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a Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164-6420
b College of Forest Resources, Univ. of Washington, Seattle, WA 98195-6613
* Corresponding author (wlpan{at}wsu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CEC, cation exchange capacity • EC, electrical conductivity
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INTRODUCTION |
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A current common disposal approach is through a combustion recovery process where black liquor is concentrated, burned for producing energy, and recycled for recovering inorganic chemicals (Grover et al., 1999). However, this disposal approach is neither cost effective nor environmentally friendly (Girovich, 1996). Traditionally, NaOH is used in straw pulping, producing Na-based black liquor. Land application of this liquor raises concern for deterioration of soil structure (Balba, 1995). Increased soil erodibility (Singer et al., 1982), diminished plant growth and reduced soil permeability have been attributed to high concentrations of Na (Jorgensen, 1970; Guerri, 1973). Reducing Na content of black liquor by substituting KOH for NaOH for pulping should render black liquor that can be used as a K fertilizer and soil amendment while offering an environmentally acceptable disposal alternative. Cox et al. (1997) observed that application of black liquor increased plant nutrients, soil organic matter, cation exchange capacity (CEC), water holding capacity, and tilth. Other effects on soil quality parameters are less well defined, but the alkalinity and soluble organic matter in the black liquor could be beneficial.
The objectives of this study were to: (i) determine the temporal impacts of KOH-based black liquor on soil biological and chemical indicators of soil quality, and (ii) distinguish the causal factors of these impacts by the black liquor between pH modifications vs. other factors.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Black liquor was obtained from wheat straw pulping with KOH instead of NaOH according to modified Universal Pulping (M. Jackson, consultant, Tolovana Park, OR, personal communication, 2000). Briefly, wheat straw was mixed with KOH, concentrated HNO3, and alum [Al2(SO4)3.14H2O] at a ratio of 10:1, 100:1, and 1000:1 on a weight basis, respectively, and water was added to keep the ratio of water to straw of 10:1. The mixture was cooked under the ambient pressure and a temperature of 90°C for 1 h. After cooling, black liquor was separated from the pulped straw by draining, and tested for the residual alkalinity based on acid titration neutralization (M. Jackson, consultant, Tolovana Park, OR, personal communication, 2000). The separated liquor was reused as part of the cooking liquor for the next cook after fortification with KOH to the target level (mass ratio of straw/KOH = 10:1). Black liquor was recycled eight times. General characteristics of this eight times, recycled liquor are shown in Table 1.
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Experiment 1
Air-dried soil was passed through a 2-mm sieve and amended with black liquor at rates of: 0 (nonamended control), 200 (recommended K rate), 400, 800, and 1200 kg K ha–1. Black liquor application rate for each treatment was calculated based on its K concentration (1.49 g K 100–1 mL). The corresponding volumes of black liquor for treatments were 0, 11.2, 22.4, 44.8, and 67.2 mL kg–1 soil, respectively, assuming a soil bulk density of 1.19 g cm–3 and a soil depth of 10 cm (1.19 x 106 kg soil ha–1). The black liquor was thoroughly mixed with a 25-g air-dried soil for each treatment. There were four replications for each treatment. Additional water was added to keep the soil moisture content at 60% of the water holding capacity.
Treated soils were placed individually in sealed 1-L Mason jars, each containing two vials, one with 5 mL of 1 M NaOH to trap evolved CO2, and one with 10 mL of CO2–free water to maintain soil moisture during the incubation period. The treated soils were incubated in a growth chamber at 24°C for 60 d using a randomized complete block design. Incubation jars were opened to refresh the incubation air and to remove and replace the CO2 trapping vials at 2, 5, 10, 20, 40, and 60 d.
Experiment 2
The same rates of black liquor used in Exp. 1 were applied to 500 g of air-dried soils with four replications per treatment. The treated soils were incubated at 24°C for 60 d in a dark growth chamber, and maintained at 60% of water holding capacity by adding additional water every 2 d over the incubation period. The treated soils were destructively sampled at Days 2, 5, 10, 20, 40, and 60 for the determination of pH, EC, microbial biomass C, and soil enzyme activities.
Experiment 3
Experiment 2 demonstrated that soil microbial activity and soil pH increased with increasing black liquor application rates, raising a question of whether pH was a controlling factor of microbial activity. To separate the chemical and pH modifying effects of black liquor, KOH was added alone to soil. Addition of black liquor at a rate of 11.2 to 67.2 mL kg–1 soil in the Exp. 1 increased soil pH by 0.1 to 0.5 units over the 60 d of incubation. A preliminary soil incubation determined that the addition of 0.40 M KOH to the same soil at a rate of 10, 30, and 50 mL kg–1 soil increased soil pH to 7.7, 7.9, and 8.0, respectively, mimicking the soil pH modifications found in Exp. 2 (data not shown).
Twenty five grams of air-dried soil samples were amended with 0.40 M KOH at a rate of 0, 10, 30, and 50 mL kg–1 soil with four replications for the determination of CO2 evolution over 60 d of incubation under the conditions as described in the Exp. 1. The same rates of 0.40 M KOH were applied to 200 g air-dried soils with four replications under conditions as described in the Exp. 2. The treated soils were destructively sampled at Days 5, 20, 40, and 60 for the determination of CO2 evolution and soil enzyme activities.
Black Liquor and Soil Sample Analysis
The pH and EC of black liquor were measured with a pH meter (211/digital pH meter, Orion Research Inc., Boston, Mass) and an EC meter (YSI Model 35, Yellow Springs Instrument, Co., Inc., Yellow Springs, OH), respectively. Black liquor subsamples (5 mL) were oven-dried at 85°C for 24 h for determination of solid content. Polysaccharides and lignin in black liquor were determined as described by Sun and Tomkinson (2001). Black liquor was digested using HNO3–H2O2 (Jones and Case, 1990). The digestion solution was analyzed for the determination of K, Ca, Mg, Si, Mn, Cu, and P concentrations. A 0.5-mL subsample of black liquor was used for the determination of total C and N by a Leco analyzer (CNS2000, LECO, St. Joseph, MI).
A water-saturated soil paste was equilibrated for 1 h for the determination of pH by a pH meter (211/digital pH meter, Orion Research Inc., Boston, MA), and EC by an EC meter (YSI Model 35, Yellow Springs Instrument, Co., Inc., Yellow Springs, OH).
Carbon dioxide absorbed in NaOH was measured by titration (Anderson, 1982). The cumulative CO2 amounts were expressed as µg CO2–C g–1 dry soil, and CO2 evolution rates as µg CO2–C g–1 dry soil d–1. The apparent percentage of black liquor-derived C evolution was calculated based on the equation:
Apparent percentage of black liquor-derived C evolution (%) = [(cumulative CO2–C evolution in the black liquor amended treatment – cumulative CO2–C evolution in the non-amended control)/total black liquor derived C] x 100.
Soil microbial biomass C was determined by the fumigation–incubation method (Horwath and Paul, 1994). Following the removal of chloroform, 25 g of fumigated dry weight equivalent, and non-fumigated soils at soil moisture with 60% of water-holding capacity were then incubated in a 0.5-L Mason jars at 24°C in the dark for 10 d. The CO2 flush from fumigated and unfumigated soils were trapped according to the method as described in the Exp. 1. The soil microbial biomass C was calculated based on the equation:
The soil microbial biomass C (µg g–1 dried soil) = (F – UF)/K, where, F = CO2 flush from the fumigated soil, UF = CO2 produced by the unfumigated soil, and K, coefficient = 0.41 (Anderson and Domsch, 1978).
Soil dehydrogenase, ß-glucosidase, and arylsulfatase activities were measured by the method of Tabatabai (1994).
Statistical Analyses
Analyses of variance (ANOVA) were conducted on each parameter. There was interaction between treatments and sampling date for all measured parameters except for soil pH and EC. Therefore, separate analyses were performed for microbial parameters at each sampling date. Protected LSD test was performed when treatments were significant at p 
0.05 (SAS Institute, 2002) to compare the means of all different treatments within the same sampling date and the same parameter.
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RESULTS AND DISCUSSION |
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Application of black liquor increased soil EC by 0.06 to 0.26 dS m–1 compared with the nonamended control over 60 d of incubation (data not shown). The high salt concentration of the black liquor (EC 27.5 dS m–1) was due to the KOH used in pulping and soluble salts released from the wheat straw.
Soil EC is an indicator of salt concentrations in soil solution. Salinity problems can arise from land application of organic wastes with high EC such as paper mill effluent. However, the low soil EC levels ranging from 0.67 to 1.04 dS m–1 due to application of this liquor at these rates, suggest that black liquor applied up to 67.2 mL kg–1 soil would not cause a soil salinity problem in most soils with initial EC 3 dS m–1 (McBride, 1994).
Carbon Dioxide Evolution
Cumulative CO2 evolution differed among the five treatments over the incubation period (Fig. 1a
), ranging from 316 µg CO2–C g–1 soil in the unamended control to 831 µg CO2–C g–1 soil in the amended soil receiving the highest black liquor rate (67.2 mL kg–1 soil). There was also a linearly increase with increasing rates of black liquor (r2 = 0.99; p < 0.01).
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A similar percentage (18.3% at Day 2 and 39.1% at Day 60) of black liquor derived C was evolved as CO2–C across all treatments during incubation period (Fig. 1b). This suggests that black liquor at 11.2 to 67.2 mL kg–1 soil had no detrimental effect on soil microbial activity in terms of CO2 evolution, with a similar proportion of total added C evolved as CO2 regardless of application rates within this range.
Microbial Biomass Carbon
Soil microbial biomass C increased with increasing application rates except at Days 20 and 40 (Table 2). Studies reported that additions of silage effluent and urban wastewater (Bardgett et al., 1995; Meli et al., 2002) increased soil microbial C, partly due to the addition of readily available nutrients into soil, and partly due to the input of microorganisms into the soil from the waste effluents. In our study, this increase in microbial biomass was probably attributed to the addition of easily decomposable substrate such as polysaccharides (Table 1).
Metabolic Quotient
The metabolic quotient (specific respiration, qCO2) is the ratio of microbial respiration CO2– C rate to microbial biomass C. In our study, the qCO2 increased with increasing application rates throughout the incubation period except at Day 60 (Table 3).
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In our study, increased qCO2 due to the increasing application rates (Table 3) may not mean that soil microorganisms were stressed. Soil microbial biomass C and CO2 evolution rates (Table 2) both increased with increasing application rates. This indicated no stress to the microorganisms (Badalucco et al., 1992). In addition, there were no differences in the proportion of black liquor derived C evolved as CO2 (Fig. 1b).
The increased soil pH induced by the black liquor rates could result in a number of changes that affected the composition of the microbial community (Dick et al., 2000). High salinity and heavy metal contents of sludge can inhibit microbial activity (Tester and Parr, 1983; Wong and Lai, 1996). Black liquor obtained by wheat straw pulping with KOH had undetectable levels of heavy metals (data not shown). Application of black liquor at rates up to 67.2 mL kg–1 soil resulted in low levels of soil EC. Therefore, increased qCO2 were not likely a result of the heavy metals or soil salinity.
Enzyme Activities
Black liquor increased soil dehydrogenase activity over the incubation period (Fig. 2a
). Soil dehydrogenase activity was the highest at the start of our incubation and decreased over time. Soil dehydrogenase activity was considered an index of total viable soil microorganisms (Taylor et al., 2002). Soil ß-glucosidase activity was relatively stable throughout the incubation, and generally increased from 10 through 40 d (Fig. 2b). Soil ß-glucosidase, an important extracellular enzyme, one of the three enzymes responsible for decomposition of cellulose, has potential as a biological indicator of soil quality (Turner et al., 2002). Similar to soil dehydrogenase and ß-glucosidase activities, soil arylsulfatase activity increased with increasing application rates except at Days 2 and 5 (Fig. 2c). Soil arylsulfatase is an enzyme that catalyzes the hydrolysis of an arylsulfate anion by cleavage of the O-S bound and is considered partly responsible for soil S cycling and also as an indicator of fungi activity in soil (Oshrain and Wiebe, 1979). The addition of organic substances due to application of organic wastes, stimulated microbial activities and subsequent enzyme synthesis (Sastre et al., 1996). Kannan and Oblisami (1990b) found that 15 yr of irrigation of paper mill waste effluent increased soil dehydrogenase, amylase, and phosphatase activities. They attributed the increases in enzyme activities to the addition of organic matter and nutrients. Increased soil enzyme activities in treated soils in our study may be related to the addition of polysaccharides.
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CONCLUSIONS |
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Application of KOH-based black liquor up to 67 mL kg–1 soil in this study, corresponds to nearly 80 000 L ha–1 in the field. Therefore, 1 ha of soil can receive KOH-based black liquor generated from production of approximately 10 tons of paper generated from 20 tons of straw. Field application rates may be higher, but potential problems due to high EC, high pH or K imbalances should be monitored. Costs of transporting this dilute material limit its range of applicability to local fields unless steps are taken to increase nutrient concentrations. Further experiments are needed to clarify upper limits of KOH-based black liquor application rates, and whether some combination of Na, K, and Ca hydroxide might provide reasonable pulping conditions that might also allow for higher rates of black liquor application.
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ACKNOWLEDGMENTS |
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Received for publication October 19, 2004.
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