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The response of soil structure to liming and controlled burning.

Abstract: Erosion following unintended forest fires or controlled burning programs designed to improve plant diversity is a major environmental concern. The loss of vegetation exposes the soil surface briefly to raindrop impact, allowing conversion of the raindrop's kinetic energy to mechanical energy and the resultant destruction of the soil structure. The loss of soil structure permits the preferential loss of organic material and clay, reducing the soil's productivity. The purpose of this investigation is to observe changes in the soil k structure and fertility following a controlled burning program and to infer if erosion is enhanced because of the controlled burning programs. A secondary objective is to determine if a liming program favorably improves the soil fertility and modifies the soil structure. The investigation was conducted on two different soils located on Taum Sauk Mountain in the St. Francois Mountains of Missouri. Following a controlled burning on a portion of the mountain, replicated plots were established on each soil to measure soil's fertility and structure. In addition, selected plots were limed to assess the effect of lime on the soil structure and fertility. The burning program did not significantly affect the size distribution of soil aggregates, suggesting that burning and rainfall did not degrade the soil structure or accelerate erosion. The root mat was not affected by the burning program, therefore the root mat provided a measure of surface protection until the vegetation was re-established. Liming increased the soil's exchangeable calcium (Ca) and the cation exchange capacity (CEC) and reduced the neutralizable acidity, an improvement of these key soil fertility components. Importantly, smaller aggregates were less acidic and contained greater levels of exchangeable Ca, inferring that the soil is not entirely homogeneous in its reactivity towards lime.

Introduction

Soils are three dimensional natural resources that support plant growth, cycle carbon and other nutrients, store and transmit water, exchange energy with their surroundings and maintain an impressive array of microbial and invertebrate populations (Brady and Weil, 1996). Each soil supports and responds differently to these phenomena because of numerous inherent physical, chemical and mineralogical properties, chief among these being pH, texture and structure. Texture is the percentage of the sand (2 to 0.5 mm), silt (0.5 to 0,002 mm) and clay (less than 0.002 mm) fractions (separates) coupled with visual estimates of the gravel, cobble and boulder contents (Brady and Well, 1996). Soil structure is the arrangement of the separates composing the fine earth fraction into peds or aggregates, which may be classified by their distinctive shapes, sizes and coherence (strength) (Kay and Angers, 2000). In surface and near-surface horizons soil organic matter (SOM) is an important cementing agent that enhances aggregate coherence and provides much of the soil's cation exchange capacity (Baldock and Nelson, 2000; Brady and Weil, 1996; Johnson and Todd, 1998; Kalisz and Stone, 1980).

In surface horizons, aggregates are frequently subdivided into microaggregates (less than 250 [micro]m) and macroaggregates (greater than 250 [micro]m), with the tacit assumption that macroaggregates are composed of microaggregate assemblages. The nature of the SOM responsible for aggregate coherence varies with aggregate size (Baldock and Nelson, 2000; Beare et al., 1994a and b; Gale et al., 2000; Kristensen et al., 2000). Water stable macroaggregates are largely stabilized by transient and relatively unstable SOM, whereas microaggregates are largely stabilized by microbially processed SOM (Beare et al., 1994a and b; Gale et al., 2000; Kay and Angers, 2000; Kristensen et al., 2000). Gale et al. (2000) proposed that microaggregates are formed within existing macroaggregates. Kristensen et al. (2000) provided evidence that N mineralization and immobilization rates are affected by the aggregate size distribution.

An important consequence of soil structure is the creation of pore space. Macropores (greater than 75 [micro]m) and mesopores (30 to 75 [micro]m) are largely responsible for the redistribution of water, whereas micropores (5 to 30 [micro]m) and ultramicropores (0.1 to 5 [micro]m) are considered to be water storage pores. Cryptopores (less than 0.1 [micro]m) are largely associated with the physical isolation and protection of soil organic carbon (SOC) from microbial decomposition (Kay and Angers, 2000). The total pore space, the distribution of pore classes and their connectivity results from an incredibly complex interaction involving soil texture, the presence of SOM, climate, vegetation, soil drainage, and human activities. Soil structure is a complex interaction among soil texture, clay mineralogy, SOM, inorganic noncrystalline materials, plants and soil organisms, soil profile depth and human activities (Kay and Angers, 2000). The purpose of this investigation is to assess the influence of controlled burning programs on soil structure and SOM levels of selected forest soils. A secondary objective is to determine the influence of liming treatments, following a controlled burning program, on the soil's structure and its physical and chemical attributes. This paper is an outgrowth of a more comprehensive study to assess the influence of controlled burning programs on erosion rates and its effect on vegetative regrowth.

MATERIALS AND METHODS

Study Area

Taum Sauk Mountain is part of the St. Francois Mountains in southeastern Missouri. These mountains are composed of Precambrian felsics that are intruded with granites and mafics. Soils of the Ultisol and Alfisol orders dominate the area, including Taum Sauk Mountain (Brown and Gregg, 1991). Typically, these soils are deep to somewhat deep, well drained, strongly acid to extremely acid, silt loam to clayey textured A - E - Bt horizon sequences overlying rhyolite. Brown and Gregg (1991) have described three soil series in the Taum Sauk Mountain study area: Knobtop (fine-silty, mixed, mesic Aquic Hapludults), Irondale (loamy-skeletal, mixed, mesic Typic Hapludults), and Taumsauk (loamy-skeletal, mixed, active, mesic Lithic Hapludults). The summer climate is generally hot and humid with July temperatures averaging 25[degrees]C, whereas January temperatures average near 0[degrees]C. Rainfall averages over 1 m, with spring receiving somewhat more rainfall; however rainfall is generally common throughout the year, causing the upland soils in the study area to maintain a udic soil moisture regime (Brown and Gregg, 1991). Deciduous forests form a partially closed canopy cover over much of the study area, with vegetation composed mostly of white oaks (Quercus alba L.), red oaks (Quercus rubra L.) and an assortment of grasses, herbaceous plants and mosses.

Experimental Design, Field, and Laboratory Protocols

Two sites representing the Knobtop series and two sites representing the Taumsauk series were located and sampled on Taum Sauk Mountain (T 33 N, R 3 E, Sec 4, 5 and 6, Iron County, MO). Soil profile characteristics are reported in Aide and Wendel (1997). For the Knobtop site, a summit landscape, four plots were established (15.3 m * 15.3 m or 50 ft * 50 ft). Two of the plots were not subjected to a controlled burn; whereas, the other two sites experienced a controlled burn approximately three months prior to plot establishment. Secondary treatments included liming one plot from each of the unburned and burned areas. Calcitic limestone [Mississippi Lime] was applied at a rate of 45.5 kg (100 lbs.) in August of 1998 to provide an equivalent surface coverage of 1943 kg [ha.sup.-1] (1742 lbs. [Acre.sup.-1]). In March of 1999, the liming application was repeated. For the Taumsauk site, a convex sideslope, four plots were established (15.3 m * 15.3 m or 50 ft * 50 ft). Three plots experienced a controlled bum, at the same time as the controlled burn for the Knobtop sites, and the remaining site served as an unburned control. Secondary treatments included liming two plots from the burned area. Calcitic limestone [Mississippi Lime] was applied identically as described for the Knobtop sites.

Whole soil samples from the O and A horizons were collected periodically. Oven dried aggregates were isolated by sieving into the following size intervals: [5 mm, to 2 mm], [2 mm, to 1 mini, [1 mm, to 0.5 mm], [0.5 mm, to 0.25 mm], [0.25, mm to 0.1 mm], and [less than 0.1 mm]. Sieving of 100g samples was accomplished using U.S. standard sieves with a shaking interval of exactly 1 minute using a gentle back and forth hand motion with a horizontal displacement of approximately 15 cm and a frequency of 60 cpm. Dry sieving was used, instead of the traditional wet sieving technique, to minimize alteration of the aggregate's chemical properties. Traditionally wet sieving provides a more reliable estimate of aggregate stability and is commonly used in studies involving soil structure changes induced by tillage practices. Analysis of whole soil and samples of each aggregate class consisted of soil pH in water, the ammonium acetate (pH 7.0) extraction of exchangeable bases, the total acidity by slow titration to pH 8.2 with 0.01 MNaOH, and SOM by loss on ignition (LOI) using methods in Carter (1993). The cation exchange capacity (CEC) was calculated from the sum of the exchangeable bases and the total acidity. The particle size distribution was estimated by Na-saturation of the exchange complex, centrifuge washing with water-methanol mixtures to remove excess electrolyte, dispersion in 0.01 M [Na.sub.2]C[O.sub.3], and centrifuge fractionation and wet sieving of the separates (Carter, 1993). The bulk density of the Knobtop sites was estimated using the ring method (Carter, 1993). The bulk density of the Taumsauk horizons were not determined because of the gravel content.

The root-mat of the O horizon was sampled by cutting a 10 x 10 inch (25.4 x 25.4 cm) soil section with a depth corresponding to the O horizon's thickness. The particulate organic matter content (POM) was dispersed by sonification in water and isolated by sieving and hand removal with a forceps. The POM was washed in water, dried at 70[degrees]C, weighed and analyzed for N using micro-Kjeldahl (Carter, 1993) and S, P, Ca, Mg, K, Na, Al, Fe, Mn, B, Cu and Zn using inductively-coupled plasma-emission spectroscopy. The root-mat was washed in water, dried at 70[degrees]C, weighed and similarly analyzed for N, S, P, Ca, Mg, K, Na, Al, Fe, Mn, B, Cu and Zn.

RESULTS

Selected Soil Properties

The surface and near-surface horizons of the Knobtop pedons consist of a thin O horizon (2-3 cm) composed of litter and partially burned residues (POM), partially to fully humified SOM and a living, dense rootmat overlying a silty A horizon (4 to 5% clay, 60-65% silt, 30-36% sand with less than 10% rhyolitic gravel). The surface and near-surface horizons of the Taumsauk pedons are similar, except the Taumsauk pedons have gravelly (25-50% rhyolitic gravel) silt loam textures. Soil pH ranges from acid to extremely acid for the O horizons and acid to strongly acid for the A horizons. Exchangeable Ca levels are particularly low, ranging from 0.47 to 1.61 [cmol.sub.p(+)] * [kg.sup.-1] for the Knobtop sites and 0.76 to 3.48 [cmol.sub.p(+) * [kg.sup.-1] for the Taumsauk sites. As expected, exchangeable Ca levels are somewhat greater for those plots receiving liming treatments. The base saturation is particularly low, consistent with the Ultisol order and the nature of the parent materials (Table 1). Selected soil physical and chemical properties of the near surface horizons after eight months of equilibration with the lime are in Table 1.

Soil Aggregates Size Distribution

The soil aggregate is distribution was relatively uniform among sites within each soil type, with microaggregates (aggregates smaller than 0.25 mm) ranging from 15 to 27% (kg/kg) in the Knobtop sites and ranging from 12 to 20% in the Taumsauk sites (Fig. 1a and 1b). Variation of the aggregate distribution over time was largely insignificant and variation within the soil type appears related to the soil's natural heterogeneity. Thus, no differences in the soil aggregate distribution between the two soil types was observed. Secondly, the effect of liming on the aggregate distribution is not statistically significant.

[FIGURE 1 OMITTED]

Chemical Properties of the Soil Aggregates

Each aggregate class was chemically analyzed for pH, POM, LOI (Table 2), and CEC (Fig. 2). The CEC varied appreciably among the aggregate classes, the soil horizon designations and the liming practices. Coarse aggregates display a lower CEC, especially in the Taumsauk sites; a feature likely attributed to differences in clay content and a higher quartz and feldspar content. The CEC expression is consistently greater in the Taumsauk sites and in the O horizons of each site. The CEC is greater in lime treated sites, a likely consequence of the pH dependent nature of SOM (Brady and Weft, 1996).

[FIGURE 2 OMITTED]

POM and LOI values from medium and fine sized aggregates are roughly equivalent, whereas coarse aggregates show markedly smaller POM and LOI values (Table 2). Unlimed aggregate pH values are extremely acidic and are roughly equivalent across the aggregate size classes; whereas limed aggregates are slightly acidic to acidic. Limed Taumsauk aggregates show little differences in pH across the aggregate size classes, whereas the fine and limed Knobtop aggregates show a higher pH. Thus, liming treatments did not uniformly alter the soil's chemical properties, rather chemical changes were altered to a greater extent in the smaller aggregate classes.

Chemical analysis of the aggregate classes show that exchangeable Ca percentages (ECaP), the percentage ratio of Ca and the CEC, are distinctly different because of soil type, horizon designation and liming practices. Calcium was indexed to the CEC to account for clay and SOM differences among the aggregate size classes (Fig. 3). Knobtop epipedons generally shows smaller ECaP values than the Taumsauk epipedons. Liming treatments dramatically increased the ECaR Fine aggregates from unlimed sites show a slight tendency to have smaller ECaP values, whereas in limed sites the fine and medium aggregates from the A horizons show greater ECaP. Interestingly, the O horizons generally have higher ECaP levels than the corresponding A horizons, possibly a combined consequence of their surface position and their initial contact with the lime and also because of Ca cycling by the vegetation.

[FIGURE 3 OMITTED]

The (NAP), the neutralizable acidity indexed as a percentage of the CEC, are distinctly different because of soil type, horizon designation and liming practices. The NAP reduces the intrinsic clay and SOM differences among the aggregate size classes, thus chemical differences attributed to the treatments are converted from capacity to intensity factors. The NAP values from unlimed sites are somewhat greater in the Knobtop sites (Fig. 4). Smaller aggregates showed a slight tendency to have a greater NAP values. Liming has the pronounced tendency to reduce the NAP and the O horizons were more responsive to the liming treatments than the A horizons. Fine and medium sized aggregates from the Taumsauk sites show the greatest liming response, whereas medium sized aggregates from the Knobtop sites show the greatest liming response.

Analysis of the Root Mat

The root-mat density varies from 0.17 to 1.2 kg-root * [m.sup.2] and the bulk density varies from 0.65 to 0.70 g-soil * [cm.sup.-3] in the Knobtop O horizons, whereas the root-mat density varies from 0.12 to 0.28 kg-root * [m.sup.-2] in the Taumsauk O horizons. The root mat density variation was considerable in the Knobtop sites, reflecting the natural soil and forest variation. Variation in the root-mat density and the bulk density over time are relatively constant and reflect site heterogeneity rather than site disturbance because of the controlled burning program or the liming practice. The elemental composition of the root mat did not reflect the controlled burning program or the liming practices.

DISCUSSION

The controlled burning program was established to stimulate plant diversity; however, one question needed to be addressed was the possible unintended consequence of accelerated erosion. As is commonly known, water erosion rates are affected by climate, soil properties, topography, vegetation and human activities (Brady and Weil, 1996). Soil properties key to influencing water erosion rates include: texture, structure, SOM, drainage and permeability, and gravel content (Brady and Weil, 1996). The controlled burning program did not influence the soil structure or SOM content. The root-mat remained intact and living, providing a living web of strong tissue to anchor the O horizon and the underlying A horizon and provide protection from the rainfall's energy. Thus, a controlled burning program in the St. Francois Mountains should not accelerate soil erosion.

The liming program did alter the soil's chemical properties, as expected and intended. However, the aggregate classes were not equally affected by the liming program, suggesting that soil horizons are not entirely homogeneous regions. The liming program did improve the soil fertility by increasing the CEC and the base saturation of the soil's exchange complex. These improvements should promote plant diversity by reducing the specter of Al-toxicity and increasing the overall soil fertility.
Table 1. Initial soil properties at Taum Sauk Mountain
immediately after imposition of liming treatments
(March, 1999).

 Exchangeable Bases

Treatment pH Ca Mg K Na

 O A [cmol.sub.(p+)] *
 [kg.sup.-1]

 Horizons

 Knobtop Sites

lime-unburned 5.8 5.3 0.96 0.29 0.13 0.15
no lime-unburned 4.0 5.7 0.47 0.25 0.12 0.14
lime-burned 5.5 5.8 1.61 0.39 0.15 0.16
no lime-burned 4.6 5.5 1.20 0.37 0.17 0.17

 Taumsauk Sites

no lime-unburned 4.3 5.3 0.74 0.36 0.18 0.17
lime-burn 5.7 5.7 1.18 0.18 0.11 0.13
no lime-burn 4.8 5.5 1.28 0.27 0.13 0.13
lime-burn 5.6 6.0 3.48 0.50 0.13 0.13

 Loss on
 Ignition

Treatment Nuetralizable Acidity O A

 % %

 Knobtop Sites

lime-unburned 6.7 12.40 2.5
no lime-unburned 6.3 6.40 2.9
lime-burned 6.5 7.10 2.6
no lime-burned 8.5 12.40 3.0

 Taumsauk Sites

no lime-unburned 11.5 11.40 6.1
lime-burn 7.3 11.20 5.0
no lime-burn 7.5 11.30 5.1
lime-burn 6.0 9.40 3.3

All values represent a composite sample taken from uniform
slices of the A horizons, except pH and LOI.

Table 2. Aggregate pH, loss on ignition (LOI), and particulate
organic material (POM) sampled August 1999.

 Aggregate
 pH ([paragraph])

Treatment Coarse Medium Fine

 A- Horizon

 Knobtop Sites

Not Burned and Limed 4.7 4.8 5.6
Not Burned and No Lime 4.5 4.5 4.6
Burned and Limed 5.2 6.1 6.5
Burned and No Lime 4.4 4.3 4.5

 Taumsauk Sites

Not Burned and No Lime 4.4 4.4 4.5
Burned and Limed 4.7 4.6 4.8
Burned and No Lime 4.4 4.3 4.4
Burned and Limed 4.7 4.5 4.7

 LOI ([section])

Treatment Coarse Medium Fine

 Percent of Whole Soil

 Knobtop Sites

Not Burned and Limed 5.9 8.7 8.9
Not Burned and No Lime 8.2 10.6 8.6
Burned and Limed 5.1 8.6 7.6
Burned and No Lime 8.9 8.7 8.9

 Taumsauk Sites

Not Burned and No Lime 4.4 7.8 9.0
Burned and Limed 4.3 6.0 4.9
Burned and No Lime 4.4 6.2 5.2
Burned and Limed 6.0 7.8 6.9

 POM

 pH ([pounds
Treatment Coarse Medium Fine sterling])

 Percent of Whole Soil

 Knobtop Sites

Not Burned and Limed 2.3 2.9 1.7 5.8
Not Burned and No Lime 2.3 1.5 0.4 4.1
Burned and Limed 2.0 1.0 0.5 5.5
Burned and No Lime 2.0 2.6 0.9 4.7

 Taumsauk Sites

Not Burned and No Lime 2.4 3.8 3.1 4.2
Burned and Limed 2.4 3.0 1.9 5.7
Burned and No Lime 1.9 2.3 2.2 4.1
Burned and Limed 7.3 5.7 3.5 5.6

([paragraph]) Coarse (>2 mm), Medium (2 mm to 0.25 mm),
Fine (<0.25 mm)

([section]) LOI was determined after POM was determined.

([pounds sterling]) pH is from O horizons.


REFERENCES

Aide, M.T., and D.W. Wendel. 1997. Soil genesis and vermiculite formation from rhyolite in the St. Francois Mountains of Missouri. Trans. MO. Acad. Sci., 31:9-18.

Baldock, J.A., and P.N. Nelson. 2000. Soil organic matter. In M.E. Sumner (ed.). Handbook of Soil Science. CRC Press: Chapter B. pp. 25-84.

Beare, M. H., M.L. Cabrera, P.F. Hendrix, and D.C. Coleman. 1994a. Aggregate-protected and unprotected organic matter pools in conventional- and no-tillage soils. Soil Sci. Soc. Am. J. 58: 787-795.

Beare, M.H., P. F. Hendrix, and D.C. Coleman. 1994b. Water-stable aggregates and organic matter fractions in conventional-and no-tillage soils. Soil Sci. Soc. Am. J. 58:777-786.

Brady, N.C., and R.R. Weil. 1996. The nature and property of soils. Prentice-Hall, N.J.

Brown, B., and K.L. Gregg. 1991. Soil Survey of Iron County Missouri.USDA-SCS in cooperation with Missouri Agriculture Experiment Station. U.S. Gov. Print Office, Washington, DC.

Carter, M.R. 1993. Methods of soil analysis. CRC Press, Boca Raton, Fl.

Gale, W.J., C.A. Cambardella, and T.B. Bailey. 2000. Root-derived carbon and the formation and stabilization of aggregates. Soil Sci. Soc. Am. J. 64:201-207.

Johnson, D.W., and D.E. Todd. 1998. Harvesting effects on long-term changes in nutrient pools of mixed oak forest. Soil Sci. Soc. Am. J. 62:1725-1735.

Kalisz, P. J., and E.L. Stone. 1980. Cation exchange capacity of acid forest humus layers. Soil Sci. Soc. Am. J. 44:407-413..

Kay, B.D., and D.A. Angers. 2000. Soil structure. In M.E. Sumner (ed.). Handbook of Soil Science. CRC Press: Chapter A. pp. 229-264.

Kristensen, H.L., G.W. McCart, and J.J. Meisinger. 2000. Effects of soil structure on mineralization of organic soil nitrogen. Soil Sci. Soc. Am J. 64:371-378.

Michael Aide, Department of Geosciences, Southeast Missouri State University, and Ken McCarty, Missouri Department of Natural Resources.
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Date:Jan 1, 2003
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