Long-term experiments are ideal for evaluating the influence of agricultural management practices on occluded organic carbon in phytoliths and its interaction with soil constituents. Limited research has examined influence of organic amendments on soil organic carbon (SOC) and silica (Si) interaction. Silica in the soil solution is taken up by plant roots in the form of monosilicic acid (H₄SiO₄) and subsequently deposited throughout the intra- and extra-cellular structures of their leaf, stem and root systems (Parr and Sullivan, 2005). Deposits of Si in plants have become known as opal phytoliths. The C occluded in opal phytoliths is highly resistant to oxidation (Wilding et al., 1967), and have been dated to 13,300 ± 450 years ago (Wilding, 1967).  Gramineae species have a Si content in the range of 1-3 %, expressed as SiO₂ per dry weight in the shoot (Parr and Sullivan, 2005). Norgren (1973) reported rates of phytolith production of 300 kg ha^-1 y^-1 in Oregon. This high formation rate was attributed to the large amount of readily weatherable volcanic silica parent material from which these soils developed (Al-Ismaily, 1997). Al-Ismaily (1997) estimated approximate phytolith production of 280 kg ha^-1 yr^-1 during the wheat cropping cycle in a Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll). At this rate, the estimated cumulative production of amorphous phytolith Si is 7,300 kg Si ha^-1 for the 59-years cropping period assuming wheat straw contains 45 g SiO₂ kg^-1 dry wt. The rate of phytolith accumulation is affected by monosilicic acid concentration in the soil solution, climate, geomorphology and soil management. Amorphous forms of Si can be dissolved rapidly under acidic soil environment (Beckwith and Reeve, 1964; Milnes and Twidale, 1983). Agricultural practices such as use of N fertilization may accelerate dissolution of phytoliths by release of H⁺ ions during nitrification of the applied NH4⁺. We will present data from two long-term experiments to illustrate the influence of agricultural management practices on SOC and its interaction with Si. The objectives were to: i) determine the effect of tillage and N fertilizer on SOC accretion and on Si dissolution; and ii) evaluate the influence of organic amendments on fine organic matter (FOM) distribution and interaction with soluble Si. A long-term fallow-wheat (Triticum aestivum L.) experiment 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) was established on a Walla Walla silt loam in 1931. The experiment is an ordered block with 2 replications. A second long-term fallow-wheat experiment with two tillages (moldboard plow, MP; and sweep, SW) and two N rates (45 and 180 kg N ha^-1) 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 OM (COM), FOM, water-soluble C (C_ws) and Si (Si_ws). The SOC storage for the NB_M was 25% higher than FB₀ in the 0-50 cm depth. The N fertilizer application (45 or 90 kg N ha^-1) decreased Si_ws by 17% while manure or pea vines application increased Si_ws by 9%. The FOM fraction for the SW was 14% higher than the MP for the 180 kg N ha^-1 in the 0- to 60-cm depth. High SOC, particularly FOM, reduced Si dissolution, illuviation and deposition at the base of the Ap horizon. Although the form of this Si_ws and SOC association was not determined, it is likely the occluded SOC in phytoliths reduced the siliceous surface available for dissolution. The role of organic C occluded within phytoliths in soil carbon sequestration should be examined because this passive C pool component of SOC is highly resistant to oxidation compared to other OC components in the soil. Extending our knowledge of phytolith behavior in soils widens our understanding of the biogeochemical cycling of Si and organic C.  

Reference:

Al-Ismaily, S.S. 1997. Genesis of silica-enriched agricultural pans in soils managed under wheat-fallow cropping systems. Master Thesis. Oregon State. Univ., Corvallis.

Beckwith, R.S. and R. Reeve. 1964. Studies on soluble silica in soils: II. The release of monosilicic acid from soils. Aust. J. Soil Res. 2:33-45.

Jones, L.H.P. and A.A. Miline. 1963. Studies of silica in the oat plant: I. Chemical and physical properties of the silica. Plant and Soil. 18:207-220.

Milnes, A.R. and C.R. Twidale. 1983. An overview of silicification in Cainozoic landscapes of arid central and southern Australia. Aust. J. Soil Res. 21:387-410.

Norgren, J.A. 1973. Distribution, form and significance of plant opal in Oregon soils. Ph.D. diss. Oregon State. Univ., Corvallis.

Parr, J.F. and L.A. Sullivan. 2005. Soil carbon sequestration in phytoliths. Soil Bio. Biochem. 37:117-124.

Wilding, L.P. 1967. Radiocarbon dating of biogenetic opal. Science. 156:66-67.

Wilding, L.P., R.E. Brown, and Holowaychuk. 1967. Accessibility and properties of occluded carbon in biogenetic opal. Soil Sci. 103:56-61.