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USDA Service Center, Pacific Southwest Research Station, 3644 Avtech Parkway, Redding, CA 96002
* Corresponding author (mbusse{at}fs.fed.us).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AWCD, average well-color development • LTSP, North American Long-Term Soil Productivity • PCA, principal component analysis • PLFA, phospholipid fatty acid • WHC, water holding capacity
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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On public lands, ameliorative treatment is generally required when soil bulk density exceeds pretreatment levels by 15% on at least 15% of a harvested area (Powers et al., 1998). Although some regions substitute total porosity, macroporosity, or soil strength as threshold parameters in place of bulk density, the intent is unchanged: to monitor and mitigate detrimentally compacted soils. Guidelines for soil compaction are empirical, based on field experience and practicality, and take a conservative approach to soil management by lumping virtually all soils as one regardless of their origin, texture, or organic matter content.
Recent evidence from the North American Long-Term Soil Productivity Study (LTSP), which examines the effects of compaction, organic matter removal, and vegetation control on forest sustainability, challenges this practice. Gomez et al. (2002a)(2002b) found highly variable responses to compaction along a soil textural gradient in ponderosa pine and mixed-conifer plantations in the Sierra Nevada Mountains of California. Compaction was detrimental to plant-water availability and conifer growth in a clay soil, even though plant N uptake and N mineralization were generally improved. Conversely, plant growth and water availability were improved due to compaction in a coarse-textured soil, while N dynamics were unaffected. A complex relationship between compaction and soil microorganisms is apparent from the variety of responses found in field studies. For example, several investigations have found decreases in microbial activity or biomass due to compaction (Dick et al., 1988; van der Linden et al., 1989; Kaiser et al., 1991; Torbert and Wood, 1992; Li et al., 2003), while others report either no relationship (Smeltzer et al., 1986; Jordan et al., 1999; Ponder and Tadros, 2002) or a positive response by microorganisms (Breland and Hansen, 1996).
Numerous physical and biological factors contribute to the ambiguous response of microorganisms to compaction. Changes in the physical habitat, particularly altered pore-size distribution, may benefit the microbial community by increasing the volume of habitable pores while providing protection from larger predators (Postma and van Veen, 1990; Hassink et al., 1993). Alternatively, compaction-induced declines in air-filled porosity can restrict O2 diffusion (Santruckova et al., 1993), increase CO2 accumulation (Conlin and van den Driessche, 2000), and favor anaerobic conditions (Linn and Doran, 1984) to the detriment of the general community. Diversity of forest soil types, regional climates, and functional groups of soil organisms adds to the complexity, making predictions of microbial responses to compaction difficult. Further, the effects of soil compaction are often confounded during harvesting by concurrent mixing and displacement of the surface horizon, inadvertently altering or reducing soil nutrient pools (Dick et al., 1988).
Federal guidelines for public lands in the USA mandate remediation of compacted soils following timber harvesting and related activities. Whether compaction is universally detrimental to soil health, however, often is assumed yet untested in many soils. We challenged this assumption by measuring microbial community responses to compaction in a sandy loam and a clay loam soil under laboratory and field conditions. Our objective was to determine whether moderate or severe compaction decreases microbial community size, activity, or diversity in differing soil textures. The underlying theme was to characterize the link between soil physical and biological properties using several indices of soil health based on the concepts presented by Doran and Safley (1997).
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Samples were collected from the edge of the LTSP plantations in untreated areas with no visible signs of vehicle traffic, soil mixing, or O horizon disturbance. We avoided collecting samples from within the adjacent mature stands to limit the number of excised roots in the soil cores. Mineral soil was collected by gently removing the O horizon, inserting a 10 cm diam. polyvinylchloride (PVC) collar to a depth of 5 cm, and carefully digging around the collar. A 3-cm headspace was left at the top of each core, and the cores were covered with aluminum foil before transport to the laboratory.
Average moisture content and WHC were determined using nine randomly selected cores of each soil type. The remaining cores were wetted to 60% WHC and allowed to equilibrate for a minimum of 2 wk at room temperature (approximately 23°C) before compaction. During the equilibration period, surface CO2 efflux was measured every 48 h with an infrared gas analyzer (LI-6200, LI-COR, Lincoln, NE) to monitor the flush of CO2 from decaying roots. Bulk densities were estimated from core weights and volumes, corrected by the average moisture content of the destructively sampled cores. Samples with CO2 efflux values or bulk densities greater than one standard deviation from the mean were discarded.
Forty-eight cores per soil type were randomly assigned to the treatments before compaction. Three levels of compaction (no, moderate, severe) were applied to the intact cores. Soils were compacted manually using a weighted and capped PVC pipe, slightly smaller in diameter (9.5 cm) than the soil cores. Preliminary testing had shown that this method produced uniform soil strength with depth. Soil volume was reduced 15% for moderate compaction and 30% for severe compaction. These levels were chosen to cover the range of bulk density increases apt to occur under operational field conditions. Sixteen replications of each treatment were arranged in a completely randomized experimental design. Soil water content was maintained at 60% WHC during the experiment by frequent weighing and additions as necessary.
Soil Physical and Microbial Measurements
Four replications of each treatment were destructively sampled on four harvest dates (4, 11, 32, or 67 d after compaction) to determine short-term changes in soil physical and biological characteristics. Measurements taken on each date included bulk density, microbial biomass, respiration, total bacteria, bacterial biomass, fungal hyphal length, fungal biomass, culturable bacteria and fungi, CO2 efflux, N mineralization, and C use (Biolog). Phospholipid fatty acids were measured only on Day 67 samples. Surface CO2 efflux was measured semi-weekly throughout the experiment as well as on harvest dates.
The bulk density of each core was determined by measuring total soil weight (wet) and volume, and correcting for the moisture content determined on subsamples. Soil from each core was then sieved (8 mm), mixed, and subsampled for microbial analyses within 30 min. of disruption. Subsamples for PLFA analysis were frozen at –20°C for subsequent analysis, and subsamples for N mineralization were stored at 4°C before analysis of inorganic N.
Pore-size distribution was determined on intact soil cores by the Division of Agriculture and Natural Resources laboratory, Univ. of California, Davis. Three replicates of each compaction treatment and soil were compared using standard pressure-plate procedures for soil moisture retention curves (Klute, 1986). Volumetric water content was determined at 0, 10, 30, 100, 500, and 1500 kPa. Corresponding maximum pore diameters are >30, 30, 12, 3, 0.6, and 0.2 µm (Papendick and Campbell, 1981).
Respiration was measured by infrared gas analysis on 25 g samples (equivalent dry weight) during the initial 4 h following core sampling. Microbial biomass was measured within 2 h of sampling by substrate-induced respiration (Anderson and Domsch, 1978), using 25 g of soil and 5 g kg–1 of glucose.
Total bacteria and fungal hyphal lengths were quantified by epifluorescent microscopy (Bottomley, 1994). Briefly, 3 g of soil (dry weight equivalent) was mixed with 27 mL of 0.15 M NaCl for 10 min on an orbital shaker. For bacteria, samples were diluted with saline, filtered successively through 8.0- and 3.0-µm filters, preserved with 95% formalin, and stored up to 14 d at 4°C before counting. Samples were equilibrated at room temperature before staining with acridine orange and filtering onto 0.2-µm blackened filters. Total number and size class (<0.5, 0.5–1.5, and >1.5 µm diam.) of bacteria were each counted on 10 fields per filter. Bacterial biomass was calculated by the method of Bottomley (1994). For fungi, samples (200-fold final dilution) were mixed with 0.2% calcofluor and allowed to stain for 30 min. before filtering on 0.4-µm blackened filters. Hyphal length and width were measured on a minimum of 30 fields per filter to estimate biomass and total length (Bottomley, 1994).
Culturable bacteria were enumerated by dilution plating (duplicate samples) on tryptic soy agar after a 14-d incubation at 28°C. Fungi were enumerated on malt extract agar following a 3-d incubation.
Monthly net N mineralization was measured as the change in NH4+ and NO3– concentrations between Day 4, 32, and 67 samples using the distillation method of Bremner (1965). Briefly, 5 g of soil were suspended in 50 mL of 2 M KCl, mixed for 1 h, and distilled following the addition of 0.2 g carbonate-free magnesium oxide. Distillates were titrated with 0.001 M HCL to determine NH4+ concentration. Nitrate concentration was determined by adding 0.2 g of Devarda's alloy to the sample, further distilling, and titrating with 0.001 M HCL.
Changes in the functional diversity of the microbial community due to compaction were measured on Biolog GN plates (Biolog, Hayward, CA). Soil inoculum was prepared by mixing 3 g of soil (dry weight equivalent) in 27 mL of 0.15 M NaCl on an orbital shaker for 10 min. Following 10 min. of settling, the supernatant was diluted 15-fold in saline, and a 0.15-mL aliquot was added to each of the 95 wells. Biolog plates were incubated in the dark at 28°C for 72 h, and bacterial growth (optical density at 590 nm) was measured three times per day. Average well color development (AWCD) of the 95 wells was corrected by subtracting the optical density of the control well (no C source).
Phospholipid fatty acid community structure was determined using a procedure modified from Frostegard et al. (1993). Soil (3 g) was extracted with a one-phase solution of chloroform, methanol, and citrate buffer (0.15 M, pH4, 1:2:0.8 v/v/v). The solution was then filtered through glass wool, and two additional chloroform rinses were used to extract lipids from the soil. Lipids were fractionated on a silica column (J and W, Folsom, CA) into neutral-, glyco-, and phospholipids. The phospholipids were then converted to methyl esters using a solution of sulfuric acid (4%) in methanol (1:1, v/v), and an internal standard (12 µg of methyl nonadecanoate) was added. Sample analysis was conducted on a Varian 3800 gas chromatograph equipped with an autosampler and coupled to a Star data analysis workstation (Varian Inc., Palo Alto, CA). An HP-5 (60 m x 0.25 mm 250 µm) capillary column was used with He as the carrier gas and a temperature program as described by Frostegard et al. (1993). Methyl esters were identified by mass spectrometer analysis using an HP 5973 mass-selective detector (Agilent, Palo Alto, CA) coupled to an HP 6890 engine. Individual PLFA content (ng g–1 soil) was determined relative to the peak size of the internal standard.
Field Validation
Soil physical and microbial characteristics were compared at the two LTSP plantation sites in May 2001. Four transect lines were established in the no, moderate, and severe compaction plots at each site. Five sample points were taken per transect line for a total of 120 sample points. Sampling protocol followed a strict progression at each sample point: (1) surface CO2 efflux was measured within a 12.6 cm2 sampling area using a LI-6200 infrared gas analyzer, (2) soil strength was measured to a minimum depth of 15 cm within the same sampling area using a CP20 recording cone penetrometer (Rimik, Australia), and (3) a soil sample (0–15 cm depth) was collected within the sampling area for laboratory analysis of microbial biomass and respiration. All sampling was completed within 20 min. at a given sample point. Soil samples were sieved (2 mm) and analyzed for microbial biomass and respiration as described above within 24 h of collection. Soil organic matter content also was determined on all samples by loss-on-ignition (Hesse, 1971).
Statistical Analyses
All measures from the laboratory experiment, with the exception of semi-weekly CO2 efflux, were analyzed using a fixed-model ANOVA and Tukey's mean comparison (SAS Institute, 1998). Soil compaction and sampling date were the main effects. Semi-weekly measurements of CO2 efflux were analyzed by repeated measures analysis using PROC MIXED (SAS Institute, 1998). For this model, compaction was a fixed effect and time was a random effect since differences in time were not expected but serial correlation of the consecutive measurements was expected. Regression analysis was used to determine the relationship between physical (soil strength) and biological (microbial biomass and respiration) characteristics measured in the field. Data from the two soils were not compared statistically in either experiment. Significance for all statistical analyses was at 
= 0.05 unless otherwise stated.
Functional diversity was compared using the area-under-the-curve data for each Biolog well (Guckert et al., 1996). Structural diversity was compared using the content (ng g–1soil) of all PLFAs. We used principal component analysis (PCA) to explore (i) compaction related changes in microbial population function and structure and (ii) potential relationships among soil type, treatments, and soil biological and physical properties measured in the laboratory experiment (all microbial and physical measures).
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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= 0.05) for changes in total porosity, macroporosity, and habitable pores (0.2–30 µm) in the clay loam, and for macroporosity only in the sandy loam.
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Microbial Community Size and Activity
Microbial characteristics varied considerably between soils, but not between compaction treatments. The clay loam generally had twice the community size and activity as the sandy loam soil (Table 1). Few differences in community characteristics were related to compaction, however. Only 3 out of 44 comparisons (6 microbial indices x 4 sampling dates x 2 soils; N-min only reported for two sampling dates) showed statistically significant treatment effects (Table 1). Fungal hyphal length was greatest in the severe treatment on Day 4 in the sandy loam and on Day 32 in the clay loam. Nitrogen mineralization also was greater following severe compaction on Day 32 for the clay loam. No negative effects with compaction were found.
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Carbon Use (Biolog)
Carbon use by bacteria varied between soils and among compaction treatments. Average well-color development was two to three times greater for the clay loam compared with the sandy loam soil (Fig. 4)
. Bacterial responses to compaction were unique for each soil. In the clay loam, severe compaction was associated with greater AWCD compared with no compaction (P = 0.052). Differences between treatments were greatest on Day 67, although the compaction x time interaction was not significant (P = 0.843). In contrast, a transient response with compaction was found for the sandy loam soil. Significantly greater C use was noted in the severe compaction treatment than either moderate or no compaction (P < 0.0001) on Day 11 only, yielding a significant compaction x time interaction (P < 0.0001). Differences among compaction treatments were consistent regardless of substrate type (carbohydrates, carboxylic acids, amino acids, polymers). Substrate richness (number of 95 compounds metabolized) also responded positively with compaction in the clay loam soil. Moderate and severe compaction treatments averaged 12% greater substrate richness compared with the control soil (P = 0.013), primarily as a result of metabolism of carbohydrates (N-acetyl-D-galactosamine, i-erythritol, D-galactose, alph-D-glucose, D-mannitol, and L-rhamnose). No differences in substrate richness were found among treatments for the sandy loam soil.
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Phospholipid Fatty Acid Profiles
Forty PLFAs were identified in each soil on Day 67 of the experiment. Total PLFA content was more than two-fold greater in the clay loam than the sandy loam soil (Table 2), presumably indicative of greater microbial biomass. No differences in individual bacterial (including actinomycetes) or fungal PLFA were found in the clay loam soil. In contrast, 10 out of 15 bacterial PLFAs increased with severe compaction in the sandy loam soil. Moderate compaction was associated with a decline in fungal and actinomycete PLFA, although the treatment effect was not significant ( = 0.05) due to considerable within-treatment variation.
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0.05.
Multivariate Analysis
Ninety-six soil cores (3 treatments x 2 soil types x 4 sampling dates x 4 reps) were analyzed for 12 biological and physical properties in this experiment. Principle component analysis was used to explore data variability related to the soil properties and to elucidate potential relationships to soil type, compaction treatment, and property type. Nitrogen mineralization and PLFA were excluded from the analysis since they were not measured on all sampling days resulting in a data matrix with 96 rows (soil cores) and 10 columns (soil properties).
This dataset was well explained by PCA: three PCs accounted for 84% of the variability. Soil type accounted for the largest portion of the variation, as the two soils clearly separated along PC1 (Fig. 5) . Soil properties with the heaviest loadings on PC1, indicative of major differences between the two soils, were CO2 efflux, bulk density, total porosity, respiration, microbial biomass, and total bacteria.
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Finally, several of the soil properties of the clay loam grouped into distinct clusters (Fig. 6), representing highly related variables (e.g., microbial biomass, respiration, total bacteria). Interestingly, CO2 efflux was clustered with total porosity and air-filled porosity, indicative of the relationship between CO2 efflux and soil porosity. Respiration, in contrast, showed little correlation to CO2 efflux. The relationship between CO2 efflux and soil physical properties was less apparent in the sandy loam soil. This likely reflects the greater tolerance of CO2 efflux to compaction in the coarser-textured soil (Fig. 3).
Field Validation
Field investigation results from the two LTSP sites corroborated our laboratory findings that a weak relationship exists between soil physical and biological properties in compacted soils. Mean bulk density ranged from 1.00 to 1.30 Mg m–3 across compaction plots at Challenge (clay loam) and from 1.2 to 1.45 Mg m–3 at Rogers (sandy loam). Soil strength also varied greatly, from 446 to 3865 kPa at Challenge and from 75 to 3391 kPa at Rogers. Correlation of these physical parameters with microbial characteristics was low, however. As an example, Fig. 7 shows the lack of relationship between soil strength and microbial biomass across 120 sample points. Soil strength also was unrelated to either respiration or surface CO2 efflux at both sites. Instead, soil organic matter content showed a stronger relationship to both microbial biomass and respiration, particularly at Challenge.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The results showed that compaction was not detrimental to microbial community characteristics. Eight indices of community size, activity, function, and structure were compared, providing a cross-section of traditional and modern techniques for comparison of coarse-level community changes. No statistically significant decline with compaction was found for any of the indices, regardless of soil type or sampling date. In fact, several indices (C use, PLFA, and fungal hyphae) increased with compaction, suggesting rapid and successful adaptation to the altered environment.
Carbon use (Biolog) and PLFA are common measures of the functional and structural diversity of soil bacteria, respectively. Although considered complimentary (Buyer and Drinkwater, 1997; Widmer et al., 2001), the methods differ with respect to their target organisms. Carbon use measures the functional ability of culturable bacteria, while PLFA provides a coarse-level profile of the total bacterial community. Greater C use and PLFA content were found with severe compaction compared with the control and moderate compaction treatments. However, the response was not universal. In the sandy loam soil, C use was greater in severe compaction samples on Day 11 only, while PLFA content of several bacterial markers was greater on Day 67. Treatment differences in C use increased with time in the clay loam soil, while no differences in PLFA markers were found. These discrepancies, plus the lack of corroboration with other microbial measures, suggest that the stimulation of bacterial diversity by compacting was both inconsistent and inconclusive.
A caveat to the general observation of microbial tolerance to compaction was a decline in surface CO2 efflux for the clay loam soil. Semi-weekly measurements averaged 50% less CO2 efflux for the severely compacted treatment compared with the control. We were particularly interested in whether this response was due to reduced microbial respiration, gas diffusion, or a combination of both. Reduced CO2 efflux in compacted soil has been observed by many (Torbert and Wood, 1992; Dulohery et al., 1996; Conlin and van den Driessche, 2000), and the primary role of restricted gas diffusion is often cited (Currie, 1984; Xu et al., 1992; Pumpanen et al., 2003). Exchange of gases between soil and atmosphere occurs mainly by molecular diffusion and requires an adequate and continuous system of air-filled pores (Currie, 1984). Evidence from our study supports this idea. A strong relationship between CO2 efflux, air-filled porosity, and total porosity was identified in multivariate analysis (indicated by the close grouping of these variables in Fig. 6). Additionally, compaction had little affect on soil respiration or microbial biomass at each of the four sampling dates. Both parameters were measured during the initial 4 h following sampling, before any appreciable recovery of the microbial community would be anticipated. Although actual rates of gas diffusion were not measured, we surmise that the reduction in CO2 efflux resulted from limited air-filled porosity, irrespective of biological activity.
Compaction favored smaller, habitable-sized pores (0.2–30 µm) at the expense of large pores. Small pore volume increased 72% in the clay loam and 39% in the sandy loam with severe compaction, while macropores were nearly eliminated in both soils. This observation helps explain the indifference shown by microorganisms. Habitat condition (available pore space) was improved by compacting, and apparently offset any detrimental effects of restricted air, water, or nutrient flow. Whether the reduction in large pores also led to fewer bacterial and fungal predators, as suggested by Hassink et al. (1993) and Postma and van Veen (1990), was not tested.
That compaction drastically reconfigured pore structure without a reduction in microbial habitat size also helps explain the results from our field study and from numerous other LTSP study sites across North America. Microbial biomass and respiration were unrelated to physical measures of soil compaction (bulk density and soil strength) at the two LTSP sites. Similarly, nominal changes in microbial biomass, PLFA structural diversity, genetic diversity of bacteria, mycorrhizae, and litter decay due to compaction have been found at other LTSP sites (Jordan et al., 1999; Kranabetter and Chapman, 1999; Li, 2000; Chow et al., 2002; Ponder and Tadros, 2002). Axelrood et al. (2002) noted some differences in bacterial composition between treatments at LTSP sites in British Columbia, although total bacterial diversity was high regardless of compaction treatment. To date, Li et al. (2003) have reported the only decline in microbial community response to compaction within the network of LTSP sites. They found reduced N mineralization in a loblolly pine plantation for 5 yr following treatment, and suggested the potential role of poor aeration or limited physical access to organic N.
We deliberately wetted all soil cores to optimum moisture content (50–60% WHC) before compacting. As a consequence, substantial differences in soil aeration were attained following treatment. This was most evident in the clay loam soil. About 40% of the pores were air filled in the control treatment, compared with <10% in the severely compacted soil. Heterotrophic activity (respiration) did not differ between treatments during the 10-wk experiment, however. Nor was an increase in PLFA markers for anaerobic bacteria (17:0cyc and 19:0cyc) found. This finding differs from the results of Linn and Doran (1984), who found heterotrophic activity declined considerably as the air-filled pore space dropped below 40% in an intensively managed agricultural soil. Whether the disparity between studies is related to differences in total soil porosity or air permeability, or is simply a reflection of inherent differences between agricultural and forest soils is unclear. From a practical standpoint, restrictive aeration is a transient condition (at most) in the summer-dry, Mediterranean climate of California. Results may differ, however, for poorly drained soils in regions of high precipitation, where anaerobic conditions are likely extended on compacted sites.
The indifference shown by the two microbial communities to compaction provides an interesting contrast to response of tree growth at the LTSP sites. Gomez et al. (2002b) found that severe compaction reduced tree growth nearly in half at the clay loam site and increased tree growth more than two-fold at the sandy loam site. The growth decline at the clay loam site was considered a function of severe mechanical impedance to root growth as the soil dried during the spring and summer months. Microbial biomass and activity, in contrast, showed little response to a wide range of soil strength values in the field study. The differential response of the two life forms to compaction underscores this concept and highlights the insubstantial link between soil physical disturbance and microbial characteristics at the LTSP sites.
Both the laboratory and field studies were designed specifically to test the physical effects of compaction on microbial communities. Skid trails, in contrast, typically confound physical compaction with soil mixing and displacement during harvesting operations. Soil organic matter and nutrient pools often are reduced in skid trails, to the detriment of microorganisms (Dick et al., 1988). At the LTSP sites, heavy equipment was kept off all plots during harvesting, and tree boles were fully suspended during removal. Soil mixing was further avoided by temporarily removing surface organic matter before compacting. The results of our study, therefore, are applicable to one of several compaction-related problems associated with harvesting.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Our results agree with most other studies of soil compaction from the network of LTSP sites in North America that show tolerance or resilience by microbial communities. We suggest the fundamental explanation for these observations lies in the reconfiguration of pores following compaction. Compacting both the clay loam and the sandy loam soils resulted in reduced total porosity and a near elimination of large pores. In contrast, habitable pore volume, accessible primarily to bacteria and fungi, increased in both moderately and severely compacted soils. Therefore, with the exception of poorly drained soils or for those regions receiving high annual precipitation where saturation is a concern, soil physical changes associated with compaction appear to be of little consequence to the microbial community.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication January 21, 2004.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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