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Published online 1 May 2008
Published in J Environ Qual 37:956-962 (2008)
DOI: 10.2134/jeq2007.0296
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

The Effect of Cattle Slurry Electroflotation Products as Fertilizers on Gaseous Emissions and Grassland Yield

S. Menéndeza,*, P. Merinob, A. Lekuonac, M. Pintob, C. González-Muruaa and J. M. Estavilloa

a Dep. of Plant Biology and Ecology, Univ. of the Basque Country, Apdo. 644. E-48080 Bilbao, Spain
b Inst. of Agricultural Research and Development, NEIKER. B. Berreaga, 1. E-48160 Derio, Bizkaia, Spain
c ADE Biotec S.L. Mikeletegi Pasealekua, 2. E-20009 Donostia, Gipuzkoa, Spain

* Corresponding author (sergio.menendez{at}ehu.es).

Received for publication June 6, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
The climatic conditions of the Basque Country (northern Spain) provide the favorable conditions for the growth of grasslands and the development of livestock enterprises. The intensification of the farms is leading to serious environmental risks due to the great generation of manures and slurries and their subsequent inefficient management. Their application involves N losses that can be pollutant. The environmental company ADE BIOTEC S.L. is developing the process called "electroflotation" with the aim of reducing the volume of slurries from intensive livestock farms. The process consists basically of an electrolysis of the slurry catalyzed by iron which leads to the flocculation of the solid particles, giving as a final result a solid and a liquid fraction. The objective of this work was to assess the usefulness of these two fractions as fertilizers. With this aim, the environmental risk of their application was determined regarding gaseous emissions to the atmosphere (i.e., of NO, NH3, N2O, and CO2) and their fertilizer capacity was investigated by determining their effects on grassland yield and N uptake in comparison to the untreated slurry. The untreated slurry and the solid and the liquid fractions were all applied at a rate of 70 kg NH4+–N ha–1. The application of the products of electroflotation did not affect N2O and CO2 losses, being of the same magnitude as those caused by the application of the original slurry. However, after their application, a reduction in NH3 volatilization losses was induced in the short term and a reduction in NO losses was caused in the long term. The solid and liquid fractions both increased biomass yield with respect to the untreated slurry. The solid fraction even induced a higher N uptake than the liquid fraction and the untreated slurry.

Abbreviations: CO2, carbon dioxide • DM, dry matter • NH3, ammonia • N2O, nitrous oxide • NO, nitric oxide • WFPS, water filled pore space


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
INTENSIFICATION of livestock production has resulted in increases in generation of animal manure, which may be a significant source of harmful nutrient emissions into the environment if handled improperly. Losses of nitrogen (N) from manure slurry emerge at all stages of slurry handling, but most of them occur in the field after slurry application (Bussink and Oenema, 1998). Because of the limited storage capacity of slurry collected on farms in winter, large quantities of slurry are normally applied well before the growing season which results in poor utilization of the nutrients and environmental pollution. Fouling of such areas with slurries may lead to nitrate leaching as well as emissions of gases of environmental concern such as ammonia (NH3), nitrous oxide (N2O), and nitric oxide (NO). Ammonia is implicated in eutrophication of fragile ecosystems (Bobbink et al., 1992) and soil acidification (Van der Eerden et al., 1998). Nitrous oxide is an important greenhouse gas involved in global warming (Badr and Probert, 1993) and contributes to the destruction of the ozone layer. It has a mean atmospheric residence time of more than 100 yr (Prather et al., 2001), while NO contributes to acid deposition (Logan, 1983) and to ozone formation in the troposphere (Crutzen, 1979). Losses of carbon (C) from manure slurry are in the form of carbon dioxide (CO2), the most important greenhouse gas causing global warming, and the concentration of which is increasing at 0.5% annually (Lal and Kimble, 1995). Concurrent measurement of emissions of the four gases is important in assessing the overall diffuse pollution after organic fertilizer applications, because strategies to reduce the emission of one gas may increase emissions of others. Few studies (Akiyama et al., 2004; Menéndez et al., 2006) have investigated emissions of all the gases together after the application of slurries.

The company ADE BIOTEC S.L. is developing the slurry treatment process called "electroflotation" with the aim of reducing the volume of slurries from intensive livestock farms and having a less polluting final product. As a consequence of the process a solid and a liquid fraction are obtained. The solid fraction is a sludge which can be either dehydrated by mechanic means (by filter, press, etc.) or directly used as an organic fertilizer. The solid fraction will presumably have a greater nutrient richness because of its movement from the liquid to the solid fraction. The liquid fraction shows characteristics to be used in ferti-irrigation programs. Both fractions could be useful as fertilizer amendments, but the risk of N and C losses should be determined during the electroflotation process itself as well as after the agronomic application of both fractions as fertilizers. This study deals with the evaluation of the agronomic part. So, the specific objectives were to determine the atmospheric pollution induced by NH3, NO, N2O, and CO2 emissions when both electroflotation fractions are applied as fertilizers with respect to the original untreated slurry, as well as to determine the effect of their application on grassland yield.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Electroflotation Process
In the electroflotation process the slurry is first filtered to eliminate the straw and reduce its dry matter content, after which it is forced to pass through iron plates to which a low tension continuous electric current is applied. This way, the electric current crosses from one plate to the next through the slurry. Iron is dissolved into the slurry in the form of Fe2+ by anodic dissolution, acting as a coagulating agent inducing the formation of floccules. A further flotation of these floccules occurs caused by the formation of hydrogen bubbles of catodic origin. In this way the two final fractions are obtained.

Site Description
The experiment was performed on a cut grassland in Derio (Northern Spain, 43°18'20'' N, 3°53'0'' W) in a valley surrounded by a hilly area (30 m a.s.l.). The region has a temperate climate with typical annual mean temperature of 12°C and high rainfall of 1200 mm yr–1. The soil was a poorly drained clay loam (34% fine sand, 3% coarse sand, 34% silt, 29% clay in the top 10 cm). A typical permanent pasture (Lolium perenne L. var. Herbus, 60%; Lolium hibridum L. var. Texi, 32%; Trifolium repens L. var. Huia, 8%) was sown at a density of 40 kg seeds ha–1 in the previous autumn.

Experiment 1
In spring of 2004 one experiment was performed to evaluate the effect of the application of electroflotation products on NH3, N2O, NO, and CO2 emissions. The original untreated cow slurry was obtained from a concrete storage pit on a dairy farm. On 9 May untreated slurry as well as solid fraction and liquid fraction were surface applied at a rate of 70 kg NH4+–N ha–1. A treatment with no fertilizer was included as a control. Four microplots (0.03 m2) per treatment were established. The characteristics of the original untreated slurry, the solid fraction, and the liquid fraction were analyzed just before their application as shown in Table 1 .


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  		Table 1. Characteristics of the untreated slurry, the solid fraction and the liquid fraction applied in both experiments.

 
One hour after treatment applications ammonia emission measurements were started for 44 h by a photoacoustic infrared gas analyzer (Model 1302 Multi-Gas Monitor, detection limit 0.03 g kg–1; Brüel and Kjær, Nærum, Denmark) using an open chamber technique as described by Menéndez et al. (2006). Opaque PVC chambers with a volume of 6.75 L and an area of 0.0314 m2 were fitted tightly onto a frame which was inserted 3 cm into the soil. The chambers had a small hole on the top for air exhausting.

Emissions of N2O and CO2 were measured using a closed air circulation technique in conjuction with a photoacoustic gas analyzer (Model 1302 Multi-Gas Monitor, detection limit 0.01 g kg–1) (Menéndez et al., 2006). Measurements were taken during 40 min after insertion of the chamber and the fluxes were calculated from the linear concentration increase in the headspace with time. Emissions were measured daily during 2 wk after fertilizer application. The measurement chambers had the same characteristics as those used for NH3 measurements, except that they were completely closed.

Emissions of NO were measured daily for 2 wk after fertilizer application using open chambers as described by Menéndez et al. (2006). The flux chambers had the same characteristics as those used for NH3 measurements. Concentrations of NO were measured at the air inlet and outlet of the chamber using an NO-NO2–NOx chemiluminescence analyzer (Model AC31M, detection limit 0.08 mg kg–1; Environnement SA, Poissy, France). Fluxes of NO were calculated from the concentration differences between inlet and outlet air, the air flow rate through the chamber, and the surface area covered by the chamber.

Cumulative gas emissions for all gases were estimated by averaging the rate of loss between two successive determinations, multiplying that average rate by the length of the period between the measurements, and adding that amount to the previous cumulative total.

Experiment 2
In the summer of 2004, a second experiment was performed on the same grassland as in experiment 1 to evaluate the effect of the products of electroflotation on grassland yield. Emissions of NO were also measured because at this time edapho-climatic conditions were expected to be optimal for NO losses. Treatments and N application rates were the same as in experiment 1, and were applied on 7 June. A randomized complete block factorial design with four replicates was established, with single plots of 4 x 3 m.

Emissions of NO began to be measured 24 h after the application in the same way as in experiment 1, although only in three plots per treatment. Measurements were taken daily for 2 wk, and afterward once a week until the end of the trial on 24 August.

After completion of NO measurements, soil (0–10 cm) was sampled for moisture content determination on each measure day and mineral N (ammonium and nitrate) determinations with a lower frequency. For mineral N determinations, 100 g moist soil were extracted with 200 mL 1 mol L–1 KCL, and ammonium and nitrate determined by segmented flow analysis (Alpkem 1986, 1987). Soil water content was determined gravimetrically as described by Merino et al. (2002) and was expressed as the percentage of water filled pore space (WFPS).

One month after treatment applications grassland yield was assessed on one randomly chosen area of 4.5 m2 per plot. Herbage was oven-dried at 70°C for at least 48 h, weighed, and ground. Nitrogen concentration in herbage was determined by the Macro Kjeldahl method. The contribution of clover, ryegrass, and other species to the total dry matter yield was also determined and expressed as a percentage. Fertilizer apparent efficiency was calculated as the ratio between dry matter yield and the fertilizer applied. Fertilizer apparent recovery was calculated as the ratio between herbage N extraction and the fertilizer applied, not taking into account clover N extraction, thus assuming that all the N extracted by clover comes from atmospheric fixation.

Data Management
The LSD test was used for multiple comparisons of mean daily gas emission rates using the SPSS 11.0 statistical package (SPSS, 2004). Differences between cumulative gas emissions and the herbage yield in the different treatments were compared by ANOVA as well as Duncan tests for separation of means between treatments. Emissions of N2O followed a logarithmic distribution and log transformations of these emissions were used for statistical analyses. Significance for ANOVAs as well as Duncan tests were conducted at p < 0.05 for cumulative gas emissions and at p < 0.10 for herbage yield.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Experiment 1
Ammonia
Figure 1 shows ammonia volatilization rates during 44 h after treatment applications. Untreated slurry showed the highest emissions after application with a maximum rate of 125 g NH3–N ha–1 h–1 immediately after application. Five hours after the application a decrease was observed in NH3 emission from the untreated slurry down to 20 g NH3–N ha–1 h–1. It is stated in the literature that ammonia volatilization after the surface application of slurries increases with temperature (Sommer et al., 1991) and decreases with rainfall (Klarenbeek and Bruins, 1991). Therefore the decrease occurring in the ammonia volatilization rate was probably due to the temperature decrease as well as to the precipitation beginning 2 h after treatment application, accounting for 8.5 mm until it stopped 20 h after treatment application, resulting in an increase of NH3 emission from this treatment. Cumulative emissions from untreated slurry were 1.17 kg NH3–N ha–1, whereas with the application of the solid and the liquid fractions the cumulative emissions were 63 and 80% lower compared to the untreated slurry, respectively (Table 2 ). The lower NH3 emission rates observed by the application of the electroflotation products with respect to the original untreated slurry were probably due to physical characteristics of these products. As reported by many authors (Moal et al., 1995; Braschkat et al., 1997; Vandré and Kaupenjohann, 1998) a higher dry matter content in the slurry would make its infiltration into the soil more difficult, enhancing NH3 volatilization losses. In this sense, de Jonge et al. (2004) suggested that a greater infiltration of the slurry into the soil could decrease NH3 volatilization. In our study both the solid and liquid fractions would have presented a higher infiltration rate into the soil than the untreated slurry because of their physical characteristics, as shown by their lower dry matter contents (Table 1). It should be expected that the application of the solid fraction would cause higher NH3 losses than the liquid fraction because of its presumable lower infiltration into the soil. After the application of the solid fraction the losses were somewhat larger than for the liquid fraction, but these differences were not significant. Furthermore, they were similar to those of the control treatment, which indicates a great efficiency decreasing N losses by NH3 volatilization.


Figure 1
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  		Fig. 1. Ammonia volatilization rates measured during the first 44 h following treatments application in spring (experiment 1). ({blacktriangleup}) Control, (•) Untreated slurry, ({square}) Solid fraction, ({circ}) Liquid fraction. The vertical bars indicate LSD between treatments for each sampling time (P < 0.05; n = 4).

 

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  		Table 2. Cumulative emissions of the different gases in spring (experiment 1) up to Day 15 after treatment applications and CO2 equivalents (CO2e) for cumulative N2O emissions as well as CO2e corresponding to N2O + CO2 (IPCC, 1996).

 
Nitrous Oxide
Emissions of N2O were always maintained below 100 g N2O–N ha–1 d–1 in the control treatment (Fig. 2A ). The other treatments always showed higher emission rates, the solid fraction showing the highest ones except for Day 7 after fertilization. Nevertheless, the different rates between the products of electroflotation and the untreated slurry never showed to be statistically significant. Fifteen days after fertilizer application no more differences were observed between fertilized treatments, but the control treatment continued to show lower emission rates than the rest of the treatments.


Figure 2
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  		Fig. 2. Nitrous oxide (A), nitric oxide (B), carbon dioxide (C) emission rates and water filled pore space (WFPS), and soil temperature (D) in spring (experiment 1). ({blacktriangleup}) Control, (•) Untreated slurry, ({square}) Solid fraction, ({circ}) Liquid fraction, ({diamondsuit}) soil temperature at 10 cm depth, ({diamond}) WFPS (0- to 10-cm depth). The vertical bars indicate LSD between treatments for each sampling day (P < 0.05; n = 4).

 
Studies by Merino et al. (2002) conducted under similar edafoclimatic and soil moisture conditions showed similar N2O emission rates close to 200 g N2O–N ha–1 d–1 after cattle slurry application. According to Menéndez et al. (2006) N2O losses through denitrification are higher than those through nitrification when the soil WFPS is higher than 50% in these edapho-climatic conditions. Taking this into account, the WFPS occurring in the soil during the present study (50–80%) (Fig. 2D) suggests that the N2O losses were mainly a product of denitrification.

Cumulative N2O emissions showed to be higher after the application of the different treatments compared to the control (Table 2), but no significant differences were observed between the untreated slurry and the products of electroflotation.

Nitric Oxide
In spring (experiment 1), NO emissions were not affected by the application of any of the treatments, and were comparable to the control (Fig. 2B). Maximum emission rates were never higher than 15 g NO-N ha–1 d–1 in any treatment, being most of the time similar or lower than 5 g NO-N ha–1 d–1. These low NO emission rates observed were possibly due to the fact that soil WFPS during the study was in the range of 50 to 80%, thus being near to field capacity (70%) when NO diffusion to the atmosphere is limited (Cárdenas et al., 1993). In fact, NO emissions are known to be highly influenced by the soil water content (Skiba et al., 1992), with the highest emissions occurring when soil WFPS is in the optimum range of 30 to 60% (Skiba et al., 1997).

Cumulative losses during the 15 d of the assay showed no statistically significant differences between treatments. Although the products of electroflotation caused slightly higher NO losses than the untreated slurry, they were not statistically different even from the control, which showed the lowest losses (Table 2).

Carbon Dioxide
Emission rates of CO2 were never higher than 315 kg CO2 ha–1 d–1 in the control treatment (Fig. 2C). On the contrary, the application of the untreated slurry and the products of electroflotation caused an increase in CO2 emissions during the first 2 d after application. Thus, the first day a maximum rate of 600 kg CO2 ha–1 d–1 was observed after the application of the untreated slurry, while the other treatments presented emission rates slightly lower than 460 kg CO2 ha–1 d–1. During the first 2 d, the observed CO2 emission rates could have been influenced by the organic matter contents of the different treatments, higher emissions being observed in the treatment with a higher content. Thus, the untreated slurry caused the highest emission rates, followed by the solid fraction and finally by the liquid fraction. The organic matter amendment, which was intrinsic to the different treatments, may have increased the soil respiration rate. In this sense, Jones et al. (2005) described an increase of soil respiration in grasslands after the application of organic manures in comparison with the application of inorganic fertilizers. Menéndez et al. (2006) also observed an increase in soil respiration during the first 4 d after cattle slurry applications in the same edafoclimatic conditions as this study.

When cumulative CO2 emissions were calculated (Table 2) the untreated slurry was shown to have caused significantly higher losses than the control, the products of electroflotation showing to be in an intermediate position between the untreated slurry and the control. When the global warming potential caused by both CO2 and N2O together was calculated expressed in terms of CO2 equivalents, no significant difference was observed between the products of electroflotation with respect to the untreated slurry. Therefore, we can conclude that the global warming effect caused by the application of slurry is at the same level as the application of the electroflotation products.

Experiment 2
Soil Mineral Nitrogen
The amounts of 80 and 55 kg NH4+–N ha–1 were detected in the soil the first day after the application of the solid fraction and the untreated slurry, respectively (Fig. 3A ). In the following 7 d these values decreased below 10 kg NH4+–N ha–1. This decrease might be due to nitrification of ammonium, immobilization, and plant uptake. With respect to the liquid fraction treatment, soil ammonium detected on the first day after application was in the same range as the control. This might be due to the physical characteristics of the liquid fraction, which penetrated better into the soil than the rest of treatments because of its lower dry matter content. Contrarily, in the solid fraction and untreated slurry treatments the applied slurry remained in a greater proportion in the soil top 10 cm. Moreover, the better penetration of the liquid fraction into the soil induced a faster immobilization of the ammonium by soil microbial biomass.


Figure 3
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  		Fig. 3. Soil ammonium (A) and soil nitrate content (B) (0- to 10-cm depth). ({blacktriangleup}) Control, (•) Untreated slurry, ({square}) Solid fraction, ({circ}) Liquid fraction. The vertical bars indicate LSD between treatments for each sampling time (P < 0.05; n = 4).

 
Directly after application soil nitrate contents did not show clear differences between treatments (Fig. 3B), showing values around 10 kg NO3––N ha–1. Thirty-two days after fertilization a small increase was observed in soil nitrate content in all treatments, coinciding with the increase in soil WFPS up to 66% (Fig. 4B ), what might have caused a general increase in the gross nitrification rate (Breuer et al., 2002) in all treatments. Nevertheless, from the first week on, the untreated slurry treatment showed slightly higher nitrate contents than the rest of the treatments reaching 20 kg NO3––N ha–1. This suggests that nitrification rate in this treatment was higher than in the rest.


Figure 4
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  		Fig. 4. Nitric oxide emission rates (A) and water filled pore space (WFPS) and soil temperature (B) in summer (experiment 2). ({blacktriangleup}) Control, (•) Untreated slurry, ({square}) Solid fraction, (•) Liquid fraction, ({diamondsuit}) soil temperature at 10 cm depth, ({diamond}) WFPS (0- to 10-cm depth). The vertical bars indicate LSD between treatments for each sampling time (P < 0.05; n = 3).

 
Nitric Oxide
The lower soil water content present in summer (experiment 2) compared to spring (experiment 1) resulted in higher NO emission rates. Compared to experiment 1, higher emission rates were detected in the fertilized treatments than in the control during the first days after fertilization (Fig. 4). Emissions in the control treatment were always lower than 8 g NO-N ha–1 d–1 except for Day 57. Maximum emissions were measured on the first day after application reaching 61 g NO-N ha–1 d–1 in the solid fraction application, and followed by the liquid fraction and the untreated slurry application. After the second day, emission rates decreased below 20 g NO-N ha–1 d–1 in the different treatments coinciding with an increase in soil WFPS from 40 to 50%. Therefore, the high emission rates of the first 2 d were probably caused by a high nitrification rate enhanced by the high availability of soil ammonium in the recently applied treatments (Fig. 3). Following Slerm and Seiler (1984), nitrification can be considered the main process in NO production in the soils fertilized with ammonium during the first days after application, when soil ammonium content is higher than nitrate content, as is the case in our study. On Day 57 a general increase in NO emissions occurred in all treatments including the control. On this occasion, it was the untreated slurry treatment which showed the highest emission rate (48 g NO-N ha–1 d–1). This new peak of emissions was probably due to the optimum soil conditions being again similar to those of the first days, with a soil temperature reaching the highest value of the experimental period (25°C) and a soil WFPS with a low value of 30%. According to Gut et al. (1999) and Pinto et al. (2004) maximum NO emission rates coincide with maximum nitrification rates. The products of electroflotation caused higher NO emission rates than the untreated slurry during the first 2 d after application (Fig. 4A). These higher emissions could be due to their physical characteristics. Both the solid and liquid fractions infiltrated better into the soil than the untreated slurry, having better contact with soil microorganisms. Nevertheless, the solid fraction showed a lower penetration into the soil than the liquid fraction. So, because of being nearer to the soil surface, the produced NO diffusion to the atmosphere could have been easier. Thus, in the first 15 d after treatment application this treatment showed the highest cumulative NO emissions (Table 3 ). From the third day on after treatment application no significant differences between treatments were observed, except for Day 57 when untreated slurry showed the highest emission rate. Apparently, the temperature increased up to 25°C on this day and caused a general increase in mineralization in all treatments, which was especially more evident in the untreated slurry treatment. The higher organic N content in this treatment would have induced a higher N mineralization rate with its subsequent higher nitrification rate leading to the higher NO emission detected. Although at this moment soil mineral N content was not measured for testing this mineralization rate increase, in a previous study Menéndez et al. (2006) indicated that NO emission measurement is a more sensitive indicator than soil mineral N measurement for detecting differences between treatments regarding the nitrification rate. When cumulative NO emissions were calculated in the long term (78 d), the untreated slurry treatment had the highest emissions (Table 3) due to the late mineralization of the organic N present in the slurry. The long-term losses in this treatment were similar to those described by Menéndez et al. (2006) under same edafoclimatic conditions. The application of both the solid and liquid fractions of electroflotation resulted, as a consequence, in a reduction of 20% in NO losses with respect to the untreated slurry. This reduction effect in the long term should be due to the lower late mineralization rate in these treatments with respect to the untreated slurry.


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  		Table 3. Cumulative NO emissions up to Days 15 and 78 after treatments application in summer (experiment 2).

 
Grassland Yield
Treatments with the solid and liquid fractions of electroflotation caused a significant increase in the grassland dry matter yield (Table 4 ), with 45 kg DM kg–1 N for the solid and liquid fraction treatments and 34 kg DM kg–1 N for the untreated slurry.


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  		Table 4. Dry matter yield of component species and total grassland, and apparent efficiencies of the N applied.

 
When the N extraction rates were calculated, it was observed that the solid fraction caused an extraction 28 to 32% higher than that of the liquid fraction or the untreated slurry (p < 0.10) (Table 5 ). This was mainly due to the higher N extraction by ryegrass in this treatment (Table 5), even though ryegrass yield in this treatment was not different from ryegrass yield in the liquid fraction or the untreated slurry treatments (Table 4). In the case of clover, it was also observed that N extraction was higher with the application of the electroflotation products than with the untreated slurry, although not significantly (Table 5). Clover is a weak competitor for resources in comparison to ryegrass. The latter having an erect behavior, a greater fine roots biomass, and less demanding climatic and nutritional requirements (Hopper and Clement, 1966). Moreover, N application increases the competitive capacity for light and nutrients in favor of ryegrass and, on the other hand, reduces nodulation and N2 fixation in the leguminous species (Dunlop and Hart, 1987). When apparent recovery of the N applied was calculated assuming that all of the N extracted by clover comes from atmospheric fixation, it was observed that both products of electroflotation (the solid and liquid fractions) caused a higher N recovery than the untreated slurry (Table 5). This, as previously commented, was translated into a significantly higher biomass yield and apparent efficiency (Table 4) in the use of the N applied in these two treatments.


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  		Table 5. Uptake of N by component species and total grassland, and apparent recoveries of the N applied.

 
The fact that the solid fraction caused a higher N extraction could be due to the lower NH3 losses by volatilization in this treatment with respect to the untreated slurry, as was confirmed in spring in experiment 1. In this sense, Mattila and Joki-Tokola (2003) described higher N contents and uptake rates when NH3 emissions were reduced. The second factor that could have induced a higher N extraction after the solid fraction application is its lower dry matter content. A lower dry matter content could thus induce a lower immobilization of N by the soil microbial biomass, which would increase N uptake by plants (Dosch and Gutser, 1996). Nevertheless, the liquid fraction caused a lower N extraction rate than the solid fraction. This could have been due to the effect of P. In the electroflotation process P is displaced mainly to the solid fraction (ADE BIOTEC S.L., personal communication, 2004) and becomes a limiting factor in the liquid fraction with respect to the solid fraction.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
The solid and liquid fractions of electroflotation can be considered useful products as fertilizers. Both products caused an increase in grassland yield with respect to the original untreated slurry, with grassland N extraction being even higher after the solid fraction application. Regarding environmental concerns, if applied under temperate conditions they do not increase the risk of global warming, with N2O and CO2 emissions caused by their application being of the same magnitude as those caused by the application of the original untreated slurry. Their application has a positive environmental effect regarding NH3 and NO emissions. In the short term a reduction is caused in NH3 volatilization losses, and in the long term a reduction is induced in NO losses.


   ACKNOWLEDGMENTS
 
This study was funded by projects AGL2003-06571-CO2-02, 9/UPV00118.310-13533/2001 and UE03/A03. S. Menéndez held a grant from the Ministerio de Educación y Ciencia of the Spanish Government (FPU, Programa Nacional).


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


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