Journal of Environmental Quality 30:602-607 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
WASTE MANAGEMENT
Stabilization of Composted Organic Matter after Application to a Humus-Free Sandy Mining Soil
Jens Leifelda,
Stefanie Siebertb and
Ingrid Kögel-Knabnerc
a Federal Research Station for Agroecology and Agriculture FAL/IUL CH 3003 Bern-Liebefeld, Switzerland
b VHE-NRW, D-40479 Düsseldorf, Germany
c Technische Universität München, Lehrstuhl für Bodenkunde, D-85350 Freising-Weihenstephan, Germany
Corresponding author (jens.leifeld{at}iul.admin.ch)
Received for publication June 21, 1999.
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ABSTRACT
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The use of mining substrates for recultivation purposes is limited due to their low organic matter (OM) contents. In a 1-yr laboratory experiment we evaluated the stabilization of biowaste compost added to a humus-free sandy mining soil to examine the suitability of compost amendment for the formation of stable soil organic matter (SOM). The stabilization process was characterized by measuring enrichment of OM and nitrogen in particle size fractions obtained after dispersion with different amounts of energy (ultrasonication and shaking in water), carbon mineralization, and amount of dissolved organic carbon (DOC). During the experiment, 17.1% of the organic carbon (OC) was mineralized. Organic carbon enrichment in the <20-µm particle size fraction at the beginning of the experiment was in the range of natural soils with similar texture. Within 12 mo, a distinct OC redistribution from coarse into fine fractions was found with both dispersion methods. The accumulation of OC was more pronounced for the size separates obtained by ultrasonication, where the carbon distribution between 0.45- to 20-µm particle size fractions increased from 30% at the beginning to 71% at the end of the experiment. Dissolved organic carbon contents ranged between 50 and 68 g kg-1 OC and decreased during the incubation. In conclusion, the exponential decrease of carbon mineralization and the OC enrichment in the fine particle size fractions both indicated a distinct OM stabilization in the mining soil.
Abbreviations: DOC, dissolved organic carbon • OC, organic carbon • OM, organic matter • SOM, soil organic matter
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INTRODUCTION
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A SERIOUS PROBLEM that has to be considered when recultivating open cast mining areas is the low OC content of the mining substrate. A possible way to increase the OM content and thus to improve the chemical and physical properties of these substrates is the addition of organic materials, such as biowaste compost, as a precondition for afforestation or agriculture (Senesi, 1989; Haubold and Katzur, 1999). Although the suitability of compost amendment as a means of OM enrichment in soils is well documented (Senesi, 1989; Hadas and Portnoy, 1994; Giusquiani et al., 1995), a more precise understanding of the processes leading to a stabilization of the added OM is still missing, especially for mining substrates with very low OM content.
Key factors affecting stabilization of SOM are the accessibility of SOM, interactions with soil minerals, and enrichment of recalcitrant residues of plants and microbes (Sollins et al., 1996). Protection of SOM by formation of organo–mineral associates with primary mineral particles or by formation of soil aggregates not only depends on biological parameters but to a considerable degree on soil texture (Christensen, 1996; Skene et al., 1996).
Several studies demonstrate an enrichment of OC in the finer primary particle size fractions of soils (Bremner and Genrich, 1990; Preston et al., 1994; Stemmer et al., 1999). Characterization of the OM in particle size fractions indicates increased humification with decreasing particle size (Baldock et al., 1992; Guggenberger et al., 1994; Hopkins et al., 1993) and lower mineralization rates of the OC in fine fractions (Gregorich et al., 1988). Consequently, the enrichment of OM in fine mineral fractions may represent a major stabilization process controlling SOM storage. Sandy soils incorporate less OC into organomineral associates than fine textured soils and therefore have only low OM protection capacities (Hassink, 1996; Skene et al., 1996).
Mining soils obtained by open cast brown coal mining in eastern Germany are often composed of tertiary sediments overlaid by quaternary sandy meltwater deposits, the latter used as the surface layer to initiate the recultivation. Stabilization of added organic materials in these loosely packed quaternary substrates may be limited due to low clay and fine silt contents of the sediments and a supposedly low protection capacity for OM. At some of these recultivated sites, proportions of clay and silt in the sediments together account only for 4% of the total mineral matter (Haubold-Rosar and Katzur, 1999).
To obtain more detailed information about the suitability of compost amendment as an approach to meliorate mining substrates by forming stable SOM, we conducted a laboratory microcosm experiment. A biowaste compost was added to a quaternary sandy mining soil in a 1-yr laboratory experiment and size fractions were obtained at the beginning and the end of the experiment. The capacity of the mining soil to stabilize the added compost material was evaluated by investigating the distribution of OM among particle size fractions and the carbon mineralization of the bulk soil. Dissolved organic carbon, an easily accessible, nonprotected OM fraction, was quantified during the fractionation.
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MATERIALS AND METHODS
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The compost was a commercially available biowaste compost, produced from biogenic waste of households and gardens and composted 21 d in a covered open windrow system. The ratio of house to garden biogenic waste was 70:30 (v/v). The biowaste compost was sieved to <10 mm and compost maturity was determined by a self-heating test in a dewar vessel, reaching a temperature of 46°C and thus representing an immature compost according to the German Federal Compost Quality Commission (Bundesgütegemeinschaft Kompost e.V., 1994). The maturity of biowaste composts determined by the self-heating test was found to be closely correlated to that characterized by the respiration intensity (Popp et al., 1998). The compost had an OC content of 232.7 g OC kg-1 dry matter, a C to N ratio of 13, and a pH of 7.8. As a mineral soil substrate, we used a sandy mining soil (quaternary sandy meltwater deposit, pH 6.7) from an open cast brown coal mining in Lusatia, Germany. Contents of OC and N of the mineral soil substrate were under the detection limit of 0.5 and 0.1 g kg-1 dry matter, respectively.
The microcosms consisted of PVC tubes with a diameter of 200 mm and length of 400 mm. For the incubation experiment, 3790 g (dry wt.) of the soil substrate and 296 g (dry wt.) compost were mixed and filled in the microcosms (layer thickness 10 cm, soil bulk density 1.3 g cm-3). This application is equivalent to 70 Mg ha-1 biowaste compost (dry wt.) in the field (calculated for the same incorporation as described above). The compost addition resulted in soil–compost mixtures with a pH of 7.5 and OC contents of 15.2 to 17.0 g OC kg-1 dry matter in the fraction <2 mm. Under controlled conditions, at 14°C and 50% maximum water holding capacity (WHC), the soil–compost mixture was incubated in a microcosm system in duplicates for 12 mo. Aliquots of the soil–compost mixtures were taken at the day of their preparation and after 12 mo of incubation and freeze-dried.
Microcosms were covered with an air-permeable film during the experiment. To quantify the carbon mineralization, they were closed airtight for 0.5 (beginning of the experiment) to 8 (end of the experiment) h every 1 to 2 wk, depending on the actual CO2 production rate. The range of linear CO2 concentration increasing with time within the headspace was tested in preceding experiments. Gas samples were taken with a gas-tight syringe through a rubber stopper and analyzed for CO2 by gas chromatography (HP [Palo Alto, CA] 5890 Series II, TCD, packed column, detector temperature 110°C, column temperature 50°C, carrier gas He 5.0). The amount of carbon mineralized was calculated based on the curve fitting of the released CO2 and the amount of OC in the soil compost mixture of each microcosm. Decay rates for four different time steps of the incubation were expressed as: mg CO2–C d-1 g-1 OC.
For the particle size fractionation, two different techniques were used. For complete disaggregation, samples of the two microcosms were mixed, 30 g dry matter of the mixed sample was suspended in 150 mL of demineralized water, allowed to settle for 3 h, and then ultrasonically dispersed with an energy of 450 J mL-1 according to Schmidt et al. (1999). In addition, 30 g of the soil sample were suspended in 600 mL distilled water and shaken end over end for 5 min. This weak dispersion allowed an estimate of potential redistribution of OC due to the higher energy input by sonication. Sand fractions were isolated by wet sieving (63 to 200, 200 to 630, and 630 to 2000 µm). Silt fractions (2 to 6.3, 6.3 to 20, and 20 to 63 µm) and clay (<2 µm) were separated by sedimentation of the remaining suspension. Size separates were freeze-dried and analyzed for OC and nitrogen (for fractions obtained after shaking only) with a LECO (St. Joseph, MI) CHN 1000 analyzer. Samples were free of carbonate. Organic carbon within each fraction as a proportion of the total carbon was calculated by: [OC content in fraction (g kg-1) x mass of fraction (%)]/total OC recovered (g kg-1).
The water extract obtained after filtering each fraction over a 0.45-µm cellulosenitrate filter (Sartorius, Göttingen, Germany) was analyzed for DOC to quantify the carbon solution losses during the fractionation procedure. The recovery of the mineral soil exceeded 98% compared with the conventional particle size analysis (soil dispersion with Na-metaphosphate followed by pipette–sieve method according to Gee and Bauder, 1986) that was carried out for the pure mineral soil substrate without compost.
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RESULTS AND DISCUSSION
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Carbon Mineralization
Carbon mineralization occurred mostly during the first 150 d, following first-order kinetics (Fig. 1)
. During the incubation, 17.1% (±1.2%) of the added OC was mineralized (Table 1). The easily degradable OC that was rapidly consumed by the microorganisms within the first 60 d accounted for approximately 14% of the added OC. The remaining proportion of OC that was not mineralized during the incubation, due to physical stabilization and chemical recalcitrance, accounted for more than 80% of the OC. The relative disappearance of decomposable compounds during the incubation is reflected by the decay rates, which decreased from 3.7 to 0.02 mg CO2–C d-1 g-1 OC at the beginning and the end of the experiment, respectively.
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Fig. 1. Cumulative CO2 production during a 12-mo incubation at 14°C. Each point is the mean of two replicates
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Table 1. Mean percentages of organic carbon (OC) mineralized and mean decay rates of the mining soil at different stages of decomposition. Standard deviations are given in parentheses
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The quantity of carbon mineralized is higher than expected for soils without organic amendments but only slightly higher than that obtained for soils amended with manure composts. Proportions of mineralized carbon in soils amended with manure composts were found to be in a range between 7 to 17% and 8 to 14% of the OC, respectively (Atallah et al., 1995; Hadas et al., 1996).
Particle Size Distribution
The particle size distribution of the mining soil following chemical dispersion showed that the sand fraction accounted for 93% and the silt + clay fraction for only 7% of the mass of mineral soil (Table 2). Differences among the chemical dispersion, the strong ultrasonic dispersion, and the weak dispersion by shaking were apparent in the >2-mm coarse fraction and in the <20-µm fractions. Disaggregation by shaking led to a higher yield of particles >2 mm and particles in the size class 20 to 2 µm. The higher amount of particles >2 mm for the shaken samples could be ascribed to visible coarse compost particles. Clay contributed only 0.9% to the sum of particles compared with 2.4 to 4% clay with chemical and ultrasonic dispersion, respectively.
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Table 2. Particle size distribution of the mining soil determined by ultrasonic dispersion with 450 J mL-1, end-over-end shaking and after chemical dispersion. Standard deviations are given in parentheses
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A higher yield of 20- to 2-µm particles with the end-over-end shaking method may reflect incomplete disaggregation. Hence, the compost application resulted in the formation of microaggregates that were resistant to shaking. The relative weakness of a 5-min end-over-end shaking compared with a strong ultrasonic dispersion is in agreement with a study of Raine and So (1997), who compared different dispersion techniques.
Carbon and Nitrogen in Particle Size Fractions and Dissolved Organic Carbon
In all samples, highest OC contents were found in the >2000-µm fraction, due to visible coarse woody compost particles, and in the <20-µm fine particle size fractions (Table 3). Comparing the two fractionation methods, maximum OC contents within the <20-µm fine fractions were found in the fine silt (6.3 to 2 µm) for the samples dispersed by ultrasonication, but in the clay fraction for the samples dispersed by shaking. In the two fractions >630 µm, OC contents were lower after ultrasonic dispersion than after shaking. Organic carbon contents of the 630- to 63-µm size fractions were low and independent of the fractionation method. Differences in OC contents of the fractions between dispersion methods might be attributable to abrasion of mineral particles in the coarse fractions that accumulated in the clay fraction or a redistribution of coarse organic particles due to ultrasonic breakdown. A redistribution of OC from the coarsest fraction into silt-sized fractions due to mechanical disruption of compost fragments during ultrasonication is also apparent in a lower proportion of the gravel-sized fraction.
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Table 3. Contents of organic carbon (OC) in the size separates of the samples dispersed by ultrasonication and end-over-end shaking at the beginning (0 mo) and the end (12 mo) of the experiment and contents of N and C to N ratios for size separates dispersed by end-over-end shaking
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Changes in OC contents of the different size fractions in 12 mo of incubation are consistent despite the different fractionation methods: OC contents decreased in the coarse and medium sand fractions, but increased in the three smallest size fractions. This change in OC content was more pronounced for the samples treated ultrasonically.
Nitrogen concentrations followed patterns similar to those of carbon, with high concentrations in the very coarse and very fine fractions and low concentrations in the medium and fine sand. Carbon to nitrogen ratios of the fractions at the beginning of the experiment decreased with decreasing particle size. The C to N ratios of 4 to 7 in the fine silt and clay may indicate a high proportion of microbial derived products as described in general for fine fractions (Christensen, 1992). At the end of the experiment, the C to N ratio of the clay fraction was considerably higher than at the beginning of the incubation. One possible reason for the increase of the C to N ratio during the experiment might be an accumulation of mineralized N in the soil (since there is no leaching and no N uptake by plants) that is removed by solution during fractionation. The soluble mineral N of the bulk soil increased from 0.4 to 8% of the total N at the beginning and the end, respectively, of this experiment (Siebert et al., 1998). Assuming that a high proportion of mineralized soluble N was located in the clay fraction, a selective enrichment of carbon over nitrogen might explain the observed shift in the C to N ratio of the clay fraction. The preferential mineralization from N in clay fractions compared with coarse fractions was shown by Cameron and Posner (1979).
The distribution of carbon among particle size fractions (percent of the sum of OC in particle size fractions) at the beginning and the end of the experiment is given in Fig. 2
for each of the dispersion techniques. For the soil dispersed by ultrasonication, 42% of the OM was located in the 63- to 2000-µm fraction at the beginning of the experiment. Particles >2000 µm accounted for approximately 19% of the total OC. Almost 24% of the OC was found in the 63- to 2-µm silt fraction and 14% in the clay fraction. During incubation, OM in silt and clay fractions increased to 73% of the OC while the >63-µm fraction decreased to 27% of OC. The process was particularly significant for the clay fraction, with an increase from 14 to 45% of OC.
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Fig. 2. Organic carbon distribution among particle size fractions in the mining soil at the begining of the experiment and after 12 mo dispersed by ultrasonication (A) and shaking (B)
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For the samples dispersed by shaking, 38% of the OC was in the gravel fraction and 25% was in the silt and clay fraction before incubation. The high proportion of OC in the >2000-µm fraction can be ascribed to coarse compost particles, as indicated by the absence of this fraction for the mineral soil without compost application. As for the samples dispersed by ultrasonication, a noticeable redistribution from the coarse to the fine size fractions occurred during 12 mo of incubation, but to a lower extent. Since the distribution of OC among particle size fractions is a function of their OC content and of the yield of each fraction, the lower yield of clay by shaking accounts for this smaller redistribution.
A comparison of OC accumulated in the clay and silt fractions with data for OC enrichment revealed by Hassink (1996) for several agricultural soils showed similar trends, although the OC located in the fine fractions of the mining soil at the end of our experiment exceeded the accumulation reported by Hassink (1996). This agreement might indicate that the accumulation of SOM in fine fractions quickly reaches levels of agricultural soils, but requires a sufficient OM input. A rapid transformation of added OM into finer particle size fractions was also demonstrated by Aita et al. (1997) for an agricultural A horizon, where 70% of residual added labeled 13C straw material was found in the <50-µm fraction within 1 yr. Also, Stemmer et al. (1999) found in a 1-yr study of different A horizons an enrichment of residual labeled maize straw in fine particle size fractions. Thus, the observed transfer of OC into the finer particle size fractions of the mining soil may represent a basic process of humification.
The proportion of 22% (shaking) and 30% (ultrasonic dispersion) of OC in the <20-µm fraction of the mining soil immediately after compost application may be explained by a redistribution of already existing finer particles or soluble compounds in the compost. In particular, microbial biomass and microbial biomass-derived products may have contributed significantly to the relatively high proportion of OC in the fine particle size fractions. This assumption was supported by the low C to N ratios of the fine fractions and the high microbial biomass, which accounted for approximately 15% of the total OC in the compost (Leifeld et al., 1999). Fine particles were found to contribute to minor proportions of composted materials by Aoyoma (1985), who obtained 4 to 7.7% of the OC in fine fractions from different composts by centrifugation. Atallah et al. (1995) investigated particle size distributions of manure composts and found proportions of 10 to 21% dry matter in the <50-µm compost fraction.
In addition to fine particles, soluble compounds of the compost may also have contributed to the OC content in the fine fractions, due to the formation of organo–mineral associates. Dissolved organic carbon contents accounted for 6.8. and 5.2% (ultrasonic dispersion) and 5.9 and 5.0% (shaking) of the total OC at the beginning and the end of the experiment, respectively. These yields indicated a slightly increased stabilization of OC in the mining soil during the incubation concerning its water extractability, but the DOC contents at the end of the experiment were still high. Schmidt et al. (1999) obtained lower DOC yields with an average of 3.3% of the OC for five A and B horizons without compost amendment but using the same approach of fractionation. For composts, Jörgensen et al. (1996) found 1.24% of K2SO4–extractable OC for a mature biowaste compost. Gonzales-Prieto et al. (1993) found amounts of extractable OC for composts in a range of 3 to 14% OC (hot water extract). The extraction of immature composts with cold water gave soluble proportions of 2 to 5% of total OC (Avnimelech et al., 1996). The observed decrease of soluble OC was low compared with the decrease of carbon mineralization. Therefore, the soluble OC was less suitable to estimate the amount of easily degradable OC and pointed toward a substantial proportion of nonprotected but chemically stabilized OM in the compost-amended soil.
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CONCLUSIONS
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In a 1-yr laboratory experiment biowaste compost application was shown to increase OC contents and to produce stabilized SOM in a sandy mining soil that was not exposed previously to any OM input. This stabilization took place within several months as indicated by an exponential decrease of carbon mineralization, the formation of microaggregates resistant to shaking, and an enrichment of OC in silt- and clay-sized particle fractions. The observed enrichment of OC in fine particle size fractions was in the upper range of natural soils and may represent a basic process of SOM stabilization. Accumulation of OC in the fine particle size fractions occurred to a considerable extent immediately after compost application, presumably accelerated by nonoccupied sorption sites in the mineral soil matrix. However, the comparable high yields of soluble organic compounds that were still present at the end of the experiment indicated a possible surplus of added OM to the sandy substrate. The consideration of the texture of mining soils may be helpful as a means to assess appropriate amounts of OM to be added for recultivation purposes.
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ACKNOWLEDGMENTS
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This work was financially supported by the German Federal Ministry of Education, Science, Research and Technology (1460638L). We thank PlanCoTec (Witzenhausen, Germany) for providing the compost material and M. Haubold-Rosar (FIB Finsterwalde, Germany) for providing the quaternary mining soil.
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ABSTRACT
INTRODUCTION
