Full length articleAssessing the roles of crops and livestock in nutrient circularity and use efficiency in the agri-food-waste system: A set of indicators applied to an isolated tropical island
Introduction
Agri-food systems (AFS) provide multiple services to human societies: human food (and more generally food security), animal feed, bio-energy (bio-fuel, dung), animal traction for transport, leisure, cultural activities, ornamental plants, construction materials and clothing (Willemen et al., 2010; Huang et al., 2015; FAO, 2021). However, AFS can also have negative environmental impacts resulting from the alteration of nutrient cycles. Inputs to agri-food systems require the extraction of limited resources, particularly phosphorus (P) from mines (Liu et al., 2010) and non-renewable fossil energy (Service, 2014). The use of non-renewable fossil energy also results in greenhouse gas (GHG) emissions (CO2), which contribute to climate change (Crippa et al., 2021). AFS are also responsible for large releases of reactive nitrogen (N) into the atmosphere, leading to N cascade effects (Gruber and Galloway, 2008) (i) increasing GHG emissions (N2O) and (ii) increasing atmospheric deposition (NOx and NH3) thereby affecting the productivity, functioning and composition of natural and cultivated ecosystems, the eutrophication of terrestrial and aquatic systems (NO3-), and global acidification. Nutrients that leave the AFS through runoff and leaching may also contribute to eutrophication (Carpenter et al., 1998). Today, the interference of human activities in N and P cycles is considered to have gone beyond planetary boundaries, i.e. beyond the safe operating space for human society to maintain the resilience of the Earth's system (Rockström et al., 2009; Steffen et al., 2015).
Reducing the negative impacts of AFS on these nutrient cycles requires reducing the nutrient use efficiency gap of the system itself, i.e. the gap between the current and achievable system nutrient use efficiency (Cui et al., 2014). It is with this goal in mind that van der Wiel et al. (2020) extended the limits of AFS to all nutrient managing activities and coined the “agro-food-waste” system, the latter being of composed of five interconnected sub-systems: crop (food) production, animal production, food and feed processing, consumption, and waste management. Based on the same reflexion, here we refer to an “agri-food-waste” system (AFWS) which combines (i) the concepts of an agri-food system (FAO, 2021) (including both food and non-food agricultural production), (ii) current connected activities in terms of material flows containing nutrients (e.g. waste management, energy production) and (iii) potential other connectable waste management activities.
By focusing on the local environmental impacts of an AFWS, without seeking to change local economic activities or human dietary habits (i.e. structural changes), the nutrient use efficiency gap of the system could be reduced by two means (i) increasing circularity between sub-systems and (ii) increasing the internal (i.e. processes) efficiency of sub-systems. These two concepts of circularity and process efficiency has not yet been considered as two separate parts of a systemic AFWS efficiency approach but are often used separately to assess or evaluate AF(W)Ss and/or their sub-systems (Zhang et al., 2015; van der Wiel et al., 2020).
In particular, circularity reflects how nutrient cycles are closed (i.e. flows circulating among the sub-systems rather than leaving the system). The immediate action that can be taken to further close the cycles, without structural changes, is recycling unused secondary products (wastes and by-products) between different sub-systems (e.g. rather than putting them in landfills or discharging them into the environment). Process efficiency reflects how the different sub-systems limit the sources of inefficiency related to their processes themselves (e.g. losses into the atmosphere). Actions that can be taken without structural changes are (i) adjusting inputs to real needs (e.g. for livestock production: adjusting feed intake to needs) and (ii) reducing losses into the atmosphere and to the sub-soil and surface water (e.g. for crop production: burying fertiliser, in-soil incorporation of manure during spreading, sustainable soil quality management). It is important to distinguish between these two aspects (circularity and process efficiency) when assessing the roles of sub-systems in the efficiency of an AFWS as they represent two means to improve its efficiency, and thus enable a systemic multi-level understanding. The sub-systems we chose to focus on are crops and livestock as they play critical roles in the environmental impact of AF(W)S (Thévenot et al., 2013; Lassaletta et al., 2014; Zanten et al., 2018; Crippa et al., 2021). We chose to assess crops and livestock separately as they are two specific types of biophysical processes. Crop production depends on plant growth processes that differ from those involved in animal production, which depend on animal demographic and growth processes.
Among AFWS, crop and livestock production can play a negative role as they are known for their limited process efficiency (Vayssières and Rufino, 2012) and are sometimes responsible for unused secondary products (Hasler et al., 2015; FAO, 2018; Walling and Vaneeckhaute, 2020). Crop production causes N losses into the atmosphere when fertiliser is spread. Croplands are also the scene of many losses to the sub-soil and to surface water due to nutrient leaching and runoff. Losses could be due to over-fertilization, but could also depend on the quality and structure of the soil. Livestock production leads to losses into the atmosphere from both manure storage and management. Livestock manure is also sometimes misused, i.e. is spread in areas where plant needs are already covered, simply to get rid of the manure. Some manure management also consists of N denitrification thereby intentionally breaking the N cycle. Livestock feeding is also often not adjusted to the animals’ real needs, resulting in more nutrients in the manure, thereby increasing the openness of the nutrient cycles.
Crop and livestock production can also play a positive role in AFWS through process efficiency and circularity. Crops are the preferred target for organic wastes, as recycling to other activities mostly requires higher levels of technology (Harder et al., 2019). Livestock enables the recycling of secondary products that would otherwise remain unused, especially crop residues, weeds and spontaneous fodder (Oosting et al., 2021; Van Zanten et al., 2019). Livestock manure can supply organic matter for crop production, with beneficial effects on soil fertility (Leinweber et al., 1999). In many places, these practices contribute to crop-livestock integration, i.e. a combination of farming practices that favours circularity and efficiency within AFWS (Herrero et al., 2010; Stark et al., 2016).
The specific isolated roles of crop and livestock production in the nutrient use efficiency of AFWS have thus already been identified. However, their systemic role has not yet been characterised (i) in relation to all other sub-systems that comprise the AFWS and (ii) by distinguishing circularity and process efficiency as two separate parts of the nutrient use efficiency of the AFWS.
The purpose of this paper is to propose a method with quantitative indicators to characterize the role of crops and livestock in the nutrient use efficiency of an AWFS, in terms of nutrient process efficiency and circularity between sub-systems. The proposed method is illustrated in an isolated insular context, tropical Reunion Island, and with nitrogen metabolism.
The Reunion Island AFWS is a case study of interest because, on one hand, the AFWS is based on intensive production systems and large imports of inputs, while on the other hand, the proximity of economic activities facilitates the recycling of secondary products. Also, as Reunion Island is a well delimited region, existing databases already contain most of the necessary flow data. We chose to focus on nitrogen metabolism to illustrate one application of the method because it is intended to be applicable to all nutrients and nitrogen included losses into the atmosphere whereas phosphorus and potassium do not. Furthermore, nitrogen is a key nutrient for life and an important limiting factor in both crop and livestock production.
We assessed the current role of crop and livestock production in nutrient use efficiency in the AFWS by re-examining the use efficiency, process efficiency, and circularity concepts. These concepts needed legitimate clarification for our method. For example circularity is sometimes used as an objective (van der Wiel et al., 2020) and sometimes a means (Tseng et al., 2019). Nutrient use efficiency sometimes only refers to process efficiency (Ma et al., 2010) but is sometimes defined as a circularity indicator (Papangelou and Mathijs, 2021). The set of indicators proposed in this paper is not specific to the crop and livestock production sub-systems, it is meant to be used to assess the role of any AFWS sub-system.
To illustrate how the method can also be used to assess the potential role of crops and livestock, i.e. to characterize the changes in the indicators depending on improvement actions, we chose to use an improvement scenario. The most relevant scenario for Reunion Island was the recycling of unused secondary products (i.e. the circularity part) as it represents ongoing multi-stakeholder dynamics in Reunion Island (Vigne et al., 2021). The scenario also offers an opportunity to identify the links between recycling actions and both circularity indicators and AFWS efficiency indicators.
In this paper, we do not consider structural changes, rather, as the first step in our reflection, we consider the actions needed to increase circularity and process efficiency in a defined economic structure. These actions (see Fig. 1) are logistic, organisational and technical, and should result in fewer sources of inefficiency and less imports. Implementing these actions in the short term is more realistic than implementing structural changes (e.g. changes in land use, changes in herd size). These actions apply in particular in the context of isolated islands, which raises particular questions due to the economic and environmental costs of long-distance imports. Disregarding structural changes particularly implies not accounting for a change in atmospheric inputs and local production, and in exports of primary products (types and/or quantities). Disregarding structural changes drove our choice of the groups of indicators (Fig. 1), in particular to define system efficiency as an objective. Importing more food instead of producing locally, or exporting more, would for example ‘artificially’ increase the system efficiency, but on the other hand, would externalize environmental impacts. Disregarding structural changes also drove our choice of the system, as considering structural changes could expand potential connectable activities beyond waste management.
Section snippets
Method
The method is based on (1) nutrient flow analysis of the AFWS, (2) selection and calculation of indicators using a detailed typology of flows, and (3) analysis of an improvement scenario based on recycling.
The AFWS in Reunion Island
This section provides an overview of the nitrogen use efficiency at the level of the AFWS. A distinction is made between its two parts: process efficiency and circularity.
Their roles in Reunion Island
The recycling receiver role of crops in Reunion Island does not seem surprising as 63% of the secondary products used by activities other than waste management are or originate from manure and sewage plant sludge, which are directly (or indirectly after energy production) only recyclable on the land. The context in Reunion Island appears to be special, with a high proportion of nitrogen from secondary products among N inputs to crops (i.e. 58%, 38% for manure alone), whereas at the scale of the
Conclusion
Considering (i) system efficiency, (ii) process efficiency and (iii) circularity indicators together made it possible to characterize the roles played by crops and livestock in the nutrient use efficiency of an agri-food-waste system (AFWS). The indicators are complementary and this paper suggests that they should be used together to provide the comprehensive understanding required for sound policy guidance. The system efficiency group of indicators allowed us to assess the objective at
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was co-funded by the French Environment and Energy Management Agency (ADEME).
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