Antibiotic use in livestock production leads to a reduction in the decomposition rate of animal manures, posing a threat to effective soil carbon sequestration on farmland without more effective manure treatments before field applications.
Applying manure to farm soils is considered to be an effective way of maintaining soil organic carbon (SOC) stocks and mitigating climate change, with global analysis showing that on average such applications increase SOC by 35.4%.
Dangers in dung
Despite this promise, concerns have been raised about the possible negative impacts of applying manures from animals that have been treated with antibiotics to soils. Antibiotic accumulation can lead to worsening of antibiotic resistance and pose threats to the wider environment — and now, researchers from Southwest University in China say that it could also risk undermining soil carbon sequestration itself.
“Although the residual antibiotics can be greatly eliminated through proper pre-treatment, storage, and settling, a significant proportion of livestock manure that is not adequately treated applied directly to fields currently,” the scientists wrote in Journal of Environmental Sciences.
“Given that, it is critically important to assess the impact of residual antibiotics on the mineralization and nutrient release of livestock manure following their land application.”
Reduced decomposition poses problems
In their study, tested the application of cattle and pig manure collected from farms in Chongqing, China. They first used a multi-step process to ensure that antibiotic levels were not quantifiable, then mixed solutions of two antibiotics — oxytetracycline and ciprofloxacin — in small amounts with the manures. They then put the mix in mesh bags and buried them in soil at depths of 10-15cm to simulate farm soil application.
Although the experiment lasted a month in total, they collected four replicates of each treatment at five different time points and measured levels of antibiotics, the composition of the manures and the activity of microbes and soil enzymes.
They found that both types of antibiotics hampered decomposition and release of the vital nutrients, nitrogen and phosphorus. This could “influence nutrient supply for crop growth, affecting food production.”
Antibiotic exposure also affected carbon release from the manure, they found, as well as the activity of enzymes associated with the formation of organic matter in soils. Further, they saw that both antibiotics affected which microorganisms were present as well as their interactions, with both drugs decreasing the amount of gram-negative bacteria present but ciprofloxacin having a greater impact on microbial community structure overall.
The findings underline the need to eliminate antibiotic residues in manure with appropriate technologies before they are applied to fields, they concluded.
