage; Siws, doi:10.2136/sssaj2004.0284 © Soil Science Society of America water soluble Si; SOC, soil organic carbon; SON, soil organic nitrogen; SW, sweep tillage. 677 S. Segoe Rd., Madison, WI 53711 USA 1102 Published online June 2, 2005