In a recent study published in ACS Sustainable Chemistry & Engineering, authors Santanu Bakshi, Chumki Banik, David A. Laird, Ryan Smith, and Robert C. Brown present a novel solution to a major agricultural problem: the excessive and inefficient use of nitrogen (N) fertilizers. Nitrogen is vital for crop production, but complexities in the soil N cycle often lead to significant environmental harm, including water quality degradation and greenhouse gas emissions. The key challenge lies in the lack of synchrony between when N is released from fertilizers and when crops actually need it. This paper addresses the challenge by developing a biochar-based slow-release fertilizer (SRF), which reduces N loss and increases N use efficiency.

The researchers synthesized several SRF formulations using pine biochar activated with H3​PO4​ (phosphoric acid, at 5% and 15%) to increase its porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More and acidity. This modified biochar was infused with urea, along with calcium lignosulfonate (as a binder) and paraffin wax (to add hydrophobicity and inhibit rapid dissolution). Characterization analyses confirmed that urea was successfully incorporated into the biochar, forming hydrogen bonds with the functional groups and utilizing the biochar’s pores for physical trapping. This intricate structure was hypothesized to stabilize the urea and NH4+​ ions, thereby regulating the nutrient release.

The performance of the developed SRFs was tested in laboratory settings against uncoated urea (the control) using aqueous dissolution tests, soil column leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More studies, and ammonia volatilization experiments. The results demonstrated a dramatic improvement over conventional urea. In accelerated aqueous release tests, while nearly all of the uncoated urea (99.6%) dissolved within just 12 minutes, the SRFs showed significantly slower release, with some formulations releasing only 61.2–90.2% of the added urea even after a prolonged 4,320 minutes. This slow release is primarily due to the paraffin wax restricting water diffusion and the urea being physically trapped and chemically bonded within the biochar’s structure.

The real-world implications were highlighted in the soil column study, which simulated the effects of rainfall. The uncoated urea quickly leached out, losing 99.5% of its mass after only four leaching events. In sharp contrast, the most effective SRF compositions (1:4 biochar:urea with 5% acid and 1:3 with 15% acid) lost only 65.4–66% of the added urea after 18 leaching events. In terms of total added nitrogen, the uncoated urea column lost 99.6% within those first four events, while the optimized SRFs demonstrated an N release rate more than 12 times slower. Crucially, the soil columns treated with SRFs retained NH4+​-N concentrations several times higher than the control columns, which suggests the SRFs successfully prolonged the availability of plant-essential ammonium. Furthermore, the SRFs significantly reduced ammonia (NH3​) volatilization loss by approximately 3–4 times compared to the uncoated urea in a greenhouse experiment, with the SRFs losing only 0.27–0.78% of applied N versus 2.24% for the control.

The production cost of the SRF (e.g., the 1:3 formulation) is estimated to be 1.75–1.9 times higher than uncoated urea. However, conventional agriculture typically loses an average of 40% of urea-N, with losses sometimes reaching 78%. Given the SRFs’ proven ability to significantly reduce these major losses, the enhanced N use efficiency has the potential to offset the higher manufacturing cost, making them economically viable. Beyond economics, the SRFs offer crucial environmental benefits: reducing the risk of eutrophication by limiting N runoff and providing a long-term role as carbon sequestration agents due to the recalcitrant nature of biochar. Further field studies are warranted to fully quantify N use efficiency and residual N reserves.

Source: Bakshi, S., Banik, C., Laird, D. A., Smith, R., & Brown, R. C. (2021). Enhancing Biochar as Scaffolding for Slow Release of Nitrogen Fertilizer. ACS Sustainable Chemistry & Engineering, 9(24), 8222–8231.