Elsevier

Biomass and Bioenergy

Volume 122, March 2019, Pages 257-269
Biomass and Bioenergy

Research paper
Integrated and spatially explicit assessment of sustainable crop residues potential in Europe

https://doi.org/10.1016/j.biombioe.2019.01.021Get rights and content

Highlights

  • Detailed theoretical, technical, environmental & sustainable spatially explicit potentials are estimated at European level.

  • Temporal variability of crop residues production is important, with potential impact on their use.

  • The CENTURY model provides sustainable removal rates, which allow the maintenance of soil organic matter at current levels.

  • Specific technical and environmental constraints determine the spatial distribution of biomass that could be collected.

Abstract

This paper provides estimates of available agricultural residues at the European level. It complements previous studies by expanding the geographical study area to 36 European countries, and improving the methodology by incorporating temporal variability of the theoretical potential resources and taking into consideration technical and environmental constraints. The amount of residues potentially obtainable from the main crops cultivated in Europe, namely wheat, rye, barley, oats, maize, rice, rapeseed and sunflower was assessed. Detailed spatial distribution maps of different crop residue potentials (theoretical, technical, environmental and sustainable) are provided at 1 km spatial resolution. The average theoretical potential was estimated at 291 and 367 Mt DM in the EU and Europe, respectively. Significant yearly variability of crop residue production has been noticed, between 209 and 365 million tonnes dry yr−1 (3673–6389 PJ) in the EU and between 253 and 483 million tonnes dry yr−1 (4434–8453 PJ) across Europe. The average technical potential of crop residue was estimated at 168 million tonnes dry yr−1 (2941 PJ) in the EU and at 212 million tonnes dry yr−1 (3715 PJ) in Europe. The average sustainable potential considering both technical and environmental constraints in each location was estimated at about 124 million tonnes dry yr−1 (2162 PJ) in the EU and at 14 Mt DM (2601 PJ) in Europe.

Introduction

The 2030 Climate and Energy Policy Framework [1] has set EU wide targets and policy objectives for 2030: a 40% reduction in GHG emissions compared to 1990 levels, a minimum share of 27% of renewable energy consumption, and at least 27% energy savings compared with the business-as-usual scenario. Later on, in June 2018, the European Council and the European Parliament have agreed a binding renewable energy target of 32% by 2030, up from the previous goal of 27%. The European Parliament approved on November 13th, 2018 the 32% binding target for renewables for 2030, an energy efficiency target of at least 32.5% and the binding target of minimum 14% for renewables in transport in 2030 that were included in the legislative proposals of the 2016 Clean Energy for All Europeans package. This includes a review clause by 2023 for an upward revision of the EU level target. First generation biofuels with a high risk of “indirect land use change” will no longer count towards the EU's renewable energy goals from 2030. From 2019, the contribution of first-generation biofuels to these goals will gradually be phased out until it reaches zero in 2030. Each Member State must present a ten-year “integrated national energy and climate plan” with national targets, contributions, policies and measures by the end of 2019.

Bioenergy is expected to contribute with 140 Mtoe to the Gross Final Energy Consumption (GFEC) in 2020, compared to 116 Mtoe achieved in 2016, corresponding to almost 60% of the GFEC covered by all renewable energy sources, and 12% of the GFEC in the EU [2]. For the target year of 2030, bioenergy could reach values between 160 and 180 Mtoe (6698–7536 PJ), representing a share around 14%–16% of the GFEC, according to authors' estimations based on the Energy Roadmap 2050 [3].

Biomass is currently the largest source of renewable energy in the EU, providing heat, electricity and transport fuels. Despite high growth rates in the photovoltaic and wind sectors, which may change the whole energy mix and contribute to the diversification of energy sources, bioenergy is expected to remain, at EU level, the major renewable energy source until 2030. The Energy Roadmap 2050, which investigated possible pathways for a transition towards a GHG emission reduction of 80% economy-wide, indicates that bioenergy will still play a major role in the decarbonisation in all analysed scenarios [3].

Biomass availability, competition between alternative uses of biomass, as well as sustainability issues are major concerns for policy development and bioenergy deployment. In the EU, there is a significant unexploited biomass potential. The environmentally compatible biomass potential for the EU (covering 25 Member States) was estimated by the European Environment Agency (EEA) for 2020 at 235 Mtoe: 39 Mtoe from forestry, 96 Mtoe from agriculture, and 100 Mtoe from waste [4]. The Biomass Futures project shows that the biomass sustainable potential might be even larger in 2020 (considering the EU-27), reaching 375 Mtoe, of which 167 Mtoe from forestry, 158 Mtoe from agriculture, and 50 Mtoe from waste. For 2030, the estimations indicate lower sustainable biomass potential, reaching 351 Mtoe, of which 163 Mtoe from forestry, 143 Mtoe from agriculture, and 45 Mtoe from waste [5].

In order to realise its potential, biomass mobilisation within the EU will have to increase substantially. In this context, biomass from waste resources (crop residues, livestock manure, municipal solid waste, industrial waste sewage sludge, etc.) could contribute significantly to energy supply without resulting in major land use change or other sustainability impacts. Indeed, agricultural residues and crop residues are potential feedstock for bioenergy production or bio-based materials and are important biomass resources leading to an enlargement of the biomass feedstock base [6]. The use of crop residues offers good opportunities for bioenergy production with low or no land use competition for biomass feedstock [7]. However, sustainability issues have to be considered in relation to crop residue collection due to potential negative impacts on soil fertility [6,7].

Straw is been collected in bales and has traditionally been utilized as fodder and bedding in animal farming, as mulch in horticulture and vegetable cultivation, for frost prevention, or as a growth substrate in mushroom production, among other uses [[7], [8], [9], [10]]. Straw can be also incorporated into the soil to maintain the soil organic matter and soil productivity or left as a mulch on top soil [8]. The residues from crop production are an important source of organic carbon and play a critical role for the maintenance of the organic matter balance and nutrient cycles in soil. Excessive residue removal from the field can reduce the soil carbon pool, which may decrease the long term productive capacity of the soils [11]. In the past, most of the straw not employed for the previously listed purposes was used as fuels in rural areas or burned in the fields. During the last years, straw burning has been banned for the air quality protection reasons and a more structured and efficient use for straw for energy production has emerged.

Currently in Europe, there are some uses of straw for energy purposes in Denmark, Spain and the United Kingdom [12], as well as other countries that have introduced a dedicated support scheme [7]. With growing interest in bioenergy, it might be expected an increasing proportion of the unutilized straw resource to be diverted to energy and an increasing pressure for the removal of straw from land [13]. The decision to build a bioenergy plant, with large capital investments entailed, requires a detailed knowledge of the biomass feedstock that is locally available and its temporal variability. Indeed, the availability of straw for bioenergy production depends on numerous factors, including crop production, crop rotation, varieties grown, the relative proportions of straw to crop biomass, harvesting practices (cutting height, direct incorporation in soil), and various competitive uses [14]. Therefore, the assessment of spatial local and regional availability of feedstock is crucial for the practical implementation of bioenergy plants [6].

This study provides a spatial explicit assessment of sustainable crop residues potential based on a Geographic Information System (GIS) approach and data from multiple sources, considering local soil, climate, and farming practices across Europe. The potential of crop residues estimated in this study could be used for different purposes: fodder, bedding, horticulture, bio-based materials, not only for bioenergy production. The final use is generally determined by the economics, which was out of scope of this paper.

This study represents an update of previous studies [9,10,12,15] with an improved methodology and extended geographical scope. In particular, in addition to the previous studies, this work provides a comprehensive assessment of the spatial explicit sustainable potential that considers the technical and environmental constraints at each location (1 km pixel). The area covered includes all the European Union Member States (Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, United Kingdom) and other European countries (Albania, Bosnia and Herzegovina, FYROM - the Former Yugoslav Republic of Macedonia, Kosovo - under United Nations Security Council Resolution 1244/99, Moldova, Montenegro, Norway, Serbia, Switzerland and Ukraine).

In this paper, the yield of straws was estimated based on the crop yield and the corresponding crop-to-residue ratios. The spatial distribution of crop residues, calculated at 1 km spatial resolution, follows the spatial distribution of the crop production areas. The availability of crop residues was obtained based on technical constraints for harvesting and on environmental requirements for the conservation of soil organic matter. This paper also analyses the impact of crop residues removal on soil organic matter taking into consideration soil data, crop rotation information and climate projections to estimate continental-scale residue availability. The estimation on the theoretical technical environmental and sustainable potentials of bioenergy production from crop residues is also discussed.

A variety of methodological approaches has been used to estimate crop residue production on regional, national, or global scale. In contrast to crop production info, statistical data on straw production is lacking, since yield data are mostly reported for crop production [14]. In fact, estimation of crop residue production and potentials is generally hampered by lack of experimental data, which causes uncertainty in the parameterization of the relation between crop and residue yields [16].

The estimation of straw resources normally relies on assumptions about the relationship between crop and straw yields i.e. by using a factor which is based on the ratio between these yields - Residues-to-Product-Ratio (RPR) [9,17]. Harvest Index (HI), defined as the crop yield expressed as the fraction of aboveground biomass production, is also used for crop residue estimations. It indicates the partitioning between grain and straw (including stem, leaves, chaff and stubble) production and allows for convenient prediction of crop residues production [18].

Harvest residue generation fraction, on the other hand, is defined as the ratio between the amount of generated residues and harvested crop. It reveals a recoverability fraction that can be used to calculate the amount of residues that realistically can be recovered. Straw production and the RPR vary greatly depending on crop yield, variety, farming practices, weather conditions, etc. [19]. Some evidence suggests that RPR is not constant but proportional to yield [9,16,20]. Present day cultivars tend to be shorter and higher-yielding, and produce a higher proportion of grain relative to straw, as result of the attempts made to increase yield [11,19,21].

Experimental data demonstrated that spring wheat straw to grain ratio decreases nonlinearly in respect to increasing grain yield, with lower variation at higher grain yield levels [22]. Similarly, HI can increase without significant changes in total biomass production, suggesting an exponential relation between crop and residue yields [16].

The RPR differs between and within cultivars, locations, and cultivation period, indicating that residue production also depends on soil type, weather conditions, fertilizer regimes, among other factors [13,16]. Some studies also reported that drought has an impact on straw to grain ratio that may either decrease or increase if drought occurs at earlier or later growth stages, respectively [18]. This variability poses difficulties in the estimation of crop residues and may lead to inaccurate estimates of straw resources in individual years. The amount of crop residues can be increased by choosing varieties with a high RPR. If crop yields are high, more straw is produced and thus more straw can be removed, as below ground biomass (roots) and stubble turnover already provide enough organic matter input to the soil. In fact, during the last decades the effort focussed on modifying the grain ratio in order to increase the usable yield (grain) of the crops.

In most studies, however, the estimations of the availability of crop residues for energy purposes are based on rough estimations and constant factors, which do not take into consideration local conditions. The residue production is usually calculated by assuming a proportional dependence on the total crop production and not on the crop yield. In some cases, constant factors are applied to estimate sustainable removal rates to account for the actual amounts that could be collected from the field [8,16,[23], [24], [25], [26], [27], [28], [29]] disregarding in this way the local conditions (soil type, soil quality, soil organic carbon climate data). Collectable straw yields are generally lower than the total straw production, since there are technical and environmental constraints that limit the amount of material that can be removed from the field in a specific location.

Several studies estimating the biomass potentials have been published with significant variations in the results. The amount of straw and other potential biomass raw materials that is available and can be collected differs among regions due to different factors, including:

  • Differences among definitions of crop residues, as well as the type of crops considered in each study;

  • Variation on assumptions and estimation methods, including on the ratio of residue to product;

  • Inclusion to different extent of technical, environmental and economic limitations and reporting of theoretical, technical and sustainable potentials;

  • Annual variability of the residues production each year in different regions, as a result of farming practices (fertilisers application, irrigation, varieties, etc.), climatic conditions (temperature, precipitations), etc.;

  • Different competing uses considered;

  • Differences among spatial and temporal scales; and

  • Uncertainties related to the varieties (cultivars) used over large areas (country, continental scales).

The estimation of crop residue potentials available on the literature can be generically grouped into four main categories:

  • Assessment of the theoretical potential: refers to the total residue production of aboveground biomass without taking into consideration any harvesting, environmental and/or economic constraints;

  • Assessment of the technical potential: refers to the physical amount of materials that could be technically removed from the field. It depends on the crop type (e.g. length of the plant), type and efficiency of the equipment (e.g. straw collection machinery, cutting height, etc.) and field management practices (e.g. crop rotation, tillage procedures, harvesting losses, cutting height, vertical distribution of biomass along the stem height, etc.), among other factors;

  • Assessment of the environmental potential: refers to the physical amount of materials that could be removed from the field without causing major adverse impacts on land (e.g. depletion of soil organic matter); and

  • Assessment of the sustainable potential: refers to the physical amount of materials that could be removed from the field considering technical constraints for harvesting and collecting and environmental related to the impacts on land as well.

There are several studies that provided estimates of the production of crop residues in a theoretical level. In a global context, Bentsen et al. [16], based on six crop types (wheat, barley, oats, maize, rice, rapeseed and sunflower), calculated the total amount of crop residues around 3700 Mt yr−1 Dry Matter (DM) and an theoretical energy potential from the selected crop residues of 65 EJ yr−1. This study considered an exponential relation between crop yield and residue production, concluding that crop residue assessments should focus on species or cultivars-specific exponential (or logarithmic) multipliers.

Using crop specific HI in contrast to RPR for each crop type, Hakala et al. [25] estimated the current global technical crop residue energy potential at 39–42 EJ and at 38–41 EJ for 2050, assuming that 70% of the crop residues can be harvested. The authors acknowledged the importance of the crop residues on soil fertility and also the difficulty of quantifying the amount needed for maintaining it.

Hoogwijk et al. [30], based on the compilation of other studies, concluded the global potential of energy production from residues for the year 2050 is in the range of 10–32 EJ yr−1.

In Europe, the Biomass Futures project has identified a straw (barley, wheat, rye, oats and other cereals combined) potential of 127 Mt DM for the EU-27 in 2020 taking into account elements of competing uses of straw (i.e. non-energy use of straw) [5].

Fischer et al. [31], based on GIS approach, estimated the total of agricultural residues around 457 Mt DM in the EU-27 and Ukraine, of which 216 Mt DM could potentially be used as bioenergy feedstock. This study considered a linear relationship for RPR values relative to the crop yield, with higher values for low yielding cultivars.

Considering regional crop yields, harvested areas and species specific factors for residue to crop product ratios, Scarlat et al. [9] established the total crop residue potential for the EU-27 between 200 and 305 Mt DM per year (258 Mt DM per year on average). Based on a spatial explicit assessment at 1 km of spatial resolution, Monforti et al. [12] spatially allocated and updated the statistical results provided by Scarlat et al. [9], estimating the theoretical potential of crop residues production at 283 Mt DM in the EU-27.

The amount of crop residues that can be collected in the field depends on the equipment used for harvesting and the condition of the straw or stover. Several studies showed that up to 70%–75% of residues could be harvested by existing machines due to current equipment harvesting limitations [18,32,33].

Some studies have estimated crop residue production based on harvest indices to provide data on straw. The harvestable straw as a fraction of grain production was estimated by McCartney et al. at 0.72 for wheat, 0.54 for barley and oats and 0.65 for rye in a conventional (combine) system [18]. Hoskinson et al. [34] defined an alternative parameter, collected stover harvest index, to determine collectable stover produced by crops with specific grain yield. The harvest index values were found 0.60–0.66 for different cutting heights, and they provide data for computing the amount of stover (including cobs) that can be collected. This approach has limitations as it does not consider variation of the harvestable straw due to the local conditions.

Additional studies show that technical factors limit the possible harvest to about 60–80% of total straw produced that can be recovered [6,35]. In other works, the technical potential was estimated at about 50% of the theoretical potential [7,8]. Stumborg and Townley-Smith [36] suggested that 15 cm cutting height allowed for harvest of approximately 0.8 of total straw for several wheat cultivars. Zając et al. [37] evaluated the straw yield losses for different cutting heights and demonstrated a linear dependence for wheat, oats and barley, accounting for 22%, 29% and 31% respectively. For spring wheat cultivars, a fraction of 0.66–0.77 of straw was collected as baled straw [38]. Smeets et al. [19], using a harvest residue generation fraction, estimated the global technical potential between 49 and 69 EJ yr−1 for 2050. Kretschmer et al. [39] quotes unpublished studies by two German institutes that give a technical potential of straw in the range of 50 and 110 Mt D.M in the EU-27.

There is a controversial discussion about how much of the straw can be used as feedstock for energy conversion. Maintaining soil fertility is a primary factor in assessing the sustainability of crop residue removal from field. The fraction of sustainable crop residue extraction is difficult to quantify because it depends on cultivated crops, soil conditions (soil type, soil organic carbon, etc.), farming practices (crop rotation, fertiliser application) and climate (temperature, precipitations), which are all very location specific [8,40]. Establishing the amount of straw that needs to be left for maintaining soil quality requires modelling of the organic matter processes in soils, organic matter balance and long term experiments to assess the changes into soil organic matter over long time periods [15,41]. However, most studies used threshold levels of residue removal without taking into account specific local conditions.

The sustainable straw extraction rates were reported to range from 25 to 75% of the total crop residue production, while data on the available straw suggests a smaller range of 25–30% after harvesting and after considering competing uses. Fischer et al. [31] assumed that up to 50% of crop residues could be removed in the EU-27 and Ukraine without significant impacts on soil fertility or soil erosion. As a result, 241 Mt DM per year of residues should be left in the field for maintaining ecosystem functions. Scarlat et al. [9] considered the sustainable collection rate of 40% for wheat, rye, barley, while 50% for maize, rice, and sunflower, during the estimate of available crop residues in the EU27.

Using a GIS based model for the assessment of sustainable crop residue potentials, Haase et al. [8] analysed the amount and the spatial distribution (1 km × 1 km grid cells) of cereal straw, root crop and oil plant residues in five European regions. This study took into consideration organic carbon content in topsoil, soil erodibility, and protected areas. The maximum sustainable residue potential was estimated to vary between 45 - 59% and 24–48% of the theoretical potential in two scenarios (Basis and Restrict), respectively.

Based on statistical data at NUTS 3 level, Weiser et al. [7] estimated that 29.8 Tg (fresh matter) of cereal straw (without maize) are produced annually in Germany. The sustainable potential was estimated from 8.0 to 13.3 Tg straw (between 24% and 44% of the theoretical potential) following the standard humus-balance model VDLUFA (Verband Deutscher Landwirtschaftlicher Untersuchungs-und Forschungsanstalten - Association of German Agricultural Analytic and Research Institutes) in Germany in various scenarios: the “lower (VLV) and upper (VUV) values” and the Dynamic Humus Unit methods (DHU).

Monforti et al. [15] applied the agro-ecosystem CENTURY model to simulate the relation between the removal of crop residues and the potential depletion of Soil Organic Carbon (SOC) stocks. The authors conclude that 146 Mt D.M per year could be used for energy production without depleting the SOC levels in the EU-27 member states. This corresponds to a potential gross energy production of about 2.3 EJ/year.

Section snippets

Materials and methods

The potential of crop residues in the EU has been estimated in previous studies [9,10,12,15]. In this paper, it was performed a spatial explicit assessment of the potential agricultural residues availability in a wider European area at 1 km spatial resolution, based on an improved methodology that reports the different potentials (theoretical, technical, environmental and sustainable) of crop residues production. The energy potential of crop residues is calculated considering a Lower Heating

Theoretical crop residue potential

The theoretical potential of crop residues, considering average values for the period 2000–2015, was estimated at 291 Mt DM (5094 PJ) in the EU-28 and at 367 Mt DM (6436 PJ) in Europe (see Table 2). This reveals the richest countries in residues: France, Ukraine, and Germany, followed by Spain, Poland, UK, Romania and Italy.

The calculations showed significant annual variability of crop residue production, directly related to the annual variation of crop production in all European countries. The

Discussion and conclusions

This paper provides estimations of the available agricultural residues at European level, considering the main eight crops cultivated in Europe. The calculations were based on 15 years of statistical data, geospatial datasets on land cover and crop distribution, and a SOC stock dataset produced by the agro-ecosystem CENTURY model.

Detailed spatial distribution maps of crop residue potentials (theoretical, technical, environmental and sustainable) are provided at 1 km spatial resolution. A

Disclaimer

The views expressed are purely those of the authors and may not in any circumstances be regarded as stating an official position of the European Commission.

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