Biosolids and Short Paper Fiber: A Natural Partnership

Transcription

1 Biosolids and Short Paper Fiber: A Natural Partnership A.F. Carpenter*, C.G. Hanson** *Soil Scientist, Northern Tilth, P.O. Box 361, Belfast, Maine U.S.A. ( andrew@northerntilth.com) **Project Manager, Resource Management, Inc., P.O. Box 1081, Ashland, NH U.S.A. ( charley.hanson@rmirecycles.com) Abstract: Biosolids are a significant source of nitrogen, phosphorous, and micro-nutrients when used as a soil amendment. However, in order to apply biosolids at rates commensurate with the crop uptake of nitrogen and phosphorous, the amount of organic matter added to soil from biosolids is relatively small on an annual basis. In contrast to biosolids, short paper fiber, the solids recovered at paper mill wastewater treatment plants, often has a very low level of all nutrients, including nitrogen and phosphorous. Due to the wood fiber content of short paper fiber, the organic matter and carbon content of short paper fiber are very high. Engineered blends of these two materials have provided the basis for organic matter-rich manufactured topsoils with balanced fertility that are used for disturbed land reclamation projects in the northeastern U.S. In this bench-scale investigation, a low fertility topsoil with low organic matter content was blended with short paper fiber to improve soil tilth. The blended topsoil was then supplemented with either commercial fertilizer or heat-dried biosolids to increase plant-available nitrogen and overall soil fertility. Soil incubations indicated that heat-dried biosolids provided the best balance between having adequate plant-available nitrogen while avoiding excessive soil nitrate levels. Biosolids additions targeted to avoid nitrogen immobilization induced by the short paper fiber did not provide excessive available soil phosphorous levels. Keywords: Biosolids, short paper fiber, manufactured topsoil, nitrogen mineralization, carbon to nitrogen ratio INTRODUCTION Short paper fiber, the solids generated during the wastewater treatment process at paper mills, is made up primarily of wood fiber, clay and lime. Because wood fiber is the major component in short paper fiber, the material, like wood, has a high organic matter content. Short paper fiber from the primary wastewater treatment process has a relatively low nitrogen and phosphorous content. Due to the low nutrient content, short paper fiber from the primary wastewater treatment process, can be added to soils at high application rates that will significantly increase soil organic matter without providing excessive nutrients. Applications of short paper fiber improve soil water-holding capacity and soil tilth more effectively than biosolids, primarily because the nitrogen-based land application rates for biosolids act as a limiting factor in the amount of organic matter that can be provided by the biosolids alone. Existing literature has demonstrated that short paper fiber is effective in increasing soil water-holding capacity and soil aggregation (Chantigny et al. 1999, Zibilske et. al. 2000, Foley and Cooperband 2002). When short paper fiber is applied at high application rates, soil microbes decomposing the organic matter in the short paper fiber, can scavenge existing plant available nitrogen (PAN) from the soil, and cause nitrogen immobilization. Four recent studies have demonstrated that due to this induced nitrogen immobilization, short paper fiber can be used to lower PAN levels in agricultural fields that would otherwise have environmentally significant levels of soil nitrate (Kirchmann and Bergstrom 2003, Rahn et al. 2003, Zebarth et al. 2002, and Vinten et al. 1998). Biosolids, on the other hand, when added to soil at high application rates have the potential to provide excessive PAN to soil. When this PAN is in the form of nitrate, which is highly soluble, the potential exists to negatively impact groundwater quality. A good balance of carbon and nitrogen contributed to soil from organic residuals would provide sufficient PAN to avoid nitrogen immobilization while limiting excess nitrate in soil. When short paper fiber is applied in combination with nitrogen-rich organic wastes, such as biosolids, 767

2 the high carbon to nitrogen ratio (C:N) of the short paper fiber provides abundant available carbon for soil microbes that are simultaneously mineralizing nitrogen from the nitrogen-rich materials. Studies investigating blends of short paper fiber with various nitrogen sources have typically found that a C:N between 25:1 and 40:1 provides a balance between nitrogen immobilization and excessive nitrogen mineralization. (Bowden et al. 1993, Catricala et al. 1996, Carpenter and Fernandez 2000, Vagstad et al. 2001). This bench-scale study, completed by Northern Tilth in 2004, was designed to compare the impacts to soil fertility of biosolids and commercial fertilizers as supplemental nutrient sources for topsoils manufactured with short paper fiber. METHODS AND MATERIALS Topsoil blends in this trial mimicked manufactured topsoil being produced by Resource Management, Inc. (RMI), based in Holderness, New Hampshire USA. The primary components of the manufactured topsoil consisted of a slightly acidic, low fertility soil and short paper fiber derived from the primary wastewater treatment process at a deinking paper mill located in Vermont, USA. The blend consisted of two parts soil to one part short paper fiber on a volumetric basis. The short paper fiber used in this study has a high C:N (97:1) and a relatively high calcium carbonate content (introduced by the precipitated calcium carbonate, or PCC, that is used as a filler in the paper-making process). The biosolids used in the study are heat-dried pellets generated at a municipal wastewater treatment plant in Massachusetts, USA. Nutrient analysis of the short paper fiber and biosolids, based on two year averages of 10 and 12 samples, respectively, are included in Table 1. Table 1 Analysis of topsoil components Topsoil and Fertilizer Blending Four topsoil blends and one control were investigated in this study. The topsoil blends consisted of 2 parts soil and 1 part short paper fiber on a volumetric basis. The blends are listed in Table 2 below. Topsoil components and supplemental fertilizers are reported on a kg-ha -1 basis using the assumption that the topsoil would be placed on the landscape at a 23 cm thickness. Two topsoil blends ( LC and LC ½ ) used commercial fertilizer to provide all of the supplementary nutrients. The fertilization rates in the LC blend provided a C:N ratio of 38:1 in the final topsoil blend, with approximately 50% of the nitrogen being supplied by sulfur-coated urea (SCU), a slow release nitrogen fertilizer. The majority of the remaining nitrogen was supplied in the form of ammonium sulfate, which is an acid-forming fertilizer, to try to neutralize, somewhat, the projected alkaline ph of the topsoil blend. Enough mono-ammonium phosphate (MAP) was added to the LC blend to provide approximately 190 kg-ha -1 phosphorous. The MAP also provided some nitrogen to LC. The LC ½ blend contained exactly ½ the amount of all of the fertilizers used in the LC blend. The calculated C:N for the LC ½ blend was 55:1. Determination of C:N in the topsoil blends was based on the contributions from the short paper fiber, biosolids and fertilizers only. It was assumed that with the low organic matter content of the control soil, contributions of that soil to the active carbon and nitrogen pools would be minimal in the blended topsoils. The LB blend used the heat-dried biosolids pellets as the only supplemental nitrogen and phosphorous sources for the topsoil. Approximately 92% of the nitrogen contained in the biosolids pellets is in the organic matter form; consequently, these biosolids pellets provide a slow-release form of nitrogen. Determination of the application rate of the biosolids pellets was based on providing a final C:N ratio of 40:1 in the topsoil. This application provided a high amount of total phosphorous (approximately 1270 kg-ha -1 ). Using the phosphorous 768

3 availability coefficients from the the Pennsylvania Phosphorous Index (2007), it was assumed that approximately 40%, or 508 kg-ha -1, of this phosphorous would be in a plant-available form. The LO topsoil blend contained no added fertilizer, and the Control topsoil was simply the low-grade topsoil with no short paper fiber or fertilizer added. The calculated C:N of the LO topsoil blend, then, was the same as the short paper fiber (97:1). Table 2. Topsoil blends and supplemental fertilization. Notes SPF short paper fiber MAP- mono-ammonium phosphate SCU- sulfur-coated urea NH4-SO4 - ammonium sulfate a- Phosphorous is calculated as P2O5 b- indicates the total amount of phosphorous added per acre; the phosphorous reported to be added in the form of MAP in LC and LC ½ is only that portion determined by the fertilizer companies to be 100% plant-available Sampling Procedure Topsoils were blended with fertilizer on June 17 th, For each topsoil, approximately 1 gallon of soil was thoroughly homogenized and blended with fertilizer (for those treatments receiving fertilizers) using a hand trowel. Nitrogen Mineralization. Immediately following homogenization and fertilization, three replicates of approximately 400 ml of each soil were placed in 1-liter mason jars and incubated in the dark at room temperature for 12 weeks. Based on an average temperature of 70 o F, the incubation period covered approximately 1700 growing degree days. The jar lids were slightly loose on the jars to allow for some air exchange. The incubation jars were checked periodically for moisture levels, and additional water was added when the moisture level was less than ½ of field capacity. Sub-samples were removed from the jars on a biweekly basis and analyzed for nitrate and ammonium concentrations. Soil Fertility. Topsoil blends were sampled and analyzed at the University of Maine s Analytical Laboratory for standard soil fertility parameters (NEC ) two times during the trial; once immediately following blending and homogenization, and the second time after 12 weeks. The 12-week samples were taken from sub-samples of the topsoil blends that had been placed in 10cm diameter plant pots, seeded to grass and kept under timed grow lights. Samples were taken from the pots in which the grass was grown, because these samples best represent the chemical and physical changes that will likely occur in the field after topsoil is placed and seeded to grass. Results from the soil fertility testing after 12 weeks in the pots seeded to grass are included in Table

4 RESULTS Nitrogen Availability Results of the nitrogen mineralization study are included in Figure 1. There was no extended initial nitrogen immobilization present in any of the topsoil blends with supplemental fertilizer (LC, LC ½ and LB). Virtually all of the nitrogen added to LC and LC ½ in the form of ammonium fertilizers was transformed to nitrate (nitrified) within the first two weeks of the incubation. Total PAN also increased within the first two weeks of the incubation for these three blends. There was a period from 40 to 60 days in the LC and LC ½ topsoil blends when total PAN decreased (Figure 1), but this was after the PAN (derived from a combination of nitrogen that had been either mineralized or remained in the inorganic form) reached levels that were more than ample for most crops that may be planted on a manufactured topsoil. This drop in PAN after 40 to 60 days may have represented a period at which the majority of the readily decomposable organic matter had already decomposed, and the soil microbes were in need of extra nitrogen in order to decompose the more recalcitrant organic matter remaining in the short paper fiber. Alternatively, high moisture conditions in the incubation jars may have been created by water liberated during decomposition, and these saturated conditions may have induced denitrification. With the exception of the final sampling period, the amount of PAN in LO was generally as low as or lower than that of the control topsoil throughout the study, indicating that without fertilization, the short paper fiber does initially immobilize soil nitrogen, and that additional fertilization is needed to provide adequate PAN. However, the PAN levels in both LO and the control, ranging from 4-20 and 3-23 mg/kg, respectively, over the course of the incubation, suggest that the control topsoil was providing some PAN, and that this contribution of PAN may have helped reduce the length of the period of nitrogen immobilization in LO. The LB topsoil blend, which provided nitrogen primarily in the organic form, showed no signs of nitrogen immobilization in this study. By 60 days, LB had slightly more PAN than LC ½. Additionally, as is evident in Figure 1, the release of nitrogen from the LB was more steady over time. This steady release of PAN would be a benefit both from the perspective of plant growth, and preserving water quality at sites where these topsoils are used. After 40 days, the PAN in LC, LC ½ and LB were equivalent to 491, 161 and 121 kg-ha -1, respectively. And after 80 days (after LC and LC ½ had gone through a period in which PAN decreased and then started to increase again), PAN for the same blends was 500, 125 and 128 kg-ha -1, respectively. For a topsoil consisting of one part short paper fiber to two parts low-grade soil, the PAN provided by the LC fertilizer blend in this trial is in excess of recommended nitrogen applications for seeding down grass (Hoskins, 1997), while the PAN provided by LC ½ and LB should be adequate for the initial establishment of grass. Based on these findings, both the amount of heat-dried biosolids used in the LB blend and the amount of nitrogen 770

5 fertilizer added in the LC ½ blend appear to provide a good balance between providing adequate PAN and not creating a large excess nitrate in soil that could be subject to leaching. It should be noted that grass is a luxuriant feeder of nitrogen, and it is possible that up to 60% of the PAN in the LC blend could be taken up by grass provided that all of the other conditions for good grass growth are present in the soil (Cherney et al. 2002). Soil Fertility Relative to the control (low-grade topsoil), available soil phosphorous, ph, cation exchange capacity (CEC), and organic matter increased in all four topsoil blends. Because the control soil was slightly acidic and generally low in nutrients, these increases represent an improvement in soil fertility and soil tilth. Exchangeable acidity, which was 38.2% in the control topsoil, was completely eliminated in each of the blended topsoils. Table 3 Soil fertility results after 12 weeks Phosphorous. The recommended optimum range for available soil phosphorous when analyzed according to the University of Maine s modified Morgan extraction is kg-ha -1 (Hoskins 1997). Phosphorous, which was below the optimum range in the control topsoil, was increased to within the optimum range in LC and LB and was also slightly increased in LO and LC ½. The difference in soil phosphorous levels between the control soil and LC indicates that adding 190 kg-ha -1 of phosphorous from MAP increased available soil phosphorous by 6 kg-ha -1. Assuming that the phosphorous in MAP is 100% plant-available according to fertilizer standards and comparing the available phosphorous levels in LC to LB it appears that approximately 18% of the 1270 kg-ha -1 of phosphorous added in the form of biosolids pellets was plant-available phosphorous. This indicates that, under the conditions of blending and sampling used in this study, the biosolids phosphorous availability coefficient of 40% from the Pennsylvania Phosphorous index overestimates the phosphorous availability in these heat-dried biosolids. Soil ph. While liming is often required to bring northeastern U.S. soils up to the optimum ph for balanced soil fertility, too much lime can reduce phosphorous and micronutrient availability. Soil fertility problems associated with high ph are generally not evident until the ph exceeds In these trials ammonium sulfate was used to help counteract the over-liming that can potentially occur due to the high PCC concentrations in the short paper fiber. In the topsoil blends that included ammonium sulfate (LC and LC ½), soil ph dropped with increasing rates of ammonium sulfate. This indicates that ammonium sulfate can provide a double benefit in topsoil fertilizer blends by both providing inorganic nitrogen and helping to bring the soil ph levels closer to the optimum range. Based on the final ph of the LB topsoil (7.6), it also appeared that the biosolids pellets were effective in reducing soil ph relative to the unfertilized LO topsoil blend. CONCLUSIONS Results of this investigation indicate that both biosolids and commercial nitrogen fertilizers can be used to avoid nitrogen immobilization when using short paper fiber as an organic matter source in in manufactured topsoil. In this study, biosolids provided a more even release of PAN over time than did commercial fertilizer. 771

6 When supplementing short paper fiber with nitrogen sources to target a C:N ratio within the range that existing literature suggests will a balance between nitrogen immobilization and excess nitrogen mineralization, biosolids resulted in less PAN in manufactured topsoil than did commercial nitrogen fertilizer. Although the total amount of phosphorous added to the manufactured topsoil in the form of biosolids greatly exceeded the crop uptake rate of phosphorous for all crops, the resulting available soil phosphorous levels in the manufactured topsoil with biosolids were not excessive. REFERENCES Bowden, W.B., C.T. Smith, W.H. McDowell and C.E. Catricala Assessing potential water quality impacts of creating topsoil with pulp and paper mill residuals. Department of Natural Resources. University of New Hampshire. Durham, N.H. Final report prepared for Scott Paper Company, Inc. and Resource Conservation Services, Inc. Carpenter, A.F. and I.J. Fernandez Pulp Sludge as a Component in Manufactured Topsoil. J. Envir. Qual. 29(2): Catricala, C.E., W.B. Bowden, C.T. Smith, and W.H. McDowell Chemical characteristics of leachate from pulp and papermill residuals used to reclaim a sandy soil. Water Air Soil Pollut. 89: Chantigny, M.H., D.A. Angers, and C.J. Beauchamp Aggregation and organic matter decomposition in soils amended with de-inking paper sludge. Soil Science Society of America Journal. 63: Cherney, D.J.R., J.H. Cherney, and E.A. Mikhailova Orchardgrass and tall fescue utilization of nitrogen from dairy manure and commercial fertilizer. Agronomy Journal. 94: Foley, B.J. and L.R. Cooperand Paper mill residuals and compost effects on soil carbon and physical properties. J. Envir. Qual. 31: Hoskins, B Soil testing handbook for professionals in agriculture, horticulture, nutrient and residuals management. 3 rd Edition. Kirchmann, H. and L. Bergstrom Use of Paper-mill wastes on agricultural soils: is this a way to reduce nitrate leaching? Acta Agriculturae Scandinavica, Section B Plant and Soil Science. 53(2): Pennsylvania Phosphorous Index, Version 2, NEC Recommended soil testing procedures for the northeastern U.S. 2 nd Edition. University of Delaware Experiment Station. Bulletin #493. Rahn, C.R., G.D. Bending, M.K. Turner, and R.D. Lillywhite Management of N mineralization from crop residues of high N content using amendment materials of varying quality. Soil Use and Management. 19(3): Vastag, N., Broch-Due, A. and L. Lyngstad Direct and residual effects of pulp and paper mill sludge on crop yield and soil mineral N. Soil Use and Management. 17: Vinten, A.J.A., E.M. Baggs, K. Castle, and R. Davies Control of nitrate leaching from a nitrate vulnerable zone using paper mill waste. Soil Use and Management. 14(1): Zebarth, B., D. Walker, S. Fahmy, P. Toner and K. Terry Use of fibre clay to reduce nitrate leaching in potato fields. Final Report, March from the Potato Research Centre, Agriculture and Agri-Food Canada, Fredericton, NB. Zibilske, L.M., W.M. Clapham, and R.V. Rourke Multiple applications of paper mill sludge in an agricultural system: soil effects. Journal of Environmental Quality. 29: