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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)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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b), water-soluble C (Cws), and water-soluble Si (Siws). 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 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, which impacts siliceous pan formation, reduces soil mechanical resistance, and enhances drainage and plant growth.
Abbreviations: b, bulk density • Cws, water soluble carbon • COM, coarse organic matter • D, soil depth • FOM, fine organic matter • Ksat, saturated hydraulic conductivity • MP, moldboard plow tillage • Siws, water soluble Si • SOC, soil organic carbon • SON, soil organic nitrogen • SW, sweep tillage
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The Walla Walla silt loam is one of many Mollisols in the Pacific Northwest that contains a high concentration of potentially mobile Si, ranging from 40 to 100 mg SiO2 kg–1 soil in the form of H4SiO4 (Douglas et al., 1984). Douglas et al. (1984) showed that the concentration of H4SiO4 in these soils increased as pH decreased, when base cations were leached out of the soil profile. Measurements of Siws, soil pH, amorphous Si, and total acidity in the same Walla Walla silt loam (Baham and Al-Ismaily, 1996) indicated that Si had leached from the upper Ap layer, illuviated into the lower Ap, and deposited in the boundary between the Ap and B horizons. Higher mechanical (penetrometer) resistance, especially in dry soils, has been reported by several investigators and was attributed to movement and deposition of Si at the 20- to 30-cm depth to form a weakly cemented siliceous plow pan (Douglas et al., 1984; Baham and Al-Ismaily, 1996; Wilkins et al., 2002).
Brown and Mahler (1987)(1988) hypothesized that acidification from long-term use of NH4+–based N fertilizers solubilized Si, which then illuviated and was retained as amorphous Si in the plow pan of Mollisols in Idaho. Water extracts of soil in well-developed plow pans had more Si than in poorly developed plow pans, while Si sorption occurred above and below the plow pans. Brown and Mahler (1988) concluded that Si concentrations in the plow pan were apparently controlled by amorphous Si deposit. These studies did not investigate the possible influence of SOC on soluble Si. Bloom and Nater (1991) showed that complex-forming ligands, such as carboxylic and phenolic acids, may cause dissolution of amorphous siliceous minerals to Siws, and that these weathering reactions are a forward dissolution.
Little or no research has examined the combined influence of tillage and N fertilization on SOC distribution, acidification profiles, and Si movement in a soil profile because of the complexity of interactions between soil constituents, climate, and soil management. Through the use of long-term field experiments, the relation of soluble Si to both SOC and acidification were examined in the context of soil management and C sequestration in this soil. Objectives of this study were to: (i) determine the effect of tillage and N fertilizer on SOC accretion in a Walla Walla silt loam, and (ii) then examine the influence of FOM and pH on Si movement and distribution in the soil profile.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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b determination. Soil cores were gently mixed with a spatula and air-dried. The oven-dry (105°C) soil mass (estimated from a subsample) and sample volume (number of cores, 2-cm increments and 18-mm diam.) were used to calculate b. All constituent properties were expressed on a volumetric basis using these b.
Coarse Organic Matter and Fine Organic Matter Separation
Air-dried soil samples were gently sieved over a 0.5-mm sieve (35 mesh), to avoid breaking up fresh and partially decomposed crop residue and to separate out this coarse crop residue (Allmaras et al., 1988b). The coarse crop residue fraction (plant residue with adhered soil particles) retained on the sieve (>0.5 mm) was further analyzed to determine the uncontaminated coarse crop residue. The residue fraction (>0.5 mm), containing some contaminant mineral soil, was air dried and weighed, a small subsample of the <0.5-mm separate was used to estimate oven-dried weight. A methodology (conservation of C procedure) developed by Allmaras et al. (1988b) was used to estimate the weights of contaminating mineral soil and coarse residue particles free of soil. This fraction is designated as COM. The term FOM is used for soil material with organic matter that passed through the sieve (<0.5 mm) to distinguish this fraction from the COM fraction containing predominantly particulate material. The air-dried crop residue fraction (>0.5 mm) and a subsample of the FOM fraction (<0.5 mm) were each ball milled with SPEX/Mixer/Miller (Model 8000, SPEX Industries, Inc. Scotch Plains, NJ)1. Total C was determined for both fractions with a Leco carbon analyzer (Leco Corp., St. Joseph, MI). Selected analysis showed that none of these soil samples contained inorganic C. Gentle sieving and the C conservation procedure minimized particulate organic matter contamination of FOM. This separation of COM and FOM was made only in the top 30 cm, which exceeds the deepest depth of residue incorporation by the MP.
Particulate material is frequently separated by flotation (Gregorich and Ellert, 1993; Carter and Gregorich, 1996) to expose sequestered C. However, such a procedure would have required additional soil sampling, compromised the integrity of the long-term plots, and lost the precision associated with multiple analyses on the same sample.
Soil pH, Water Soluble Carbon, and Silica Measurements
Part of the air-dried and ball milled soil fraction (<0.5 mm) used for the determination of SOC in FOM was used for additional soil measurements. Soil pH was measured in duplicate for each soil sample using a 1:2 (w/w) soil to 0.01 M CaCl2 ratio.
To analyze for Cws and Siws, 5 g of air-dried soil was added to 25 mL of deionized water in a flask, shaken for 30 min, allowed to settle overnight, centrifuged (12000 x g) for 10 min, and filtered (Whatman No. 40). Filtrate aliquots were used for soluble C (Cws) and Si (Siws) determinations. Water-soluble C was analyzed as oxidizable C (Technicon Industrial Systems, 1976; Douglas et al., 1984) present in the aqueous filtrate. An aliquot of the filtrate in the auto analyzer stream was exposed to a high velocity stream of CO2–free air in a turbulent liquid film to remove any inorganic C contaminant. The carbonate free stream was then mixed with an acid and persulfate stream, before exposure to ultraviolet radiation to convert organic C to CO2. The CO2 was collected into a turbulent liquid film stream and reacted with a weakly buffered phenolphthalein indicator. The original C concentration was colorimetrically determined. One aliquot per soil sample was analyzed for Cws.
Silicic acid in the aqueous filtrate was determined using a modified heteropoly blue method (Rand et al., 1975; Technicon Industrial Systems, 1976; Douglas et al., 1984). Ammonium molybdate at pH 1.2 reacts with Si and any phosphate contaminant to produce heteropoly acids. First a 0.03 M HCl solution was introduced into the auto analyzer sample stream to dilute the aliquot, and oxalic acid was added to eliminate phosphate interference before ascorbic acid addition. Silicomolybdate in acidic solution formed a molybdenum blue color and was assayed colorimetrically.
Soil Organic Nitrogen Estimation
The data sets for SON in the top 26 cm used here are from an experiment reported earlier (Rasmussen and Rohde, 1988). They reported total N as organic N, because inorganic N was <1% of total N. Soil organic N (g N kg–1) was converted to mg cm–3 using b profiles measured as described above. Any change in SON between 1983 and 1984/1986 was assumed to be negligible.
Statistical Procedures
Nested ANOVA and GLM procedures (SAS Institute, 1988) were used for statistical analysis. The nested ANOVA was used to compute a root mean square error from which the standard error of the mean was computed from duplicate composites (replications) corresponding to treatment x soil property x depth. Significant differences among treatment means (P < 0.05) were tested using t tests. Multiple linear regression and ANOVA in GLM were used to estimate parameters and statistical significance.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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b were characteristic of primary tillage tools (Fig. 1a) . Moldboard plowing with secondary tillage produced a b profile, which had a maximum at the 20-cm depth, minimum at 10- to 18-cm depth, and a 2- to 10-cm layer that was uniformly repacked by secondary tillage. Sweep tillage produced a 5- to 20-cm layer with nearly uniform b, which did not distinguish the sweep shear plane from soil repacked by secondary tillage. The overall b values below the 25-cm depth were not sensitive to tillage systems.
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b at about the same depth. Moldboard plowing inverts residue, such that the largest concentration of fresh residue occurs below 15 cm (Allmaras et al., 1988a, 1996), depending on tillage depth. Residue retention for the SW system was highest near the surface and declined exponentially with soil depth (Fig. 1b), somewhat similar to a chisel tillage system (Allmaras et al., 1988a, 1996). An integrated value of COM (Table 2) is about 11% of FOM and is about 75% of the C in the shoot residue produced at the last harvest.
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The MP system significantly (P < 0.05) decreased soil pH in the B horizon by an average of 0.54 units compared with the SW system even down to 55 cm (Fig. 1c). Rasmussen and Rohde (1989) attribute the greater MP effect on pH values, as compared with SW, to a greater denitrification rate in the SW than the MP system. The overall decrease in mean pH due to higher fertilizer rate (data not shown in Fig. 1c) was only 0.02 and 0.30 units for the SW and MP systems, respectively. However, the small pH responses to N rate for the two tillage systems in the B horizon were reversed from their juxtaposition in the Ap horizon.
Management Effects on Fine Organic Matter, Water-Soluble Carbon, and Water-Soluble Silica
Tillage had a consistently larger effect on FOM than did N rate (Fig. 2a)
, even though the 180 kg N ha–1 rate increased wheat residue production approximately 25% compared with the 45 kg N ha–1 rate (Table 1). The 180 kg N ha–1 treatment had consistently more FOM in the Ap horizon than did the 45 kg N ha–1 treatment, however the difference was not statistically significant (Fig. 2a). The profile distribution of FOM in the top 12 cm was similar to the profile distribution of COM (Fig. 1b) with respect to the primary tillage treatment.
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Primary tillage influenced the Siws content with depth in the Ap horizon. The Siws substantially increased with depth in all treatments, however the high N rate significantly (P < 0.05) decreased mean Siws for both tillage systems as much as 16% compared with the low N rate (Fig. 2c, Table 2). A maximum concentration just above the B horizon implies solubilization, movement and ultimately deposition within the lower Ap horizon. Tillage treatment effect on Siws in the 20- to 26-cm layer was unusually small. No treatment effects on Siws were noted in the B horizon, where the overall mean concentration was relatively unchanged as a function of depth, and was significantly (P < 0.05) greater than in the Ap horizon (Table 2). No tillage differential was observed (Table 2), even though the depth distribution of Siws was markedly sensitive (Fig. 2c). At the 45 kg N ha–1 rate, Siws concentrations in the B horizon were 34 and 39% greater than in the Ap horizon for the MP and SW systems, respectively.
Tillage and N fertilizer rate had a significant (P < 0.05) effect on SON content (Table 2). The SON contents in the MP system (0.27 and 0.30 kg N m–2 for low and high N rates, respectively) were lower than in the SW system (0.31 and 0.33 kg N m–2).
Soil Organic Carbon Storage in the Fine Organic Matter Fraction
Primary tillage systems had a stronger effect on SOC storage than N fertilization in both Ap (0–26 cm) and B horizons (26–60 cm). The SW system had a significantly (P < 0.05) and consistently greater FOM storage than the MP system in the Ap (especially above 15 cm) (Fig. 2a and Table 2). Mean FOM was 3.46 and 3.86 kg C m–2 in the Ap horizon, and 2.26 and 2.60 kg C m–2 in the B horizon, for the MP and SW systems, respectively (Table 2). These FOM differences were statistically (P < 0.05) significant for both comparisons. Greater FOM in the B horizon for the SW compared with the MP system suggests more below ground biomass under the SW system. We suggest that this could be due to more moisture and a deeper rooting system under the SW than the MP tillage system, or because of an accelerated decomposition rate under the MP system. Soil water-storage is greater in the SW than MP treatments when the soil is moldboard plowed in the fall (Douglas et al., 1992). This effect is also likely when spring moldboard plowed.
Mean FOM for the 45 and 180 kg N ha–1 rates were 3.60 and 3.72 kg C m–2 in the Ap horizon, respectively; the difference was statistically (P < 0.05) significant. However, the SOC storage of 2.43 and 2.45 kg C m–2 for the two N rates were not different in the B horizon. Greater SOC storage in the B horizon for tillage system compared with N rate cannot be explained by shoot growth because the mean crop residue at harvest in 1981 was 25% greater due to the high N rate (Table 1). Accelerated residue decomposition with high N application could have resulted in lower SOC storage.
The SW system significantly (P < 0.05) increased the mean SOC storage in the FOM fraction, for both fertilizer rates, in the Ap and B horizons by 11.9 and 15.0%, respectively, compared with the MP system (Table 2). Mean SOC storage in the FOM fraction for the 180 kg N ha–1 treatment was 6.6 and 5.8 kg C m–2 for the SW and MP system, respectively, in the 0- to 60-cm soil depth (Table 2). Total FOM (6.6 kg C m–2) in the SW system was 14% higher (5.8 kg C m–2) than in the MP system. These are lower than the estimated SOC storage of 6.9 kg C m–2 in the upper 50 cm of soil in Haploxerolls reported by Kern et al. (1997).
Interaction of Soil Organic Carbon Components
Three components of SOC are COM, FOM, and Cws. Both COM and FOM distribution profiles were sensitive to primary tillage and N rate (Fig. 1b and Fig. 2a). A multiple linear regression (F-ratio, P < 0.05) model depicted the general relationship of these two SOC components in the Ap horizon as follows:
 | [1] |
b is in Mg m–3; D is soil depth in cm; 
= 0.26; ß1 = 0.28; ß2 = 13.33; and ß3 = –0.199.
There was a positive relation between FOM and COM (ß1 = 0.28), because long-term use of the same tillage system controls COM position. This in turn controls FOM distribution in the soil profile subject to the influence of wheat rooting biomass. The b decreased as COM increased (Fig. 1a). The COM burial did not reach the base of the Ap horizon, where b was greater because of a tillage pan. Greater COM and lower b were observed between the maximum tillage depths, as secondary tillage with a cultivator may redistribute residue toward the surface (Allmaras et al., 1996). Bulk density has decreased as much as 12% in small soil volumes that contained COM (Allmaras et al., 1996).
The third component of SOC, Cws, was measured from the same soil samples used to measure FOM. The Cws increased in the Ap horizon of both tillage treatments as the N fertilizer rate increased. The mean ratio of Cws to FOM within the Ap horizon was 0.023. The Cws values were likely a maximum because soil samples were obtained during April, when soil moisture and temperature were suitable for residue decomposition (Douglas et al., 1980). A larger fraction of FOM was in the Cws form in the B relative to the Ap horizon (Table 2). Higher Cws in the B than Ap horizon could be due to leaching. Douglas et al. (1984) observed that Cws leached from the 0- to 15-cm layer and accumulated below 15 cm in a controlled leaching study using soil from the same field plots.
Treatment-Induced Changes in Fine Organic Matter and pH Related to Water-Soluble Silica Responses
Soil management that changes SOC storage and soil pH can impact Siws. Profiles of Siws were sensitive to tillage and N rate in the Ap horizon (Fig. 2c and Table 2). A multiple linear regression was used to examine the influence of soil constituents on Siws. The dependent variable, Siws, was regressed to the independent variables, pH, FOM, b, and D:
 | [2] |
b units are in Mg m–3; D units are in cm; ' = –139.51; ß'1 = 20.52; ß'2 = –1.46; ß'3 = 115.70; and ß'4 = 0.43. This equation accounted for 83% of Siws variability. All regression coefficients were statistically significant (P < 0.01) except that for D was significant (P < 0.20). The positive coefficient for pH is consistent with acidification of amorphous Si, due to N fertilization, as necessary for Siws leaching from the upper portion of the Ap horizon. The negative coefficient for FOM indicated more crop residue and sequestered SOC reduced Siws content, which is consistent with Douglas et al. (1984). They reported less leaching of Siws as FOM increased. The regression coefficients for both b and D are indicative of their influence on Siws in the lower parts of the Ap horizon.
Simple correlations (Table 3) between Siws and pH were large and positive. The correlation between Siws and FOM was large and negative, and that between Siws and b was positive and small. The large negative correlation between pH and FOM (Table 3) is consistent with effects of N fertilization. The large correlations of D with pH and D with FOM are consistent with Fig. 1c and Fig. 2a, respectively.
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The general shape of the Siws profile (Fig. 2c) is similar to those of Brown and Mahler (1987)(1988) in Idaho, as well as Baham and Al-Ismaily (1996) on the Walla Walla silt loam. Lower Siws concentrations were found in the Ap horizon than in the B horizon, with the highest concentration at the boundary between horizons at about the 20-cm depth. Brown and Mahler (1987)(1988) reported a high concentration of polymerized amorphous Si in the plow pan zone. Baham and Al-Ismaily (1996) found a large amorphous Si concentration in the Ap horizon of Walla Walla silt loam that was insensitive to several long-term N fertilizations, but their water soluble Si concentration was much lower and sensitive to the N fertilizations.
Our measurements in producer's fields within 50 km from the research plots (not shown) showed soil profiles of Siws, FOM, and pH similar to those in this study. These sites, mostly MP system, were more intensively managed with respect to N fertilization, various crop sequences, and supplemental irrigation.
A constant but limited supply of Siws in this soil can be expected because the less soluble siliceous minerals (biogenic opal, volcanic glass, and other amorphous minerals) can undergo dissolution in the presence of organic ligands. Higher Siws concentrations found in the B horizon than in the Ap horizon, with the highest concentration at the boundary between horizons, is most likely the result of dissolution of amorphous Si to Siws and subsequent leaching. Bloom and Nater (1991) showed that complex-forming ligands, such as carboxylic and phenolic acids, might cause dissolution of amorphous siliceous minerals to Siws. However, the decrease in pH associated with N fertilization did not increase Siws in the presence of high FOM concentrations produced by the increased biomass (Fig. 1c and Fig. 2c).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Profiles of COM, FOM, and b were all characteristic of tillage systems. Profiles of pH, Cws, and Siws were related to tillage and N fertilizer. The pH profiles with SW system at the 0- to 12-cm depth were more acidic than with MP system for both fertilizer rates. The MP treatment decreased soil pH by an average of 0.54 units compared with the SW treatment in the B horizon. Lower Siws concentrations were found in the Ap horizon than in the B horizon, with the highest concentration at the boundary between horizons. A decrease in pH associated with N fertilization did not increase Siws in the presence of high FOM concentrations. Interaction of tillage and N with Siws suggests that SOC provides a mechanism to reduce Si solubility. Therefore, crop residue management has an important impact on Si solubility and movement. This may be especially important at the interface of the Ap and B horizons, where siliceous pan formation increases mechanical resistance to root penetration, impairs drainage and reduces plant growth and production.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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retired. 
1 Mention of trade names or commercial products in this manuscript is solely for the purpose of providing specific information, and does not imply recommendation or endorsement by the USDA.
Received for publication August 24, 2004.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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