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Land Resource Science, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
* Corresponding author (trashid{at}uoguelph.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CMR, chlorophyll meter readings • MERN, maximum economic rate of nitrogen application • PAON, potentially available organic N • PPNT, preplant nitrogen soil test • PSNT, Presidedress soil test for nitrogen • TSN, total soluble nitrogen • WWC, winter wheat covercrop
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The most immediate impact of high C/N ratio organic materials application is on the availability of N to the subsequent crop, as a consequence of mineralization–immobilization processes. Patterns of N immobilization and mineralization and high rates of decomposition were observed in studies involving different oils (Smith, 1974; Higuchi and Kurihara, 1980), volatile fatty acids (Kirchman and Lundvall, 1993; Sorensen, 1998), and oily food waste (Plante and Voroney, 1998). Plante and Voroney (1998) reported that the N immobilized during decomposition of oily food waste is expected to be subsequently mineralized and available to the succeeding crop. Net soil N immobilization was observed due to oily food waste application in spring; however, immobilized N was remineralized where oily food waste was applied in previous fall (Rashid and Voroney, 2003). Corn grain yields were significantly reduced due to oily food waste application in spring compared with application in the previous fall. Supplemental fertilizer N was required for the plots amended with oily food waste in spring to maintain crop yields (Rashid and Voroney, 2004).
Since additions of oily food waste may affect the short- and long-term soil N dynamics differently than net N-supplying organic wastes, recommendations based on conventional methods may not be applicable. There is a need to identify a reliable method for making N fertilizer recommendations for corn grown on soils amended with oily food waste.
Several soil and plant tests have been proposed over time to convert basic soil fertility data into soil and plant testing programs that evaluate variable response to applied nutrients (Schroder et al., 2000). Assessment of several soil and plant test methods that could improve N management for corn production under a wide range of field and crop management conditions has been reported in the literature (Magdoff et al., 1984; Blackmer et al., 1989; Fox et al., 1989; Hong et al., 1990). Once a soil or plant tissue test is identified, its validity can only be accepted after a considerable experimental evidence of its ability to assess the N sufficiency for plant growth across a wide range of field conditions (Balkcom et al., 2003).
Presidedress soil test for N (PSNT) in cornfields is based on NO3–N concentrations in the surface 30-cm layer of soil when plants are 15- to 30-cm tall (Magdoff et al., 1984). This sampling time is late enough to reflect the effects of spring weather conditions and early enough to apply fertilizer if needed. The PSNT allows farmers to adjust N fertilization rates and can take into account factors such as soil type, weather conditions, and management needs (Magdoff, 1991). The PSNT has been shown to be a useful in evaluating N availability for corn under various crop, and soil management conditions in humid regions of Northeast and Midwest of USA (Blackmer et al., 1989; Fox et al., 1989; Magdoff et al., 1990; Meisinger et al., 1992; Roth et al., 1992) and in Ontario, Canada (McGonigle et al., 1996).
Measurement of soil N mineralization potential proposed by Stanford and Smith (1972) has widely been evaluated as a soil N availability index. The method is time-consuming, requiring >20 wk of incubation under optimum temperature and moisture conditions (Campbell et al., 1994). A rapid chemical method to determine the potentially available organic nitrogen (PAON) proposed by Gianello and Bremner (1986) has recently received renewed attention and has been recommended for use as an index of N mineralization potential (Jalil et al., 1996; Campbell et al., 1997).
Transmitting light through a leaf has been used to assess the greenness of the plant by using SPAD meter (Minolta, 1990). The meter detects how much red and near infrared light is transmitted by a leaf. The amount of absorbed red light indicates the amount of chlorophyll, whereas the amount of absorbed near infrared serves as an internal reference to compensate for leaf thickness and moisture content (Schroder et al., 2000). Nitrogen concentrations, chlorophyll concentration, and CMRs are strongly correlated (Wood et al., 1992; Waskom et al., 1996). Corn plants under N stress show chlorosis or yellowing, and this deficiency can easily be detected at the 5 to 6 leaf stage, which is close to the PSNT sampling period. Chlorophyll meter was considered to be a convenient tool for evaluating the N status of a corn crop in the early part of the crop-growing season (Piekielek and Fox, 1992; Schepers et al. 1992a, 1992b; Dwyer et al., 1995; Binder et al., 2000; Rashid et al., 2005).
Soil and plant indices of soil fertility status have traditionally been developed using conventional soil and crop management practices. Published research resulting in N fertilizer management recommendations for corn produced on soils amended with C-rich organic materials such as oily food waste is scarce.
The research reported in this manuscript is part of a program to develop a protocol for the disposal of oily food waste to agricultural soils as a C source, and to develop N fertilizer recommendations for corn grown on amended soils. Experiments were conducted at different locations representing different soil fertility status and crop and field management conditions. Oily food waste was applied at variable rates, at different times and at different slope positions within a field. The specific objective of this research was to evaluate different soil and plant indices for making N recommendations for corn production on fields receiving oily food waste.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Timing of Oily Food Waste Application (Exp. 2)
Experiment 2 was also conducted at Elora Research Station in 1996 and 1997 on two separate fields. The experiment was set out in a split-plot design with four replications. The main plot treatments were control (no oily food waste), oily food waste in fall (Fall-waste), oily food waste in spring (Spring-waste), and winter wheat cover crop (WWC). Main plots were 22.5 m by 10 m. Oily food waste at 10 Mg ha–1 was applied on October third and fifth (in fall) and on April 25th and 28th (in spring) during 1996 and 1997, respectively. The WWC treatment was another source of readily decomposable C and provided a reference in addition to the control for the effect of time of oily food waste application on soil NO3–N levels.
The winter wheat cover (Triticum aestivum L. cv. Harus) was planted on the same day that oily food waste was applied in the fall and was incorporated in the soil on the same day that oily food waste was applied in spring. Four samples of the cover crop (1 m2) from each replication were taken on May 20th and 22nd in 1996 and 1997, respectively, before its incorporation in soil and oven dried (60°C) to determine the quantity of cover crop residue retained. The dry weight of WWC biomass (CN = 14 ± 2) incorporated in soil were 1.48 and 1.26 Mg ha–1 for 1996 and 1997, respectively. Nitrogen fertilizer application treatments (0, 50, 100, 150, and 200 kg N ha–1) were assigned to subplots (4.5 m by 10 m) and were applied at the time of field preparation for corn seeding during both years. The same hybrid of field corn planted in Exp. 1 was also planted (65000 plants ha–1) at two locations for this experiment during the third week of May in 1996 and 1997.
Slope Positions within a Field (Exp. 3)
The experiment was conducted at two different locations on a farm in Bellwood, ON during 1995 and 1996. The soils at these locations were classified as Hillsburg fine sandy loam (Typic Hapludalf). Topography was rolling with gradients up to 5% in 1995 and 9% in 1996. The experiment was randomized complete block design with four replications at each of three slope positions: upper (summit), mid (back slope), and lower (foot slope). Oily food waste was stored in a liquid manure tank at the farm and was well mixed by using a tractor-operated mixer before it was applied to the field by using a commercial liquid manure application tanker. Oily food waste was applied at 10 Mg ha–1 on April 26th in 1995 and April 28th in 1996. The surface soil was allowed to dry (2–4 h) before the oily food waste was incorporated by moldboard plow. Urea N fertilizer was applied at the time of field preparation at 0, 50, 100, 150, and 200 kg N ha–1 to plots (4.5 m x 10 m) during both years. Field corn (hybrid DK-306) was planted at 65000 plants ha–1 on May 21st in 1995 and May 24th in 1996.
Oily Food Waste Analysis
Oily food waste used in these studies was 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 fat, oil, and grease, water and food-derived solids. Without mixing, the fluid quickly separates into four distinct layers from top to bottom: (i) a dark oily layer, (ii) suspended solids, (iii) yellow-tinted water, and (iv) settled solids. Grab samples of oily food waste were taken at four different times during its application in the field each year for chemical analysis.
The non-aqueous content (oil + solid contents) of the waste was determined by freeze drying at –30°C. The nonaqueous contents are organic because no grit or other fixed solids were found after ignition at 550°C. Total C, N, and oil contents were determined on waste samples that were freeze-dried at –30°C. 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 85% of the molecular weight (Plante, 1996). The C content of the residue and oil C were summed for oily food waste C content. Total Kjeldahl N, pH, and electrical conductivity of oily food waste were also determined by following the methods described by McGill and Figueiredo (1993), Peech (1965), and Bower and Wilcox (1965), respectively. 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 to calculate the oil contents in oily food waste samples (American Public Health Association, American Water Works Association, and Water Environment Federation, 1995). Chemical composition of oily food waste applied at different locations of two different sites is presented in Table 2.
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Soil samples (field moist) were thawed and sieved through a 2-mm sieve and were extracted with 2 M KCl (1:5 ratio) at room temperature (Keeney and Nelson, 1982) and NO3–N and NH4–N were determined using a Braun + Lubbe TRAACS 800 instrument (Tel and Heseltine, 1990). Soil samples were heated for 4 h at 100°C in 2 M KCl (Gianello and Bremner, 1986) and the NH4–N released was determined as described previously. Potentially available organic N was determined by subtracting the NH4–N extracted with 2 M KCl at room temperature from the NH4–N extracted by 2 M KCl after heating as described by Gianello and Bremner, (1986). Soil total soluble N (TSN) was determined by heating soil samples in 0.05 M K2SO4 at 100°C for 4 h and analyzing N colorimeterically using a Technicon Industrial Systems auto analyzer (Voroney et al., 1993). Data regarding the soil sample analysis taken at preplant time of sampling and presiddress time of sampling for different forms of N in soil (NO3–N, NO3–N + NH4–N, hot KCl NH4–N, POAN, and TSN are presented in Table 3 and 4, respectively. Table 4 also contains the chlorophyll meter readings data as it was only taken at presidedress time of sampling.
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At maturity, two central corn rows (1.52 m x 5 m; 7.6 m2) of each plot were hand harvested at all experimental locations to determine the grain yields. Ten subsamples of cobs were collected and oven dried at 65°C. Oven-dried cobs were shelled and corn grain yield was calculated (expressed on 155 g kg–1 moisture content basis). The maximum economic rate of N fertilizer application (MERN) was calculated from the quadratic yield response in Eq. [1] and a fertilizer to corn price ratio as described below.
 | [1] |
The derivative of the quadratic response equation:
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The ratio "R" of the price of 1 kg of fertilizer N to the price of 1 kg of corn grain was equal to 7.
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Statistical Analysis
Statistical analysis of MERN calculated for different experiments was performed by PROC GLM procedure of SAS (SAS Institute, 1996). The analysis of variance for the change in MERN due to oily food waste application was performed by dividing the data into three data subsets, representing the rate, time, and landscape position of oily food waste application. Correlation coefficients were also calculated for the relationship between MERN and different forms of soil N at PPNT and PSNT time of soil sampling and CMRs. Analysis of the regression lines (MERN vs. soil and plant indices) was performed to determine if the slopes are significantly different from zero.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil N index values were affected by the oily food waste management conditions at both sampling times. Soil N index values were lower where oily food waste was applied in spring compared with the control, fall-applied oily food waste, or to lower slope positions in the field. Incorporation of organic materials in soil cause a rapid increase in the soil microbial biomass (Fauci and Dick, 1994; Jensen, 1997), which acts as a sink and source for plant nutrients and is an active participant in nutrient recycling (McGill et al., 1986). Soil inorganic N depletion during initial stages of decomposition of edible oils, fats, volatile fatty acids, and oily food waste have been reported (Smith, 1974; Higuchi and Kurihara, 1980; Kirchmann and Lundvall, 1993; Sorensen, 1998; Plante and Voroney, 1998; Rashid and Voroney, 2003).
Soil N index values in plots received oily food waste in fall were also lower after its application, however, later in the next spring season higher soil N index values were observed. Soil N immobilized during the decomposition of oily food waste was remineralized (Plante and Voroney, 1998) and become available to growing corn crop (Rashid and Voroney, 2004).
Higher contents of soil N at the lower slope position might be due to the fact that this position typically has high amounts of organic matter and total N compared with soils in upper slope positions (Gregorich and Anderson, 1985). The lower slope position receives N-rich topsoil from higher slope positions as a result of redistribution (Pennock et al., 1994). The greater soil available N contents at the lower slope position would have accelerated the decomposition of applied oily food waste, as soil available N influences the decomposition rate of high C/N organic materials (Mary et al., 1996; Recous et al. (1995).
Maximum economic rates of N application were different for each oily food waste management treatment and location and ranged between 0 to 182 kg ha–1 (Table 5). MERN data was divided in three sub sets: (i) rate of oily food waste, (ii) time of oily food waste, and (iii) slope positions, to perform the statistical analysis. The MERN values were significantly (P < 0.05) increased with the increasing rate of oily food waste application. The MERN values were also significantly higher where oily food waste was applied in spring compared with control, WWC, and fall-applied oily food waste. Lower slope position had significantly lower MERM values compared with Mid and Top slope positions.
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The MERN was used as agronomic indicator to evaluate different soil and plant indices for predicting N fertilizer requirements of corn grown on fields receiving oily food waste. The correlations between soil (samples taken at PPNT and PSNT) or plant indices and MERN are illustrated in Fig. 1 and 2 . Regression coefficients of individual trend lines were significantly different from zero at P < 0.001. Soil NO3–N, NO3–N + NH4–N, and NO3–N + PAON at PPNT were highly correlated (inversely correlation) with MERN, showing high coefficients of correlation (r = –0.78, –0.80, and –0.78, respectively). Correlation between hot KCl NH4, hot KCl POAN, and TSN were lower (–0.58, –0.55, and –0.49) compared with soil NO3–N, NO3–N + NH4, and NO3–N + POAN at PPNT time of sampling.
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The variability in MERN was better explained by soil NO3–N at PSNT sampling time (r2 = 0.88) compared with some other soil N indices (hot KCl NH4–N, hot KCl– PAON, and hot K2SO4 TSN). The high inverse correlation between NO3–N at PSNT sampling time and MERN indicates that soil NO3–N content is a more reliable indicator of soil N availability compared with hot KCl NH4–N, PAON, and hot K2SO4 TSN. A high correlation between PSNT soil NO3–N has been reported for corn grain yields (Binford et al., 1992; Meisinger et al., 1992; Vyn et al., 1999), and N supplying capacity of the soil (Hong et al., 1990) and MERN (Kachanoski et al., 1996; McGonigle et al., 1996).
The lower inverse correlation between MERN and 2 M hot KCl-NH4–N or 2 M hot KCl PAON compared with correlation between PSNT soil NO3–N and MERN is in agreement with that reported by Hong et al. (1990). They found a poorer relationship between N supplying capacity of soil (NSC) and 2 M hot KCl extractable organic N (r2 = 0.48) compared with that with soil NO3–N, at PSNT (r2 = 0.75) for corn. Our findings are contradictory to those reported by Jalil et al. (1996) and Campbell et al. (1997) as they found a high correlation between 2 M hot KCl NH4–N and wheat grain yields. Smith and Li (1993) and McTaggart and Smith (1993) also reported a high correlation between 2 M hot KCl PAON and grain yields for barley (Hordeum vulgare L.), ryegrass (Lolium multiflorum L.), and oats (Avena sativa L.). Likely reasons for disagreement between our results and those reported by these researchers are the difference in crop, type of organic waste applied, and the crop response indicator used to correlate with soil indices. Their results are from conventional crop management conditions, while we applied a C-rich waste material to soil. Crop grain yields were used in their studies while we used MERN as a crop response indicator to correlate with the soil and plant indices.
We expected that the sum of potentially available organic N to soil NO3–N (NO3–N + PAON) as an index should improve the correlation between N index and MERN, but the correlation was not improved. It implies that the method we used for PAON, suggested by Gianello and Bremner (1986), does not prove to be a good indicator of soil N mineralization potential for soils amended with C-rich organic materials like oily food waste.
Chlorophyll Meter Readings and their Correlation with Maximum Economic Rate of Nitrogen Application
Chlorophyll meter readings were taken during the same period when soil samples were taken at PSNT time. Lower CMR values were observed for N deficient plots, (oily food waste applied at higher rate or applied in spring) and higher CMR were observed for N sufficient plots, (control, fall applied oily food waste, and lower field slope positions). Highest CMR (37 and 40) were recorded where PSNT soil NO3–N contents were high (20.4 and 21.5 mg kg–1; lower slope positions) and lower CMR values were recorded where soil NO3–N contents in plots were lower (5.9, 6.1, and 2.1, 2.3 mg kg–1; oily food waste was applied at 10 and 20 Mg ha–1 in spring; Table 4). Leaf greenness is influenced by a number of factors (hybrid, stage of growth, and nutrients), but soil N availability most probably has the greatest effect within a field (Blackmer and Schepers, 1995; Rashid et al., 2005). Chlorophyll meter readings correspond to plant N levels due to effects on chlorophyll content (Girardin et al., 1985; Wolfe et al., 1988; Wood et al., 1992).
An inverse linear correlation was observed between CMR and MERN (Fig. 2). Chlorophyll meter reading had a lower inverse correlation with MERN (r = –0.80) compared with soil NO3–N at PSNT sampling time (r = –0.88). Vetsch and Randall (2004) showed that leaf CMRs at different corn growth stages are influenced by crop as well as N management practices. Attempting to diagnose potential N deficiency at 5 to 6 leaf stage was problematic in all years, with r2 ranging between 0.50 and 0.77. They suggested that determining leaf chlorophyll content at the 5 to 6 leaf stage as an index was too early to develop a satisfactory relationship for predicting N deficiency. Piekielek and Fox (1992) reported that the chlorophyll meter method was similar to several proposed soil N availability tests and soil NO3–N in its accuracy for separating N responsive sites from non-responsive sites. They further reported that CMRs were not well enough correlated with soil N supplying capability (r = 0.59) to determine sidedress N fertilizer rates for N responsive sites.
As organic matter including crop residues, manure, composts, cover crops (Magdoff, 1991) and oily food waste (Plante and Voroney, 1998) decomposes, available N is released. During decomposition process N in organic molecules is converted into mineral (mostly NH4–N) forms and most of the NH4–N is quickly converted to NO3–N. The NO3–N is highly mobile in soil and primarily reaches plant roots by moving with the flow of water (mass flow). Most of the N that corn will take up from soil will be as NO3–N whether it comes from fertilizer or from the decomposition of soil organic matter.
Synchrony between NO3–N release and N demand by crop and is one of the most important keys to understanding N management options. By applying N close to the time of the crop's greatest need (and when soil moisture tends to be below field capacity and evapotranspiration exceeds precipitation) there is a little possibility for loss by denitrification or leaching (Magdoff, 1991).
Soil samples for PSNT NO3–N are taken when corn is 15 to 30 cm tall and is corn crop has a very high demand for N for its growth. The rate of N uptake by corn is relatively slow before the plant enters the period of rapid growth at about the 6-leaf stage. This stage usually occurs in early June (Corn Belt in USA) to early July (Ontario, Canada). As the plant grows in height and new leaves emerge the rate of N uptake increases rapidly to about 2.5 to 4.5 kg ha–1 d–1 (Karlen et al., 1988). The PSNT NO3–N test performed during this high N demand by corn crop is considered to be the most reliable soil N test to make N fertilizer recommendation to corn (Magdoff, 1991; Binford et al., 1992, Vyn et al., 1999). Furthermore soil NO3–N content at that stage represents the net balance between production (mineralization from soil organic matter, manure, and/or fertilizers) and loss (leaching, denitrification, and immobilization) because little or no N uptake occurs before that stage (Meisinger et al., 1992).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication January 21, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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