1 Introduction
Sustainable and resilient agriculture is critical to tackling climate change whilst delivering food security and reducing dependence on finite resources such as fossil fuels (FAO
2018). Within the European Union, the Common Agricultural Policy (CAP) is a major driving force that influences practice in the agricultural sector (Europe Commission
2018). Grain legumes are supported under CAP within ecological focus areas, agri-environmental schemes and greening requirements, and also promoted within organic farming (Behera et al.
2012). Despite being encouraged by these policies, Zander et al. (
2016) argue that legume system development is limited by other stronger market and policy incentives, such as the policies that boost oilseed rape designated to biofuel production (European Parliament
2009). Current European cropping systems rarely include legumes in their rotations. Only 1.5% of arable land is dedicated to cultivating legumes, compared with 14.5% worldwide (FAOStat
2016). This situation contributes to a deficit of 70% of high-protein crop commodities for animal feed in Europe, which is compensated by imports from North and South America (Watson et al.
2017). In addition to raising concerns over food security, large-scale import of protein to the EU (European Parliament
2018), especially soybean, is related to environmental concerns such as deforestation and associated habitat loss and greenhouse gas (GHG) emissions (Nemecek et al.
2008). In this context, one of the priorities for European policy is to reduce the dependence on imported protein (European Commission
2018b).
Legumes are an important source of protein for feed and food. These crops have the ability through symbiotic microbial associations to fix atmospheric nitrogen (N) which is eventually returned to the soil, leading to a reduction in N fertilization needs, not only for their own production but also for the following grain crop in the order of 60 kg of N ha
−1 annually (Preissel et al.
2015). These values can vary according to the soil and cultivar species, for example peas can provide a N credit of 40–49 kg N/ha for the following wheat crop (Plaza-Bonilla et al.
2017). Yields of subsequent cereal crops have been measured at 0.2 to a 1.6 t/ha greater following legumes, and agrochemical use 20–25% lower (Zander et al.
2016). Hence, incorporating legumes into typical cereal rotations across Europe could bring benefits in terms of reducing environmental burdens across multiple crops and derived products, with significant potential to reduce GHG emissions (especially from fertiliser production and use), acidification, terrestrial and aquatic ecotoxicity burdens, among others (Nemecek et al.
2008). However, a possible trade-off of legume cultivation is higher rates of nitrate leaching (Nemecek et al.
2008; Watson et al.
2017). Overall, agricultural experiments and life cycle assessment (LCA) studies suggest that increasing legume production in Europe could be an effective strategy to improve protein security whilst reducing environmental impacts (Nemecek et al.
2008; Karlsson et al.
2015; Stoate et al.
2015; Plaza-Bonilla et al.
2018).
From an economic perspective, legumes are typically regarded as inferior to cereals (Foyer et al.
2016). This perception is challenged by Preissel et al. (
2015) who studied 53 legume rotation models in Europe and concluded that 66% of them present competitive gross margins compared with non-legume systems. In addition, Zander et al. (
2016) highlight the importance of external effects of legumes which are usually not taken into economic consideration, such as the enhancement of biodiversity and improvement of soil quality and soil organic carbon specifically (Yao et al.
2017; Goglio et al.
2018b).
LCA consists of analysing the environmental aspects of a product or service over the entire value chain of production, use and end-of-life, considering upstream and downstream processes (ISO 14040
2006). According to Klöpffer (
2003), ‘Life cycle thinking is the prerequisite of any sound sustainability assessment’. The author cautions that modifying a specific production step based on information for only one impact category can bring about negative consequences for other impact categories and other steps of the system. When applied to agriculture, many LCA studies draw boundaries or focus around a single crop or its (co-)product(s) (Bevilacqua et al.
2014; Hedayati et al.
2019). Thus, since the focus of these studies is on one cropping cycle, important interactions across crops and over years within crop rotations may be neglected. Recently, numerous authors have emphasised the importance of analysing whole cropping systems rather than individual crops in those systems (Brankatschk and Finkbeiner
2015; Brankatschk
2018; Peter et al.
2017). Therefore, new LCA methods, calculators and approaches are being proposed to evaluate the environmental impacts of changes to agricultural systems (Brankatschk and Finkbeiner
2015; Stoate et al.
2015; Reckling et al.
2016; Brankatschk
2018; Peter et al.
2017; Carof and Godinot
2018; Goglio et al.
2018b).
Representation of legume rotations are just one example of cropping system challenges in LCA studies. Brankatschk and Finkbeiner (
2017) simulate production of wheat bread, cow milk, rapeseed biodiesel and straw for bioethanol by modelling them as discreet annual cultivations or as crop rotations (through attributional LCA), where straw is treated as either a residue or a co-product of the system. Treating straw as a co-product within rotation LCA, the carbon footprints of bread, milk and rapeseed can be 11%, 22% and 16%, lower, respectively, compared with a simple LCA of an annual cultivation cycle, whilst the footprint of bioethanol can be up to 80% higher.
This review aims to understand how LCA has been applied to assess legume rotations (rather than legume crops in isolation). More specifically, it investigates how various inter-crop rotation effects are taken into account and the main barriers representing these effects accurately in LCA. To do this, we ask the following questions:
(i)
Which functional units are appropriate for legume rotation systems?
(ii)
Where are the optimal system boundaries delineated through space and time (e.g. a single cropping cycle or a crop rotation)?
(iii)
How are carbon, nitrogen and wider nutrient cycling effects best represented?
(iv)
How and when should allocation be applied?
(v)
Which impact categories are most relevant?
2 Method
A review was conducted to assess how legume cropping systems are represented in LCA. The literature review was completed in June 2019, based on evaluation of publications from peer-reviewed journals. The search engines used were ScienceDirect and Web of Science. LCA studies for legume rotations and intercropping were assessed by searching the following code: (‘life cycle assessment’ OR ‘carbon footprint’ OR ‘environmental impact’ OR ‘environmental footprint’) AND (‘legume’ OR ‘pulse’ OR ‘leguminous’ OR ‘peas’ OR ‘chickpeas’ OR ‘beans’ OR ‘lentils’ OR ‘lupin’ OR ‘vetch’ OR ‘alfalfa’ OR ‘clover’) AND (‘Rotation’ OR ‘integration’ OR ‘intercropping’ OR ‘cropping system’ OR ‘farming system’). Next, studies were selected where they matched the theme of LCA for legumes within rotation or intercropping systems by screening for compliance with all of the following requirements: (i) reporting results based on LCA or life cycle inventory methodology; (ii) inclusion of (a) legume(s); (iii) the legume(s) is/are analysed within the context of a wider cropping system (i.e. rotation or intercropping). There was no time restriction, since the number of older articles regarding this subject is limited compared with other themes. Two years was the minimum rotation length considered. Soybean was the only legume crop not included, unless it occurred with other legume varieties in the rotation. This decision was taken as soybean is often grown in industrialised mono-cultures or in very short rotations in major producing countries such as the USA and Brazil (WWF
2014). These systems involve fewer crop interactions and are mostly outside of Europe. Leguminous tree species were also outside the scope. European rotations were the main focus of this study, although Canadian and Australian rotations were also considered, as these countries have a high share of their arable land dedicated to legume cultivation (FAOStat
2016).
The articles obtained were analysed according to their main LCA structure. The first step was to understand the goals of each study and how they were translated into a functional unit. We categorised the functional units according to how many functional variables analysed per study. We further investigated whether these variables were based on independent criteria (e.g. kilogram of product, energetic potential), or combined in a dependent metric where the total amount of product is corrected by a product characteristics such as the fat and protein correct milk value FPCM (European Commission
2018b).
As a second step, the system boundaries were classified according to the main activities included, and excluded, in the LCA studies. The classification varied from simple to more complex approaches. We investigated which phases of the life cycle were included in each study. For example, from cradle to farm gate (until the harvesting of the grain) where activities associated with extraction and manufacturing of the majority of inputs were considered alongside their use on the farm, but nothing more. The other classification varied according to the activities included after the farm gate (downstream processes), such as transport and storage of grains before processing; the industrial phase (up to industry gate); or distribution and retail of the products. Another classification was added when avoided processes or consequential scenarios were considered, involving the expansion of boundaries to include, e.g. the avoidance of the use of a specific fossil fuel in favour of biodiesel.
We further analysed if the authors explicitly considered any soil organic carbon changes (SOC) or N fixing, whether by demonstrating the specific amount of N fixed or by considering any reduction of fertiliser use on the following crop. The penultimate step entailed the study of the allocation methods used in the following instances: (i) between the final products and considered co-products; (ii) allocation of specific upstream processes, such as production of farm machinery; and most importantly, (iii) the allocation of the nutrient flow between legumes and following crops. A final step involved the analysis of the impact categories presented in the studies, including a broad definition to capture critical inventory results, such as land use in square meters per year.
4 Discussion
A crop rotation is multifunctional in that it produces a range of products for different purposes, such as animal feed, food for direct human consumption, energy or fibre. Introducing more legumes into European rotations has been proposed to improve the sustainability of European food and feed production (Watson et al.
2017). However, changes to rotation sequences, nutrient cycling and yields of crops within rotations mean that simple attributional LCA of the individual legume crops introduced into rotations does not adequately represent consequences for the environmental efficiency of rotations and related food systems, nor of individual crops within modified rotations. The solutions to better representation of rotation-level effects of legume integration within LCA lie in either (i) attributing an environmental footprint to each product in the rotation, taking into account their interaction with the preceding and following crops; or (ii) defining a rotation level FU that can meaningfully represent multiple products (and services) delivered by rotations.
Of the four types of legume rotation LCA studies we categorised, type I is the most prevalent in the wider literature, but many such studies were filtered out of this particular review which focuses on rotations. Type I attributional LCA studies of discreet cultivation systems underpin widely used large-scale datasets (Blonk Consultants
2018; Moreno-Ruiz et al.
2018), and usually present footprints per kg of a crop (product) excluding rotation interactions. Type I LCA studies often ignore crop sequence interactions and draw the boundaries around a single cropping year—neglecting N fertiliser substitution benefits associated with legume residue N carryover, or representing this fertilisation credit in a reduced footprint for following (non-legume) crop(s). Meanwhile, leaching burdens are often attributed to the legumes. Thus, eutrophication and global warming burdens may be over-allocated to legumes, and under-allocated to following (cereal) crops (Cai et al.
2018). Type II studies involve assessment of whole rotation systems, often with a simple aggregating FU, often based on area over time. These were the most prevalent type of study reviewed here, but their interpretation has little significance from a production efficiency perspective—results may be used to draw conclusions about land management rather than the environmental efficiency of food production (Schau and Fet
2008). Thus, the most widely applied types of crop rotation LCA have important deficiencies that constrain their usefulness in informing more sustainable food production.
The amount of N carryover is strongly influenced by the incorporation of legumes in rotations (Kayser et al.
2010), whilst soil carbon content is influenced by specific management practices (Vestberg et al.
2002). These factors can significantly influence the environmental footprint of crops and derived products, but the type of allocation method employed determines the extent to which legumes are credited with fertiliser avoidance credits or leaching and N
2O emission burdens (Naudin et al.
2014). Representing these factors is important to draw out potential effects of legumes in order to accurately inform stakeholders (Kayser et al.
2010), but typically requires field-scale modelling. Procedures to avoid allocation were encountered in this review. Peter et al. (
2017) demonstrate the sensitivity of crop footprints to allocation methods through a legume cover crop case study. If alfalfa is not harvested but used as a green manure, the following crop can have a 7–8% higher carbon footprint, and an 11–13% higher cumulative energy demand, but if the environmental impact is attributed to the harvested alfalfa crop, the legume crop (alfalfa in this example) has a 99% larger carbon footprint, whilst the following crop has a 1% smaller footprint. In the first situation, alfalfa was first considered as a green manure, in other words, an input (nutrient provider) for the following crop (product), so its impact will count towards the succeeding crop footprint. In the second situation, alfalfa is considered as an individual crop, which contributes towards delivering the functional unit chosen by the practitioner (dry matter, energy, etc.), and therefore a product that has impacts associated with it. This approach is valid, but misses the potential multifunctionality of alfalfa in providing N fertilisation to the next crop (and the fact that a significant share of the leaching burden of alfalfa is biophysically related to this additional function). Other studies employed sub-process division to avoid allocation, considering rotations as a composition of annual crop cultivations (Nemecek et al.
2011; Prechsl et al.
2017; Goglio et al.
2018b).
The CU (Brankatschk and Finkbeiner
2014), based on the digestible energy content of animal feed commodities, is a useful metric to aggregate multiple products from crop rotations. It does not affect the system boundaries and brings robustness to the LCA (Brankatschk and Finkbeiner
2015). However, the method is constrained to rotations primarily producing animal feeds, and to only one dimension of animal nutrition, and could reinforce the lock-in of European rotations to cereal dominance promoted by public polices, market demand and specialization based on agrochemical paradigms (Magrini et al.
2016). Therefore, using the CU might not be appropriate for studies focussing on the production of crops for direct human consumption, or indeed, for other uses, including protein-rich animal feeds of which there is a deficit within Europe (Watson et al.
2017). Other kinds of physical or biophysical allocations for rotation systems have been proposed. Martínez-Blanco et al. (
2014) recommend N release as a parameter to allocate compost effects across crops, though this requires reliable estimation of mineralisation rates. Alternatively, the authors recommend allocation based on N (or phosphorus/potassium) uptake by the plant (Martínez-Blanco et al.
2014). Knudsen et al. (
2014b) also discuss different allocation methods for green manures and other catch crops. They suggest allocating based on N residual effect (as used by Tuomisto et al. (
2012)) or by area. At present, no consensus for allocation methods in rotation systems has been achieved, which can lead to highly variable results and interpretation (Goglio et al.
2012; Martínez-Blanco et al.
2014; Brankatschk and Finkbeiner
2015). Sensitivity analysis is rarely applied in LCA studies. Given the variances outlined above, we propose that attributional LCA studies on legume cropping systems should apply sensitivity analyses at least to allocation methods.
Defining a FU for multifunctional cropping systems is challenging, since several products with different fates arise from these systems. No consensus definition of FU for legume rotations or intercropping systems was found from the types investigated in this review. However, awareness of the complexity of representing crops within crop rotations in LCA is increasing. Numerous authors have already applied multiple FU in order to understand systems from the perspective of an entire rotation (Nemecek et al.
2011; MacWilliam et al.
2014; Yang et al.
2014; Prechsl et al.
2017), especially in type III studies (Röös et al.
2016). Recent studies have proposed FUs that address the delivery of different functions (type III). For example, a multi-product approach has been proposed by Röös et al. (
2016) and Costa et al. (
2018). Costa et al. (
2018) propose a FU based on a population demand for five food and energy products over a period of 7 years. This approach enables agricultural systems and rotations producing a range of different products to be compared in terms of their delivery of a proportion of overall human consumption. Allocation is fully avoided whilst the study captures important interactions across the years and elements (crops, trees and livestock) of rotations. The difference between the multi-variable and land use approaches is that the multi-variable FU allows a comparison of a mix of products versus their independent production. In other words, this is a way of measuring the efficiency of integrating the products into a cropping system compared with producing them by their traditional mode, such as mono-cropping. As with the area-based FU, the disadvantage of such multifunctional FUs is that they do not provide a single product environmental footprint as required for labelling and evaluation of diet choice among other goals.
Final consumption and human nutrition FUs are often used to compare diet choices (Willett et al.
2019). However, due to the amount and complexity of data, most diet studies use international datasets rather than undertaking farm LCAs. To compare the nutritional footprints of alternative diets, potentially hundreds of footprints of food products are needed (Willett et al.
2019). To counter this situation, FUs that only cover one nutritional aspect are becoming common, such as protein content (MacWilliam et al.
2014; Karlsson et al.
2015). These FUs are not representative of other key nutrients. Furthermore, protein quality varies considerably depending on the source, with different amino acid compositions affecting human (and animal) nutrition (Sonesson et al.
2017; Leinonen et al.
2019). In developed countries, protein quality is less pertinent considering that the population largely over consumes protein, and net protein utilisation from various sources is similar for the adult population (WHO
2007). Notarnicola et al. (
2017) recommend a careful analysis of nutritional values comprising not only fat, protein and energy but also other relevant nutrients. Van Dooren (
2017) proposes a nutrition density unit as a FU, considering more than one nutritional aspect. However, Notarnicola et al. (
2017) highlighted the limitation of such a FU when considering products that are consumed for a social aim, such as wine, beer, and coffee. Establishing human nutrition as a FU can bring additional limitations, especially when applying a cradle-to-gate boundary. First, it can be difficult to define nutritional composition for each product at the farm gate, in terms of specific elements (proteins, fatty acids, carbohydrates, vitamins, etc.) owing to the influence of soil type, climate and management (e.g. level of fertilisation) on concentrations of these elements. Additionally, nutritional FUs are usually intended for application to prepared foods ready to eat, following processing and cooking. In farm-level LCAs (most common approach for types I, II and III), nutritional aspects are difficult to define because the grains cultivated on the farm have different and sometimes unknown fates. The grains can supply different value chains across the food and feed industries requiring different levels of processing and therefore exhibiting different final nutritional values (FAO
2016). For example, cultivated chickpeas can be processed into flour, pasta, hummus, canned grains or just dried grains to be soaked and consumed. Therefore, assuming a nutritional value for chickpeas at farm level could be misrepresentative. Second, the FU could limit the boundaries of diverse agriculture systems, where co-products intended for energy or textile uses would need to be allocated off. Therefore, the best solution identified in this review is by Goglio et al. (
2018a). Recognising the aforementioned limitations, they suggest a dual approach for crop rotations, simultaneously providing results for the rotation as a whole and for each product in the cropping system.
Assessing multiple impact categories can be also complicated in regional studies with wider boundaries, such as those integrating regional or international consequential analyses. In a consequential analysis, used by Knudsen et al. (
2014a), the overall impact of producing more grain legumes in Europe was revealed to have a small climate benefit compared with importing soybeans. However, their study did not address nutrient carryover or other consequences at farm level, and only assessed GWP. One of the key potential advantages of introducing more legume cropping in Europe is the delivery of ecosystem services promoted by grain legumes (Karlsson et al.
2015). The choice of impact categories varies among the studies, and global warming potential is by far the most adopted impact category across all studies, which neglects potentially important co-benefits and trade-offs. For example, Costa et al. (
2018) showed that complex crop-animal-tree rotation systems had a lower global warming potential but very high abiotic depletion (due to more use of animal feed compounds) compared with conventional (separate) systems. Regarding the calculators and tools designed to address LCA cropping system interactions, the Crop.LCA tool (Goglio et al.
2018b) is the only one that provides acidification potential, eutrophication and energy demand alongside global warming potential.
Following international guidelines such as ILCD (EC- JRC -IES
2011), or the more recent Product Environmental Footprint (European Commission
2018b), could be challenging for entire crop rotations owing to high data requirements. Impact categories and methods that assess soil quality, structure and biodiversity are not commonly reported in LCA (Gabel et al.
2016; Teixeira et al.
2016). Soil is often analysed at inventory level, e.g. accounting for the amount of land in the life cycle rather than a factor representing quality of land in terms of, e.g. SOC (Milà i Canals et al.
2007) or biodiversity (Koellner and Scholz
2007).
Product substitution and inclusion of consequential scenarios are found in studies performing product system expansion (type III). A common practice of product substitution is when organic fertilisation, including via legume residue incorporation, leads to credits from avoided synthetic
fertilizer use (Nemecek et al.
2011; Brockmann et al.
2018). However, the inclusion of multiple avoided products and consequential analyses could be questionable due to the lack of standardisation and multiple speculative possibilities that can be evaluated (Mackenzie et al.
2017). Despite these limitations, the consequential approach has value in its ability to capture important indirect and intersystem effects (Ekvall and Weidema
2004). This is pertinent when the goal of LCA studies is to evaluate the consequences of introducing more legumes in to European rotations.
Consequential LCAs (type IV) are rarely applied to analyse legumes. However, the approach is pertinent when the goal is to understand cropping system changes at a regional scale. Compared with attributional LCA, consequential LCA could avoid the need for allocation through application of expanded system boundaries, whilst also capturing important potential (indirect) displacement effects in other supply chains. The lower yields of legume crops compared with cereals could mean that (cereal) production is displaced elsewhere, causing indirect land use change and international ‘leakage’ of environmental impacts (Styles et al.
2017). Meanwhile, legumes have an important role to play in diet change (providing quality plant protein to replace animal protein) and, as discussed, can enhance yields of subsequent crops. Therefore, legume deployment could also indirectly lead to carbon sequestration via, e.g. afforestation on spared land (Lamb et al.
2016). The balance of the aforementioned effects requires holistic evaluation of legume rotations and downstream (avoided) value chains. Consequential LCA has an important role to play here.