Energy yields in intensive and extensive biomass production systems
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Biomass and Bioenergy 22 (2002) 159167
Energy yields in intensive and extensivebiomass production systems
IVEM, Center for Energy and Environmental Studies, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands
Received 14 May 2001; received in revised form 12 November 2001; accepted 15 November 2001
As for agricultural crops, biomass crops can be grown in intensive production systems (external inputs such as pesticides andarti,cial fertilisers) or extensive systems with few external inputs. The choice between an intensive or extensive productionsystem has consequences for yields. A method is presented to estimate biomass yields in intensive and=or extensive productionsystems. This method is applied to a poplar coppice production system. Results of the method are used to evaluate severalintensive and extensive production systems with respect to bioenergy yield and fossil fuel use e/ciency. The energy yield(GJ=ha) of the intensive systems was highest, while the extensive systems show the better fossil fuel use e/ciency (GJoutput=GJ fossil energy input). ? 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Energy crops; Inputoutput relations; Production systems; Energy yields; Energy use e/ciency; Europe
The growing of agricultural crops require land,machinery, plant material, fertilisers and crop pro-tection agents for the maintenance of the crops. Theproduction method varies between extensive produc-tion systems with minimal inputs to intensive systemswith large inputs including arti,cial fertilisers andagri-chemicals. In general yields produced from ex-tensive systems are lower than those of intensivesystems.The inputs used require energy both for their pro-
duction (indirect energy) and for their application(direct energy such as diesel to power the tractor).During the last decade many studies have been under-taken to calculate the use of fossil energy in food crop
Tel.: +050-363-4611; fax: +050-363-7168.E-mail address: firstname.lastname@example.org (S. Nonhebel).
production systems . These studies includedcomparison between organic agricultural productionversus high-input production, impact of technologicaldevelopments in agriculture on energy use, and com-parison of energy use in agriculture in di?erent coun-tries. All studies showed that energy use in systemswith limited inputs is less than in high-input systems.Absolute energy use (GJ=ha) and relative energy use(expressed as energy requirements per unit of prod-uct) were both higher in the high-input systems. Inaddition, fossil fuel energy was used most e/cientlyin low-input crop production systems which producedthe most biomass per unit of energy applied.There is no physiological di?erence between the
production of plant material for food or for energy.Sometimes even the same crops are used such as oilseed rape. This implies that reduction of energy usee/ciency with increasing inputs that is observed infood crops would also be expected for energy crops.
0961-9534/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S 0961 -9534(01)00071 -X
160 S. Nonhebel / Biomass and Bioenergy 22 (2002) 159167
For food production, energy use e/ciency is of limitedinterest since the value of the harvested material is notdetermined by its heating value. For biomass crops,fossil fuel energy use e/ciency and energy yield areimportant parameters since they determine the poten-tial for growing these crops. Only crops that yield sig-ni,cantly more energy than is required to grow themare suitable as energy crops. Greater potential existsfor crops with increasing net yields leading to greaterdi?erences between energy inputs and energy outputs.This boundary condition does not exist for food crops,and many examples exist where energy required toproduce the crop is higher than the energy that couldbe obtained from it, such as the production of tomatoesin greenhouses. The importance of the energy ratio pa-rameters with respect to energy crops justi,es furtherinvestigation.The objective of this paper is to compare energy
yields and energy use e/ciencies of extensive andintensive biomass production systems. A method todetermine these parameters is presented and resultsare evaluated.
2. Material and methods
To determine energy use e/ciencies informationon energy inputs and energy outputs (crop yield)is required. For food producing systems this infor-mation can be obtained from annual agriculturalstatistics (published by organisations such as Foodand Agricultural Organisation (FAO) of the UN, andnational statistical o/ces like Statistics Netherlands,but also in the reports of agricultural research insti-tutes like Agricultural Economics Research Institute(LEI) in The Netherlands. For energy crops, simi-lar sources of data are lacking since these crops arepresently only grown in ,eld experiments. There-fore, in this paper the target oriented approach wasused , whereby the yield level (output) is ,rstde,ned and then the required energy inputs to reachthis yield level are determined. To determine theyields of intensive and extensive production sys-tems in this paper the following assumptions weremade.The yield of the intensive production system was
de,ned as the potential production level of the cropin a certain region. The potential production is (by
de,nition) the production that can be obtained whena crop is optimally supplied with water and nutrientsand free from pests and diseases . This implies thatonly crop characteristics, air temperature and solar ra-diation determine this yield. Potential production canbe determined with crop growth simulation models,where crop growth is simulated in relation to air tem-perature and solar radiation. The potential yield levelis a measure for what can be obtained under optimalgrowing conditions in a region. Sometimes this yieldlevel can be reached in well-designed ,eld experi-ments with irrigation.Irrigation requires a certain infrastructure, fur-
ther application of water involves extra inputs(labour, pumps, fuel, etc.) and irrigation of cropsis often not feasible. This makes that the calcu-lated potential production has limited to do withthe actual yield possibilities in a region. Thereforethe so-called water-limited production level isrecognised. This production level is by de,nition:the production of a crop that is optimallysupplied with nutrients and free from pests anddiseases, but yield is limited by the availability ofwater .The magnitude of the water-limited production is
also determined with crop growth simulation modelsbut in these models a soilwater balance is incorpo-rated to simulate water availability to the crop. Thewater-limited yield is often interpreted as the attain-able yield level for individual farmers. Good cropmanagement can reduce e?ects of nutrient shortagesand pest damage. Irrigation, however, may requireinvestments on a regional scale; an individual farmercannot establish this. It should be realised that theactual yields obtained in Europe lay far below thesepotential levels; as will be discussed later. In regionswith relative large amounts of precipitation nowater shortage occurs during the growing seasons ofthe crops. In these regions, the calculated potentialyields and the calculated water-limited yields are thesame.To determine the yield in the extensive systems
forests are taken as a starting point, where inputsare applied only during planting and harvesting. Inthese systems, the only available nitrogen originatesfrom natural sources after decomposition and depo-sition. The available amount of nitrogen from thenatural sources is very small in comparison with
S. Nonhebel / Biomass and Bioenergy 22 (2002) 159167 161
Fig. 1. Simulated potential production (tonne=ha=yr) of short rotation poplar systems in various regions in Europe .
nitrogen applications in intensive production sys-tems. It is assumed that nitrogen is the limiting factorin the extensive systems and that yield is linearlyrelated to available nitrogen. When the nitro-gen content of the harvested material (Cn[kg=kg])is known, the yield (kg=ha) dependent on theavailable nitrogen (N[kg=ha]) can be calculatedas
2.1. Description of the biomass productionsystems studied
In principle all crops with a positive energy ratiocould be used for energy supplies. Comparative stud-ies between a range of crops showed that productionof biomass in short rotation forestry systems was mostpromising . Therefore, this type of biomass produc-tion system is evaluated here.The system concerned a short rotation poplar sys-
tem. The trees were planted in spring with a densityof 1 tree=m2 (10; 000 trees=ha), every fourth year thecrop is harvested and chipped. It was assumed that theplantation has a total lifespan of 20 years, so that 5harvests can take place before replanting. It was as-sumed that the production is similar each year.
2.2. Determination of yields
2.2.1. Intensive production systemsYield potentials derived in a study on produc-
tion possibilities for biomass crops grown in variousEuropean regions were used as a starting point . Inthat study a crop growth simulation model was de-veloped to simulate production of biomass crops. Themodel was based on the linear relationship betweenintercepted radiation and above ground biomass pro-duction, being light use e/ciency . A soil waterbalance was incorporated to simulate the e?ects ofwater shortage on crop production. The model re-quired monthly averages of global radiation, tem-perature and precipitation and crop characteristics tosimulate production . In what publication, informa-tion can be found on both potential- and water-limitedyields of several biomass crops in