Emissions of N2O and CH4 from agricultural soils amended with two types of biogas residues

b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 1 2 e1 1 6

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Emissions of N2O and CH4 from agricultural soils amended with two types of biogas residues M. Odlare a,*, J. Abubaker b, J. Lindmark a, M. Pell b, E. Thorin a, E. Nehrenheim a a b

School of Sustainable Development of Society and Technology, Ma¨lardalen University, Box 883, SE 721 23 Va¨stera˚s, Sweden Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, SE 750 07 Uppsala, Sweden

article info

abstract

Article history:

Biogas residues contain valuable plant nutrients, important to the crops and also to soil

Received 5 May 2010

microorganisms. However, application of these materials to the soils may contribute to the

Received in revised form

emission of greenhouse gases (GHG) causing global warming and climate change. In the

18 April 2012

present study, incubation experiment was carried out, where the emission rates of N2O and

Accepted 11 May 2012

CH4 were measured after amending two soils with two types of biogas residues: (1)

Available online 8 June 2012

a regular residue from a large scale biogas plant (BR) and (2) a residue from an ultrafiltration membrane unit connected to a pilot-scale biogas plant (BRMF). The emissions

Keywords:

of N2O and CH4 were measured at two occasions: at 24 h and at 7 days after residue

Agricultural soil

amendment, respectively. Amendment with filtered biogas residues (BRMF) led to an

Biogas residues

increase in N2O emissions with about 6e23 times in organic and clay soil, respectively, in

Emission

comparison to unfiltered biogas residues (BR). Methane emission was detected in small

Methane

amounts when filtered biogas residue was added to the soil. Amendment of unfiltered

Nitrous oxide

biogas to the organic soil resulted in net consumption. In conclusion, fertilization with BRMF can be combined with risk of an increase N2O emission, especially when applied to organic soils. However, in order to transfer these results to real life agriculture, large scale field studies need to be carried out. ª 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

The increasing concentration of nitrous oxide (N2O) and methane (CH4) in the atmosphere has gained much attention since these gases have detrimental effects on the stratospheric ozone layer and the global warming [1]. Even though N2O is present only at small levels its contribution as a greenhouse gas is significant and has in fact a more than 200-fold stronger global warming potential (GWP) than CO2 [2]. Methane has, per mole, a global warming potential 3.7 times that of carbon dioxide [3]. The soil is a major source of N2O and it is produced mainly by microbial processes under semi-anaerobic and anaerobic

condition, as a by-product of nitrification and an intermediate product of denitrification [4,5]. Methane is also produced in water logged soils in anaerobic environments by obligate anaerobic microorganisms through either CO2 reduction or transmethylation processes [6]. Therefore, in agriculture ecosystems the risks for high emissions of N2O and CH4 can be expected after fertilization, especially at wet condition [7]. The risks are further accentuated if organic fertilizer containing both nitrogen and easy available carbon, such as pig slurry or biogas residue, are used [8e10]. Biogas residue is a by-product of the anaerobic digestion process of organic material (such as ley crop, household wastes and slaughter house wastes) in which biogas is

* Corresponding author. Tel.: þ46 21 101611; fax: þ46 21 101370. E-mail addresses: [email protected] (M. Odlare), [email protected] (J. Abubaker), [email protected] (J. Lindmark), [email protected] (M. Pell), [email protected] (E. Thorin), [email protected] (E. Nehrenheim). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2012.05.006

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produced [11]. The number of biogas plants in the world has increased in the last decade and, hence, the amount of biogas residues produced. Therefore, finding a sustainable, economical and safe usage of these residues is of great urgency. Applying biogas residues to agricultural land has a number of beneficial effects, such as supplying the soil with nutrients and organic carbon. However, to be able to recommend biogas residues as an ecological sound fertilizer it is important to investigate its impact on the environment. Process water from a biogas plant is often recirculated and mixed with the incoming feed. The dry matter content of the final mixture should be as high as possible to maximise the use of the capacity of the plant without exceeding the capability of the pumps. An advantage with a thicker product is less cost for transporting and spreading the residue in the fields. One way to reduce the dry matter content of the recirculated process water is to pass it through a ceramic ultrafiltration (UF) membrane [12]. After installation of a membrane the dry matter content was reduced, leading to that 29% more new material could be loaded to the process. The filter installation therefore seems to have two beneficial effects: (1) lower the dry matter content of the recirculated process water and (2) yielding a biogas residue with less water content. The objective of the present study was to investigate the emissions of N2O and CH4 after two different types of biogas residues were amended with soil: (1) a regular residue from a large scale biogas plant and (2) a residue from an ultrafiltration membrane unit connected to a pilot-scale biogas plant. Both residues originated from the same incoming raw material. The experiment was designed as laboratory incubation, amending the biogas residue into a clay and organic soil, respectively, and gas emissions measured after 24 h and 7 days.

2.

Materials and methods

2.1.

Experimental set-up

Two different soils and biogas residues with two different qualities were used in the study (Table 1). The incubation experiment was performed in flasks (1 L) with airtight lids provided with rubber septa for headspace sampling. All treatments were run in triplicates. In each flask 500 g of soil was placed. The soil moisture was 40 and 66% of the water holding capacity (WHC) of clay and organic soil, respectively. To simplify comparison of the different treatments, the same amount of soil (i.e. 500 g based on fresh weight) was used for

Table 1 e Treatments used in the incubation experiment. Abbreviation CS þ BR CS þ BRMF CS OS þ BR OS þ BRMF OS

Soil type

Treatment

Clay soil Clay soil Clay soil Organic soil Organic soil Organic soil

Biogas residue (non filtered) Biogas residue (membrane filtered) No treatment (control) Biogas residue (non filtered) Biogas residue (membrane filtered) No treatment (control)

both clay soil and organic soil. Biogas residue (70 g kg1d.w. soil) was then carefully sprinkled on top of the soil. The bottles were then placed in a dark room with a constant temperature of 22  0.5  C. During the incubation the lids of the flasks were kept loosely to allow gas exchange with room atmosphere. The emission of N2O and CH4 was measured at two occasions: after 24 h and after 7 days. At each sampling time the lids of the flasks were closed after which a 60 ml gas sample was taken with a syringe at 15 and 60 min. The samples were directly transferred into 12 ml glass vials, allowing 5 ml excess pressure to avoid air contamination. The sampling procedure was repeated after 7 days. The gas samples were analyzed by a gas chromatograph (Clarus 500, PerkinElmer, Waltham, MA) and a headspace sampler (TurboMatrix 110, PerkinElmer). The GC was equipped with two channels one provided with an electron capture detector (ECD) to determine the concentrations of N2O and the other with a flame ionization detector (FID) to determine the concentration of CH4. All analyses were made within a week after sampling.

2.2.

Soil and fertilizer characteristics

The soil was collected from Nibble farm (59 670 N, 16 740 E) located outside Va¨stera˚s in central Sweden. Two different soil types were collected; a clay soil (CS) and an organic soil (OS). Soil characteristics are presented in Table 2. The farm is run as a traditional agriculture with a crop rotation consisting of cereals and oil seeds. At the time of the soil sampling, in late autumn, no crop was grown. Before experimental set-up the soil was mixed and sieved (4 mm). Soil dry matter (DM) and organic carbon (Org-C) was measured according to SS-EN 13137. Extractable phosphorous (P) and potassium (K) was analyzed in an ammonium lactate/ acetic acid solution (AL) to estimate exchangeable P or K and in a hydrochloric acid solution (HCl) to estimate nonexchangeable P and K [13]. Mineral N (NH4eN and NO3eN) was analyzed with dialysis on an AutoAnalyzer TRAACS 800

Table 2 e Soil chemical, physical and microbiological characteristics of the experimental soils prior to the incubation experiment expressed on dry soil basis. Parameters Chemical analyses Soil solid content (g kg1) pH Org-C (g kg1) 1 NHþ 4 eN (mg kg ) 1 NO 3 eN (mg kg ) PeAL (g kg1) PeHCl (g kg1) KeAL (g kg1) KeHCl (g kg1) Soil analysis (g kg1 of mineral fraction) Clay Silt Sand Microbial analysis 1 (mg kg1) N min NHþ 4 eN 10 day 1 1 eN min (mg kg ) PAO NO 2 1 1 PDA N2OeN min (mg kg )

Clay soil

Organic soil

750 6.7 0.6 2.67 7.3 0.12 0.9 0.2 4.34

590 4.9 2.0 2.87 30.9 0.18 1.7 0.28 3.06

6.6 2.6 0.7

5.8 3.7 0.4

37  1.4 4.5  0.14 19  1.5

138  0.3 1.8  0.2 31  1.9

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(Kontram, Stockholm, Sweden). NH4eN was analyzed according to the method ST9002-NH4D and NO3eN according to the method ST9002-NO3D. The potential ammonia oxidation rate (PAO) in 25 g of soil was assayed as accumulated nitrite according to the short-incubation chlorate-inhibition technique [14e16]. The rate of NOe 2 formation was determined by linear regression. The potential denitrification activity (PDA) was assayed in 25 g of soil according to the modified short-incubation C2He 2 inhibition method [17]. The assay substrate contained 2 mol dm3 of glucose and 2 mol dm3 of KNO3. During the assay, seven gas samples were withdrawn from the headspace and analyzed for N2O on a gas chromatograph provided with an electron capture detector (Clarus 500, PerkinElmer). The rate of N2OeN formation increased with time and the data were fitted to a product formation equation that takes exponential growth into consideration [18]. Two parameters, the initial product formation rate (PDA) and the specific growth rate (mPDA), were derived by nonlinear regression. The nitrogen mineralization capacity (N min) was measured with the anaerobic incubation method [19]. Slurry with 10 g of soil was anaerobically incubated for 10 days at 37  C before extraction and analysis of NHþ 4 . The rate of NHþ 4 eN formation was determined as the difference in product between the start and end of a 10 day incubation period. Biogas residue was obtained from a biogas plant in Va¨stera˚s (59 600 N, 16 54E) where source-separated household waste is co-digested with restaurant waste, liquid waste (grease trap removal sludge) and ley crop. The microbial process in the digester is mesophilic and the suspension is mixed with compressed process gas. Membrane filtered biogas residue was collected from a pilot plant situated close to the large scale biogas plant. The filtration process is described below.

2.3.

Ceramic ultra-filtration membrane

The details of the filtration procedure have been described previously [14]. Briefly, the membrane element used in the experiment is produced by Atech Innovations Gmbh and is of the type 37/3.8e1200, meaning that it has 37 channels each with an internal diameter of 3.8 mm and a length of 1200 mm.

This membrane has an area of 0.53 m2 and a pore size of 50 nm and like most membrane filters it is set-up, for crossflow filtration with the feed flowing parallel to the membrane to avoid clogging. The membrane experiments were conducted at 100  C in a pilot plant. After filtration permeate was recirculated to the inlet tank or withdrawn to an outlet tank for analysis, whereas the concentrate was collected and used for the present experiment Fig. 1.

3.

Results and discussions

After filtration the dry matter content of the process water in the biogas plant was reduced from 40 to 16 kg m3 corresponding to a 29% increase in the amount of new material that could be added to the process. This leads to a more than doubling of the dry matter content of the filtered residue compared to the unfiltered (Table 3). In addition, the pH of the residues was considerable higher in filtered BR due to the fact half of NHþ 4 was removed after filtration. The amount of energy required to heat the membrane when using heat recovery is small compared to the energy of the additional methane produced from the additional added substrate. Higher concentrations of dry matter were added to the soil by the filtered residue. A biogas residue with higher dry matter content is desirable for the farmers since the residue circulation is more efficient if less amounts of water needs to be transported. Some of the water soluble compounds, such as NHþ 4 , are likely to stay in the liquid phase and is hence recirculated to the biogas plant, whereas most of the P and the organic N remain in the solid phase.

3.1.

Nitrous oxide emission

The emission of N2O was higher after 24 h compared to day 7 for both soils. In addition, the emission of N2O was 6 times higher in organic soil than in the clay soil (Fig. 2). After 24 h, fertilization of BRMF and BR emitted N2OeN at rates of 27 and 20 mg kg1 h1 (dry basis), respectively, for the organic soil while the rates were 4 and 0.5 mg N2OeN kg1 h1 (dry basis) respectively, for the clay soil. These results were expected

T

Inlet pipe

P

F

Inlet tank Membrane

Water cooler Outlet tank

P 6 kW

Oil heater

T T

P Concentrate outlet

Circulation pump

P

P: Pressure meter T: Thermometer F: Flow meter

Fig. 1 e Schematic description of the pilot-scale biogas plant used for the ceramic ultra-filtration membrane experiments.

b i o m a s s a n d b i o e n e r g y 4 4 ( 2 0 1 2 ) 1 1 2 e1 1 6

Table 3 e Characteristics of the unfiltered biogas residue (BR) and the membrane filtered biogas residue (BRMF) used in the experiment expressed on dry matter basis. Parameter

Biogas residue (BR)

Biogas residue (BRMF)

42.4 7.5 730 270 51 70 8000 4.4 27 57 5.5 4.8 0,29 49 9.6 0.055 9.9 150

93.5 10.1 762 238 22 77 7500 4.5 21 43 4.4 11 0.65 130 39 0.11 53 360

Solid content (g kg1) pH Loss on ignition (g kg1) Residue on ignition (g kg1) NH4eN (g kg1) ToteN (g kg1) ToteP (mg kg1) S (g kg1) Ca (g kg1) K (g kg1) Mg (g kg1) Pb (mg kg1) Cd (mg kg1) Cu (mg kg1) Cr (mg kg1) Hg (mg kg1) Ni (mg kg1) Zn (mg kg1)

g N2O-N min-1 kg-1dry soil

30 25 20 24 h

15

denitrification process, where nitrate is first reduced to nitrite and then to N2O. Denitrifying organisms use organic C compounds as electron donors and therefore denitrification is dependent on the availability of carbon and mineral nitrogen. Therefore, an increase in N2Oe emissions is likely to take place after soil amendments with organic manure with a low C/Nratio, such as biogas residues. Considerable higher amounts of N2O emissions were measured after 24 h compared to 7 days. These results are in agreements with other studies [9,27,28] reporting a decrease in emission rates after ca 10 days. Probably, most of the mineral nitrogen and easy available carbon was consumed by the microorganisms during the first days which in turn slowed down the emission rates. The results from the present study indicate that application of filtered biogas residue significantly increased the emission rates of N2O in both organic and clay soil compared to unfiltered biogas residue. However, in order to transfer these results to real life agriculture, large scale field studies need to be carried out.

3.2.

since the organic soils contained high amounts of NOe 3 and organic material and also had high rates of microbial activity (Table 2). The nitrogen mineralization capacity in the organic soil was 3 times higher than clay soil, which will increase the mineral nitrogen content. Furthermore, denitrification activity was 2 times higher in organic soil than clay soil. Nitrification activity was low in the organic soil compared to clay soil, which was probably due to the sensitivity of nitrifying bacteria to humic acids and low pH in the organic soil [20,21]. Furthermore, the low pH in the organic soil will lead to increased N2O emission [22]. Several studies have shown that organic soils are greater N2O sources than mineral soils [23,24]. When these soils are tilled, oxygen is transferred to the microorganisms which degrade the organic material. In nitrification, ammoe nium is oxidized to nitrite (NOe 2 ) and nitrate (NO3 ). However, under anaerobic condition, the process will lead to a production of N2O. This may occur in wet soils after fertilization or soils with high amounts of organic material [25,26]. The biogas residue supplied the soil with carbon and organic nitrogen, which promotes the heterotrophic

7 days 10

115

Methane emission

Methane emission was only observed from the clay soil. Application of filtered biogas residue resulted in emission rates ranging between 70 and 8 ng kg1 h1 of CH4eC (dry basis) after 24 h and 7 days, respectively. On the other hand, application of unfiltered biogas residue showed net consumption after 24 h and a small net production at 7 days. The organic soil acted as sink at all treatments and this negative flux occurred at both sampling days. Consumption of CH4 in the organic soil was probably caused by the presence of the methane oxidation bacteria. The high porosity of the organic soil will increase the oxygen concentrations, which in turn may stimulate methane oxidation. Methane oxidizing bacteria are common inhabitant of agricultural soils ecosystem and, hence, consumption will occur wherever methane and oxygen are present together [29,30].

4.

Conclusions

Ceramic membrane filtration of biogas process water is a promising method to decrease the dry matter content of the recirculated water in a biogas process and to increase the dry matter content of the biogas residue. Fertilization with filtered biogas residue may lead to an increased risk of N2O emission compared to unfiltered biogas residue, especially in organic soils. Application of biogas residues (both filtered and unfiltered) is likely to produce higher emission rates of N2O and higher consumption of CH4 in organic soils compared to clay soils.

5 0 CS

CS + BR CS + BRMF

OS

OS + BR OS + BRMF

Fig. 2 e Emissions of N2O measured from a clay soil (CS) and an organic soil (OS) after application of biogas residue (BR), filtered biogas residue (BRMF) and control with no application. Error bars indicate standard deviations.

Acknowledgments This work performed within the BiogasOpt-project and financial support by the Knowledge Foundation is gratefully acknowledged. The authors also like to express their grateful

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thanks to Mantas Astrovas, Benoit Mace and Quentin Provost for their help with the experimental work.

references

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