Livestock, soil health, biochemical flows, and water quality

Unsustainable animal husbandry and feed production practices disrupt natural nitrogen and phosphorus cycles, releasing excess nutrients into ecosystems, leading to acidification, eutrophication, and biological damage. Protecting waterways from contamination by animal manure and fertilizers used in feed production is critical for ecosystem health. Manure leaching can also contaminate water with pathogens and antibiotic residues. In the EU, agriculture contributes to 7-8 million tonnes of reactive nitrogen losses annually, primarily as ammonia, nitrous oxide, and nitrates, with livestock responsible for over 80% of these losses. Plant-based foods have significantly lower nitrogen losses per unit of protein compared to animal-source foods, with beef having 25 times higher losses and monogastric meat, eggs, and dairy having 4-8 times more. However, the solution is not as simple as shifting from livestock to plant-based diets.

Reactive nitrogen losses in livestock production vary significantly depending on climate, rainfall, soil type, fertilizer use, livestock housing, and manure storage. For example, in the US, nitrogen losses per kg of beef carcass weight range from 100 g in the arid Southwest (driven by ammonia) to 300 g in the wetter Southeast (due to higher fertilizer use and nitrate leaching). Differences also exist between industrialized systems with high nutrient input and systems in low- and middle-income countries with nutrient undersupply. Soil type plays a crucial role in nitrogen leaching, with sandy soils typically having higher leaching rates than clay soils. The presence of macropores, such as those created by earthworms or plant roots, can also influence leaching by promoting faster flow. Additionally, nitrate leaching is affected by how water moves through the soil, with preferential flow leading to greater leaching compared to bypass flow, which leaves nitrates undisturbed in soil aggregates.

Policy suggestions to halve the production of animal-source foods to reduce nitrogen losses overlooks the broader impacts on public health, culture, and livelihoods. It also fails to address the environmental trade-offs associated with shifting to plant-based diets, including the impacts of ultra-processed foods, which contribute significantly to fertilizer use. Plant production, like livestock farming, can lead to environmental issues such as ecotoxicity, eutrophication, and acidification, depending on practices used. In regions like Brittany, livestock farming has not worsened nitrate contamination in water, while intensive crop areas have seen higher nitrogen leaching. As another example, horticulture in New Zealand uses significantly more nitrogen fertilizer than livestock farming, leading to higher nitrogen runoff. Reducing livestock without improving integration with crop systems could increase reliance on synthetic fertilizers, which are major contributors to greenhouse gas emissions. Manure, despite its challenges, provides valuable organic carbon, microbial diversity, and nutrients, making it a better fertilizer than synthetic alternatives for improving soil health and crop yields. Sustainable livestock management, integrated with crop agriculture, may offer a more balanced solution.

Livestock farming often disrupts nutrient cycles and contributes to water pollution, but these issues can be mitigated through better practices like improved feed strategies, precision feeding, and better manure management. Herd management, particularly for ruminants, offers additional opportunities to enhance nutrient cycling. Grasslands, which are highly efficient in biogeochemical cycling, benefit from the presence of herbivores that consume vegetation and return nutrients through manure. Multi-paddock grazing systems that mimic natural patterns can improve soil health, biodiversity, and water retention, as well as reduce runoff into waterways. While these practices can yield positive results, their effectiveness varies depending on context. Adaptation to local conditions is key to ensuring consistent and beneficial outcomes in grazing management.