Modern agriculture runs on nitrogen (N) fertilizers. While essential for feeding a growing population, their overuse pollutes waterways and releases potent greenhouse gases. A primary goal for sustainable agriculture is to improve Nitrogen Use Efficiency (NUE)—getting plants to grow more with less fertilizer. One promising tool is biochar which can be used to replace unsustainable peat in horticultural substrates. But how biochar helps plants has been unclear. A new study in Plant and Soil by Alvaro F. Garcia-Rodriguez and colleagues provides a clear answer: biochar acts less like a simple nutrient sponge and more like a metabolic stimulant, supercharging the plant’s own nitrogen-processing engine.

The researchers conducted a greenhouse experiment growing Batavia lettuce for 31 days in substrates containing different levels of vineyard-pruning biochar. The control group (B0) had no biochar (0:70:30, BC:peat:vermiculite), while two other groups had 15% (B15) or 30% (B30) of the peat replaced with biochar. The results were striking. Plants grown in the 30% biochar mix produced 44.2% more total biomass and had a 23.2% larger leaf area than the control plants. These plants also showed beneficial morphological changes, such as higher succulence (meaning denser, more water-rich leaves), all without any signs of physiological stress.

The most fascinating finding came from digging into the mechanism. The common theory is that biochar’s porous structure simply adsorbs and holds onto nitrogen, acting as a slow-release fertilizer. This study found the opposite. The B30 substrate and roots actually had lower concentrations of available inorganic nitrogen (both NO3−​ and NH4+​). This wasn’t because the plants were starved; it was because they were consuming and processing the nitrogen with incredible speed and efficiency. The B30 plants were pulling nitrogen from the substrate faster (a 31.8% increase in N uptake flux) and, once it was inside, were far better at converting it into growth.

The researchers found the “smoking gun” in the plant’s own metabolic machinery. The B30 lettuce showed a 34.8% increase in N Utilization Efficiency (NUT​E). This metabolic boost was directly linked to the enhanced activity of key enzymes responsible for N assimilation. The activity of Nitrate Reductase (NR)—the first critical step in the N-assimilation pathway—was significantly higher in B30 plants. Likewise, the enzymes further down the line, Glutamine Synthetase (GS), Glutamate Synthase (GOGAT), and Glutamate Dehydrogenase (GDH), all showed progressively higher activity with more biochar.

This coordinated upregulation of the entire enzymatic pathway means the B30 plants were rapidly converting raw nitrogen into valuable organic compounds. The proof was in the final product: despite having lower concentrations of raw N in their tissues, the B30 plants produced significantly more total soluble protein than the controls. The plants were not just accumulating N; they were using it more effectively to build protein. This study demonstrates that biochar is more than a passive soil conditioner; it’s an active enhancer of the plant’s physiological N metabolism. By stimulating the plant’s internal N-cycle, biochar offers a powerful strategy to boost crop productivity while optimizing fertilizer use for a more sustainable future.

Source: Garcia-Rodriguez, A. F., Moreno-Racero, F. J., Álvarez, R., Colmenero-Flores, J. M., Knicker, H., & Rosales, M. A. (2025). Biochar enhances nitrogen use efficiency in lettuce by promoting its metabolic assimilation. Plant and Soil.