Does shifting to plant-based diets really reduce emissions?

Livestock reduction is generally considered an effective strategy for lowering anthropogenic greenhouse gas (GHG) emissions. For this reason, reducing animal protein consumption is often advised to mitigate GHGs, particularly methane emissions from livestock enteric fermentation. Previous studies have mainly focused on livestock-related emissions and food production, overlooking the implications for soil emissions. Consequently, the impact of dietary changes on the GHG balance of agricultural soils has not been thoroughly evaluated.

This gap has been addressed in a recent study, which reveals that shifting towards more plant-based diets may not automatically result in net reductions in agricultural emissions. On the contrary, reducing livestock production could have unintended consequences, such as negatively affecting soil organic carbon (SOC) storage and potentially increasing nitrous oxide (N₂O) emissions.

Denmark as a case study

This study, published in Agricultural Systems, Elsevier’s journal, by the Joint Research Centre (JRC), examines the case of Denmark, which reduced its cattle population to lower overall livestock emissions. The study evaluates the impact of shifting to healthier diets, such as the one recommended by the EAT-Lancet Commission, on the greenhouse gas balance of agricultural soils, using Denmark as a case study. The EAT-Lancet diet encourages a higher consumption of vegetables, fruits, whole grains, legumes, and nuts while advocating for moderate fish and poultry consumption and a significant reduction in red meat and processed foods.

Surprisingly, the study revealed that reducing the number of cattle, which is in line with the nutritional guidelines of the EAT-Lancet diet, could have significant negative impacts on greenhouse gas emissions. The researchers conducted a comprehensive assessment of the EAT-Lancet diet’s effect on the net GHG balance of agricultural soils at the national level, using an advanced, process-based modelling framework with a dual approach.

First, they applied the economic general equilibrium model MAGNET to quantify the demand-driven changes in food consumption and land use across the European Union following the adoption of the EAT-Lancet diet. Next, they used the DayCent process-based biogeochemical model to evaluate how these changes would impact the GHG balance of agricultural soils in Denmark.

Less livestock, more emissions

The findings indicate that, compared to business-as-usual diets, full adoption of the EAT-Lancet reference diet would lead to significant carbon losses, up to 480 Gg CO2e per year, and a 2.1% increase in N₂O emissions, amounting to 50 Gg CO2e per year, from 2030 to 2100 in agricultural soils. The main cause of this outcome is the reduced availability of animal manure as a natural fertilizer, which decreases the production of organic material that could otherwise be partially stored in the soil. The shift from permanent grasslands to cultivated land also contributed to the emissions.

The increased application of mineral fertilizers, required to compensate for the reduced use of manure and the conversion of grasslands to croplands, negatively affected soil organic carbon (SOC) storage and amplified nitrous oxide emissions, a potent GHG, especially during the initial years of transition. SOC losses were particularly significant in sandy soils, which have a lower capacity for carbon storage.

As a result, the net GHG balance of agricultural soils worsened, showing higher emissions compared to the business-as-usual scenario. The negative contribution of soils to emissions was estimated to be an 18% increase in net GHG emissions under the full adoption scenario.

The importance of fully accounting for soil GHG emissions

These findings highlight the importance of fully accounting for soil GHG emissions, as they offset some benefits of adopting a plant-based diet. While reducing the number of animals can be a limited strategy to lowering overall livestock emissions, these contrasting secondary effects could partially alter the overall picture and should be incorporated into existing models.

Reducing animal-based product consumption has unintended negative effects on soils and may lead to increased GHG emissions from agricultural soils. These effects vary depending on soil and climate conditions and should be carefully considered in emission reduction strategies.

Policies to reduce consumption of animal products in some countries must consider potential demand increases in other countries to meet the global demand. This implies that decreasing livestock production in one country could be more than compensated by increases in GHG emissions in another, leading to higher net GHG emissions worldwide.

Therefore, future studies should cover the entire food system’s GHG emissions with a more integrated approach, considering the interactions between diet, land use, fertilizers, and farming practices. Targeted strategies, such as improved fertilizer management and carbon sequestration, could help mitigate the negative effects of soil emissions. Livestock farming can play a key role in these practices, such as incorporating animal manure, agroforestry, and rotational grazing, contributing positively to tackling climate change.