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Quantifying the yield of perennial grasses grown as a biofuel for energy generation
Renewable Energy, Vol.5, Part 1], pp. 762-766, 1994 Elsevier Science Ltd Printed in Great Britain 0960-1481/94 $7.1~-i-0J~0
Pergamon
Quantifying the yield of Perennial Grasses Grown as a Biofuel for Energy Generation D.G. CHRISTIAN Crop Management Department, IACR Rothamsted Experimental Station, Harpenden, Herts. AI.5 2JQ, UK.
ABSTRACT Combustible biomass fuels may be produced on set-aside land by growing crops that produce a heavy yield of dry matter. Two promising candidate species are Miscanthus and Switchgrass. Establishment of both species was slow and weeds may cause problems. Yields were low becausse of the short vowing period in the first season. Nitrate leaching under Miscanthus suggested that N levels were too high relative to the crops development.
KEYWORDS: Miscanthus sinensis, Panicum virgatum, yield, nitrogen, leaching.
INTRODUCTION In 1992 more than 500,000 hectares of arable land was set aside, being no longer required for food production because of structural changes in the European Unions common agricultural policy. There is much interest in using set-aside land to produce biomass as a fuel for the generation of electricity. Biomass offers the prospect of a clean and renewable source of energy however many crops are unsuitable because they may not have sufficient dry matter content at harvest or require a too high level of inputs to achieve sufficient yield of dry matter. Planting trees at t igh density (10,000/ha) has been studied elsewhere as a biomass production system and is now on the point of commercial exploitation in Britain. However this system has a 2-5 year harvest cycle depending on species (Forster 1992). A perennial crop with annual harvest would make a useful alternative and this might be achieved with some perennial grasses. In 1993 experiments were established at Rothamsted to quantify the yield of two candidate species, Miscanthus sinensis and Panicum virgatum. Miscanthus is an oriental grass and the plant of most interest, M. sinensis "Giganteus" is propagated by rootstock cuttings or micropropogation. Panicum (switchgrass) from North America is established by seed. Both species are expected to remain productive for ten harvests or more although in the UK no experiments have been of sufficient duration to verify this.
762
763 In both crops shoots emerge from underground rhizomes in April and growth ceases with onset of frosts in the autumn. When senescence is complete, which is between December and March, the crop can be harvested with machinery already available on farms for the collection of silage or baling of hay and straw. In this paper, the early results of the establishment phase of the crops is presented along with assessment of the loss of nitrogen under Miscanthus during its dormant phase.
MATERIALS AND METHODS The experiments were established in May 1993 on a silt loam soil at Rothamsted Farm. The Miscanthus experiment used micropropogated plants planted at 50 crn x 50 cm spacing in 10 m x 10m plots. Three levels of nitrogen are tested; 0, 60 and 120 Kg N ha 1. The Panicum experiment was established using seed, sown at 10 Kg ha "1. Seven cultivars were sown, viz., Cave-in-rock, Kanlow, Pathfinder, Sunburst, Forestburg, Nebraska 28 and Dacotah. The number of plants established six weeks after sowing was estimated by counting plants in two adjacent 50 cm lengths of row in three places on each plot. At harvest, two areas each 1.8 m 2 were cut and weighed to assess fresh weight, then a sub-sample taken and dried at 90 ° C to constant weight to measure dry matter. On the Miscanthus experiment five plants were tagged on each plot and at intervals they were used to record changes in shoot number and shoot height which was measured as the peak of the natural curve of the last fully expanded leaf above ground level. At harvest, plants were cut and weighed from a 6 m x 6 m area on each plot. The cut material was weighed in-situ and sub-sampled for dry matter assessment as described for the Panicum experiment. In each plot on the Miscanthus experiment a single ceramic suction cup was installed to a depth of 90 cm and were used to collect a sample of soil solution at intervals during the winter. These were analysed for nitrate NO3 content and estimates of loss by leaching nutrient below 90 crn were made using meteorological data.
RESULTS Panicum Study Plant establishment was assessed seven weeks after sowing and the results are given in Table 1. Germination and emergence varied with variety and slow growth was common to all. As a result of slow growth weeds were very competitive and probably suppressed crop growth. The application of herbicides did not start until the beginning of July because it was felt that if applied earlier there was a risk that the young plants would be injured. Weed competition prevented further crop assessments being made. When shoots had senesced in December yield and moisture content were measured. Table 1. Population of Panicum seven weeks after sowing Variety
Plants, m ~
Cave-in-rock Kanlow Pathfinder Sunburst Forestburg Nebraska 28 Dacotah SED
283.8 188.8 285.7 218.6 267.0 275.0 301.2 36.18
764 Table 2 shows that there was considerable variation in yield between varieties and with five of the varieties yield was heavier where no nitrogen was applied than where 60 Kg N was given to the crop. This effect may have been influenced by increased competitive effect of weeds where N was applied. Table 2.
Yield and moisture content of Panicum harvested after shoot senescence Dry Matter, gm2
Variety Cave-in-Rock Kanlow Pathfinder Sunburst Forestburg Nebraska 28 Dacotah SED Variety x N LSD Variety P > 0.05
Moisture Content %
Nitrogen 0
Kg ha1 60
Nitrogen 0
Kg h a "1 60
165.7 215.2 241.9 134.2 152.7 155.2 110.6
165.9 163.8 206.1 103.0 146.9 151.4 134.9 51.71 NS
60.2 68.2 57.9 58.5 62.2 67.3 60.0
64.9 68.1 57.5 60.3 59.9 69.8 64.8 3.57 6.19
The moisture content of Kanlow and Nebraska 28 was significantly greater than the other varieties which were not statistically different from each other. The variation in moisture content indicates differences in the rate of drying of biomass and that in future a common harvest date may be inappropriate. The slow growth of the crop in the spring may have been affected by the lower temperatures of the maritime climate of the UK for a warm-season grass. However, weed control is important. Vogel (1987) suggests that a pre-emergence application of Atrazine can provide adequate weed control except where post application rainfall is limited. Miscanthus Study Crop Development. Both plant height and shoot number increased between June and November. The measurement in June took place shortly before nitrogen was applied and the measurement in November was after the first frosts occurred. The results of the measurements are given in Table 3. Table 3. Changes in shoot height and shoot number between June and November 1993. 25 June Nitrogen level kg ha" 0 60 120 SED
27 August
3 November
Height (cm)
Shoots (m2)
Height (cm)
Shoots (m2)
Height (cm)
Shoots (m2)
19.4 17.7 14.6 3.60
14.4 14.1 11.5 1.51
46.0 43.9 39.2 2.05
74.4 73.9 69.3 11.67
77.4 77.0 68.3 4.38
132.5 133.3 120.8 16.93
No treatment differences were found at any of the three sampling occasions but plants on plots that received 120 Kg N ha "1 were shorter and had fewer shoots than on plots receiving 0 or 60 kg N ha"1 where crop height and shoot number were closely similar. The relative change in both crop parameters
765
between the first and last measurement were greater following the application of 120 Kg N compared with the other treatment which were similar. When crops were harvested in January 1994 shoot number on the different treatments were similar to the number recorded in November indicating that shoot production ceased at about the time of the November sampling. There were 33, 34 and 29 shoots per plant on the No, N6o and N120plots respectively.
1.7
1.6 1.5-
~
. ..
&.
-.°...
"
~..
1.4-
o..
°. "- -.. •
Nitrogen
Leaf
-°-m ""'A'"
i
"°...-o°..
..--""
"
" Stem
1.3-
aO~N~N~
.11. 1 ~ N , ~ -- e---O~N~.
1.2-
-- ~--I~N~ 1.1
1 0 o.
°
°°0"
0.9 O.B
I 22/10~g3
I
I
I
I
03/11/93
25/11/94
0~17293
0e~01~4
Dale
Fig. 1. Changes in leaf and stem weights of Miscanthus Shoot Drv Weight. Measurements started on 12 October before frost occurred so that maximum shoot weight was recorded. Figure 1 shows that between November and January there was little change in the weight of the stem and most of the decrease in weight can be accounted for by changes in leaf weight. Moisture content of stem and leaf declined from an average of 81%, when sampling started on 12 October, to 59% on 9 December. Nitrogen treatment had little effect on the moisture content of shoots. Sometime after 9 December moisture content of biomass began to increase as the dry stems and leaf were re-wetted by rain, but dried down to 59% moisture content at the time of harvest. Yield. Harvestzble yield (collection of standing biomass excluding litter) was 1.47, 1.41, 1.82 dry tonnes ha" on the No, N~o and Nn0 plots respectively. On each treatment the weight of leaf exceeded the weight of stem. The harvest was earlier than planned because brackling damage (shoots kinked higher than base node) resulted in many leaves lying on the soil surface where they could not dry and were deteriorating. In a mature crop this type of damage is less likely because stems are taller and stiffer. Yield was predicted to be low because of the short growing season as a result of the May planting and the crop becoming established. Nitrate Leaching. Over the winter period losses of nitrate occurred on all treatments. The calculated losses between October 1993 and March 1994 were 110 kg, 130 kg, 175 kg ha' of Nitrate-N on the No, N6o and N,:o trea~alents respectively, Fig. 2. Loss from No represents soil source nitrogen. The site used for t~]e experiment had previously been used to grow winter beans and the loss is high probably because of the contribution made by the winter bean residues. By deducting losses that could be attributed to soil sources from the calculated losses from plots receiving nitrogen suggests that about 28% of the N applied to N6o and 38% applied to Nt20 had been lost by March. It is possible that too much nitrogen was applied to plants that were small and immature. These data indicate that losses
766 were not proportional to the amounts applied to the two N treatments, more information is required about the rate and timing of nitrogen to newly established crops.
12.00 14.0010.I00 e~
[ " NO I
z
= N120
t - * - " Nl:,o-no 4.00
---
-"
2.00
0oo
-~-~-~- ,-~" g ~ ' ~ ;
~ , °" ~", . . . . . .
,
.... ,
,
28/0~9318/10/9307/11/9327/11/9317/12/9306/01/9426/01/9415/02/9407/03/94 Date
Fig. 2. Nitrate-N Cumulative Leaching Losses ACKNOWLEDGEMENTS Thanks are expressed for the technical assistance of colleagues, N. Yates, A Riche and A Todd. The work was carded out with funding provided in part by the European Union DGVI and the Energy Technology Support Unit of the Department of Trade and Industry. REFERENCES Foster, C. (1992). Wood as a fuel, short rotation coppice; opportunities and implications. BCPC Monograph 50, Set-aside. BCPC Publications, Bracknell, UK pp. 215-222. Vogel, K.P. (1987). Seeding rates for establishing big Bluestem and Switchgrass with pre-emergence atrazine applications. Agron. J. 79, 509-512.
Pergamon
Quantifying the yield of Perennial Grasses Grown as a Biofuel for Energy Generation D.G. CHRISTIAN Crop Management Department, IACR Rothamsted Experimental Station, Harpenden, Herts. AI.5 2JQ, UK.
ABSTRACT Combustible biomass fuels may be produced on set-aside land by growing crops that produce a heavy yield of dry matter. Two promising candidate species are Miscanthus and Switchgrass. Establishment of both species was slow and weeds may cause problems. Yields were low becausse of the short vowing period in the first season. Nitrate leaching under Miscanthus suggested that N levels were too high relative to the crops development.
KEYWORDS: Miscanthus sinensis, Panicum virgatum, yield, nitrogen, leaching.
INTRODUCTION In 1992 more than 500,000 hectares of arable land was set aside, being no longer required for food production because of structural changes in the European Unions common agricultural policy. There is much interest in using set-aside land to produce biomass as a fuel for the generation of electricity. Biomass offers the prospect of a clean and renewable source of energy however many crops are unsuitable because they may not have sufficient dry matter content at harvest or require a too high level of inputs to achieve sufficient yield of dry matter. Planting trees at t igh density (10,000/ha) has been studied elsewhere as a biomass production system and is now on the point of commercial exploitation in Britain. However this system has a 2-5 year harvest cycle depending on species (Forster 1992). A perennial crop with annual harvest would make a useful alternative and this might be achieved with some perennial grasses. In 1993 experiments were established at Rothamsted to quantify the yield of two candidate species, Miscanthus sinensis and Panicum virgatum. Miscanthus is an oriental grass and the plant of most interest, M. sinensis "Giganteus" is propagated by rootstock cuttings or micropropogation. Panicum (switchgrass) from North America is established by seed. Both species are expected to remain productive for ten harvests or more although in the UK no experiments have been of sufficient duration to verify this.
762
763 In both crops shoots emerge from underground rhizomes in April and growth ceases with onset of frosts in the autumn. When senescence is complete, which is between December and March, the crop can be harvested with machinery already available on farms for the collection of silage or baling of hay and straw. In this paper, the early results of the establishment phase of the crops is presented along with assessment of the loss of nitrogen under Miscanthus during its dormant phase.
MATERIALS AND METHODS The experiments were established in May 1993 on a silt loam soil at Rothamsted Farm. The Miscanthus experiment used micropropogated plants planted at 50 crn x 50 cm spacing in 10 m x 10m plots. Three levels of nitrogen are tested; 0, 60 and 120 Kg N ha 1. The Panicum experiment was established using seed, sown at 10 Kg ha "1. Seven cultivars were sown, viz., Cave-in-rock, Kanlow, Pathfinder, Sunburst, Forestburg, Nebraska 28 and Dacotah. The number of plants established six weeks after sowing was estimated by counting plants in two adjacent 50 cm lengths of row in three places on each plot. At harvest, two areas each 1.8 m 2 were cut and weighed to assess fresh weight, then a sub-sample taken and dried at 90 ° C to constant weight to measure dry matter. On the Miscanthus experiment five plants were tagged on each plot and at intervals they were used to record changes in shoot number and shoot height which was measured as the peak of the natural curve of the last fully expanded leaf above ground level. At harvest, plants were cut and weighed from a 6 m x 6 m area on each plot. The cut material was weighed in-situ and sub-sampled for dry matter assessment as described for the Panicum experiment. In each plot on the Miscanthus experiment a single ceramic suction cup was installed to a depth of 90 cm and were used to collect a sample of soil solution at intervals during the winter. These were analysed for nitrate NO3 content and estimates of loss by leaching nutrient below 90 crn were made using meteorological data.
RESULTS Panicum Study Plant establishment was assessed seven weeks after sowing and the results are given in Table 1. Germination and emergence varied with variety and slow growth was common to all. As a result of slow growth weeds were very competitive and probably suppressed crop growth. The application of herbicides did not start until the beginning of July because it was felt that if applied earlier there was a risk that the young plants would be injured. Weed competition prevented further crop assessments being made. When shoots had senesced in December yield and moisture content were measured. Table 1. Population of Panicum seven weeks after sowing Variety
Plants, m ~
Cave-in-rock Kanlow Pathfinder Sunburst Forestburg Nebraska 28 Dacotah SED
283.8 188.8 285.7 218.6 267.0 275.0 301.2 36.18
764 Table 2 shows that there was considerable variation in yield between varieties and with five of the varieties yield was heavier where no nitrogen was applied than where 60 Kg N was given to the crop. This effect may have been influenced by increased competitive effect of weeds where N was applied. Table 2.
Yield and moisture content of Panicum harvested after shoot senescence Dry Matter, gm2
Variety Cave-in-Rock Kanlow Pathfinder Sunburst Forestburg Nebraska 28 Dacotah SED Variety x N LSD Variety P > 0.05
Moisture Content %
Nitrogen 0
Kg ha1 60
Nitrogen 0
Kg h a "1 60
165.7 215.2 241.9 134.2 152.7 155.2 110.6
165.9 163.8 206.1 103.0 146.9 151.4 134.9 51.71 NS
60.2 68.2 57.9 58.5 62.2 67.3 60.0
64.9 68.1 57.5 60.3 59.9 69.8 64.8 3.57 6.19
The moisture content of Kanlow and Nebraska 28 was significantly greater than the other varieties which were not statistically different from each other. The variation in moisture content indicates differences in the rate of drying of biomass and that in future a common harvest date may be inappropriate. The slow growth of the crop in the spring may have been affected by the lower temperatures of the maritime climate of the UK for a warm-season grass. However, weed control is important. Vogel (1987) suggests that a pre-emergence application of Atrazine can provide adequate weed control except where post application rainfall is limited. Miscanthus Study Crop Development. Both plant height and shoot number increased between June and November. The measurement in June took place shortly before nitrogen was applied and the measurement in November was after the first frosts occurred. The results of the measurements are given in Table 3. Table 3. Changes in shoot height and shoot number between June and November 1993. 25 June Nitrogen level kg ha" 0 60 120 SED
27 August
3 November
Height (cm)
Shoots (m2)
Height (cm)
Shoots (m2)
Height (cm)
Shoots (m2)
19.4 17.7 14.6 3.60
14.4 14.1 11.5 1.51
46.0 43.9 39.2 2.05
74.4 73.9 69.3 11.67
77.4 77.0 68.3 4.38
132.5 133.3 120.8 16.93
No treatment differences were found at any of the three sampling occasions but plants on plots that received 120 Kg N ha "1 were shorter and had fewer shoots than on plots receiving 0 or 60 kg N ha"1 where crop height and shoot number were closely similar. The relative change in both crop parameters
765
between the first and last measurement were greater following the application of 120 Kg N compared with the other treatment which were similar. When crops were harvested in January 1994 shoot number on the different treatments were similar to the number recorded in November indicating that shoot production ceased at about the time of the November sampling. There were 33, 34 and 29 shoots per plant on the No, N6o and N120plots respectively.
1.7
1.6 1.5-
~
. ..
&.
-.°...
"
~..
1.4-
o..
°. "- -.. •
Nitrogen
Leaf
-°-m ""'A'"
i
"°...-o°..
..--""
"
" Stem
1.3-
aO~N~N~
.11. 1 ~ N , ~ -- e---O~N~.
1.2-
-- ~--I~N~ 1.1
1 0 o.
°
°°0"
0.9 O.B
I 22/10~g3
I
I
I
I
03/11/93
25/11/94
0~17293
0e~01~4
Dale
Fig. 1. Changes in leaf and stem weights of Miscanthus Shoot Drv Weight. Measurements started on 12 October before frost occurred so that maximum shoot weight was recorded. Figure 1 shows that between November and January there was little change in the weight of the stem and most of the decrease in weight can be accounted for by changes in leaf weight. Moisture content of stem and leaf declined from an average of 81%, when sampling started on 12 October, to 59% on 9 December. Nitrogen treatment had little effect on the moisture content of shoots. Sometime after 9 December moisture content of biomass began to increase as the dry stems and leaf were re-wetted by rain, but dried down to 59% moisture content at the time of harvest. Yield. Harvestzble yield (collection of standing biomass excluding litter) was 1.47, 1.41, 1.82 dry tonnes ha" on the No, N~o and Nn0 plots respectively. On each treatment the weight of leaf exceeded the weight of stem. The harvest was earlier than planned because brackling damage (shoots kinked higher than base node) resulted in many leaves lying on the soil surface where they could not dry and were deteriorating. In a mature crop this type of damage is less likely because stems are taller and stiffer. Yield was predicted to be low because of the short growing season as a result of the May planting and the crop becoming established. Nitrate Leaching. Over the winter period losses of nitrate occurred on all treatments. The calculated losses between October 1993 and March 1994 were 110 kg, 130 kg, 175 kg ha' of Nitrate-N on the No, N6o and N,:o trea~alents respectively, Fig. 2. Loss from No represents soil source nitrogen. The site used for t~]e experiment had previously been used to grow winter beans and the loss is high probably because of the contribution made by the winter bean residues. By deducting losses that could be attributed to soil sources from the calculated losses from plots receiving nitrogen suggests that about 28% of the N applied to N6o and 38% applied to Nt20 had been lost by March. It is possible that too much nitrogen was applied to plants that were small and immature. These data indicate that losses
766 were not proportional to the amounts applied to the two N treatments, more information is required about the rate and timing of nitrogen to newly established crops.
12.00 14.0010.I00 e~
[ " NO I
z
= N120
t - * - " Nl:,o-no 4.00
---
-"
2.00
0oo
-~-~-~- ,-~" g ~ ' ~ ;
~ , °" ~", . . . . . .
,
.... ,
,
28/0~9318/10/9307/11/9327/11/9317/12/9306/01/9426/01/9415/02/9407/03/94 Date
Fig. 2. Nitrate-N Cumulative Leaching Losses ACKNOWLEDGEMENTS Thanks are expressed for the technical assistance of colleagues, N. Yates, A Riche and A Todd. The work was carded out with funding provided in part by the European Union DGVI and the Energy Technology Support Unit of the Department of Trade and Industry. REFERENCES Foster, C. (1992). Wood as a fuel, short rotation coppice; opportunities and implications. BCPC Monograph 50, Set-aside. BCPC Publications, Bracknell, UK pp. 215-222. Vogel, K.P. (1987). Seeding rates for establishing big Bluestem and Switchgrass with pre-emergence atrazine applications. Agron. J. 79, 509-512.