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a Inst. of Nat. Resour. and Environ. Sci., NARC, Park Road 45500, Islamabad, Pakistan
b Land Resour. Sci., Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
* Corresponding author (trashid{at}uoguelph.ca).
Received for publication April 3, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: FallFOG, fall-applied fat, oil, and grease • FOG, fat, oil, and grease • MERN, maximum economic rate of nitrogen application • MEY, maximum economic yield • SOC, soil organic carbon • SpringFOG, spring-applied fat, oil, and grease
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Oily food waste (FOG) is traditionally discharged to sanitary sewers or stabilized with concrete and disposed to landfills. However, municipalities across Canada now strictly regulate the sewer discharge of FOG, and many municipalities have also closed sanitary landfills to FOG as part of their organic waste diversion plans. With the continued generation of oily food wastes, the need for proper disposal or utilization strategies is imperative.
Oily food waste provides an easily decomposable substrate for the soil microbial biomass and has the potential to be an agricultural soil amendment that provides a beneficial source of organic matter. Higuchi and Kurihara (1980) reported in a 5-wk incubation study that decomposition of lard, cow fat, soybean oil, and rapeseed oil ranged from 23 to 70% when added to soil at 5 mg C g–1 soil (10 Mg C ha–1). The rate of decomposition of cooking oils depends on the amount of oil applied to soil. Smith (1974) reported that the extent of biodegradation of soybean and palm oil after 12-wk incubation was 76% at 2.2 Mg ha–1 and 44% at 112 Mg ha–1. The author further suggested that slower biodegradation at the higher oil application rate may have been due to low N contents. In a 4-wk laboratory study, Plante and Voroney (1998) observed 40 and 31% decomposition of oily food waste applied at 11.5 and 23 Mg C ha–1, respectively.
It has long been established that N availability can be a limiting factor for soil microorganisms responsible for decomposition of organic materials (Mary et al., 1996). When organic materials having a wide C/N ratio undergo microbial decomposition, the microorganisms can become N limited (Kay and Hart, 1997). Additional N is required to support microbial growth and to promote decomposition, and these N requirements can be fulfilled by application of N fertilizers (Green et al., 1995).
Corn production in conjunction with FOG application to soil requires that sufficient N be applied for decomposition of the FOG waste and for corn growth. When a decomposable substrate with a high C/N ratio (>25:1) undergoes microbial decomposition in soil, microbial growth can be limited by lack of N. Additional N can come from soil mineral N (Blackmer and Green, 1995) or added fertilizer N (Green et al., 1995). Residual soil N in fall, which is otherwise subject to leaching, could be conserved by an application of FOG (a rich C source), causing N immobilization. The immobilized N would subsequently be available to the succeeding crop by remineralization (Plante and Voroney, 1998) in spring.
The maintenance of adequate levels of organic matter in soils has been identified as a key to productive, sustainable agricultural systems (Doran and Smith, 1987). Carbon sequestration can be enhanced by the application of organic materials to soil (Janzen et al., 1998). Management practices should promote C sequestration in soil within short periods of time, typically 1 to 5 yr (Kay, 1990). Adopting management practices that increase C inputs relative to C losses can prompt gains in soil organic C (SOC), and application of FOG to agricultural soils as a C source could be one of them.
Information about crop production in conjunction with FOG management is not currently available. Concern exists that application of FOG, which has a high C/N ratio (90:1), may affect the availability of N to crops due to soil N immobilization during its decomposition. Therefore, three studies were conducted at the Elora Research Station, University of Guelph, ON, Canada, in 1995, 1996, and 1997 to determine (i) the effect of FOG application on corn grain yields, (ii) N requirements of corn grown on FOG-amended soils, (iii) its contribution to soil C, and (iv) its accumulation in soil after continuous application.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Oily food wastes used in these studies were collected from grease interceptors located in commercial and institutional food services outlets in the Greater Toronto area, Canada. The waste consists of heterogeneous mixtures of animal and vegetable FOG, water, and food-derived solids. Without mixing, the fluid quickly separates into four distinct layers: (i) a dark oily layer (ii) over floating solids, (iii) over yellow-tinted water, and (iv) over settled solids. The nonaqueous content of the waste ranges between 170 and 190 g kg–1, and these solids are organic because no grit or other fixed solids were found after ignition at 550°C. The pH, electrical conductivity, oil content of solid material, C and N content, and C/N ratio of the material applied to different experimental locations are presented in Table 1. Oily food waste (FOG) applied had high C contents and very low N content, with C/N ratio between 88:1 and 97:1. Soil fertility analysis of all experimental locations (1995–1997) at Elora Research Station is presented in Table 2.
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Amounts of N fertilizer applied along with the application of oily food waste at different rates were estimated based on the following assumptions: (i) C in waste = 85%, (ii) extent of FOG decomposition = 100%, (iii) C use efficiency = 40%, (iv) microbial biomass C/N = 10, and (v) N remineralization after FOG decomposition = 0%. Fertilizer N as urea was applied at 170, 340, and 510 kg N ha–1 to plots receiving 5, 10, and 15 Mg ha–1 to provide sufficient N to fulfill the requirements of microbial decomposition of oily food waste and crop growth. The urea was broadcast before FOG incorporation in soil. These plots received repeated applications of FOG and supplemental N at the same rates for 3 yr (1995, 1996, and 1997). A control, 0 FOG + 100 kg N ha–1 (soil test N fertilizer recommendation; OMAFRA, 1996), was also included in the experimental design.
Experiment 2
In Exp. 1, the amounts of N applied (170, 340, and 510 kg N ha–1) along with 5, 10, and 15 Mg FOG ha–1 were relatively high. Substantial amounts of residual N may remain in soil after crop harvest with these high N application rates. A second experiment (optimal FOG application rate, three locations) was initiated to refine N management for FOG-amended soils. Oily food waste was applied at 0 and 10 Mg ha–1 at three different locations during the 1996 cropping season. Experiment 2 was a randomized complete block design with four replications (plot size 4.5 by 10 m).
The oily food waste decomposition experiments conducted in our lab as part of this project showed that the 40% of FOG added to soil was decomposed and 50% of immobilized N during FOG decomposition was remineralized (Plante and Voroney, 1998). On the basis of these results, the assumptions used to calculate the amount of N fertilizer required for FOG decomposition in Exp. 1 were changed to (i) C in waste = 85%, (ii) extent of FOG decomposition = 40%, (iii) C use efficiency = 50%, (iv) microbial biomass C/N = 10, and (v) N remineralization after FOG decomposition = 50%. Nitrogen fertilizer application rate was calculated by adding the estimated amount of N required for decomposition of FOG applied at 10 Mg ha–1 (85 kg N ha–1) to the N requirement of corn without oily food waste (112 kg N ha–1) observed during the 1995 cropping season.
During initial planning of our experiments, we also decided to include N response plots (N was applied at 0, 50, 100, 150, and 200 kg ha–1, no FOG application) in Exp. 1 to calculate the maximum economic rate of N application (MERN). The MERN from Exp. 1 during 1995 cropping season was 112 kg N ha–1. Soil samples (0–30 cm depth) were taken in the spring to calculate the soil test–based N fertilizer recommendations. The N fertilizer recommendation based on the soil N test (OMAFRA, 1996) in 1995 was 105 kg N ha–1, which was also very close to the actual MERN.
Urea fertilizer at 200 kg N ha–1 was applied along with FOG application at 0 and 10 Mg ha–1 to meet the N requirements for FOG decomposition and crop growth. Oily food waste at 0 and 10 Mg ha–1 along with 200 kg N ha–1 was applied at three different locations having different amounts of available N (56, 38, and 26 kg NO3–N ha–1, Table 2) measured in late May. Several locations were sampled at Elora Research Station, and soil samples were analyzed for NO3–N contents to select suitable locations having reasonable differences in soil available N contents. The specific objective of this study was to determine the effect of soil available N on corn grain yields grown on soils amended with FOG at 10 Mg ha–1.
Experiment 3
The third experiment (time of FOG application, 2 yr) was a split-plot design with four replications, with the objective of further defining the N requirements of corn grown on FOG-amended soils. The main plots (22.5 by 10 m) received the following treatments: control, fall-applied FOG (FallFOG), and spring-applied FOG (SpringFOG). Oily food waste was applied at 10 Mg FOG ha–1 in fall (3 Oct. 1995 and 1996) or in the spring (25 Apr. 1996 and 1997). Urea was applied at 0, 50, 100, 150, and 200 kg N ha–1 to subplots (4.5 by 10 m) of each main treatment plot. Specific objectives of this study were to determine the effect of time of FOG application on corn grain yields and to calculate the MERN for FallFOG and SpringFOG application treatments.
At maturity, two central rows (1.52 by 5 m; 7.6 m2) of each plot were hand-harvested at all experimental locations to determine the corn grain yields. At harvesting time, a subsample of cobs (10) was collected for oven drying. Oven-dried cobs were shelled, and corn grain yield was calculated (expressed on 155 g kg–1 moisture content basis). MERN was calculated by the equation (McGonigle et al., 1996):
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Nitrogen was then replaced by MERN in the quadratic response equation to calculate the maximum economic yield (MEY):
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Fat, Oil, and Grease and Soil Analysis
Three subsamples of oily food waste were collected at each time of application for analysis of total C, total N, and oil contents. Total C, N, and oil contents were determined on waste samples that were freeze-dried at –30°C. Oily food waste samples (5 g each) were placed in cellulose thimbles and extracted by hexane for 24 h. Preweighed flasks containing the extracted oil were left open overnight to allow volatilization of the remaining hexane and then reweighed (Greenberg et al., 1995).
Determination of total C content of the waste by direct combustion was not possible as samples exploded in the analyzer when ignited. Instead total C in the residue left from soxhlet extraction was measured by dry combustion (Tiessen and Moir, 1993), and oil C was calculated by assuming C in oil to be 90% of the molecular weight (Plante, 1996). The C content of the residue and oil C were summed for FOG C content. Total Kjeldahl N, pH, and electrical conductivity of FOG waste were also determined by following the methods described by McGill and Figueiredo (1993), Peech (1965), and Bower and Wilcox (1965), respectively. Soil samples for the soil N test were extracted with 2 M KCl (Keeney and Nelson, 1982), and the extract was analyzed for NO3–N by using Braun and Lubbe TRAACS 800 instrument (Tel and Heseltine, 1990).
Soil samples were also taken (0–20 cm soil depth) from plots at the completion of Exp. 1 that had received FOG for 3 yr (1995–1997) at 5, 10, and 15 Mg ha–1 yr–1 to determine the SOC contents. Soil organic C was determined by potassium dichromate oxidation (Tiessen and Moir, 1993). Residual FOG content was determined using a soxhlet apparatus (Greenberg et al., 1995). Air-dried soil samples (15 g) were placed in cellulose thimbles and extracted with hexane for 24 h. Flasks containing extracted FOG were left open overnight to allow volatilization of the remaining hexane and reweighed to the nearest 0.1 mg. Residual FOG contents were calculated by subtracting the oil content of soil from control plots from that determined for soil from FOG-amended plots.
Statistical Analyses
Statistical analyses of the corn grain yield data from Exp. 1 were performed by PROC GLM procedure of SAS (SAS Inst., 1990). Contrast comparisons were made to determine the effect of FOG application rates within a year, and year within a FOG application rate, on corn grain yields. Corn grain data from Exp. 2 were also statistically analyzed by PROC GLM, and contrast comparisons were made to determine the effect of FOG application rates within the location, and locations within application rate, on corn grain yields. Statistical analyses of corn grain yield data obtained from Exp. 3 were performed for each year using the PROC MIXED procedure to compare components (intercept, linear, and quadratic) of the response curves. Contrast comparisons were used to distinguish the effects of FOG application time on corn response to applied N. Treatment effects on SOC and residual FOG were tested using the Tukey's HSD test at 5% confidence level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Organic Carbon
Total amounts of FOG C applied to soil annually after 3 yr (5, 10, and 15 Mg FOG ha–1) were 12, 24, and 36 Mg ha–1, respectively. Annual applications of FOG for 3 yr at 10 and 15 Mg ha–1 yr–1 significantly increased SOC contents compared with the control (Table 3). A significant increase in SOC (9 and 19%) was observed over control after 3 yr where FOG was applied at 10 and 15 Mg ha–1 yr–1. Voroney and Angers (1995) reported that short-term effects of management practices on SOC levels are usually not easy to detect in field conditions because the SOC contents of the surface layer are highly variable. However, assuming that the annual increase in SOC with FOG application at 5 Mg ha–1 yr–1 is constant (0.73 Mg C ha–1 yr–1), we expect to be able to detect a significant increase in SOC content after 6 yr of continuous FOG application at this rate. An increase in SOC due to alfalfa (Medicago sativa L.) (Angers, 1992), cereal straw incorporation (Nyborg et al., 1995; Liang et al., 1998), and manure additions (Angers and N'dayegamiye, 1991) has been detected within 5 to 11 yr. Liang et al. (1998) indicated that more than 7 yr were required before the changes in SOC due to crop residue additions were detectable.
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Residual Fat, Oil, and Grease
Applications of FOG to agricultural soils may result in its accumulation, which can cause hydrophobicity. Ma'shum et al. (1988) have shown that palmitic acid (an animal fatty acid) can cause water repellency at concentrations of more than 400 mg kg–1 soil. The maximum amount of residual FOG in 0- to 20-cm soil depth after 3 yr (372 kg ha–1 equal to 143 mg kg–1 soil, Table 4) was lower compared with the level reported by Ma'shum et al. (1988). Others have reported that lipids would not induce water repellency in soil (Van'T Woudt, 1959; Bond, 1968).
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Experiment 2
The effects of FOG application at 10 Mg ha–1 on corn grain yields compared with the control at the same rate of N application, i.e., 200 kg N ha–1 (application of FOG at an optimal rate; Exp. 2), are presented in Fig. 2
. Corn grain yields were significantly affected by the interaction between location and the FOG application rate. Corn grain yields obtained from the FOG-amended plots were similar (P < 0.05) to the yields obtained from control plots at Location 1. The corn grain yields obtained from FOG-amended plots at Location 2 and 3 were significantly lower (P < 0.05) than those obtained from Location 1. A decline in corn grain yields (23 and 14%) due to FOG application was recorded at Location 2 and 3, respectively. Further, a significantly greater decrease in corn grain yields was observed at Location 2 compared with Location 3.
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The availability of inorganic N in soil has been shown to affect the decomposition of organic materials such as wheat (Triticum aestivum L.) straw (Recous et al., 1995; Mary et al., 1996). Mary et al. (1996) reported that decomposition was slower with lower soil inorganic N contents compared with higher soil inorganic N contents. They further noted that N immobilization lasted longer under N-limiting conditions compared with conditions where N was not limiting. However, the soil NO3–N content at Location 1 in the upper-30-cm soil layer (56 kg ha–1) in combination with the N fertilizer seemed to be sufficient to fulfill the N requirements during FOG decomposition and crop growth as no significant difference between corn grain yields of control and FOG-amended plots was observed at this location. The other possible reason for nonsignificant difference in corn yields of FOG-amended and control plots could be that FOG might also have been decomposed rapidly at this location. An enhancement in crop residue decomposition at higher soil N contents and after the addition of N has been reported previously (Mary et al., 1996; Jensen, 1997; Henriksen and Breland 1999a, 1999b). On the basis of these results, we suggest that soil NO3–N content should be determined at the time of FOG application as it can affect the decomposition of FOG and hence the availability of N for crop growth.
Experiment 3
Corn grain yield response curves at different N application rates under different FOG management practices during 1996 and 1997 (time of FOG application) are presented in Fig. 3
and Fig. 4
, respectively. Maximum corn grain yields without N fertilizer application were obtained from control and were not significantly higher than FallFOG in 1996. The lack of a significant difference between corn grain yield in FallFOG and control in 1996 suggests that FallFOG did not significantly affect the availability of soil N for plant growth. Fall-applied FOG decomposed and the immobilized N was remineralized in time for crop uptake (Rashid and Voroney, 2003). Furthermore, similar amounts of soil NO3–N (35 mg kg–1) were measured at sidedressing time under both treatments. However, the corn grain yields obtained from control and FallFOG plots were significantly higher than corn grain yields obtained from SpringFOG plots.
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Maximum corn grain yields without N fertilizer application in 1997 were obtained from control plots followed by FallFOG- and SpringFOG-amended plots. The maximum increases in corn grain yields in 1997 due to incremental applications of N fertilizer (50 to 200 kg ha–1) were observed (57 to 150% increases over 0 N during 1997) in SpringFOG-amended plots followed by control and FallFOG.
Soil NO3–N contents in 0- to 30-cm soil depth (33 and 25 kg ha–1) were decreased to 4 and 2 kg N ha–1 1 wk after SpringFOG application both in 1996 and 1997, respectively. At the time of N sidedressing (27 June; both 1996 and 1997), soil NO3–N contents were still low (12 and 6 kg N ha–1) and may have contributed to significantly lower corn grain yields compared with control and FallFOG. Soil NO3–N was immobilized during the decomposition of FOG. Data on effect of FOG application on soil NO3–N is reported in a separate paper (Rashid and Voroney, 2003). When readily decomposable organic matter with a high C/N ratio is added to soil, the heterotrophic microorganisms become N limited (Kay and Hart, 1997), and soil inorganic N is immobilized during the decomposition of these materials (Jackson et al., 1989; Whitmore and Handayanto, 1997).
The MERN as affected by FOG management in 1996 and 1997 is presented in Table 5. Spring-applied FOG had higher MERN compared with control and FallFOG in both years. The MERN calculated for FallFOG-amended plots was lower (108 kg ha–1) compared with control (114 kg ha–1) in 1996. However, the MERN for FallFOG-amended plots was higher (132 kg ha–1) compared with control plots (119 kg ha–1) in 1997. The difference in MERN between SpringFOG and control plots in 1996 and 1997 (62 and 59 kg N ha–1, respectively) shows that supplemental N was required for SpringFOG plots to meet the microbial N demand during FOG decomposition and crop growth. Maximum economic yield for FallFOG was similar to that of control in 1996; however, it was lower compared with control in 1997. The MEY values for SpringFOG were lower compared with control and FallFOG plots both in 1996 and 1997.
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A lower MEY with SpringFOG compared with control and FallFOG treatments revealed that SpringFOG affected N availability presumably due to N immobilization. The corn crop was seeded less than a month after SpringFOG application, and N immobilization during initial stages of FOG decomposition (Rashid and Voroney, 2003) might have reduced the N available for corn plant growth.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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