Xinyue Cao

The application of livestock slurry in soils can lead to nitrogen (N) losses through ammonia (NH₃) emission or nitrate (NO₃⁻) leaching. Oxidized biochar has great potential to mitigate N losses due to its strong adsorption capacity; however, the effects of oxidized biochar in different soils treated with slurry are currently unclear. Here, we investigated the effect of untreated and oxidized biochar (applied at a rate of 50 kg C m⁻³ slurry) on reducing N losses in a laboratory experiment with three different soils (loamy sand, sandy loam, loam) amended with cattle slurry at an application rate of 73 kg N ha⁻¹. Oxidized biochar reduced NH₃ emissions by 64–75% in all soils, whereas untreated biochar reduced NH₃ emissions by 61% only in the loamy sand. Oxidized biochar significantly reduced the NO₃⁻ content in the soil solution of the loamy sand in the early phase of the incubation and led to a significantly higher NO₃⁻ concentration in the same soil compared with the slurry‐only treatment at the end of the experiment, indicating a significant increase in NO₃⁻ retention in this organic C–poor soil. We conclude that oxidized biochar can reduce N losses, both in the form of NH₃ emission and NO₃⁻ leaching, from cattle slurry applied to soil, particularly in soil with soil organic carbon content <1% and pH <5 (i.e., oxidized biochar can serve as a means for improving the quality of marginal and acidic soils).

Nitrogen (N) losses during fertilization with livestock slurry, mainly in the form of ammonia (NH₃), can cause environmental problems and reduce fertilizer efficiency. Leonardite, which is characterized by oxygen‐rich functional groups and low pH, has been found to decrease losses of slurry N. However, leonardite, as a byproduct of open‐cast lignite mining, is not a renewable resource. The objective of this study was to modify biochar by chemical surface oxidation in order to find a sustainable but similarly effective substitute for leonardite. Biochar was produced from spruce sawdust in a pyrolysis oven at a maximum temperature of 610 °C. Then the biochar was oxidized using the Fenton reaction, with a ratio of Fe²⁺/H₂O₂ of 1:1,000, as a source of highly reactive HO· radicals to introduce oxygen‐rich functional groups to the biochar surface. The ammonium (NH₄⁺) adsorption capacity of biochar, oxidized biochar, and leonardite was tested in ammonium sulfate [(NH₄)₂SO₄] solution, pH‐adjusted (NH₄)₂SO₄ solution, and cattle slurry. The results showed that biochar had the highest total NH₄⁺ adsorption of 1.4 mg N g⁻¹ in (NH₄)₂SO₄ solution, whereas oxidized biochar had the highest reversible NH₄⁺ adsorption of 0.8 mg N g⁻¹. In the pH‐adjusted ammonium solution, all materials reduced NH₃ emissions by ≥90%, and oxidized biochar reduced NH₃ emissions by 99.99%. In contrast, leonardite reduced NH₃ emissions the most in cattle slurry, and oxidation of biochar increased the reduction in NH₃ emissions from 22 to 67% compared with non‐oxidized biochar. In conclusion, biochar oxidized by means of the Fenton reaction greatly decreased NH₃ emissions by increased adsorption of NH₄⁺ in cattle slurry compared with non‐oxidized biochar, indicating the great potential of oxidized biochar for reducing N losses during slurry application.

Excess nitrogen (N) after animal slurry application is a persistent problem of intensive agriculture, with consequences such as environmental pollution by ammonia (NH₃) and nitrous oxide (N₂O) emissions and nitrate (NO₃⁻) leaching. High-carbon organic soil amendments (HCAs) with a large C:N ratio have shown the potential of mitigating unintended N losses from soil. To reduce gaseous and leaching N losses after the application of slurry, a laboratory incubation study was conducted with silt loam soil. We tested the potential of three different types of HCA—wheat straw, sawdust, and leonardite (application rate 50 g C L⁻¹ slurry for each of the three HCAs)—to mitigate N loss after amendment of soil with pig and cattle slurry using two common application modes (slurry and HCA mixed overnight with subsequent addition to soil vs. sequential addition) at an application rate equivalent to 80 kg N ha⁻¹. Compared to the control with only soil and slurry, the addition of leonardite reduced the NH₃ emissions of both slurries by 32–64%. Leonardite also reduced the total N₂O emissions by 33–58%. Wheat straw reduced N₂O emissions by 40–46%, but had no effect on NH₃ emission. ¹⁵ N labeling showed that the application of leonardite was associated with the highest N retention in soil (24% average slurry N recovery), followed by wheat straw (20% average slurry N recovery). The mitigation of N loss was also observed for sawdust, although the effect was less consistent compared with leonardite and wheat straw. Mixing the slurry and HCA overnight tended to reduce N losses, although the effect was not consistent across all treatments. In conclusion, leonardite improved soil N retention more effectively than wheat straw and sawdust.