Environmental accounting of closed-loop maize production scenarios: Manure as fertilizer and inclusion of catch crops

Highlights

• The study presents a comparative LCA of conventional and alternative “closed-loop” maize production scenarios.

• Climate change and resource depletion potential can be offset using digestate fertilizer from a manure-based biogas plant.

• Best available fertilizer application practices can help reduce high air acidification potential associated with digestate.

• Catch crops can reduce freshwater eutrophication potential, but increase climate change potential due to machinery use.

Abstract

The agri-food sector has moved towards a more linear production economy, partly caused by worldwide food demand. One clear example is the intensification of livestock production, with consequent manure-management and feed-production challenges, the effects of which have led to large environmental problems. Currently, efforts are being made to move the agricultural sector towards closed-loop alternatives. To ensure high environmental performance of these alternatives, realistic quantification of environmental impacts is needed. Thus, using Life Cycle Assessment (LCA) tools, we analyzed the environmental profile of six closed-loop maize scenarios focusing on different combinations of mineral fertilizer, digested organic fertilizer (digestate) from a manure co-digestion biogas plant, and rotation with (or without) catch crops (CCs) as a strategy to prevent nitrate leaching to groundwater and as a co-substrate in the biogas plant.

Results demonstrated that replacing a large portion of the mineral fertilizers with digestate could help offset much of the total potential impact of global warming (by 25–35 %), resource depletion (by 94–96%), photochemical ozone formation (by 17–22 %), ozone depletion (by 96–99%) or even avoid it entirely as in freshwater eutrophication. However, digestate production and application contributed greatly to acidification (51%) and particulate matter (51–52%) categories, with minor differences depending on the species of CC used. An optimal combination of both digestate and mineral fertilizers is recommended. The incorporation of CCs in a maize rotation can reduce freshwater eutrophication impacts but increase global warming potential. Conclusions were drawn suggesting better management strategies to decrease environmental impacts of maize production.

Introduction

In Europe and other developed countries, animal production is intensifying due to growing livestock populations, shrinking farm numbers, and consequently higher livestock density. For example, Spain’s pig population increased by 14% from 2006 to 2017, while its total number of pig farms declined by 55% from 2005 to 2013 (Eurostat, 2018), concentrating most of the population in seven Spanish provinces – Lleida, Huesca, Zaragoza, Barcelona, Murcia, Segovia and Badajoz. Lleida and Barcelona are located within the larger region of Catalonia, which holds 25% of the total pig population (MAPA, 2016). In 2016, livestock production in Catalonia represented approximately 62% of Catalonia’s total agricultural income, of which pigs, poultry, cows and crops represented 37%, 10%, 6%, and 34%, respectively, making livestock the main agronomic activity in the region (MAPA, 2016). Areas with high livestock density produce a large amount of excess livestock manure with high nutrient contents, due not only to farm intensification but also the lack of nearby agricultural land on which to apply it.

Improper management and lack of technological resources to treat manure have led to several environmental problems, including (1) excess nutrients and pathogens in the soil when manure is over-applied or illegally dumped onto cropland as fertilizer (Gagliardi and Karns, 2000); (2) elevated nitrate (NO₃⁻) concentrations in local drinking water supplies (ACA, 2016); (3) eutrophication of water bodies that has led to the death of fauna (Camargo and Alonso, 2006); (4) ammonia (NH₃), particulate and odor emissions; (5) emission of greenhouse gases (GHG) such as methane (CH₄) from storage ponds; (6) accumulation of phosphorus (P) and heavy metals (copper, zinc) in soils and (7) leaching of micro-pollutants such as antibiotics from manure-based fertilizers (Thorsten et al., 2003).

These problems represent a clear example of a linear production economy within the agri-food sector. To address the problem of linearity, this study puts into practice the European Commission’s circular economy action plan (EC, 2015) by valorizing slurry and crop residues through waste-to-product synergy between livestock farms and crop fields, and complies with the Nitrates Directive (EC, 1991) by using catch crops (CCs) to absorb nitrates. This synergy could also help build a collaborative community of farmers and industry professionals and add value to the main crop. In this study, the slurry was valorized by recovering its nutrients in the form of fertilizers and biogas through anaerobic digestion. The feasibility of biogas as an energy source is marked by its manageability, storability, its equivalence to natural gas (if purified to biomethane) and the ability to continuously operate the plant producing it.

To ensure high environmental performance of alternatives, realistic quantification of environmental impacts is needed. Thus, this study performed a life cycle assessment (LCA) with the aim to (i) identify the hotspots in circular maize production using its associated environmental impacts, (ii) compare the environmental performance of different closed-loop maize scenarios to conventional ones, and (iii) suggest improvements that may decrease the environmental impacts. Conventional scenarios were considered as those that use only mineral fertilizers as opposed to digested organic, manure-based fertilizers (digestates). Among other methods (Bockstaller et al., 2009; Lebacq et al., 2013), LCA methodology was adopted for this assessment since it is the most comprehensive approach that uses multi-criteria analysis and a perspective of the entire value chain. It is also a standardized approach that follows ISO standards (ISO-14040, 2006). To our knowledge, this is the first LCA that has analyzed the implementation of CC rotation and manure treatment in crop production compared to conventional scenarios, across multiple impact categories. Like the present study, Bacenetti et al. (2016) have compared similar fertilization strategies for maize, but they excluded: CC rotations, NH₃ emissions from production and storage of digestate in a biogas plant, electricity and fertilizer credit, and biogas plant infrastructure. They also used background datasets for mineral fertilizer production instead of regionalized datasets, which is used in the present study. Similarly, an LCA of wheat with integrated grass/clover rotation and digestate application was performed (Tidåker et al., 2014), but only analyzed digestate fertilizers and included only a few impact categories.

Maize was chosen as the main crop for this LCA study due to its widespread cultivation in Spain and importance as livestock fodder; maize covers 10% of irrigated land in Spain and contributes 6.5% of total maize production in the European Union (EU) (Eurostat, 2016). This study analyzed six different maize production scenarios with integrated CC rotation and different fertilization systems as a strategy for reducing N leaching after fertilizing the main crop, collecting environmental data on emissions, addressing manure management challenges and ultimately closing the energy and nutrient loops in the feed-livestock system. Following the LCA perspective, this study analyzed not only N-derived environmental impacts but also the other midpoint impact categories recommended by the International Reference Life Cycle Data System (ILCD) (EC-JRC, 2010).