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Nitrogen (N) leaching from coarse-textured soils frequently leads to productivity losses and negative environmental consequences. Historically, clay amendment has been used on coarse-textured soils to decrease water repellence and nutrient leaching. More recently, biochar has been proposed as an alternative soil amendment to decrease N leaching while simultaneously storing carbon. As biochar has a greater nutrient-retention capacity, we hypothesised that biochar derived from Eucalyptus marginata would be a more effective amendment than clay at minimising N leaching. The soil used was a coarse-textured agricultural sand with the following treatments: (1) biochar incorporated homogenously into the 0–10 cm soil layer, (2) clay incorporated similarly, (3) biochar added as a layer at 10 cm depth, (4) clay added similarly, or (5) a control. Amendments were added at 25 t/ha and watered periodically over 21 days and watered with the equivalent to 30 mm. Clay and biochar amendments significantly decreased cumulative NH4+ leaching by ~20% and NO3– leaching by 25%. Biochar decreased NO3– leaching significantly more than clay, possibly due to decreased nitrification. Dissolved organic N leaching was not influenced by any treatment. Leaching of N was unaffected by amendment application method. We conclude that to decrease N leaching, land managers should apply the most readily available of the amendments in the most convenient manner.

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Summary We investigated the impact of biochar application on fungal (acetate incorporation into ergosterol) and bacterial (leucine incorporation) growth rates in two case studies: a temperate UK pasture soil and a Mediterranean Australian agricultural soil. We added biochar at similar rates per unit of soil organic carbon (SOC) and monitored both the immediate (after 1 week equilibration) and longer‐term (1–3 years) effects. The immediate effect of the biochar applied to the UK soil was a decreased fungal‐to‐bacterial growth ratio, driven by greater bacterial growth. The immediate effect of biochar application to the Australian soils was subtle, only slightly increasing the fungal‐to‐bacterial growth ratio. In both case studies, the biochar effects were transient, and no long‐term effects (1–3 years) on microbial growth rates could be detected in either soil. The bacterial growth increase in the UK soil was probably related to a release of large amounts of labile C from the biochar, or as C released from the resident SOM caused by the biochar‐induced pH increase. The increase in fungal‐to‐bacterial growth ratio could be related to a release of poor quality C in the Australian soils. There were immediate effects of biochar application on microbial growth in agricultural soils, but they were disparate between cases, making any generalization of mechanisms difficult. However, the microbial responses were consistently transient. Taken together, biochar application to agricultural soil appears to have an impact upon the decomposer community, suggesting limited resistance. However, the microbial functioning appeared resilient to these effects, stabilizing microbial communities to their initial state within 1–3 years of application.