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a Agriculture and Agri-Food Canada, Research Centre, 5403 1st Avenue South, Lethbridge, AB, Canada T1J 4B1
b Agriculture and Agri-Food Canada, Research Centre, P.O. Box 1000A, Brandon, MB, Canada R7A 5Y3
* Corresponding author (larneyf{at}agr.gc.ca)
Received for publication November 23, 2005.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AP, available phosphorus • BMP, best management practices • DM, dry matter • GHG, greenhouse gas • IN, inorganic nitrogen • OM, organic matter • PAP, percent available phosphorus • PIN, percent inorganic nitrogen • TC, total carbon • TN, total nitrogen • TP, total phosphorus; WC, water content
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Traditionally, in western Canadian feedlots, pens are cleaned in spring and solid manure is hauled directly to nearby fields for immediate land application. Manure is generally wet (up to 75% water, wet weight) following spring snowmelt, which rules out long-distance transport. Odor emissions are an additional downside (McGinn et al., 2003), often leading to complaints from nearby residents.
Another widespread handling option is stockpiling, where manure is heaped into stockpiles or stacks, either inside or outside feedlot pens, to await reloading, hauling, and spreading (Sweeten, 1996). Stockpiling permits regular pen cleaning, even when spreader trucks or recipient cropland are unavailable. It is also known as "static pile" handling (Burton and Turner, 2003) or somewhat ambiguously as "passive composting" (Rynk, 1992). Lopez-Real and Baptista (1996) described stockpiling as a minimal intervention technique, akin to normal handling procedures, and felt that referring to it as a form of "passive composting" elevated it to an undeserved status. In a dairy manure management survey in California, Morse Meyer et al. (1997) reported that 95% of producers included stockpiling in their manure collection practices. Although stockpiling is a common practice, little is known about its effects on manure properties.
Composting is a relatively recent manure handling practice in the feedlot industry (DeLuca and DeLuca, 1997; Kashmanian and Rynk, 1996; Wang and Sparling, 1995). Janzen et al. (1999) proposed that improved effectiveness in the use of manure was achieved by processing (e.g., composting) to increase nutrient retention and reduce transport costs. In southern Alberta, composting emerged in the mid-1990s and currently handles approximately 5 to 10% of the manure generated (Larney, unpublished data, 2006). Its benefits include reduced haulage requirements (Larney et al., 2000), and elimination of viable weed seeds (Larney and Blackshaw, 2003), coliform bacteria (Larney et al., 2003), and human parasites such as Giardia and Cryptosporidium (Van Herk et al., 2004). Additionally, composted feedlot manure has been successfully used for soil reclamation (Larney et al., 2005a).
In western Canada, few data exist comparing feedlot manure nutrient levels for traditional handling treatments (direct land application of fresh manure, stockpiling) with newer ones such as composting. Also, information on nutrient mass balance estimates, as affected by handling treatment, is lacking. Feedlots in western Canada are located at the northern limit of the beef industry in North America and as such represent management practices in a cold climate. One of these practices is use of straw bedding during winter to mitigate cold stress on the animals. Therefore the material removed at spring pen cleaning has a manure to bedding ratio (dry weight basis) of approximately 5:1 (Larney, unpublished data, 1999) which is different than the situation in neighboring North Dakota (Anderson et al., 2004), or further south in the United States, where little or no bedding is used.
This project examined nutrient concentration changes and mass balances as fresh beef feedlot manure was stockpiled or composted in two different Canadian ecoregions: southern Alberta, in the semiarid western prairie and southwestern Manitoba, in the more humid eastern prairie. Land application and haulage scenarios were developed to demonstrate the effect of manure handling treatment on soil nutrient inputs for crop production.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Precipitation and air temperature during the periods manure was in stockpiles or compost windrows/curing piles was measured at weather stations < 1 km from the study sites. Water was not added to the compost treatment except on 18 Sept. 2001 when 2000 L was added at Lethbridge due to very dry conditions.
Sampling
Initial sampling of fresh manure (Table 1) was conducted at various stages of depositing each truckload of manure into stockpiles or windrows. Eight approximately 1-kg (wet weight) samples were taken with a shovel and composited and mixed (four samples per bag) into two plastic bags. Depending on year, treatment, location, and amount loaded onto the truck, the number of truckloads varied from 8 to 13, hence the number of samples of fresh manure varied from 16 to 26 (two sample bags per truckload). Final sampling of stockpiled and composted manure (Table 1) coincided with land application, as the materials were loaded into a tractor-pull manure spreader (using the same weight and number of samples as described for fresh manure). Again, depending on year, treatment, location, and amount loaded onto the manure spreader, the number of spreader loads varied from 5 to 13, hence the number of stockpiled or composted samples varied from 10 to 26.
Analyses
As soon as possible after sampling, each sample bag of fresh, stockpiled, or composted manure was subsampled (10 g wet weight) for inorganic N (IN = NO3–N + NH4–N) which was determined colorimetrically after extraction with 200 mL of 2 N KCl. A larger subsample (approximately 1.5 kg wet weight) was taken for water content (WC) determination (expressed on a wet weight basis) by oven-drying at 60°C for 5 d (Peters et al., 2003). The remaining analyses were performed on oven-dried samples. Total C and TN were determined on finely ground (<150 µm) samples using dry combustion–gas chromatography at Lethbridge (Carlo Erba, Milan, Italy) and Brandon (Leco Corporation, St. Joseph, MI). Total P was determined after wet digestion with H2SO4 and H2O2 and available phosphorus (AP) using a modified Kelowna extract (Ashworth and Mrazek, 1995). Ash concentration (kg kg–1) was defined as [1 – organic matter (OM), kg kg–1] where TC concentration was used to estimate OM from an equation:
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Mass Balance Estimates
In the following estimates, "initial" refers to fresh manure and "final" to stockpiled or composted manure. All mass parameters are expressed in kg and all concentrations in kg kg–1. Dry matter (DM) mass balance was estimated by assuming (i) mass losses were wholly attributed to OM and (ii) the mass of ash (mineral matter) was conserved during the stockpiling or composting process (Bernal et al., 1998b; Petersen et al., 1998). Therefore:
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Water (H2O) mass balance was estimated as:
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Total mass balance (DM + water) was calculated as:
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Mass balance estimates for nutrient components (C, N, P) were calculated as:
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Statistical Analyses
Data were analyzed using PROC MIXED (SAS Institute, 2005) with handling treatment (fresh, stockpiled, composted) and location as fixed effects and year as a random effect. Therefore years (n = 3) were used as replicates. Data were also tested for a treatment x location interaction which, if significant (P < 0.05), indicated that the behavior of treatments, relative to each other, differed with location. Data for the fresh treatment was the average of fresh material which eventually became stockpiled manure and fresh material which became compost.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Mean air temperatures during stockpiling and, by approximation, the thermophilic phase of composting varied from 10.7°C at Lethbridge in 2002 to 16.9°C at Brandon in 2001. While air temperature may not directly affect the composting process, high evaporation rates associated with high air temperatures accelerate loss of water and inhibit microbial decomposition. On average, mean air temperatures during the entire composting period (thermophilic + curing) were cooler at Brandon than at Lethbridge (Table 1).
Water Content
A significant treatment x location interaction showed that WC of fresh manure (571 g kg–1) was significantly higher than stockpiled (459 g kg–1) which was in turn significantly higher than composted manure (336 g kg–1) at Lethbridge (Fig. 1). However, at Brandon, there was no significant difference in WC between fresh (731 g kg–1) and stockpiled manure (684 g kg–1), while composted manure (384 g kg–1) was significantly drier. The similarity of the fresh and stockpiled treatments at Brandon was likely due to higher initial water content and higher summer rainfall (Table 1). Also the difference in WC between stockpiled manure and compost was much wider at Brandon (300 g kg–1) than at Lethbridge (123 g kg–1).
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In a land application scenario, stockpiled manure supplied an average of 79% of the TC of fresh manure, and compost an average of 51% of the TC of fresh manure (Table 2). However, the stable C in compost (comprised largely of lignin, cellulose, and hemicellulose), with low mineralization potential, may have a longer residence time once incorporated into soil. Helgason et al. (2005) found that up to 98% of compost C remained after a 168-d incubation experiment, compared with values of 55 and 70% for fresh feedlot manures. Larney et al. (2005a) reported that 3.5 yr after soils were amended with compost, fresh manure, or alfalfa (Medicago sativa L.), 65% of compost C remained, compared to 45% for fresh manure and 28% for alfalfa.
Nitrogen
There was a significant treatment x location effect on TN concentration (Table 2) with no treatment differences at Lethbridge, but significantly lower TN (12.4 kg Mg–1 vs. 15.5–16.7 kg Mg–1) in the composted treatment at Brandon, likely reflecting greater N losses. Nitrate N (Fig. 2a) and NH4–N (Fig. 2b) were significantly affected by treatment but not by location or treatment x location. Therefore, averaging across locations, there was no significant difference in NO3–N between fresh (50 mg kg–1) and stockpiled manure (240 mg kg–1) while composted manure (377 mg kg–1) was not significantly different than stockpiled manure but was significantly higher than fresh manure. Both fresh (3948 mg kg–1) and stockpiled manure (4193 mg kg–1) had significantly higher levels of NH4–N than compost (311 mg kg–1). The higher NO3–N and lower NH4–N concentrations in compost compared with fresh manure is reflective of nitrification, which is a step in the degradation of OM during composting (Fauci et al., 1999; Peigné and Girardin, 2004). The greater NH4–N concentration in stockpiled compared with composted manure was linked to the static nature of stockpiles which (i) allowed a dry crust to form at the surface, thereby preventing NH3 volatilization, and (ii) promoted anaerobic conditions within the pile which hindered the aerobic process of nitrification.
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The percent inorganic nitrogen (PIN), calculated as [(NH4–N + NO3–N)/TN x 100], that is considered readily available for plant uptake, was significantly lower for compost (5.1%) than both fresh (24.7%) and stockpiled (28.9%) manure (Table 2). This was due to the loss of NH4–N during composting via (i) conversion to NH3 which is lost by volatilization and (ii) nitrification to NO2–N and NO3–N (which is prone to leaching or runoff losses from windrows), resulting in a final product with approximately 95% of its N in organic form. Unlike compost, stockpiled manure had only 71% of its N in organic form. The large PIN value (29%) was likely due to anaerobic decomposition of organic N, producing NH4–N which is stable under anaerobic conditions.
The effect of manure handling on inorganic nitrogen (IN) concentration (Table 2) followed the trend of PIN with compost being significantly lower (0.7 kg Mg–1) than both fresh (4.1 kg Mg–1) and stockpiled (4.5 kg Mg–1) manure. In a land application scenario, even though there were no significant differences in TN supplied, compost should not be viewed as an amendment to meet immediate plant N demand as it supplied only 15 to 17% of the IN of fresh or stockpiled manure.
Phosphorus
Total P concentrations were affected by a significant treatment x location interaction (Table 2). Composted manure (5.5 kg Mg–1) had a significantly higher TP concentration than fresh (4.5 g kg–1) or stockpiled manure (4.6 g kg–1) at Lethbridge, while at Brandon, stockpiled manure had a significantly higher P concentration (6.5 g kg–1) than fresh (4.7 g kg–1) or composted manure (4.9 g kg–1). The increase in P concentration in compost at Lethbridge was related to simultaneous DM losses during composting. It is unclear why composting did not significantly increase TP concentration at Brandon.
Handling treatment did not have a significant effect on percent available phosphorus [PAP, (AP/TP) x 100] (Table 2), with all three treatments having PAP values of 61 to 70% (Table 2). This in turn led to a lack of significant differences in AP concentration with handling treatment values ranging from 3.4 to 3.7 kg Mg–1. For land application, even though composting would significantly increase TP addition at Lethbridge, the supply of AP for the three handling treatments was not significantly different.
Component Ratios
Carbon to nitrogen ratio (Table 2) was also affected by a treatment x location interaction, which showed no significant difference between fresh and stockpiled manure at Lethbridge (17.6 vs. 16.3) while C to N ratio of stockpiled manure (15.4) was significantly lower than fresh manure (21.8) at Brandon. Composting led to significant declines in C to N ratio (11.7 at Lethbridge, 10.9 at Brandon) compared to fresh or stockpiled manure. This demonstrates the stabilization of manure C with composted manure approaching a typical soil C to N ratio of 10:1.
Nitrogen to phosphorus ratio is often used as an index of potential soil P loading for organic amendments (Sharpley et al., 2001). Agricultural plants typically have N to P ratios of 4.5 to 6 and therefore remove 4.5 to 6 times more N from the soil than P (Eghball, 2002). In contrast, livestock manures have substantially lower N to P ratios than plants and since manures are often applied on an N basis to meet crop N demand, P may be over-applied (Olson et al., 2005). The lower the N to P ratio, the more magnified the P over-application effect. Both stockpiled and composted manure had N to P ratios (2.7–2.9) that were significantly lower than fresh manure (3.5). Therefore application of compost or stockpiled manure at equivalent dry weight rates may lead to greater buildup of soil P. Eghball et al. (1997) also found that composting reduced N to P ratio of feedlot manure in Nebraska from an average of 2.6 in fresh manure to 1.8 in compost. However, since compost is generally purchased for land application, rather than donated like most fresh manure, low application rates are typical [approximately 10 Mg ha–1 yr–1 (wet weight) in southern Alberta] which helps restrict soil P buildup. The N to P ratio was the only parameter in Table 2 that had a significant location effect. Manure (average of all three treatments) had a significantly lower N to P ratio at Brandon (2.8) than at Lethbridge (3.2).
Mass Balance Estimates
Dry Matter
Average DM losses (Table 3) were significantly higher (P = 0.04*) with composting (39.8%) than stockpiling (22.5%). This was associated with greater aerobic decomposition stimulated by incorporation of air during composting. Tiquia et al. (2002) reported DM mass losses of 52% for turned compost windrows and 37% for unturned windrows (similar to our stockpiles) for a study with pig (Sus domesticus) manure in Iowa. Sommer (2001) measured DM losses of 39 to 43% during composting of solid cattle manure in Denmark. Eghball et al. (1997) reported DM losses of 20% for feedlot manure composting in Nebraska which were deemed lower than the normal range of 35 to 50% due to high ash content (590 kg Mg–1) of the manure. For our six compost treatments (three years x two locations), fresh manure with the lowest ash content going into the process (306 kg Mg–1 at Brandon in 2002) resulted in the highest DM loss (58.4%, Table 3). In contrast, the fresh manure with the highest ash content (556 kg Mg–1 at Lethbridge in 2002) resulted in a DM loss of 26.9% (Table 3). High ash contents in fresh manure reflect contamination with soil during pen scraping and hence a dilution of OM.
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Water
Water content of solid feedlot manure is one of the main factors limiting transport distance. Heavy wet fresh manure is less economical to transport as 65 to 75% of its weight is water. Therefore manure handling treatments that enhance water loss encourage longer-distance transport away from soils with high nutrient loadings. Water mass losses ranged from 17.4% with stockpiling at Brandon in 2000 to 93.7% with composting at Brandon in 2002 (Table 3). Low water losses during stockpiling at Brandon in 2000 may be related to the very wet conditions (337 mm precipitation) during that time period (Table 1). On average, composting led to a water mass loss of 79.9% which was significantly higher than 42.8% from stockpiling (Table 3). Compost turning brought wetter material to the windrow surface for evaporative drying while undisturbed stockpiles remained wet at the center. The water mass loss of 79.9% from composting was similar to the 83% value reported by Larney et al. (2000) for summer composting at Lethbridge. Michel et al. (2004) reported water mass losses up to 89% for dairy manure in Ohio. Robinzon et al. (2000) reported that 56 to 79% of water mass loss was due to evaporation from the windrow surface, 21 to 31% evaporated via air flow by natural ventilation, and only 6 to 10% was attributed to turning during municipal solid waste composting in Israel. The location and treatment x location effects on water mass balance were nonsignificant (Table 3).
Total (Dry Matter + Water)
Overall, composting resulted in a total mass loss of 65.6% which was significantly higher than 35.2% for stockpiling (Table 3). These values are equivalent to reductions in haulage requirements. The total mass loss for composting (65.6%) compared favorably to the 64.3% for summer composting at Lethbridge in 1997 (Larney et al., 2000). Michel et al. (2004) found total mass losses of 41 to 83% for dairy manure composting. The location and treatment x location effects on total mass balance were nonsignificant (Table 3).
Carbon
Carbon losses are associated with all forms of manure management as microbial decomposition breaks down readily available C in manure and bedding material. Carbon losses ranged from 27.1% for stockpiled manure at Brandon in 2000 to 87.4% for composted manure at Brandon in 2002 (Table 4). Averaged over locations, C losses were significantly higher (P = 0.007) with composting (66.9%) than with stockpiling (37.5%), due to greater microbial decomposition with turning. With solid pig manure, Tiquia et al. (2002) reported C losses of 64% for turned compost windrows which were significantly higher than 50% for unturned windrows (similar to our stockpiles). Carbon losses from other composting studies include 45 to 62% for feedlot manure in Nebraska (Eghball et al., 1997), 45 to 74% from sawdust-amended and 54 to 79% from straw-amended dairy manure in Ohio (Michel et al., 2004), and 44% (Sommer and Dahl, 1999) and 40 to 49% (Sommer, 2001) for deep-litter dairy cow manure in Denmark. Composting C losses at Brandon (>80% in two of three years) were higher than the above ranges of reported values. The location and treatment x location effects on C loss were nonsignificant (Table 4).
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With cattle manure in the UK, Parkinson et al. (2004) reported N losses of 24.7% for an unturned static pile (very similar to our stockpile value of 26.8%), 30.4% for one-turn, and 36.8% for three-turn compost treatments. Tiquia et al. (2002) found no significant difference between N losses from turned (50%) or unturned windrows (54%). Other reported N losses during composting include 19 to 43% for feedlot manure in Nebraska (Eghball et al., 1997), 8 to 26% for sawdust-amended and 15 to 43% for straw-amended dairy manure in Ohio (Michel et al., 2004), 15 to 42% for deep litter pig manure in Denmark (Møller et al., 2000), 25% for sheep (Ovis aries) manure in Italy (Solano et al., 2001), 59% for chicken (Gallus gallus domesticus) litter in Hong Kong (Tiquia and Tam, 2000), and 47 to 77% for livestock manures in Germany (Martins and Dewes, 1992). As with C losses, the location and treatment x location effects on N loss were nonsignificant (Table 4).
The relationship between N losses and C losses (Fig. 3) using all 12 experimental combinations (two treatments x two locations x three years) showed a significant positive quadratic fit (R2 = 0.86, P < 0.001) even though stockpiling and composting represent different handling treatments. The regression equation predicted that N loss was <0 until C loss exceeded 7.8%. On a percent of initial mass basis, predicted N losses were lower than C losses. For example, a C loss of 40% gave a predicted N loss of 23%. Figure 3 demonstrates that although N mineralization is directly linked to C mineralization during OM decomposition, the N and C loss pathways are subsequently governed by different parameters, leading to differential N and C mass losses. The N vs. C loss relationship was opposite to that of Barrington et al. (2002) who reported that N losses were significantly negatively related (r = –0.84) to C losses for in-vessel composting of liquid pig manure mixed with various bulking agents. In contrast, Curtis et al. (2005) found no measurable N losses, despite C losses of 40 to 45%, during turned windrow and passively aerated composting of a manure-straw mixture in California.
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Nutrient Haulage Scenarios
Manure is hauled "as is" for land application. Therefore water content plays a major role in haulage requirement where the aim is to transport nutrients rather than water. Given the choice of transporting 1 Mg of fresh, stockpiled, or composted manure, Table 5 shows the amounts and forms of nutrients hauled on a wet weight basis. Statistically, the DM data followed the trend of WC (Fig. 1) since DM (kg Mg–1) = (1000 – water content). For C haulage, there was no significant difference between the handling treatments with all three falling between 104 and 108 kg Mg–1. The drop in TC concentration during stockpiling and composting counterbalanced increased DM.
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For P haulage (Table 5), composting led to significantly higher levels of TP (3.3 kg Mg–1) than stockpiling (2.3 kg Mg–1) which was in turn significantly higher than fresh manure (1.6 kg Mg–1). Because of the lack of variation in P availability due to handling treatment (all had 61–70% as PAP, Table 2), the haulage of AP followed the same trend as TP: composted (2.3 kg Mg–1) > stockpiled (1.2 kg Mg–1) > fresh (0.6 kg Mg–1) manure.
The location effect was significant for DM, TC, TN, TP, and AP (Table 5). In all cases, higher amounts of these components were present on an "as is" basis at Lethbridge simply as a factor of drier materials compared to Brandon.
Manure Management Choices
Based on the results of our study, if N fertilizer is required, fresh or stockpiled manure may be favored, since composting reduces the value of manure as an N fertilizer. However if an organic soil conditioner is required to replenish soil OM with stable C, then composting may be worthwhile. Ultimately, however, the choice of handling treatment may by dictated by regulatory change. If future environmental regulations require the adoption of P-based land application of manure, individual farmers will have to greatly increase the amount of land they use for spreading (Ribaudo et al., 2003; Olson et al., 2005). Therefore the radius of manure nutrient haulage distance will increase, making the economics of composting more attractive. Vervoort and Keeler (1999) showed that composting becomes more viable as the land base for application becomes smaller and as environmental constraints become stricter.
We looked at one component of manure management: handling treatment effects on nutrient levels and mass balances. In the larger context, Grusenmeyer and Cramer (1997) recommended a total systems approach which expanded the focus on the production, collection, storage, and field application of manure to include human and animal health, odor and fly control, nutrient import and export, diet manipulation and ration balancing, and the broader economic impacts of environmental regulation, compliance, and enforcement. Nicholson et al. (2002) assigned environmental ratings to manure storage systems in categories of water pollution, gaseous emissions, and microbial pathogens.
Sandars et al. (2003) advised the use of life cycle assessment to follow mass and energy balances of manure management practices from "cradle to grave" to ensure that improvements at one stage corresponded to an overall improvement and did not simply move problems up or down the chain. For example, in our study, composting dramatically reduced haulage requirements of manure nutrients for land application, but also led to substantial N losses. Obviously, reduced haulage requirements accompanied by minimal N losses would be more desirable. Recent research in southern Alberta points to reduction of N losses by co-composting feedlot manure with phosphogypsum, an acidic by-product of phosphorus fertilizer production (Zvomuya et al., 2005).
Greenhouse gas (GHG) emissions are associated with storage and handling of livestock manure (Desjardins et al., 2001). In Alberta, GHG emissions during feedlot manure composting, as affected by aeration method (Hao et al., 2001), pen bedding material (Hao et al., 2004), or phosphogypsum addition (Hao et al., 2005) have been quantified. However, GHG emissions associated with fresh handling or stockpiling have not been compared with those for composting as a potential component of a life cycle assessment. Similarly, the energy requirements for composting (fossil fuels used for hauling to compost site instead of field and for multiple windrow turning events), and to a lesser extent for stockpiling, have not be compared with those for fresh manure handling. While composting may be the handling system of choice for long-distance haulage of manure N and P, the energy expended in making compost should be factored in as part of the "bigger picture." For example, the net energy (kJ) expended (starting at pen cleaning) in transporting 1 kg of N (dry weight) to the field in the form of fresh, stockpiled, or composted manure would be a legitimate comparison.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Composting allowed transport of two times as much P as fresh manure and 1.4 times as much P as stockpiled manure on an "as is" or wet weight basis, which has implications if future regulations dictate compliance with P-based manure application rates. Hauling P in the form of compost would be most advantageous. On the downside, composting led to substantial losses of C, most notably at Brandon (>80% in two of three years), and N. Future composting research should aim to minimize C and N losses. The location effect (Lethbridge vs. Brandon) was largely non-significant which verifies our results for the Canadian prairie region as a whole and therefore represents the northern limit of intensive beef feedlot production in North America.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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