NOTE
CARBON-TO-NITROGEN
RATIOS IN AGRICULTURAL
RESIDUES
J.O. ILUKOR and S.O. OLUKA Department of Physics, Makerere University, P.O. Box 7062, Kampala, Uganda
Abstract. Agronomic crop residues produce greenhousegas emissions. Crops that produce residues at harvest and during processingmay vary from country to country. These residues, which can be in the form of peels, husks, stalks, or straw, are generally considered to be waste products. The carbon (C) and nitrogen (N) content of 19 different agronomic and grass crops common in Uganda were determined using standard laboratory methods. The C and N content of the sampleswere calculated fromtwo separateequations containinga moisture correction factor. The crop residue C/N ratios were similar to UNEPIOECD/IEA/IPCCvalues. 1. Introduction The I P C C Guidelines f o r National Greenhouse Gas Inventories (UNEP et al., 1995) provide a default methodology for quantifying sources and sinks of key anthropogenic greenhouse gases (GHGs). Because this methodology is generic, default values for specific sectors (for example, energy or land use) can lead to significant errors in GHG emission estimates. Emissions from the land-use sector (agriculture and forestry) are particularly difficult to estimate because of the complex biogeochemistry of ecosystems (Dixon et al., 1994). Combustion of biomass and biomass chemistry are two factors that are especially difficult to characterize (Levine, 1991). Biomass varies in its carbon (C) and nitrogen (N) content, depending on the plant species, edaphic factors, climate, and cultivation practices in any given region (Seiler and Crutzen, 1980). This variation influences the net heating values of biomass combustion as well as the characteristics and intensity of GHG emissions (Crutzen and Andreae, 1990). The purpose of this study was to evaluate C and N content of plant tissues common in Uganda and compare values to UNEP/OECD/IEA/IPCC methodology.
2. Materials and methods A variety of agronomic crop and grassland ecosystem biomass commonly combusted by wildfire was analyzed (Tables I and II). Samples were collected using a stratified sampling system for representative ecosystems. Nineteen different plant types common in Uganda were sampled. The samples from each plant residue were powdered. The C content of each powdered sample was determined by oxidizing the sample in a digestion tube at 120 degrees celsius by adding both potassium dichromate solution and concentrated sulfuric acid (Crutzen and Environmental Monitoring and Assessment 38:271-275, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.
272
J.O. 1LUKORAND S.O. OLUKA
TABLE I Biofuel parameters for grass and crop residues found in Uganda ecosystems
Plant tissue
Energy Output Residue/Product Gj/Mg Ratio sd x sd
C Content ~ sd
N Content ~ sd
C/N Ratio ~ sd
0.347 0 . 0 7
0.0035 0.003
99.1 10.2
8.8
1.1
0.3
0.1
0.381 0 . 0 8 0.0175 0.002
21.8
2.2
8.9
2.3
0.8
0.2
12.5
1.2
Sugar cane (husks)
(Saccharum L.) Coffee (husks)
(Coffea L.) Cotton (stalks)
(Gossypium L.)
0.342 0 . 0 6 0.0192 0.002
17.8
1.9
9.1
1.9
Grass (mixed) (Gramineae) Papyrus
0.305 0 . 0 8 0.0183 0.007
16.7
3.1
7.9
1.9
(Cyperus)
0.314 0 . 0 3
0.0099 0.004
31.7
2.8
7.3
1.3
0.269 0 . 0 6 0.0140 0.005
19.2
1.9
8.0
1.6
Elephant grass
(Typha)
Andreae, 1990). The C content (C%) of the sample was then calculated using the following equation: C% = {[3(b - s)]/b/W} x M x 100 where b
=
ml of FeSO 4 used for the blank titration.
s
=
ml of FeSO 4 used for the sample titration.
3 W
= =
equivalent weight of C. weight of sample.
= moisture correction factor. M The N content of biomass was similarly evaluated. Compounds containing N in the biomass sample are reduced to NH 3 compounds by Kjeldhal standard digestion procedure (Vitousek, 1982). The N content of the sample was calculated using the following equation: N% = [(b - s) m/W] x 14 x (50/V) x M x 100 where b S M 14
= = = =
ml of KH(103)2 used for blank titration. ml of KH (103)2 used for sample titration. molarity of the KH(103)2. atomic weight of N.
50 V
= =
ml volume of digestion tube. ml volume of the digest.
166
CARBON-TO-NITROGENRATIOS1NAGRICULTURALRESIDUES
213
TABLE II Biofuel parameters for agronomic crop residues in Uganda
Plant tissue
C Content ~ sd
N Content K sd
C/N Ratio
N/C Ratio
~
sd
sd
Energy Output GJ/Mg ~ sd
Bananas
(Musa L.)
0.312
0.08
0.0090
0.003
34.7
9.3
0.0288
0.01
7.6
1.3
0.326
0.04
0.0160
0.003
20.4
2.6
0.0491
0.01
8.4
1.1
0.329
0.06
0.0053
0.002
62.1
0.016t
0.01
8.9
1.2
0.284
0.07
0.0110
0.003
25.8
6.3
0.0387
0.03
8.3
1.1
0.248
0.07
0.0120
0.003
20.7
4.1
0.0484
0.01
8.1
1.2
0.314
0.02
0.0140
0.002
22.4
3.1
0.0446
0.01
7.2
1.3
0.365
0.08
0.0123
0.004
29.7
6.4
0.0337
0.01
7.9
1.4
0.332
0.09
0.0230
0.004
14.4
2.5
0.0693
0.12
8.3
2.1
0.297
0.06
0.0070
0.002
42.4
8.4
0.0236
0.01
7.7
1.6
0.297
0.05
0.0330
0.003
9.0
1.0
0.1110
0.01
7.6
1.3
0.296
0.08
0.0140
0.003
21.1
5.2
0.0473
0.12
8.3
1.2
0.335
0.04
0.0210
0.002
16.0
1.7
0.0627
0.01
7.8
1.7
Millet
(Panicum L.) Maize (Zea L.)
12
Sorghum
(Sorghum L.) Rice
(Oryza L.) Wheat
(Triticum L.) Potatoes
(Solanum L.) Cassava
(Manihot) Beans
(Vicia) Peas
(Pisum) Peanuts
(Arachis) Soybeans
(Glycine L.)
W = weight of the sample. M = moisture correction factor. Dry matter weight, as well as the residue-to-crop-product ratio, was determined from measurements of dry biomass samples (von Malitz and Scholes, 1995). The C content in dry matter is higher than that of wet matter. For the agricultural crop products sampled, an average value of C content per unit of dry matter was determined to be 0.33. Therefore, to convert from dry matter to C content, a mean value of 0.35 was used for estimates in cases where crop-specific C content data were not available.
i67
274
J.O. ILUKOR AND S.O. OLUKA
To determine the net caloric content of each of the agricultural crop residue, quantities of samples were completely burned in a combustion chamber. The net heating value (equal to the grass heating value less the heat carried away by the products of combustion) was estimated by the calorimetric method (Crutzen and Andreae, 1990). The aboveground biomass was determined by specific field measurements of dry matter. This allowed the residue-to-crop-product ratio to be estimated for the agronomic crops. In Uganda, about 32% of the total crop residue surveyed is burned annually. This proportion compares with the average value of 25% for developing countries and 10% for developed countries (UNEP et al., 1995). The main source of sampling error is the intrinsic uncertainty of the water content of the crop residue. There are also errors in the determination of what exactly constitutes residue compared to the crop product when there are harvest losses. Subsamples enabled the standard deviation to be calculated to two significant digits for each crop.
3. Results The C content of grass tissue ranged from 0.25 to 0.38 (Tables I and II). Among agronomic crops, C content was very homogeneous. The N content of the plant tissue ranged widely. As a result, C/N ratios varied from 99 for sugar cane to 17 for mixed grass. The energy content was similar for all plant tissues sampled. The crop residue ratio and N/C values from the current study and UNEP et al. (1995) N/C default factors are compared in Table IlL Crop residue C values from this study are similar to UNEP/OECD/IEA/IPCC values. In contrast, the UNEP/OECD/IEA/IPCC values for N/C differ considerably from the current study.
4. Discussion and conclusion The C/N ratios and other biomass parameters were evaluated in this study to characterize agricultural crop and grass residues in Uganda. Country-specific C- and N-content data are needed to estimate GHG emissions from savanna burning and agricultural crop residues in Uganda. This information on ecosystem C and N biogeochemistry will help improve estimates of GHG emissions from land-use change in developing countries (Dixon et al., 1994). Future country GHG inventories should include efforts to generate specific country emission factors to validate or calibrate UNEP et al. (1995) default values.
Acknowledgments We greatly appreciate the support from the UNEP GHG Emission Inventory Study, which was coordinated by the Uganda Department of Meteorology, Ministry of Natural Resources, Uganda. Our thanks go to Makerere University for the laboratory facilities that were used for sample analysis.
168
CARBON-TO-NITROGENRATIOSIN AGRICULTURALRESIDUES
275
TABLE III Crop residue ratio and N/C values from current study and UNEP/IPCC/OECD/IEA
Crop
Residue Ratio
Uganda Residue Ratio
IPCC N/C Ratio
Uganda N/C Ratio
1.3
1.2
0.012
0.0446
1
1
0.02
0.0161
1.4
1.4
0.014
0.0484
1.4
1.1
0.016
0.0491
1.4
1~2
0.02
0.0387
2.1
0.7
0.05
0.0627
1
1
N/A
0.0473
0.4
0.4
N/A
0.0507
Wheat
(Triticum L.) Maize (Zea L.) Rice
(Oryza L.) Millet
(Panicum L.) Sorghum
(Sorghum L.) Soybeans
(Glycine L.) Peanuts
(Arachis) Potatoes
(Solanum L.)
References Crutzen, P.J., and Andreae, M.O.: 1990, Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles, Science 250, 1669-1678. Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., and Wisniewski, J.: 1994, Carbon pools and flux of global forest ecosystems, Science 263, 185-190. Levine, J.S. (ed.): 1991, Global Biomass Burning: Atmospheric, Climatic and Biospheric Implications, Cambridge, Massachusetts: MIT Press. Seiler, W., and Crutzen, EJ.: 1980, Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning, Climatic Change 2, 207-247. UNEP, OECD, IEA, IPCC (United Nations Environment Programme, Organisation for Economic Cooperation and Development, International Energy Agency, Intergovernmental Panel on Climate Change): 1995, IPCC Guidelines for National Greenhouse Gas Inventories, IPCC, Bracknell, 3 Volumes. Vitousek, EM., Gogz, J.R., Grief, C.C., Melillo, J.M,, Reiner, W.A.: 1982, A comparative anlaysis of potential nitrification and nitrate mobility in forest ecosystems, Ecological Monograph 52, 155-177. von Malitz, G.P., and Scholes, R.J.: 1995, The burning of fuelwood in South Africa: When is it sustainable? Environmental Monitoring and Assessment. in press.
169
CARBON-TO-NITROGEN
RATIOS IN AGRICULTURAL
RESIDUES
J.O. ILUKOR and S.O. OLUKA Department of Physics, Makerere University, P.O. Box 7062, Kampala, Uganda
Abstract. Agronomic crop residues produce greenhousegas emissions. Crops that produce residues at harvest and during processingmay vary from country to country. These residues, which can be in the form of peels, husks, stalks, or straw, are generally considered to be waste products. The carbon (C) and nitrogen (N) content of 19 different agronomic and grass crops common in Uganda were determined using standard laboratory methods. The C and N content of the sampleswere calculated fromtwo separateequations containinga moisture correction factor. The crop residue C/N ratios were similar to UNEPIOECD/IEA/IPCCvalues. 1. Introduction The I P C C Guidelines f o r National Greenhouse Gas Inventories (UNEP et al., 1995) provide a default methodology for quantifying sources and sinks of key anthropogenic greenhouse gases (GHGs). Because this methodology is generic, default values for specific sectors (for example, energy or land use) can lead to significant errors in GHG emission estimates. Emissions from the land-use sector (agriculture and forestry) are particularly difficult to estimate because of the complex biogeochemistry of ecosystems (Dixon et al., 1994). Combustion of biomass and biomass chemistry are two factors that are especially difficult to characterize (Levine, 1991). Biomass varies in its carbon (C) and nitrogen (N) content, depending on the plant species, edaphic factors, climate, and cultivation practices in any given region (Seiler and Crutzen, 1980). This variation influences the net heating values of biomass combustion as well as the characteristics and intensity of GHG emissions (Crutzen and Andreae, 1990). The purpose of this study was to evaluate C and N content of plant tissues common in Uganda and compare values to UNEP/OECD/IEA/IPCC methodology.
2. Materials and methods A variety of agronomic crop and grassland ecosystem biomass commonly combusted by wildfire was analyzed (Tables I and II). Samples were collected using a stratified sampling system for representative ecosystems. Nineteen different plant types common in Uganda were sampled. The samples from each plant residue were powdered. The C content of each powdered sample was determined by oxidizing the sample in a digestion tube at 120 degrees celsius by adding both potassium dichromate solution and concentrated sulfuric acid (Crutzen and Environmental Monitoring and Assessment 38:271-275, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.
272
J.O. 1LUKORAND S.O. OLUKA
TABLE I Biofuel parameters for grass and crop residues found in Uganda ecosystems
Plant tissue
Energy Output Residue/Product Gj/Mg Ratio sd x sd
C Content ~ sd
N Content ~ sd
C/N Ratio ~ sd
0.347 0 . 0 7
0.0035 0.003
99.1 10.2
8.8
1.1
0.3
0.1
0.381 0 . 0 8 0.0175 0.002
21.8
2.2
8.9
2.3
0.8
0.2
12.5
1.2
Sugar cane (husks)
(Saccharum L.) Coffee (husks)
(Coffea L.) Cotton (stalks)
(Gossypium L.)
0.342 0 . 0 6 0.0192 0.002
17.8
1.9
9.1
1.9
Grass (mixed) (Gramineae) Papyrus
0.305 0 . 0 8 0.0183 0.007
16.7
3.1
7.9
1.9
(Cyperus)
0.314 0 . 0 3
0.0099 0.004
31.7
2.8
7.3
1.3
0.269 0 . 0 6 0.0140 0.005
19.2
1.9
8.0
1.6
Elephant grass
(Typha)
Andreae, 1990). The C content (C%) of the sample was then calculated using the following equation: C% = {[3(b - s)]/b/W} x M x 100 where b
=
ml of FeSO 4 used for the blank titration.
s
=
ml of FeSO 4 used for the sample titration.
3 W
= =
equivalent weight of C. weight of sample.
= moisture correction factor. M The N content of biomass was similarly evaluated. Compounds containing N in the biomass sample are reduced to NH 3 compounds by Kjeldhal standard digestion procedure (Vitousek, 1982). The N content of the sample was calculated using the following equation: N% = [(b - s) m/W] x 14 x (50/V) x M x 100 where b S M 14
= = = =
ml of KH(103)2 used for blank titration. ml of KH (103)2 used for sample titration. molarity of the KH(103)2. atomic weight of N.
50 V
= =
ml volume of digestion tube. ml volume of the digest.
166
CARBON-TO-NITROGENRATIOS1NAGRICULTURALRESIDUES
213
TABLE II Biofuel parameters for agronomic crop residues in Uganda
Plant tissue
C Content ~ sd
N Content K sd
C/N Ratio
N/C Ratio
~
sd
sd
Energy Output GJ/Mg ~ sd
Bananas
(Musa L.)
0.312
0.08
0.0090
0.003
34.7
9.3
0.0288
0.01
7.6
1.3
0.326
0.04
0.0160
0.003
20.4
2.6
0.0491
0.01
8.4
1.1
0.329
0.06
0.0053
0.002
62.1
0.016t
0.01
8.9
1.2
0.284
0.07
0.0110
0.003
25.8
6.3
0.0387
0.03
8.3
1.1
0.248
0.07
0.0120
0.003
20.7
4.1
0.0484
0.01
8.1
1.2
0.314
0.02
0.0140
0.002
22.4
3.1
0.0446
0.01
7.2
1.3
0.365
0.08
0.0123
0.004
29.7
6.4
0.0337
0.01
7.9
1.4
0.332
0.09
0.0230
0.004
14.4
2.5
0.0693
0.12
8.3
2.1
0.297
0.06
0.0070
0.002
42.4
8.4
0.0236
0.01
7.7
1.6
0.297
0.05
0.0330
0.003
9.0
1.0
0.1110
0.01
7.6
1.3
0.296
0.08
0.0140
0.003
21.1
5.2
0.0473
0.12
8.3
1.2
0.335
0.04
0.0210
0.002
16.0
1.7
0.0627
0.01
7.8
1.7
Millet
(Panicum L.) Maize (Zea L.)
12
Sorghum
(Sorghum L.) Rice
(Oryza L.) Wheat
(Triticum L.) Potatoes
(Solanum L.) Cassava
(Manihot) Beans
(Vicia) Peas
(Pisum) Peanuts
(Arachis) Soybeans
(Glycine L.)
W = weight of the sample. M = moisture correction factor. Dry matter weight, as well as the residue-to-crop-product ratio, was determined from measurements of dry biomass samples (von Malitz and Scholes, 1995). The C content in dry matter is higher than that of wet matter. For the agricultural crop products sampled, an average value of C content per unit of dry matter was determined to be 0.33. Therefore, to convert from dry matter to C content, a mean value of 0.35 was used for estimates in cases where crop-specific C content data were not available.
i67
274
J.O. ILUKOR AND S.O. OLUKA
To determine the net caloric content of each of the agricultural crop residue, quantities of samples were completely burned in a combustion chamber. The net heating value (equal to the grass heating value less the heat carried away by the products of combustion) was estimated by the calorimetric method (Crutzen and Andreae, 1990). The aboveground biomass was determined by specific field measurements of dry matter. This allowed the residue-to-crop-product ratio to be estimated for the agronomic crops. In Uganda, about 32% of the total crop residue surveyed is burned annually. This proportion compares with the average value of 25% for developing countries and 10% for developed countries (UNEP et al., 1995). The main source of sampling error is the intrinsic uncertainty of the water content of the crop residue. There are also errors in the determination of what exactly constitutes residue compared to the crop product when there are harvest losses. Subsamples enabled the standard deviation to be calculated to two significant digits for each crop.
3. Results The C content of grass tissue ranged from 0.25 to 0.38 (Tables I and II). Among agronomic crops, C content was very homogeneous. The N content of the plant tissue ranged widely. As a result, C/N ratios varied from 99 for sugar cane to 17 for mixed grass. The energy content was similar for all plant tissues sampled. The crop residue ratio and N/C values from the current study and UNEP et al. (1995) N/C default factors are compared in Table IlL Crop residue C values from this study are similar to UNEP/OECD/IEA/IPCC values. In contrast, the UNEP/OECD/IEA/IPCC values for N/C differ considerably from the current study.
4. Discussion and conclusion The C/N ratios and other biomass parameters were evaluated in this study to characterize agricultural crop and grass residues in Uganda. Country-specific C- and N-content data are needed to estimate GHG emissions from savanna burning and agricultural crop residues in Uganda. This information on ecosystem C and N biogeochemistry will help improve estimates of GHG emissions from land-use change in developing countries (Dixon et al., 1994). Future country GHG inventories should include efforts to generate specific country emission factors to validate or calibrate UNEP et al. (1995) default values.
Acknowledgments We greatly appreciate the support from the UNEP GHG Emission Inventory Study, which was coordinated by the Uganda Department of Meteorology, Ministry of Natural Resources, Uganda. Our thanks go to Makerere University for the laboratory facilities that were used for sample analysis.
168
CARBON-TO-NITROGENRATIOSIN AGRICULTURALRESIDUES
275
TABLE III Crop residue ratio and N/C values from current study and UNEP/IPCC/OECD/IEA
Crop
Residue Ratio
Uganda Residue Ratio
IPCC N/C Ratio
Uganda N/C Ratio
1.3
1.2
0.012
0.0446
1
1
0.02
0.0161
1.4
1.4
0.014
0.0484
1.4
1.1
0.016
0.0491
1.4
1~2
0.02
0.0387
2.1
0.7
0.05
0.0627
1
1
N/A
0.0473
0.4
0.4
N/A
0.0507
Wheat
(Triticum L.) Maize (Zea L.) Rice
(Oryza L.) Millet
(Panicum L.) Sorghum
(Sorghum L.) Soybeans
(Glycine L.) Peanuts
(Arachis) Potatoes
(Solanum L.)
References Crutzen, P.J., and Andreae, M.O.: 1990, Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles, Science 250, 1669-1678. Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., and Wisniewski, J.: 1994, Carbon pools and flux of global forest ecosystems, Science 263, 185-190. Levine, J.S. (ed.): 1991, Global Biomass Burning: Atmospheric, Climatic and Biospheric Implications, Cambridge, Massachusetts: MIT Press. Seiler, W., and Crutzen, EJ.: 1980, Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning, Climatic Change 2, 207-247. UNEP, OECD, IEA, IPCC (United Nations Environment Programme, Organisation for Economic Cooperation and Development, International Energy Agency, Intergovernmental Panel on Climate Change): 1995, IPCC Guidelines for National Greenhouse Gas Inventories, IPCC, Bracknell, 3 Volumes. Vitousek, EM., Gogz, J.R., Grief, C.C., Melillo, J.M,, Reiner, W.A.: 1982, A comparative anlaysis of potential nitrification and nitrate mobility in forest ecosystems, Ecological Monograph 52, 155-177. von Malitz, G.P., and Scholes, R.J.: 1995, The burning of fuelwood in South Africa: When is it sustainable? Environmental Monitoring and Assessment. in press.
169
