Biochar enhancement of nitrification processes varies with soil conditions

Application of inorganic nitrogen (N) fertilizers in agriculture can increase emissions of nitrous oxide, a potent greenhouse gas, leaching of nitrate (NO3-), a groundwater contaminant hazardous to human health, and soil acidification. Soil amendment with biochar potentially mitigates these losses and undesirable outcomes. However, there have been considerable inconsistencies in reported impacts, likely owing to variable physiochemical characteristics of the biochar materials and/or the soil environment. This study methodically evaluated the impact of biochar soil incorporation on N transformation and underlying microbial processes using soils with varying biochar types, soil texture, soil moisture, and manure compost co- amendments. Laboratory incubations were conducted to monitor the fate of urea fertilizer N spiked in biochar amended and unamended soils by assaying soil ammonium (NH4+), nitrite (NO2-), and NO3- concentrations, pH, and abundances of soil nitrifiers; ammonia oxidizing bacteria and archaea (AOB and AOA) and Nitrospira with the capacity to perform complete ammonia oxidation (comammox). Soil moisture was a critical factor affecting N transformation processes, more so than biochar, but biochar did result in significantly different concentrations of N species in response to urea application. Biochar enhanced nitrification, more significantly in drier conditions and in sandy soil. Biochar offered some buffering potential in the neutral-alkaline, unsaturated soils, preventing >1 unit drop in pH compared to unamended soils. Co-application of biochar with manure composts enhanced nitrification slightly, which was evidenced by higher abundances of some soil nitrifiers at 4 weeks, although increases in nitrification rates were not statistically significant. Soil nitrifier populations tended to increase in response to a pinewood biochar, but trends differed for saturated soil, in soils of differing textures, or when different biochar materials were evaluated. Thus, when evaluating implications of biochar on the fate of mineral N fertilizer, soil moisture and other environment conditions should be considered.

Automated summary

Application of inorganic nitrogen fertilizers in agriculture can lead to increased emissions of nitrous oxide, nitrate leaching, and soil acidification. However, soil amendment with biochar has the potential to mitigate these negative effects. This study evaluated the impact of biochar soil incorporation on nitrogen transformation and microbial processes, taking into account the variability of biochar materials and soil conditions. The findings showed that soil moisture was a critical factor affecting nitrogen transformation, and biochar had a significant influence on nitrogen species concentrations and nitrification rates. The study also observed that the co-application of biochar with manure composts slightly enhanced nitrification. Overall, this research highlights the importance of considering soil moisture and environmental conditions when assessing the implications of biochar on the fate of nitrogen fertilizers.