Published in J. Environ. Qual. 32:1851-1856 (2003).
© 2003 ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Biosolids-Derived Nitrogen Mineralization and Transformation in Forest Soils
Hailong Wang*,
Mark O. Kimberley and
Mirko Schlegelmilch
Forest Research, Sala Street, Private Bag 3020, Rotorua, New Zealand
* Corresponding author (hailong.wang{at}forestresearch.co.nz).
Received for publication June 16, 2002.
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ABSTRACT
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Utilization of biosolids through land application is becoming increasingly popular among wastewater managers. To minimize the potential contamination of receiving waters from biosolids-derived nitrogen (N), it is important to understand the availability of N after land application of biosolids. In this study, four secondary biosolids (two municipal and two pulp and paper industrial biosolids) were used in a laboratory incubation experiment to simulate N mineralization and transformation after land application. Municipal biosolids were from either aerobically or anaerobically digested sources, while pulp and paper industrial biosolids were from aerated wastewater stabilization lagoons. These biosolids were mixed with two New Zealand forest soils (top 100 mm of a volcanic soil and a brown soil) and incubated at two temperatures (10 and 20°C) for 26 wk. During incubation, mineralized N was periodically leached from the soil–biosolids mixture with 0.01 M CaCl2 solution and concentrations of NH4 and NO3 in leachate were determined. Mineralization of N from aerobically digested municipal biosolids (32.1%) was significantly more than that from anaerobically digested biosolids (15.2%). Among the two pulp and paper industrial biosolids, little N leached from one, while as much as 18.0% of total organic N was leached from the other. As expected, mineralization of N was significantly greater at 20°C (average 22.8%) than at 10°C (average 9.7%). It was observed that more N in municipal biosolids was mineralized in the brown soil, whereas more N in pulp and paper industrial biosolids mineralized in the volcanic soil. Transformation of NH4 to NO3 was affected by soil type and temperature.
Abbreviations: AnM, anaerobically digested secondary municipal biosolids • AeM, aerobically digested secondary municipal biosolids • PP1 and PP2, two pulp and paper industrial secondary biosolids from two aerated wastewater stabilization lagoons
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INTRODUCTION
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STRINGENT WASTEWATER discharge standards employed in many countries have improved wastewater treatment, but have also increased production of biosolids that need to be managed. As an alternative to landfilling and ocean disposal, beneficial utilization of municipal biosolids is receiving increased attention from wastewater managers in New Zealand (Naylor et al., 2000; Olsen, 2000). In the pulp and paper industry, aerated stabilization lagoons are the predominant biological treatment technology in New Zealand. Periodical dredging of biosolids from the lagoons is essential to maintain treatment efficiency. Increased tipping fees and regulatory pressure against current landfill practices have also encouraged the industry to seek more sustainable alternatives for biosolids management (Smith et al., 2003). Land application of biosolids is in line with the New Zealand government strategy on waste minimization and utilization: the diversion of municipal biosolids by 2007 and commercial organic wastes (e.g., pulp and paper industrial biosolids) by 2010 from landfill to beneficial use is expected to have exceeded 95% (Ministry for the Environment, 2002).
Pastoral farming is the largest land use in New Zealand. As a precaution to protect the New Zealand dairy product exporting industry, the application of human and pulp and paper industrial wastes to dairy farm land and the feeding of material from other operations that have received these wastes is prohibited (New Zealand Dairy Board, 2001). The dairy industry's policy has enhanced the significance of applying biosolids to forestry land. Application of biosolids to plantation forests not only reduces the chances of contaminants entering the human food chain, but also can increase tree growth (Bockheim et al., 1988; Henry et al., 1993).
To minimize operational costs, maximizing loading rates of biosolids without causing detrimental effect on the receiving environment is a common practice in land application schemes. Surveys have indicated that New Zealand municipal biosolids generally contain low concentrations of heavy metals and other potential contaminants, such as persistent organic compounds (Ogilvie, 1998), which is in line with trends in other countries (Carpenter, 2000; Smith, 1996). However, excess N becomes a limiting factor when designing a land application program (Cogger et al., 2001).
Most N in biosolids is in an organic form. Before becoming available for plant uptake, organic N must be transformed to inorganic forms of NH4 or NH3 through mineralization, a biochemical process in soils caused by soil enzymes excreted by microbes (Jansson and Persson, 1982). The resulting NH4 may be oxidized to NO3 through nitrification (Schmidt, 1982; Sierra et al., 2001). In comparison with NH4, negatively charged NO3 is more likely to be found in soil solution where it is susceptible to be leached to ground water. In contrast to mineralization of organic N, microorganisms use inorganic N to build up their bodies, resulting in N immobilization (Jansson and Persson, 1982). The net production of mineral N is dependent on the outcome of the mineralization–immobilization turnover.
To minimize the risk of contamination of water bodies by biosolids-derived N, it is important to match the pattern of biosolids application and N mineralization to the assimilating capacity of the ecosystem. It has been reported that N mineralization of municipal biosolids varies greatly between different sources (Parker and Sommers, 1983) and is influenced by external factors, such as soil properties (Garau et al., 1986) and temperature (Barbarika et al., 1985; Campbell et al., 1994; Zibilske, 1997). While a number of studies have been conducted on municipal biosolids (Gavi et al., 1997; He et al., 2000; Magdoff and Amadon, 1980; Parker and Sommers, 1983) and pulp and paper industrial biosolids from activated sludge treatment systems (Zibilske, 1997), there is little information on mineralization and transformation of N from land-applied pulp and paper biosolids generated from aerated wastewater stabilization lagoons.
The objective of this study was to evaluate, in a controlled environment, N mineralization and transformation from some typical municipal and pulp and paper industrial biosolids in two representative New Zealand soils as affected by temperature. Our hypothesis was that the mineralization and transformation process of biosolids-derived N would be dependent on the biosolids properties and accelerated at warmer temperatures, but would be less affected by soil type.
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MATERIALS AND METHODS
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Description of Biosolids and Soils
Two municipal biosolids and two pulp and paper industrial biosolids were used to study the mineralization rate of organic N in biosolids in a laboratory incubation study. Municipal biosolids included an anaerobically digested and dewatered sample (AnM) and a liquid and stabilized sample from an autothermal thermophilic aerobic digestion process (AeM). Two pulp and paper industrial biosolids were from aerated wastewater stabilizing lagoons at two New Zealand pulp and paper mills (PP1 and PP2).
Soils used in this incubation experiment included topsoil (0–100 mm) of a stoney silt loam that is classified as pallic orthic brown soil (Hewitt, 1998) from a Canterbury pine plantation in the South Island, New Zealand (the brown soil), and a sandy tephric recent soil (Hewitt, 1998) from a pine plantation near Kawerau township in the central North Island, New Zealand (the volcanic soil) (Table 1). The brown soil may be classified as a Dystrochrept and the volcanic soil as a Typic Udivitrand under the U.S. system (Soil Survey Staff, 1998).
Freshly collected biosolids in polyethylene bottles and soils in plastic bags were stored at 4°C until use. Concentrations of carbon in biosolids and soils and N in soils were analyzed using an Elementar Analysensysteme (Hanau, Germany) combustion analyzer after an acid pretreatment to remove carbonates if present (Nelson and Sommers, 1982). The Kjeldahl method was used for analysis of total N in biosolids. Concentrations of NH4 and NO3 were determined using automated colorimetry after extraction with 2 M KCl solution (Mulvaney, 1996). The pH in water was determined for soils and AnM biosolids at a 1:1 ratio of water to fresh solid samples. The pH values of AeM, PP1, and PP2 biosolids were measured using fresh liquid samples without dilution.
Incubation Experiment
Mineralization of biosolids was evaluated using sequential leaching of aerobically incubated columns containing biosolids and soils, following the procedures developed by Gavi et al. (1997).
Fresh topsoil (<2 mm) equivalent to 10 g oven-dried was mixed with perlite (N-free, 1–2 mm in size) at a 1:1 ratio by volume to improve drainage. Biosolids (equivalent to 4 mg N per column, or 400 kg N ha-1) were mixed with the topsoil and perlite, and then loosely packed into a 50-mL (27-mm i.d., and 140-mm length) Plastipak medical syringe (with piston removed) (Becton, Dickinson and Company, Franklin Lakes, NJ). Pads made from Whatman (Maidstone, UK) glass fiber filter paper were placed above and below the amended soil mixture to stop movement of particulates during the leaching process. Each soil type was also incubated without biosolids addition (control treatment). All treatments had three replications. The columns were incubated at two temperatures: 20°C, representing summer temperature, and 10°C, representing winter temperature. In total there were 60 columns, including 48 biosolids-treated columns and 12 control columns.
The columns were placed in frames designed to allow collection of leachate in bottles directly below each column. All columns were arranged in a randomized complete block design and incubated for 26 wk in the dark. Each column was leached with 30 mL 0.01 M CaCl2 at Weeks 0, 1, 2, 4, 8, 12, 19, and 26. Before incubation (Week 0) columns were leached to remove existing inorganic N from the mixture. Leachate from each column was analyzed for NH4 and NO3 (including NO2) using a colorimetric method (Mulvaney, 1996). Organic N in the leachate was not analyzed, as previous studies indicated that leaching of organic N from biosolids–soil mixture was not significant.
Data Analysis
We presumed that there was no "priming effect" (Jansson and Persson, 1982) due to addition of biosolids to soil, that is, mineralization of N in soil was not increased by the addition of biosolids. To calculate mineralization of organic N in biosolids, we subtracted the amount of N mineralized in the control treatments (soil only). The residual inorganic N in biosolids–soil mixture that had not been leached before incubation (Week 0) was also subtracted based on a mass balance of N. Concentrations of NH4 and NO3 in leachates were added together and expressed as percentage of the initial organic N in biosolids. Subsequently mineralized N was presented as cumulative mineralization, as a percentage of initial organic N in biosolids.
An analysis of variance was used to compare the effects of temperature, biosolids, and soil types on N mineralization in biosolids using the general linear model procedure of the SAS program (SAS Institute, 1989).
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RESULTS AND DISCUSSION
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Effect of Biosolids Properties on Nitrogen Mineralization
Mineralization of biosolids-derived N was monitored by measuring inorganic N concentrations in leachate during a six-month period at two temperatures, representing cool- and warm-season temperatures in New Zealand. It was hypothesized that not only temperature, but also the properties of the biosolids, would influence their mineralization in soil.
Chemical analysis indicated that municipal biosolids had greater concentrations of N and lower C to N ratios than the pulp and paper industrial biosolids (Table 2). Concentrations of NO3 in all biosolids were very low (Table 2). A large proportion of N in municipal biosolids was in the NH4 form (e.g., 33% N in AeM biosolids was NH4–N). However, pulp and paper industrial biosolids, particularly PP2, contained low concentrations of NH4–N. Whereas 3.7% of total N in PP1 biosolids were in the form of NH4, nearly all N in PP2 was organic N (Table 2). Another distinctive property of the PP2 biosolids was that it contained a high concentration of calcium (12.5% Ca) due to residual lime mud derived from the chemical recovery process.
Treatments amended with AnM and AeM biosolids contained large quantities of NH4 in the initial leachings (up to Week 4, data not shown). It is likely that because of the high initial concentration of NH4 in these municipal biosolids (Table 2), not all NH4 material was leached in the first leaching (Week 0), and some may have been leached in subsequent leachings.
There were significant differences in N mineralization between biosolids (Fig. 1
; Table 3). After 26 weeks of incubation, the largest proportion of N mineralized was from AeM biosolids (Fig. 1). Mineralization of organic N in AeM biosolids was significantly greater than in the other biosolids. Nitrogen in AeM biosolids appeared to be more readily mineralized than AnM biosolids when applied to soil. This result agrees with previous observations made by Garau et al. (1986), who found that aerobically treated biosolids are more rapidly decomposed than anaerobic ones. Therefore, AeM biosolids not only contained relatively high concentrations of NH4 (Table 2), but also the organic N was readily mineralized.
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Fig. 1. Effect of biosolids type on mineralization of organic N in soils amended with biosolids during 26 weeks of incubation (the means across two soils and two temperatures, % of initial organic N). Error bars are 95% confidence intervals. AnM, anaerobically digested municipal biosolids; AeM, aerobically digested municipal biosolids; PP1 and PP2, two pulp and paper industrial biosolids from two aerated wastewater stabilization lagoons.
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Concentrations of N in both pulp and paper industrial biosolids were low (Table 2), but there was a great difference in N mineralization rates between these two biosolids. Nitrogen in PP1 biosolids appeared very recalcitrant with a small amount being mineralized during incubation regardless of temperature or soil (Table 4, Fig. 1). These low values indicated that the N in PP1 biosolids was in such a stable form that soil microbes could not release N from the material, and also had to immobilize some soil-derived inorganic N during decomposition. This implies that there is little risk of NO3 leaching from PP1 biosolids after land application. However, a considerable amount of N was mineralized from PP2 biosolids in this study (Fig. 1), although the C to N ratio was higher than PP1 biosolids (Table 2). This indicated that the C to N ratio may not always be a good indicator of N mineralization, probably due to the difference of bioavailability of organic C and N in these biosolids (Reinertsen et al., 1984). The net mineralization of the organic N is calculated as the difference between N mineralization and immobilization (Jansson and Persson, 1982). Janssen (1996) compared some independent studies and suggested that the high concentrations of lignin and polyphenol in an organic material may prevent its N mineralization, regardless of the C to N ratio. As high concentrations of lignin and polyphenol are typical of pulp and paper wastewater, they are also commonly found in the resulting biosolids (Vidal et al., 2001). The PP1 biosolids in the current study may have contained high concentrations of lignin, polyphenol, or other compounds, resulting in the low rate of N mineralization.
Our results indicate that availability of N varies greatly with types of biosolids. Thus, when land application systems are designed, application rates and frequency must be determined specifically for the biosolids to be applied.
Effect of Temperature on Nitrogen Mineralization
The high-temperature (20°C) treatment significantly increased the proportion of N mineralized in biosolids, compared with the 10°C treatment (Fig. 2
; Table 3). It was expected that, as a microbiological process, the rate of mineralization of organic N would increase with temperature before it reaches an optimum (Barbarika et al., 1985; Campbell et al., 1994; Zibilske, 1997). This suggests that much more N from land-applied biosolids may be mineralized in warm, summer months than in cold, winter conditions. In practice, when biosolids are applied in winter (about 10°C), most organic N will not be mineralized, but when applied in summer (about 20°C), a much higher rate of N mineralization could occur. If N mineralization exceeds plant uptake, NO3 can build up in the soil and become available for leaching loss.
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Fig. 2. Effect of temperature on mineralization of organic N in soils amended with biosolids (the means across two soils and four biosolids, % of initial organic N). Error bars are 95% confidence intervals.
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Effect of Soil Type on Nitrogen Mineralization
Although the overall difference in N mineralization between soils was not significant, there was a significant interaction between biosolids and soil (Fig. 3
; Table 3). Mineralization of N in the two municipal biosolids was significantly greater in the brown soil than in the volcanic soil. However, PP2, one of the pulp and paper industrial biosolids, released significantly more inorganic N in the volcanic soil than in the brown soil (Fig. 3). There was also an indication of a greater rate of mineralization of PP1 biosolids in the volcanic soil than in the brown soil, although the difference was not significant. These effects were much more evident at the higher temperature. At the lower temperature, there were no significant differences in mineralization between soils (Table 4). This shows that soil properties can significantly influence the release pattern of organic N from biosolids. It has been reported that a soil with higher pH value seems to result in more N mineralization in biosolids (Garau et al., 1986). Similar results were found in the current study in the pulp and paper industrial biosolids, but not in the municipal biosolids (Tables 1 and 4). The mechanisms that led to a higher N mineralization rate in the brown than the volcanic soil for municipal biosolids, but a lower mineralization rate for pulp and paper industrial biosolids in the brown soil (Table 4), have yet to be investigated. We hope to improve our understanding of this through our current study examining the soil microbial community changes after land application of biosolids.
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Fig. 3. The influence of soil on mineralization of different types of biosolids (the means across both 10 and 20°C, % of initial organic N). Error bars are 95% confidence intervals. AnM, anaerobically digested municipal biosolids; AeM, aerobically digested municipal biosolids; PP1 and PP2, two pulp and paper industrial biosolids from two aerated wastewater stabilization lagoons.
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Transformation of Mineralized Nitrogen
Nitrification of mineralized N, or transformation of NH4 to NO3, was greatly affected by soil type (Fig. 4)
. It took much longer in the brown soil than the volcanic soil to establish nitrification activity (Fig. 4).
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Fig. 4. Proportion of NO3 as percentage of total mineralized N in leachate after each incubation period at (A) 10 and (B) 20°C. AnM, anaerobically digested municipal biosolids; AeM, aerobically digested municipal biosolids; PP1 and PP2, two pulp and paper industrial biosolids from two aerated wastewater stabilization lagoons.
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In the first week of incubation, NH4 was the dominant form of mineralized N in all treatments in the brown soil. The AnM and AeM biosolids treatments in the volcanic soil were also dominated by NH4, which could be attributed to the high concentrations of residual NH4 from these municipal biosolids. After the first week, the proportion of NO3 leached from biosolids treatments in the volcanic soil increased quickly and peaked at the fourth week, regardless of temperature and biosolids type (Fig. 4). Substrate NH4, O2, CO2, pH, and temperature are the main factors that control the nitrification process in soils (Schmidt, 1982). The slower buildup of nitrification activity in the brown soil in this study may be attributed to the lower soil pH than in the volcanic soil (Table 1). Sierra et al. (2001) suggested that soil pH could control the nitrification process by controlling the number of nitrifiers that are responsible for nitrification.
Nitrate was the dominant form of mineralized N in the volcanic soil treatments incubated at 20°C after four weeks (Fig. 4B). However, the proportion of NO3 from the volcanic soil treatments incubated at 10°C markedly decreased after four weeks, particularly the AnM biosolids treatment (Fig. 4A). A decreased nitrification rate in a low-temperature environment has been well recognized (Schmidt, 1982). The much lower nitrification rates at 10°C than at 20°C for both volcanic and brown soils in this study imply that a low temperature could reduce NO3 leaching by limiting conversion of NH4 to NO3.
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CONCLUSIONS
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Mineralization of organic N in soils amended with biosolids was not only determined by the properties of the biosolids and temperature, but also strongly influenced by soil type. Over a period of 26 weeks of incubation, significantly more N was mineralized from aerobically digested municipal biosolids than that from anaerobically digested biosolids. Among the two pulp and paper industrial biosolids sampled from aerated wastewater stabilization lagoons, little N was leached from one sample, while as much as 18% of total organic N was leached from the other. Increasing temperature from 10 to 20°C not only greatly increased mineralization rate of organic N in biosolids but also enhanced nitrification process. More N in municipal biosolids was mineralized in the brown soil than in the volcanic soil, whereas the reverse occurred for pulp and paper industrial biosolids. The results show that an incubation study may provide useful information on N mineralization and transformation in biosolids before N loading rates are determined for a land application scheme. This information would help to maximize the beneficial effect of biosolids and minimize potential leaching loss of biosolids-derived N.
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ACKNOWLEDGMENTS
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The authors wish to thank the New Zealand Foundation for Research, Science and Technology for providing research funding, and G.N. Magesan and M.R. Davis for reviewing the manuscript.
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ABSTRACT
INTRODUCTION
