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TECHNICAL REPORTS

Waste Management

Effects of Moisture and Temperature on Carbon and Nitrogen Mineralization in Mine Tailings Mixed with Sewage Sludge

Department of Soil Sciences, SLU, Uppsala 750 07, Sweden

^* Corresponding author (par.wennman{at}mv.slu.se)

Received for publication April 29, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Digested sewage sludge mixed with copper mine tailings was incubated for 3 mo at 16 combinations of temperature (–1, 5, 10, and 15°C) and soil moisture content (2, 8, 14, and 24% by weight). Carbon dioxide evolution and net N mineralization were measured at increasing time intervals. A two compartment first-order kinetic model (refractory and labile C) was fitted to the time series of measured CO₂ fluxes using nonlinear regression analysis. The dependencies of the rate constants on moisture and temperature could be well described by log-linear functions. The estimated Q₁₀ value (the factor by which the rate is increased as temperature is increased 10°C) was 2.55. Within the range of temperature and moisture considered here, which correspond to conditions occurring naturally in Sweden, CO₂ evolution was more strongly controlled by moisture than by temperature. Less mineral N accumulated during the experiment at the lowest moisture or temperature. However, the dependency of net N mineralization on moisture and temperature in the remaining treatments was less clear than for C evolution, presumably due to denitrification at the higher temperatures and moisture contents. Nitrate was formed after around 2 wk but only at 10 and 15°C.

Abbreviations: Q₁₀, the factor by which the rate is increased as temperature is increased 10°C • WHC, water holding capacity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
RECLAMATION of mine tailings through plant establishment is desirable to reduce environmental impacts, for example the prevention of wind and water erosion and leaching of metals, specifically from pyritic tailings. However, some kind of organic amendment such as sewage sludge has to be applied to ensure successful establishment (Gemmell, 1977; Seaker and Sopper, 1988a, 1988b; Daniels and Haering, 1994). The growth substrate formed by mixing tailings with sludge may itself constitute a pollution problem through its effect on surface water and ground water. First, pyrite in the tailings readily oxidizes to sulfate under aerobic conditions, resulting in a pH drop to levels at which metals are mobilized (Ferguson and Erickson, 1988). Second, nitrogen and phosphorus are released by biological and chemical transformations of the sludge.

Seasonal and diurnal changes in weather conditions such as wind, sun, and precipitation affect temperature and moisture in the uncovered tailings. After application and mixing or mulching of organic material such as sewage sludge into the top layer, variations in temperature and moisture are still expected. However the amplitudes of the fluctuations are reduced through increased energy consumption in warming the substrate, but also via increased water holding capacity and changed pore size distribution (Unger, 1978; Tate, 1987, p. 260–280).

Microbial activity and N mineralization generally increased with temperature (Waksman and Gerretsen, 1931; Kirschbaum, 1995), which together with soil moisture is one of the most important factors for decomposition (Stanford and Epstein, 1974; Andrén et al., 1990; Antonopoulos, 1999). Roughly, a doubling of the activity with each 10°C temperature increase can be expected in the range from 5 to 35°C (Stanford et al., 1973; Kätterer et al., 1998). Microbial activity also occurs outside this temperature range, but decomposition rates are reduced both under warmer (Harmsen and Kolenbrander, 1965) and colder conditions (Lomander et al., 1998a; Schmidt et al., 1999). Decomposition rates usually increase with water potential, from about –5 to about –0.05 MPa (Wilson and Griffin, 1975; Lomander et al., 1998a), and then decrease in wetter conditions due to oxygen deficiency (Campbell, 1978). Nitrification only takes place when oxygen contents are adequate, which occurs at low and intermediate soil moisture contents, and when temperatures are above 5°C (Anderson and Boswell, 1964; Sierra et al., 2001).

Although reclamation of mine spoil with sewage sludge has been performed for more than 30 years (Haering et al., 2000), knowledge of carbon and nitrogen transformations is still limited. The main objective of this experiment was to quantify the effects of temperature and moisture on C and N mineralization rates in pyritic mine tailings mixed with digested sewage sludge, across ranges reflecting the conditions expected in the field. These relationships could be used in models for predicting C and N dynamics during decomposition of sludge (e.g., Schmied et al., 2003). Together with information on crop N uptake potential, this should allow better precision in determining sludge application rates before establishment.

	MATERIALS AND METHODS

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Substrate Properties
Sewage sludge stabilized by digestion at 33 to 37°C, taken from the municipal sewage treatment plant at Henriksdal, Stockholm, together with pyritic mine tailings from the copper mine Aitik, near Gällivare, Sweden (67°0.06' N, 20°0.8' E) were used as the substrate in an incubation experiment. Iron sulfate was used to precipitate phosphorus in the sewage water and the digested sludge was dewatered to a water content of 65 to 75%. Properties of the sludge and tailings and the mixture of both are presented in Table 1.

The tailings produced at Aitik consist mainly of fine sand (87–89%), but also of small amounts of silt (7–8%) and clay (4–6%), measured according to Sheldrick and Wang (1993). The total porosity, measured in field samples in an earlier study, varied between 43 and 50%, dominated by pores with diameters between 5 and 300 µm (Stjernman-Forsberg and Ledin, 2003). The tailings are mainly composed of muscovite, biotite, and traces of pyrite. Data on metal contents in the sludge and tailings can be found in Stjernman-Forsberg and Ledin (2006).

The mixture of tailings and sludge possesses a moderate buffering capacity in the first months, since the pH decreased from around 7 down to just above 5 over a 3-mo period (Ledin and Wennman, 2005).

The tailings were stored in shaded plastic bags outdoors for approximately 3 mo at 15 to 20°C and thereafter at –18°C together with the sludge before the start of the experiment. Dry matter was determined by drying at 105°C for 24 h and water holding capacity (WHC) was determined gravimetrically by placing 100 g of the tailings–sludge mixture in a funnel with a fine mesh at the bottom. The samples were saturated and the mass was measured after 24 h of free drainage (Table 1).

Experimental Design
Twenty grams of moist tailings–sludge mixture were adjusted to four different moisture contents (2, 8, 14, and 24% by weight corresponding to 10, 39, 69, and 118% of WHC) and incubated at four different temperatures (–1, 5, 10, and 15°C) in a factorial design with a total of 16 treatment combinations. The moisture contents are referred to as % by weight throughout the article. Carbon dioxide evolution was determined on 64 samples (four replicates per treatment) at increasing time intervals throughout the experiment. Nitrogen mineralization was measured on a further 20 samples per treatment by destructive sampling: on each of five occasions (after 4, 11, 21, 50, and 97 d), mineral N was extracted from 4 of the 20 samples allocated to each treatment. At the end of the experiment, mineral N was also determined in the samples used for CO₂ evolution measurements. Soil moisture contents were chosen to cover the range of conditions occurring at the Aitik mine tailings site, that is, from slightly beneath wilting point up to optimal activity according to a previous pilot study. The temperatures were chosen to cover the natural range at Aitik, from winter frost to summer temperatures.

Preparation of Samples
All samples were prepared by mixing 4 volumes of moist tailings with 1 volume of fresh sewage sludge to a homogeneous substrate, with a water content of 19%. Sludge content, as deduced from carbon concentrations in the sludge and substrate, was approximately 6% by weight and the rate of sludge was equal to that used in a field experiment (Ledin and Wennman, 2005). The water content of the substrate was kept stable during sample preparation by covering the vessels with tight lids.

Field moist (16.2 g dry matter) homogenized substrate samples were transferred to 50-mL beakers (samples for CO₂ measurements) or to centrifuge tubes (samples for N measurements) before adjusting them to the required moisture content by drying or wetting. Samples for which the target moisture content was below field moisture were dried at low temperature (<30°C) and at high air exchange, while those above field moisture level were wetted with deionized water. According to water retention measurements conducted on field samples, 2% moisture content corresponds to a water tension of approximately 1.7 MPa, which is slightly less than the wilting point. According to a pilot study where moisture contents between 16 and 30% were tested, optimal activity occurred at a moisture content of 24%, which is larger than WHC. This is probably because the substrate in the vessels prepared for the experiment (including these in the pilot study) was less compacted than those prepared for determination of WHC (see Linn and Doran, 1984). Intermediate moisture levels of 8 and 14% were also chosen for the present experiment.

After moisture adjustment, the beakers were placed into tight 500-mL plastic vessels, each together with a vial filled with 5 mL 0.5 M NaOH as a carbon dioxide trap. In each chamber, two blank samples were incubated to correct for the initial concentration of CO₂ in the vessels. The centrifuge tubes used in the N mineralization experiment were sealed with rubber bungs and placed in the chambers at the same time as the samples prepared for the CO₂ measurements.

Incubation Procedure
A series of samples, that is, 16 (plus 2 blanks) for the CO₂ measurements and 80 for the N measurements containing the four different moisture contents were incubated in separate chambers at each of the four temperatures. Unfortunately, the incubation chamber at 5°C broke down after 37 d, and no measurements were made thereafter. All samples were aerated once a week at the beginning of the experiment and every second week toward the end. A constant moisture content (±0.1 g) was maintained by adding deionized water every second week corresponding to the loss of soil water determined by weighing. These adjustments were performed at room temperature (about 20°C), and took less than 30 min for any given temperature. The loss of water at 8 and 14% moisture content was generally negligible (<0.1 g per sample). At a moisture content of 24%, the loss of water was often larger than 0.1 g but almost never larger than 0.2 g. The tubes prepared for the N measurements were more air-tight than the CO₂ beakers, and generally lost less water.

Measurements of Carbon Dioxide, Ammonium Nitrogen, and Nitrate Nitrogen
Carbon dioxide fluxes (C_flux, mg g^–1 substrate d^–1) were measured by taking two subsamples of 1 mL NaOH solution from the trap and adding 2 mL of 1 M BaCl₂, followed by titration with 0.1 M HCl (Stotzky, 1965). The term C_flux at time t refers to the mean rate of CO₂ evolved during the time interval (i) between t – i/2 and t + i/2. Mineral N was extracted by shaking each sample with 50 mL 2 M KCl for 1 h, followed by centrifugation at 2500 revolutions min^–1 for 6 min. All extracts were stored at +4°C before colorimetric analysis for NH₄⁺–N and NO₃^––N on a TRAACS 800 auto analyzer (Bran+Luebbe, Norderstedt, Germany).

Modeling of Carbon Dioxide Fluxes
Carbon mineralization was assumed to proceed from two independent substrate fractions, that is, labile and refractory, decaying in parallel. A two-compartment, first-order kinetic model was fitted to the measured CO₂ fluxes from each treatment using nonlinear regression analysis (Gauss–Newton method, procedure NLIN in SAS; SAS Institute, 2000) according to Lomander et al. (1998b):

[1]

where C₀ is the initial amount of carbon in the substrate, and 1 – are the fractions of labile and refractory C, respectively, and k_l and k_r (d^–1) are the corresponding rate constants. The rate decomposition constants are both functions of temperature (T) and moisture (M), and ß is the ratio k_r/k_l. Thus:

[2]

and:

[3]

where k_ref corresponds to k_l at the reference temperature (T_ref = 15°C) and moisture (M_ref = 24%) and r(T) and r(M) are response functions normalized for these conditions:

[4]

and:

[5]

where s_T and s_M are parameters of the response.

By inserting Eq. [4] and [5] into Eq. [3] and, in turn, Eq. [2] and [3] into Eq. [1], we obtain the total model describing C fluxes from the incubated samples in all treatments. This model was fitted to the measured C fluxes by estimating all free parameters simultaneously using nonlinear regression analysis. By integrating Eq. [1], the model was used to calculate the accumulated carbon losses during the incubation.

	RESULTS

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Carbon Mineralization
The total amount of carbon mineralized during the experimental period increased significantly in most of the treatments with temperature and moisture and varied from 2% of the initial C mass at –1°C and 2% moisture to nearly 20% at 15°C and 24% moisture (Table 2). The model predictions agreed well with the measured C accumulation. Within the range considered here, the moisture content had a twofold greater effect on amounts of C mineralized than temperature. At all temperatures, but especially at 10 and 15°C, instantaneous C mineralization and accumulated CO₂ evolution was lower at 2% moisture than at higher moisture contents (Fig. 1 and Table 2).

View larger version (23K): [in this window] [in a new window]   Fig. 1. Carbon dioxide evolution rates (Cflux; mg C g–1 initial substrate d–1) at –1, 5, 10, and 15°C, and 2% (), 8% (), 14% (), and 24% (x) moisture.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Simulated versus measured CO2–C fluxes (mg C g–1 initial substrate d–1). Error bars correspond to standard deviations of the measurements.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Response surface kl(T,M)/kref according to Eq. [5]. Parameter values are presented in Table 3.

View this table: [in this window] [in a new window]   Table 3. Estimated parameter values (see Eq. [1]–[5]) and corresponding approximate standard errors (SE) and 95% approximate confidence intervals (CI).

Net Nitrogen Mineralization
The maximum amount of mineral N accumulated during the whole experiment was smaller than the corresponding amount of mineralized C, that is, 15.1 and 19.5% of total organic N and total C, respectively (Tables 4 and 2). In contrast to C, the effects of temperature and water content on net N mineralization were not obvious. Nitrogen mineralization at –1, 10, and 15°C increased significantly from 2 to 8% moisture, and in addition at 10 and 15°C from 8 to 14% moisture. At 14 and 24% soil moisture, increased temperature significantly increased N mineralization up to 10°C. However, above 10°C, the effect of temperature was not significant.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Mineral N (—) and nitrate (- - -) dynamics during 97 d at –1, 5, 10, and 15°C and 2% (), 8% (), 14% (), and 24% (x) moisture.

	DISCUSSION

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At –1°C, unexpectedly high activities were consistently found over the range of moisture contents investigated. Some of this activity may originate from the short periods at room temperature, during which the samples were taken out from the chambers to regulate moisture contents. However, we consider this methodological impact on the result to be of minor importance. Several decomposition studies have shown microbial activity at sub-zero temperatures (Flanagan and Veum, 1974; Clein and Schimel, 1995). The results presented by Schimel and Mikan (2005) suggest a shift in microbial metabolism when the temperature approaches 0°C, by a stepwise change in substrate use from SOM to microbial biomass and their products.

The CO₂ evolution measured at 2% moisture in our experiment was on some sampling occasions close to zero and varied greatly between the replicates (Table 2). The water content in some of these samples was also higher than targeted. This was most likely due to water condensing within the vessels. When the 2% soil moisture treatment was excluded, temperature changes were more important than moisture in the intervals considered here. Similar conclusions were presented by Zak et al. (1999) measuring both C and N mineralization between 10 and 25°C and between –1.85 and –0.01 MPa, and by Lomander et al. (1998b) comparing low soil moisture content and low temperatures.

Our model assumption that the Q₁₀ value was constant within the range of temperatures considered here can be considered reasonable (Fig. 2). This is in contrast to several other studies, where Q₁₀ is reported to decrease with temperature (e.g., Lomander et al., 1998b). Kirschbaum (1995), who surveyed the literature on widely different soils, found that the Q₁₀ values as a function of temperature decreased from around 7.5 at 0°C to about 1 at 35°C. Kätterer et al. (1998), who compiled data from 25 different experiments conducted under controlled conditions that were analyzed using the same approach as in our study, reported Q₁₀ values ranging from 1.35 to 2.88. However, for the whole dataset, a Q₁₀ value of 2 was found to adequately represent the intermediate temperature range (about 5–35°C). Whether the temperature response differs between different quality fractions of organic material or not is still being debated (e.g., De Neve et al., 1996; Liski et al., 1999; Ågren, 2000; Magid et al., 2001; Knorr et al., 2005; Reichstein et al., 2006). The constant Q₁₀ suggested from our results is probably related to a narrower quality range of the organic material in sludge compared with soil organic matter.

Net N mineralization was highest at 15°C and 24% moisture during the first weeks of the experiment but its rate of increase declined thereafter compared with the treatment at 14% moisture (Fig. 4 and Table 4). This decline may have been due to denitrification, caused by oxygen deficiency in samples with high respiration rates and nitrate supply (Davidson et al., 1986). The changes in mineral N with temperature (Table 4) can be compared to those reported by Zak et al. (1999), that is, a 60 to 70% larger increase in mineral N content at 10 than 5°C, irrespective of water content. However, temperature responses to N mineralization vary widely between different studies (Stanford et al., 1973; Knoepp and Swank, 2002). Net immobilization was significant in some of the treatments during the first 3 wk of the experiment, but its timing varied somewhat between treatments (Fig. 4). A possible explanation could be that the responses of gross immobilization and mineralization to temperature and moisture are different, as suggested by Andersen and Jensen (2001) and Jansson and Persson (1982).

After about 2 wk, nitrate began to accumulate in the five treatments where CO₂ evolution was highest (Fig. 1 and 4). For these five treatments, nitrate accumulation increased with temperature and moisture. The initial lag phase was probably due to a low initial population density of nitrifiers. Nitrification rates at both 10 and 15°C and 24% moisture exceeded the rate of net N mineralization during the second half of the experiment. This has also been shown for soils amended with clover as green manure at similar temperatures (Cookson et al., 2002). At higher temperatures than measured here, faster transformation to nitrate can be expected. Premi and Cornfield (1969) found that all, or almost all, of the mineralized N was transformed to NO₃^– within 2 wk at 30°C in a soil amended with sewage sludge. Based on our results and the approximate conditions at the field site, we estimate that about 10% of the organic N in the applied sludge will mineralize during the first year after application.

The amount of sludge applied in the field (65 Mg ha^–1) corresponds to 2.2 Mg nitrogen, comprising 0.4 Mg mineral N and 1.8 Mg organic N. Thus, about 580 kg ha^–1 mineral N will be available for plant uptake during the first year. However, nitrogen uptake in the crops (barley and red fescue) at the site was less than 100 kg N ha^–1. Consequently, potential N losses from the system through leaching are high without additional pre-treatment of the sludge applied.

	CONCLUSIONS

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From the results of this experiment we conclude that:

• C mineralization increased with temperature and soil moisture.
  
• The dynamics of C evolution rates could be well described by a two-compartment model with rate constants exponentially and logarithmically dependent on temperature and moisture, respectively.
  
• The dependence of net N mineralization on temperature and moisture was less straightforward than for C mineralization.
  
• Nitrate began to accumulate after about 2 wk at the highest temperatures and moisture levels.
  
• Potential N leaching losses in the field during the first year after a sludge application of 65 Mg ha^–1 may be as high as 0.5 Mg N ha^–1.
  

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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